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Mechanism of Silurian hydrocarbon pool formation in the Tarim Basin

Mechanism of Silurian hydrocarbon pool formation in the Tarim Basin Petroleum Science VolA NoA Liu Luofu'<, Chen Yuanzhuang ', Chen Lixiu'r', Pang Xiongqi'<, Xie Qilai ,2, Huo Hong ,2, Zhao Suping'r', 1 1 Li Chao ,2, Li Shuangwen'<, Guo Yongqiang'<, Zhao Yande ,2 and Li Yan ,2 (1. Key Lab for Hydrocarbon Accumulation Mechanism, Ministry ofEducation, China University ofPetroleum, Beijing 102249, China) (2. Basin and Reservoir Research Center, China University ofPetroleum, Beijing 102249, China) (3. Exploration and Production Research Institute, Sinopec, Beijing 100083, China) Abstract: There are three formation stages of Silurian hydrocarbon pools in the Tarim Basin. The widely distributed asphaltic sandstones in the Tazhong (central Tarim) and Tabei (northern Tarim) areas are the results of destruction of hydrocarbon pools formed in the first-stage, and the asphaltic sandstones around the Awati Sag were formed in the second-stage. The hydrocarbon migration characteristics reflected by the residual dry asphalts could represent the migration characteristics of hydrocarbons in the Silurian paleo-pools, while the present movable oil in the Silurian reservoirs is related to the later-stage (the third-stage) hydrocarbon accumulation. Key words: Oil-gas pool, stage of pool formation, petroleum migration, mechanism of pool formation, model of pool formation, Silurian, Tarim Basin where the early formed oil pools suffered oxidation, 1. Introduction bio-degradation, water-washing and deasphalting, and The Tarim Basin in northwestern China covers an asphalts were formed. Therefore, it is difficult to study 4 2 area of 56x 10 km and possesses a large amount of the mechanism and models of the formation of Silurian hydrocarbon resources (Fig. I), and now is the main pools, and integrated methods should be used in order place for oil and gas exploration in China. The Tarim to provide scientific data for hydrocarbon exploration Basin is a multi-cycle superimposed basin, and during of the Silurian in the Tarim Basin. the geological periods, it underwent tectonic 2. Distribution characteristics of Silurian oil destruction for multiple times and thus its strata and gas in the Tazhong area distributions and tectonic characteristics are extremely complicated. In the Tazhong area, 57 exploratory wells have At present, the discovered oil and gas fields are been targeted at the Silurian, and there are 8 with mainly located in the areas of cratonic basin (the commercial oil, 6 with low-production oil, 19 with Tazhong area in central Tarim and the Tabei area in oil and gas shows, and 14 with asphalt shows. The northern Tarim). The Tarim Basin has several sets of five discovered oil pools are mainly of faulted petroliferous horizons, one of which is the Silurian. anticline type, and secondarily of lithologic type The Silurian strata are distributed in the North Uplift, controlled by the structural background. The Silurian Manjiaer-Awati Sags and the Central Uplift, and the petroliferous region in the northern slope of the distribution area is ca. 24x 10 km'. In recent years, the Tazhong area covers an area of 6,000 km". The Silurian is the focus of exploration, and a breakthrough geology of hydrocarbon pool formation in the of exploration has been made in the Tazhong area and Silurian in the Tazhong area is characterized by large five commercial oil pools, including Tazhong 11, distribution area, low hydrocarbon abundance and Tazhong 47, Tazhong 12, Tazhong 50 and Tazhong structural control. These Silurian oils and gases are 15-16, have been found, which demonstrates that the mainly distributed in favorable traps in TZ lOWell Silurian of the Tarim possesses a large petroleum structural belt, being apparently controlled by the potential. The Silurian reservoirs are characterized by faults (Fig. 2). Vertically, these oils and gases mainly multi-stage hydrocarbon migration, accumulation and occur in the 1st and 3rd sub-members of the upper pool formation, resulting in a variety of models of pool member of the Kepingtage Formation, being formation and coexistence of different forms of apparently controlled by the regional cap rocks of the hydrocarbon (asphalt, heavy oil, conventional oil, light lower member of the Tataaiertage Formation and the oil and gas). Influenced by Caledonian movement, the 2nd sub-member of the upper member of the Tarim Basin uplifted and the Silurian strata in the Kepingtage Formation (Table 1). The vertical reservoir Tazhong and Tabei areas were weathered and denuded, heterogeneity is serious, and oil and gas shows are 2007 2 Petroleum Science Imi")I [?j] I~"\I l._. -JL·· [Q] "-...G ~...-' Range ofthe Range of source Range of source Range of source Division line of Silurian asphaltic rock ofthe later rock of the later rock of the structural units sandstone Hercynian Yanshan-Himalayan Caledonian [Z] I~I Silurian movable Oil migration Oil migration Hydrocarbon migration oil pool direction ofthe direction of the direction of the later Caledonian later Hercynian Yanshan-Himalayan a-Tazhong 47 oil pool; b-Tazhong II oil pool; c-Tazhong 15-50 oil pool; d-Tazhong 16 oil pool Fig. 1 Distribution of hydrocarbon resources of Silurian in the Tarim Basin mainly distributed in the reservoir intervals with good destroyed oil pools which were formed by the expelled physical properties. The better the physical properties of hydrocarbons generated from the Cambrian source a reservoir, the more common are oil and gas shows. rocks of the Awati Sag during the Hercynian. The The horizontal stratigraphic correlations indicated that source kitchens of different stages of pool formation in the Silurian oil and gas shows in the western part of the the Tarim Basin are shown in Fig. 1. In the Tarim Basin, northern slope are mainly within the 3rd sub-member of two sets of commercial marine source rocks, the the upper member of Kepingtage Formation, and lower-middle Cambrian and the upper-middle secondarily within the 1st sub-member of the upper Ordovician, with a thickness of 5-7 km, can be member of the Formation, whereas in the higher part of identified from the marine Cambrian and Ordovician the northern slope (towards east), the Silurian oil and series. The correlation between oil and source rock gas shows are mainly located in the 1st sub-member, demonstrated that the preserved commercial and secondarily in the 3rd sub-member (Fig. 3). hydrocarbons of marine origin in the Basin were mainly from the marls of the middle-upper Ordovician (Liang, 3. Source rocks of the Silurian hydrocarbons et aI., 2000). By analyzing a large number of GC-MS in the Tazhong and Tabei areas and GC-MS-MS data, Li, et al. concluded that the source rocks and oils of the Cambrian-lower Ordovician It is commonly acknowledged that the Silurian possess the following characteristics: high contents of asphaltic sandstones in the Tazhong and Tabei areas are dinosteranes, 4-methyl-steranes, C , 24-norcholestanes, the remnants of destroyed oil pools which were formed 26 C , 24-nordiacholestanes, C steranes and by the expelled hydrocarbons generated from the 26 28-regular gammaceranes, but a low content of diasteranes, while Cambrian source rocks of Majiaer Sag during the late Caledonian, and similarly the Silurian asphaltic the source rocks and oils generated from the sandstones around the Awati Sag are the remnants of middle-upper Ordovician are just the opposite (Li, et al., VolA NoA Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin 1999; Li, 2000) (Table 2). According to these data, Li, middle-upper Ordovician, but the oils of a few oil pools et al. (1999) proposed the parameters of source rock came from the Cambrian-lower Ordovician source type division and oil derivation, and also made sure that rocks. the main manne source rocks discovered are the Table 1 Strata division of the Silurian in the Tarim Basin Strata Reservoir-cap Oil rocks Lithological column Lithologic description shows System Series Formation Member combination .- '" c: ::: ILM of ... 8~ GR ILO Mainly brown-red and green mudstones and silty Yimugantawu Cap rocks Middle mudstones, interbedded with Formation thin-layer gray and brown siltstones and fine sands c: Mainly light gray and brown­ .~ Upper red fine sands, interbedded with brown silty mudstones and Reservoir rocks E member ::; - mudstones t: <l) .~ r- Lower Mainly middle-thick Cap rocks member layer brown mudstones ~: Silurian Lower 1st Light gray siltite and fine Reservoir --.:i 5200 ~~ ... : sub­ sands, interbedded with silty rocks ..r - member mudstones and mudstones ~ ...... * 2nd Mainly gray and graygreen Cap rocks sub­ mudstones 2 member 5300 - Mainly light gray siltite and fine sand interbedded with Reservoir gray -green silty mudstones rocks and mudstones Gray and green-gray mudstones, interbedded with green-gray thin layer argillaceous siltstones 55001-:-· .. 'u .~ Upper "E o Petroleum Science 4 2007 TZ49\i> II» TZ 33 TZ32 @ OTZ9 III Well with Well with Well with low-prodution hydrocarbon industrial oil shows oil Dry well Well absent Well with of Silurian asphalt Pinchout line Igneous rock Fault of Silurian [§] Oil migraton directon Oil migraton directon oflater Caledonian ~of y anshan-Him~aYan 1-I4-methyl 1,8-/2,4-dimethyl 1,8-/Nex's Trimethyl carbazole(Al/(Cl carbazole carbazole dimethyl carbazole Fig.2 Distribution ofthe Silurian hydrocarbon pools of the Tazhong area in the Tarim Basin Elevation, m TZI5 TZl6 TZJ61 Elevation, m TZ47 well -2700,----t-------+----;------, -3850 -r-,---=r-IT:::::::=:::=-;----, -3900 ·2900 -4000 ·3 J00 j::::-......;;;t::::::;;;I' ·3300 -4100 A B Elevation. m TZJ2 TZ50 Elevation, mTZlITZl17 TZlll TZl12 -3200r------:;;::F:=---- --;----~ -3200 -3300 -3400 -3500 ·3350 c D CDLower member of the Tataaiertage Formation; ®Upper member of the Kepingtage Formation; ®the Yimugantawu Formation; ®the Tataaiertage Formation; @lst sub-member of the upper member of the Kepingtage Formation; ®Upper member ofthe Tataaiertage Formation; (1)2nd sub-member of the upper member of the Kepingtage Formation; @3rd sub-member of the upper member ofthe Kepingtage Formation Fig. 3 Sections of hydrocarbon reservoirs of the Silurian in the Tazhong area in the Tarim Basin Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 5 Table 2 Characteristics of biomarkers of oils from different source rocks in the Tarim Basin (Li, et aI., 1999) Strata Cambrian-lower Ordovician source rocks Middle-upper Ordovician source rocks Higher in dinosteranes, Dino/(Dino+3-M) ~ 0.2 Lower in dinosteranes, Dino/(Dino+3-M)< 0.15 Higher in 4-methyl-steranes, 4-M/(4-M+3-M) ~ 0.35 Lower in 4-methyl-steranes, 4-M/(4-M+3-M) < 0.25 '" .~ Higher in C 24-/(24-+27-) > 0.3 Lower in C 24-norcholestanes, 24-/(24-+27-) < 0.2 .~ 26,24-norcholestanes, 26, ... Higher in C 24- nordiacholestanes, diaC Lower in C 24- nordiacholestanes, diaC diaC 26, 24-/(diaC 24-+ 24-/( 24-+ 26, ..<:: o diaC > 0.25 diaC < 0.2 27-) 27-) ... Q) ~ Higher in gammaceranes, gammaceranes/Cj, H> 0.1 Lower in gammaceranes, gammaceranes/Cj-H < 0.05 OS Higher in C steranes, Lower in C steranes, 28-regular 2s-regular C28/(C27+C28+C29) > 20% C28/(C27+C28+C29) < 20% Lower in diasteranes, diaC < I Higher in diasteranes, diaC > 2 27/regC27 27/regC 27 In this study, all the Silurian movable oil also generated from the Cambrian-lower Ordovician. samples of the Tazhong area were analyzed by By comparison with the above-mentioned oils, the GC-MS, and some of them by GC-MS-MS. The Silurian oil samples from TZ 16 Well (4,125-4,129 results demonstrated that some of the Silurian oils m), TZ 30 Well (4,244.5-4,260 m) and TZ 50 Well originated from the Cambrian-lower Ordovician (4,378-4,385 m), which contain relatively low source rocks, some of them from the middle-upper contents of dinosteranes and 4-methyl-steranes, are Ordovician source rocks, and some from both. Four thought to be from both of the Cambrian-lower Silurian oil samples, which were collected from the Ordovician and middle-upper Ordovician source Tazhong area, TZ 11 Well (4,301-4,307 m), TZ 16 rocks (mixed oils). The oil at the depth of Well (4,125-4,129 m), TZ 30 Well (4,244.5-4,260 m) 4,417-4,435 m of TZ 11 Well are typically and TZ 50 Well (4,378-4,385 m) respectively, are characterized by very low contents of dinosterane and 4-methyl-sterane, and it was generated characterized by higher gammaceranes, higher C steranes, and lower diasteranes (Table 3), obviously from the middle-upper Ordovician source 28-regular rocks. The biomarkers of different oils are shown in and these oils are considered to originate from the Cambrian-lower Ordovician source rocks. These Fig. 4. four Silurian oils, together with an Ordovician oil 4. Stages and characteristics of the formation sample from TZ 16 Well (4,248-4,268 m) and a Silurian oil from TZ 11 Well (4,417-4,435 m), were of Silurian hydrocarbon pools in the basinal also analyzed by GC-MS-MS, and the results facies areas of the Tarim Basin showed that the Silurian oil from TZ 11 Well (4,301-4,307 m) possesses the following Some scholars have done much work on formation characteristics: high content of dinosteranes mechanism of asphaltic sandstones (Evans, et al., 1971; [Dino/(Dino+3-M)=0.29]; high content of 4-methyl­ Bailey, et al., 1973; Lafargue and Barker, 1988; Price, steranes [4-M/(4-M+3-M)=OAO]; and high content 1980). Also, much work on the formation stages of of C26,24-norcholestanes [24-/(24-+27 -)=0.69]. asphalts, heavy oils and normal oils, which widely Based on these characteristics, it is certain that the occur in the Silurian in the Tarim Basin, has been oil at the depth of 4,301-4,307 m of TZ 11 Well, carried out by Chinese scholars (Huang and Liang, 1995; which is light oil formed in the later stage, was Xiao, et al., 1997; Lu, et al., 1996; Lu, 1997; Liang, et al., 1998; Liu, et al., 2000a; 2000b; 2001; Li, et al., generated from the Cambrian-lower Ordovician 2002). In this study, the following methods have been source rocks. The Ordovician oil at the depth of 4,248-4,268 m of TZ 16 Well contains a large used to determine the formation stages of the Silurian amount of dinosterane and 4-methyl-sterane and was hydrocarbon pools. Petroleum Science 6 2007 Oil sources TZII(S,4301-4307m) TZ 16(0,4248-4268m) Oil sources £--0, TZ 16(S,4l25-4129m) Oil sources £~O, and 0'+1 Oil sources £-0, and 0,.1 TZ30(S,4244.5-4260m) Oil sources £-0, and 0'+1 Oil sources TZll(S,4417-4435m) o., I =4u,23,24-trimethylcholestane 3=3a-methyl-24-ethylcholestane 4=4u-methyl-24-ethylcholestane 2=413-methyl-24-ethylcholestane Fig.4 GC-MS-MS analytic results of the Silurian oils of the Tazhong area Table 3 Biomarker parameters of the Silurian oils of the Tazhong area Depth Gammaceranes Diasteranes Regular steranes Determination of oil Well No. m IC diaC C28/(C27+C2S+C29) source rock 30H 27/regC27 TZll 4301-4307 0.25 0.15 23.5% TZ16 4125-4129 0.17 0.48 59.6% TZ50 4378-4385 0.19 0.55 20.3% TZ30 4244.5-4260 0.27 0.73 23.4% 4248-4268 TZ16 (Ordovician oil) TZll 4417-4435 Notes: C C diaC "F C diasteranes; regC regular steranes 30H= 30Hopane; 2 27 27=C27 Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 7 4.1 Determination of the formation stages of the (Table 4), and these inclusions are distributed within Silurian movable oil pool by use of the fluorescence quartz-dissolved fissures and secondary overgrowth spectrum parameters and homogenization margins of quartz. While the fluorescence of temperature of hydrocarbon inclusion in hydrocarbon inclusions at other depths is in yellow tone combination with sedimentary burial history (Amax=600-640 nm) and they occur in the later stage There are two kinds of fluorescence spectra for the pores of quartz grains. In light of the fluorescence hydrocarbon inclusions in TZ 117 Well, i.e. green-dark spectrum characteristics and the hydrocarbon inclusion green and light yellow-yellow-bright yellow, with occurrences, we conclude that there were at least two yellow being the major one. The hydrocarbon inclusions stages of hydrocarbon migration and charging occurring fluorescence at the depth of 4,417.80 m and 4,436.10 m in this area, which took place mainly in the later stage of TZ 117 Well is in green tone (Amax=520-540 nm) with normal oils. Table 4 Fluorescence spectrum parameters of the Silurian hydrocarbon inclusions in TZ 47 Well and TZ 117 Well Depth, m Q6501500 Tone Spectrum characteristics Types Well No. Amax,nm Yellow Double peaks Gas + Liquid 4892.23 600 1.54 TZ47 4992.83 630 1.32 Yellow Double peaks Gas + Liquid 4415.20 640 1.77 Yellow Double peaks Liquid 0.72 Green Sharp single peak Liquid 4417.80 540 4421.25 590 1.96 Yellow Double peaks Liquid Light yellow Liquid 4431.64 640 1.78 Multiple peaks TZ117 4431.64 620 1048 Bright yellow Multiple peaks Liquid 4436.10 520 0.39 Deep Green Sharp single peak Liquid 4441.42 Yellow Double peaks Liquid 630 1.34 4453.00 640 1.68 Yellow Double peaks Liquid group, more than 150°C, is actually non The analysis results of the hydrocarbon inclusions of homogenization temperatures, and it is necessary to the Silurian asphaltic sandstones at the depth of 4,419.2-4,447.5 m of TZ 11 Well indicate that the point out that at the end of the Silurian the hydrocarbon asphaltic sandstones and the oil sands contain rich charging (the first hydrocarbon migration) could not hydrocarbon inclusions, and the homogenization form any hydrocarbon inclusions because the burial temperatures of these inclusions can be divided into three depth was shallow. groups. The first group ranges from 62 to 74°C and its Table 5 Hydrocarbon inclusion homogenization average is 67°C. temperatures of the Silurian asphaltic sands and oil sands in Based on the assumption that the temperature of the TZ11 Well of the Tarim Basin paleo-surface of the Tazhong area is 15°C and its Depth Hydrocarbon inclusion homogenization average paleogeothermal gradient is 3 °C /100m, the temperature, ·C depths of reservoir beds when the second hydrocarbon 4419.2 62,64,65,63.5 migration took place should range from 1,566 to 4423.7 72,74, 103 1,966m then, which correspond to the burial depth of the upper member of the Silurian Kepingtage Formation 4428.6 62,66,68 from the Permian to the end of the Triassic. At that time, 4433.0 125,135,155,188,>200 the migrating hydrocarbon was mainly normal oil. The 4447.5 67,69,155,>150 second group ranges from 100 to 135°C and it corresponds to the present formation temperatures of the upper member of the Kepingtage Formation, which The Silurian hydrocarbon inclusions of TZ 47 Well, represents the temperatures of the third-stage with yellow fluorescence (Table 4) and red/green color hydrocarbon migration of mainly light oil. The third ratio (Q6soIS00) more than 1, are normal mature oil, and Petroleum Science 8 2007 activities, which was possibly related to the Permian they indicate that at least one episode of hydrocarbon migration took place in this area. The size of these igneous rock intrusions. In this study, the formation inclusions varies considerably and their shapes mainly temperatures of the analyzed hydrocarbon inclusions contain rice-grain, circle, ellipse, polygon and irregular from TZ 47 Well are 100°C, 110°C, 125°C, 130°C and forms. Most of these inclusions occur in quartz, calcite, 140°C, respectively, which accord with the above two gypsum and cement, along the suture lines, in beaded groups of temperatures of salt-water inclusions of TZ 47 style, which demonstrates that the micro pathways of Well (l00-120°C and l20-140°C), and these data the hydrocarbon migration are microfractures. The indicate that in the two main temperature intervals large major homogenization temperatures of the salt-water scale hydrocarbon migration occurred. Based on the inclusions of TZ 47 Well contain three groups, data above and combined with the sedimentary burial 100-120°C, 120-l40°C and 160-210°C, among which history, one can conclude that the formation of Silurian the group with the highest temperature (l60-210°C) hydrocarbon pools of TZ 47 Well had two stages: the reflects that the Silurian reservoirs in TZ 47 Well were late Hercynian-IndoChinese stage and the Himalayan stage (Fig. 5). affected by the deep-seated hydrothermal liquid O"'r\·· ..·..·..A 60"C\········· \ v"' .. , ~~ 80·C\··.... \ '::":':=-~: ~.,,~~-=>, 100"C .... 120"C .. 140·C\ .... ·........···,,>==-c.... ... E 4000 160'C 180"C ..c OJ a 6000 600 500 400 300 200 100 o Age, Ma Fig. 5 Characteristics of thermal evolution and determination of hydrocarbon pool formation stage in the Silurian ofTZ 47 Well in the Tazhong area 4.2 Determination of the formation stages of the 117 Well at the depths of 4,300.13 m and 4,408.62 m, Silurian movable oil pool by use of GC data of respectively, for the study of the Silurian hydrocarbon different hydrocarbon components in the Silurian pool formation. The oil at the depth of 4,300.13 m, reservoirs charged in the first-stage (hydrocarbon inclusion and Pan, et al. (2000) carried out a study on the formation bound hydrocarbon), is normal oil, with maximal GC stages of hydrocarbon pools, hydrocarbon sources and peak being nC , content of high carbon number alkanes charging stages based on GC data of different comparatively high (nC and the w/nC22+=0.74-0.66), hydrocarbon components of the reservoirs, and achieved similar distribution characteristics and rich contents of good results. Using the same method, we analyzed the terpane and sterane (Fig. 6). The carbon numbers with reservoir samples from the Tazhong Silurian sandstones. highest abundance' of hydrocarbons charged in the The biomarkers of different hydrocarbon components, second-stage (free state) are nC , and the content of I 5 WnC the fluid inclusion homogenization temperatures of the lower carbon number alkane in this oil is comparatively reservoirs and the fluorescence spectrum parameters of high (nC Compared with the first-stage 21-/nC 22+=8.5l). organic inclusions in the analyzed 16 samples of the hydrocarbons, the distribution characteristics and Silurian oil pools indicate that the formation of Silurian contents of terpane and sterane of the second-stage have pools in the Tazhong area has mainly two stages, and a distinct difference. The characteristics of the oil sample only a minority of pools showed one stage of at the depth of 4,408.62 m are similar to those of the hydrocarbon charging (Table 6). sample at 4,300.13 m, and the former also shows two Two sandstone samples were collected from stages of hydrocarbon charging. The characteristics of oil-production intervals of the Silurian reservoir of TZ alkane, terpane and sterane of each stage of hydrocarbon Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 9 Table 6 Determination of the stage of hydrocarbon charge into the Silurian reservoirs of the Tazhong area by use of GC data Well No. Strata* Depth, m Oil properties Hydrocarbon charging 3770.1 Heavy oil Single stage TZ4 3819.55 Normal oil TZ6 Two stage 4303.18 Light oil Two stage rzn 4447.70 Heavy oil Two stage TZ12 4409.31 Heavy oil Single stage 4126.1 Heavy oil Two stage TZ16 ® TZ20 4699 Heavy oil Two stage TZ30 4257.5 Heavy oil Two stage TZ31 4594.5 Heavy oil Single stage TZ32 3792.80 Heavy oil Two stage TZ35 4945.17 Heavy oil Two stage Carboniferous 4389 Light oil Two stage TZ47 4980040 Light oil Two stage 4300.13 Light oil Two stage TZll7 4408.62 Heavy oil Two stage TZ161 4225.7 Heavy oil Two stage Notes: ® 1st sub-member of the upper member ofKepingtage Formation; ® 3rd sub-member of the upper member ofKepingtage Formation charging for the sample of 4,408.62m are similar to those 5.1 Migration direction of hydrocarbons in the for the sample of 4,300. 13m. This result of two-stage Silurian in the Tazhong area charging is in agreement with the conclusions made from the above-mentioned analysis based on hydrocarbon (1) Hydrocarbon migration direction during the inclusion fluorescence. first-stage pool formation at the end of Silurian in the Tazhong area 5. Migration directions of hydrocarbons in the During the formation of Silurian paleo-pools in the Silurian reservoirs Tazhong Uplift at the end of the Silurian, the There are three formation stages of Silurian hydrocarbons were mainly generated from the hydrocarbon pools in the Tarim Basin. The widely middle-lower Cambrian source rocks in the Manjiaer distributed asphaltic sandstones in the Tazhong and Sag, and migrated towards the southwest-south, Tabei areas are the results of destruction of hydrocarbon entering the Tazhong Silurian reservoirs first. Then, pools formed in the first-stage, and the asphaltic controlled by the fault distribution and structural shapes, sandstones around the Awati Sag were formed in the the hydrocarbons further migrated along the high second-stage. The hydrocarbon mig ration structural positions of the Tazhong No.1 Faulted characteristics reflected by the residual dry asphalts Structural Belt, the TZ 10 Well Structural Belt and the could represent the migration characteristics of Central Faulted Horst Belt, along unconformity surfaces hydrocarbons in the Silurian paleo-pools, while the or within the Silurian reservoir beds from northwest to present movable oil in the Silurian reservoirs is related southeast towards the Silurian up-dip direction in the to the later-stage (the third-stage) hydrocarbon Tazhong area, namely, from TZ 23 Well, TZ 37 Well to accumulation. TZ 4 Well, from TZ 47 Well, TZ 11 Well to TZ 15 Well, Petroleum Science 10 2007 GC graphs of saturates of different compositions Free state nC" Mass chromatograms of terpanes of different compositions 29HC,,,H Free state Bound state Hydrocarbon inclusion Mass chromatograms of steranes C pregnane of different compositions Free state C pregnane C pregnane Hydrocarbon inclusion Fig.6 GC of saturates and mass chromatograms of terpanes and steranes in the Silurian oil at the depth of 4300. 13m of Tazhong 117 Well in the Tazhong area and from TZ 67 Well, TZ 31 Well, TZ 30 Well to Well Manjiaer Sag to the Tazhong area (Fig. 1). However, TZ 44. In addition, hydrocarbon migration occurred probably, a few hydrocarbons, generated from the also from the Manjiaer Sag to the Silurian pinchout Cambrian source rocks in the Tazhong area, migrated boundary in the Tazhong area in the SWS direction, vertically through faults up to the Silurian reservoirs in namely from TZ 32 Well to TZ 44 Well (Fig. 2). a short distance. The above understandings of Generally speaking, horizontally, the oil and gas hydrocarbon migration directions were obtained from migration is characterized by a long distance from the the distribution characteristics of nitrogen compounds Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 11 extracted from asphaltic sandstones. The ratios of benzocarbazole [a] and alkyl carbazole migrated faster trimethyl carbazole (A)/(C), benzocarbazole [a]/[c] and than trimethyl carbazole (C), benzocarbazole [c] and alkyl carbazole/(alkyl carbazole +benzocarbazole) of benzocarbazole, respectively (Liu and Xu, 1996; Liu the Silurian asphaltic sandstone extracts of the Tazhong and Mao, 1996; Liu, et al.., 1997; 1998; Liu and Kang, area are comparatively high, and they are 2.9-34.7, 1999; Liu, 1997; 1998; Li, et al., 2000; 2001; Chen, et 0.6-5.3 (average 1.6) and 0.4-1.0 (0.85 for most al., 2004). Therefore, these three indices increase with samples), respectively, and these figures are much increasing migrating distance (Fig. 7). Also, in light of higher than those ratios of the oils from Putaohua oil the total contents of carbazole-type compounds, it could layer and Heidimiao oil layer in Xinzhan Oilfield of the be clearly shown that the hydrocarbons of the Silurian Songliao Basin, northeast China where the paleo-pools of the Tazhong area migrated from hydrocarbons had a short migration distance [trimethyl northwest to southeast, because the amounts of the carbazole (A)/(C), benzocarbazole [a]/[c] being 1.3-1.7, carbazole-type compounds, which have larger polarity et al., 2000)]. Because of than saturated and aromatic hydrocarbons, decrease 0.95-1.2 (average 1.1) (Wang, with increasing migrating distance (Fig. 8). the differences in molecular polarity, shape and size, during hydrocarbon migration, trimethyl carbazole (A), 8 30 TZ23 TZ37 TZ4 TZ47 TZllTZ15 lZ67 TZ31 TZ30 TZ44 TZ32 Fig. 7 Average values of3-methylcarbazoles (A)/3-methylcarbazoles (C) of the Silurian dry asphalts in different wells of the Tazhong area 2: o 14 u.J ~ 12 ::i. if 10 0- ~ 8 "0 ~ 6 0; 4 .... 2 ~O+-L,-----r--I"".---r-"'''''''''L..,--L.r--''''''''''---.---.........,..-L., ;: TZ23 TZ27 TZ4 TZ47 TZII TZ15 TZ67 TZ31 TZ30 TZ44 TZ32 Fig. 8 Average values of total carbazole-type compounds contents of the Silurian dry asphalts in different wells of the Tazhong area (2) Hydrocarbon migration direction during the contents of the movable oil pools which were formed during the third-stage hydrocarbon charging, it can be third-stage pool formation in Yanshan-Himalayan stage concluded that the hydrocarbon migration directions in the Tazhong area and pathways of the present movable oil pools are Based on the study of carbazole-type compound Petroleum Science 12 2007 basically the same as the ones of the first-stage pool amount of hydrocarbons. At the end of the Permian, formation in the Tazhong area. Former researches when the Silurian traps were charged and the showed that the content parameter of carbazole-type paleo-pools in the marginal areas around the Awati Sag compounds and the indices of trimethyl carbazole were formed, hydrocarbons were mainly generated and (A)/(C), 1-/4-methyl carbazole, 1,8-/2,4-dimethyl came from the lower-middle Cambrian source rocks in carbazole and 1,8-INEX's-dimethyl carbazole are useful the Awati Sag. Investigation of nitrogen compounds indicators for determining migration direction. indicates that the hydrocarbons from the source center Generally, the content of carbazole-type compounds migrated along the favorable pathways towards the decreases with the increase of migration distance, marginal areas of the Awati Sag in all directions within whereas the latter four ratios increase with the increase the Silurian reservoir beds, and accumulated to form of migration distance. The results of this study hydrocarbon pools in the structural highs or favorable demonstrated that the migration of the Silurian movable traps (Fig. 1). These hydrocarbon pools rose up to the oils in the Tazhong area is mainly in the lateral direction. surface and the present widely distributed dry asphaltic By making use of the above-mentioned indices, it can sandstones were formed due to the late Hercynian be seen that the later-stage (the third-stage) movement. hydrocarbons migrated from northwest to southeast, 6. Models of hydrocarbon pool formation and mainly from Tazhong 47, Tazhong 35, Tazhong 111, prediction of favorable oil-gas accumulation Tazhong 30 to Tazhong 16 Wells (Fig. 2). However, a areas of the Silurian distinct vertical migration occurred for the Silurian On the basis of the theory and methods of effective movable oil in the Tazhong area. That the indices of source rock, the amounts of hydrocarbon generated and trimethyl carbazole (A)/(C), 1-I4-methyl carbazole, expelled from the marine source rocks in the Tarim 1,8-12,4-dimethyl carbazole and 1,8-INEX's dimethyl basinal areas were calculated in this project. The result carbazole increase with decreasing sampling depth tells that during the whole geological ages, the reflects the vertical migration of hydrocarbon. cumulative amount of hydrocarbons expelled from the 5.2 Migration direction of hydrocarbons during the Cambrian-Ordovician source rocks is 4,301 x 10 t, first-stage pool formation at the end of the Silurian 8 8 x10 xI0 among which oil is 1,670 t and gas is 2,631 in the Tabei area tons oil equivalent. In the total hydrocarbon expulsion In the Tabei area, during the first-stage pool amount, contributions of the lower-middle Cambrian formation, the hydrocarbons also experienced a and middle-upper Ordovician source rocks are 81% and long-distance migration. Generated in the middle-lower 19%, respectively, and the expulsion amount of the Cambrian source rocks in Manjiaer Sag, the Cambrian is further more than that of the Ordovician, hydrocarbons migrated towards the northwest into the which is consistent with the characteristics of large area, Tabei Silurian reservoir beds first, then continued to huge thickness and high organic abundance of the migrate up-dip towards the northwest within the Cambrian source rocks. From Fig. 9, it can be seen that Silurian reservoir beds (or along the unconformity by the amounts of hydrocarbon expulsion in different surfaces) and finally entered the traps (Fig. 1). As in the geological stages, there exist three peak stages of Tazhong area, the carbazole-type contents and the other expulsion in the basinal areas, i.e. middle-late ratios could well indicate that hydrocarbons of the Caledonian (02+3-S), late Hercynian-Indo-Chinese, and Silurian paleo-pools in the Tabei area, originated from Yanshan-Himalayan stages. the source areas of the Manjiaer Sag, migrated In summary, combining tectonic cycles with northwest, and it is unnecessary to go into details here. sedimentary characteristics and source rock thermal 5.3 Migration direction of hydrocarbons during the evolution history of the basin, we believe that there are second-stage pool formation in the areas around the three stages of Silurian hydrocarbon pool formation in Awati Sag the Tarim Basin. In the first- and the third-stages, there During the second-stage hydrocarbon pool were plenty of hydrocarbons charging into the Silurian, formation of the basin, the Manjiaer Sag continuously and the late stage (the third-stage) is the major phase for rose up, while the Awati Sag continuously subsided. The forming today's Silurian oil and gas pools (Fig. 10). latter's major development stages were in the The first-stage is in the late Caledonian period (at Carboniferous-Permian, during which a huge thickness the end of the Silurian) and the Cambrian-lower of sediments were deposited and they provided Ordovician source rocks (especially in Manjiaer Sag) favorable conditions for the generation of a large started to expel numerous hydrocarbons. These Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 13 t: t: 0 0 t: Basin -€ '" ~§ t> ee .S (,).- ::l evolution g ~ r.n Age £1+2 HEA, lOSt £ +0 HEA, lOSt 0'+3 HEA, 10"1 2$ ~ stages ~::l0J) -5OE~ 2 c, .£ 8.<8 ~ £~~ 0 500 1000 1500 0 100 200 300 0 500 1000 1500 Complex a ~:::: foreland _0 ~~ ,,~ basin ,<:- ~ " t:- ,,~ '" ",OJ) OJ) " '" t: a '" "'~- (1).- 0 ~'" "'~ .... a 0 ~:r: .... '" ~,Q, Faulted ,,~ ,co .... basin ....1~ '" Foreland "0 Q, basin oj '" '" '" Cratonic e-V :t ",OJ) " ::c marginal '0 ,,;9 ... depression and - '" 'co " " ::g~ a .- t: ....1 intracratonic ir: ::E~ rift Peripheral t: <: D foreland 1;; '" basin ~~ '" ~" §1?'o "'OJ) CIl~ § 0; " S ....,,~ '" .~ 0._ lZl .- '" U Cratonic ... ~~ .;~g8 maginal "'-l~ t;; " aulacogen ....1 ~~~= <:" ".- .o~o Fig. 9 Division of stages of the Silurian hydrocarbon pool formation of the Tarim Basin Hydrocarbons migrated towards the southwest to the formation of asphalts and heavy oils, and the Tazhong Uplift, and towards the northwest to the Tabei bio-degradation and water washing grade of this stage Uplift, and were entrapped in the favorable structures of is much lower than that of the late stage of the the Ordovician and Silurian in the Tazhong and Tabei Caledonian. In the Carboniferous-Permian, the Tarim areas, to form large-sized oil/gas pools. Influenced by Basin was in the stage of Cratonic marginal depression the late Caledonian movement, the basin rose up and the and intracratonic rift. During the late stage of earlier strata in the Tazhong and Tabei Uplifts suffered Permian period, with collision activity of the south weathering and erosion, where the shallow parts of the margin of the Tarim plate and closing of the south hydrocarbon pools were exposed to air and underwent Tianshan Mountain, the eruption of intermediate oxidation and bio-degradation forming asphalts, while volcanic rocks and intrusion of diabase took place in the deep parts experienced bio-degradation, water the Tazhong Uplift, which resulted in strong tectonic washing and oil-deasphalting forming asphalts and activity playing a certain role in destructing pools, and heavy oils. simultaneously also provided the migration pathways The second-stage is in the late Hercynian period. for late hydrocarbon accumulation. In the Awati Sag, The source rocks of the Cambrian-lower Ordovician in the Cambrian-lower Ordovician source rocks were on the Manjiaer Sag were in the late-mature stage, and the large-scale hydrocarbon expulsion stage, and they they still expelled a few hydrocarbons; the source supplied hydrocarbons for forming oil/gas pools rocks of the middle-upper Ordovician were in the around the Awati Sag. mature stage with maximum expulsion of hydrocarbon. The third-stage is in the Yanshan-Himalayan period These expelled hydrocarbons migrated towards the (mainly in the Tertiary). The source rocks of middle­ favorable structures of the Tazhong Uplift, and upper Ordovician were at the peak of hydrocarbon accumulated to form oil-gas pools. The surface water expulsion, and there was also a gas expulsion peak with permeated the Silurian oil pools through the the Cambrian-lower Ordovician source rocks. These Carboniferous and Permian strata and bio-degradation hydrocarbons migrated towards the favorable structures and water washing took place, resulting in the through faults and sandstone carrier beds and then Petroleum Science 14 2007 C: Yanshan-Himalayan period Tazhong Uplift Tabei Upift TZ4 MXI H04 THI LN44 LNI N Elevation, m K, -----~ -10000 B: Late Hercynian period C--O 02+3 ~ £ -0, <::LI:::t=CO ---""::::::::::::=:===zz.===:::====== A: Late Caledonian period EJ Hydrocarbon kitchen Migration direction of hydrocarbon Oil pools Fig. 10 Evolution model of the Silurian hydrocarbon reservoirs in the Tarim Basin the present hydrocarbon pools of the Silurian were and stratigraphic-overlap reservoirs might be discovered formed. Owing to the increase of gas content, near the pinchout line of the main horst area in the deasphalting occurred in the pools, which increased the Tazhong area. (2) In the Tabei area, oil-sealing amount of oil and thus condensate gas reservoirs were conditions for the Silurian are poor, so that the Silurian formed. In the preservation process of the pools, if a stratigraphic-overlap and lithological pinchout pools large amount of light hydrocarbons were charged into formed during the Yanshan-Himalayan might be found the pools in the late stage, evaporative fractionation in the areas where the Carboniferous mudstones overlay would take place, and then condensate gas and asphalt directly the Silurian sandstones. (3) In the slopes around mattress would be formed. the Awati Sag, fault-sealing or anticlinal reservoirs Considering the research achievements above and formed in the Hercynian might be discovered. (4) The based on the formation stages of oil pools and the areas from the Tazhong northern slope to Gucheng assemblage relationship of pool formation, the nose-uplift are favorable for exploration of residual following areas are suggested to be favorable for paleo-pools formed during the late Caledonian. Silurian oil pool formation: (l) Structural reservoirs 7. Discussion and conclusions formed in the Yanshan-Himalayan period and lithological reservoirs formed in the late Caledonian I) In the Tazhong area of the Tarim Basin, the period could be discovered mainly in the Tazhong area, discovered Silurian oil pools are mainly faulted VolA NoA Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin 15 anticlinal pools, and lithological pools controlled by the Source Rocks and Oil Sources in the Tarim Basin. Research structural background are less common. The Silurian report of the state key project of the Ninth Five-Year Plan (in hydrocarbon shows of the basin are mainly distributed Chinese) in favorable traps of the TZ lOWell Structural Belts, Liang D. G, Zhang S. C., Zhang B. M., et al. (2000) being apparently controlled by faults. Horizontally, the Understanding on marine oil generation in China basined on Silurian oil and gas are mainly located in the 1st and 3rd the Tarim basin. Earth Science Frontiers (China University of sub-members of the upper member of Kepingtage Geoscience, Beijing), 7(4),534-547 (in Chinese) Formation, being apparently controlled by the regional Li M. W. (2000) Quantification of petroleum secondary cap rocks of the lower member of Tataaiertage migration distances: Fundaments and case histories. Formation and the 2nd sub-member of the upper Petroleum Exploration and Development, 27 (4),"11-17 (in member of Kepingtage Formation. Vertically, the Chinese) oil-gas shows are located in the Silurian intervals with Li M. w., Wang P. R., Xiao Z. Y., et al. (1999) Age Specific good physical properties (porosity and permeability). Biomarker Profile in NW China and Determination of Major The horizontal stratigraphic correlations indicate that Marine Petroleum Source Rocks in the Tarim Basin. Internal the Silurian oil and gas shows in the west part of the report to PetroChina Tarim Oil, CNPC (in Chinese) Li S. M., Liu L. F. and Wang T. G (2000) A comparison study on northern slope of the Tazhong area are mainly in the 3rd sub-member of the upper member of Kepingtage the effectiveness of using biomarker and nitrogenous Formation, and secondarily in the 1st sub-member of compounds as indexes indicating oil and gas migration. this upper member; whereas in the east part of the Petroleum Exploration and Development, 27(4), 95-98 (in northern slope (higher than the west), the Silurian oil Chinese) and gas shows are mainly in the 1st sub-member, and Li S. M., Liu L. F., Wang T. G, et al. (2001) Application of secondarily in the 3rd sub-member. non-hydrocarbon technique to charging pattern in E31 pool, 2) The hydrocarbons of the Silurian dry asphalts Gasikule oilfield, Qaidam Basin. Earth Science-Journal of originated from the source rocks of the lower-middle China University of Geosciences, 26(5), 621-626 (in Chinese) Cambrian, while the present movable oils were mainly Liu L. F., Zhao J. Z., Zhang S. C., et al. (2000a) Hydrocarbon generated from the source rocks of the middle-upper filling ages and evolution of the Silurian asphalt sandstones in Ordovician. There are three formation stages of Tarim Basin. Acta Sedimentologica Sinica, 18(3),475-479 (in hydrocarbon pools for the Silurian reservoirs of the Chinese) Tarim Basin. In the first and the third stages, there were Liu L. F., Zhao J. Z., Zhang S. C., et al. (2000b) Genetic types plenty of hydrocarbons charging into the Silurian, and and characteristics of the Silurian asphaltic sandstones in the late stage (the third-stage) is the major phase for Tarim basin. Acta Petrolei Sinica, 21(6), 12-17 (in Chinese) forming the today's Silurian oil and gas pools. Liu L. F., Zhao J. Z., Zhang S. c., et al. (2001) The depositional and structural settings and the bituminous sandstone References distribution characters of the Silurian in Tarim basin. Acta Bailey N. J. L., Krouse H: R., Evans C. R., et al. (1973) Petrolei Sinica, 22(6), 11-17 (in Chinese) Alteration of crude oil by water and bacteria-Evidence from Liu L. F. and Xu X. D. (1996a) Nitrogen compounds and study of Geochemical and Isotope studies. AAPG Bulletin, 57 (7), oil migration. Petroleum Explorationist, 1(2), 33-37 (in 1276-1290 Chinese) Chen Y. Z., Liu L. F., Chen L. X., et al. (2004) Hydrocarbon Liu L. F. and Mao D. F. (1996b) New method of oil migration migration of Silurian paleo-pools in the Tazhong and Tabei study. Advance in Earth Sciences, 11(6), 607-610 (in Areas of the Tarim Basin. Earth Science-Journal of China Chinese). University of Geoscience, 29(4), 473-482 (in Chinese) Liu L. F., Xu X. D., Mao D. F., et al. (1997) Application of Evans C. R., Rogers M. A. and Bailey N. J. L. (1971) Evolution carbazole compounds in study of hydrocarbon migration. and alteration of petroleum in western Canada, in Chinese Science Bulletin, 42 (23), 1970-1973 contributions to petroleum geochemistry. Chemical Geology, Liu L. F. (1997) Investigation on pyrrolic nitrogen compounds in 8,147-170 Qun-4 well oil of the Tarim basin. Acta Sedimentologica Huang D. F. and Liang D. G (1995) Generation and Evolution of Sinica, 15(2), 184-187 (in Chinese) Oil and Gas in the Tarim Basin, Research report of the state Liu L. F. and Kang Y. S. (1998) Investigation on secondary key project of the Eighth Five-Year Plan (in Chinese) migration of oils in central Tarim, Tarim Basin using pyrrolic Lafargue E. and Barker C. (1988) Effect of water washing on nitrogen compounds. Geochimica, 27(5), 475-482 (in crude oil compositions. AAPG Bulletin, 72 (3), 263-276 Chinese) Liang D. G, Zhang S. C., Wang F. Y., et al. (1998) Study on Liu L. F. (1998) Distribution and significance of carbazole Petroleum Science 16 2007 compounds in Paleozoic oils from the Tazhong Uplift, depth rule. Chemical geology, 28, 1-30 Tarim Basin. Acta Geologica Sinica (English Edition), 72 Wang T. G, Li S. M., Zhang A. Y, et al. (2000) Oil migration (1),87-93 analysis with pyrrolic nitrogen compounds. Journal of the Liu L. F. and Kang Y. S. (1999) Study on secondary migration of University of Petroleum, China (Edition of Natural Science), hydrocarbons in the Tazhong area of Tarim basin in terms of 24(4),83-86 (in Chinese) carbazole compounds. Chinese Journal of Geochemistry Xiao Z. Y, Zhang S. c., Zhao M. J., et at. (1997) A brief analysis (English Edition), 18 (2), 97-103 on forming periods of Silurian pools in the Tazhong A wei!. Acta Sedimentologica Sinica, 15(2), 150-154 (in Chinese) Li Y. P., Wang Y, Sun Y S., et al. (2002) Two accumulation stages of the Silurian hydrocarbon reservoir in central area of About the first author the Tarim Basin. Chinese Journal of Geology, 37 (Supp!.), 45-50 (in Chinese) Liu Luofu was born in 1958 and Lu X. X., Zhang Y W. and Jin Z. J. (1996) Preliminary study on received his PhD degree in cycling forming pools of Tarim Basin. Chinese Science geochemistry from the University of Bulletin, 41(22), 2064-2066 (in Chinese) Bristol, UK in 1992. He is a Lu X. X. (1997) Preliminary investigation on the formation professor and doctoral student mechanism of Silurian reservoir in the Tazhong low-uplift of adviser in the School of Resources Tarim Basin. Experimental Petroleum Geology, 19(4), and Information, China University 328-331 (in Chinese) of Petroleum (Beijing). His main research interest is in geochemistry. E-mail: Pan C. C., Fu J. M. and Sheng G Y (2000) Continuous extracts of oil and gas-bearing reservoirs and analysis of oil and gas liulf@cup.edu.cn inclusion ingredients in Kuche depression, Tarim basin. (Received September 4, 2006) Chinese Science Bulletin, 45(Supp!.), 2750-2757 (in Chinese) (Edited by Yang Lei) Price L. C. (1980) Crude oil degradation as an explanation on the http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

Mechanism of Silurian hydrocarbon pool formation in the Tarim Basin

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Springer Journals
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Copyright © 2007 by China University of Petroleum
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
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1995-8226
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10.1007/BF03187450
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Abstract

Petroleum Science VolA NoA Liu Luofu'<, Chen Yuanzhuang ', Chen Lixiu'r', Pang Xiongqi'<, Xie Qilai ,2, Huo Hong ,2, Zhao Suping'r', 1 1 Li Chao ,2, Li Shuangwen'<, Guo Yongqiang'<, Zhao Yande ,2 and Li Yan ,2 (1. Key Lab for Hydrocarbon Accumulation Mechanism, Ministry ofEducation, China University ofPetroleum, Beijing 102249, China) (2. Basin and Reservoir Research Center, China University ofPetroleum, Beijing 102249, China) (3. Exploration and Production Research Institute, Sinopec, Beijing 100083, China) Abstract: There are three formation stages of Silurian hydrocarbon pools in the Tarim Basin. The widely distributed asphaltic sandstones in the Tazhong (central Tarim) and Tabei (northern Tarim) areas are the results of destruction of hydrocarbon pools formed in the first-stage, and the asphaltic sandstones around the Awati Sag were formed in the second-stage. The hydrocarbon migration characteristics reflected by the residual dry asphalts could represent the migration characteristics of hydrocarbons in the Silurian paleo-pools, while the present movable oil in the Silurian reservoirs is related to the later-stage (the third-stage) hydrocarbon accumulation. Key words: Oil-gas pool, stage of pool formation, petroleum migration, mechanism of pool formation, model of pool formation, Silurian, Tarim Basin where the early formed oil pools suffered oxidation, 1. Introduction bio-degradation, water-washing and deasphalting, and The Tarim Basin in northwestern China covers an asphalts were formed. Therefore, it is difficult to study 4 2 area of 56x 10 km and possesses a large amount of the mechanism and models of the formation of Silurian hydrocarbon resources (Fig. I), and now is the main pools, and integrated methods should be used in order place for oil and gas exploration in China. The Tarim to provide scientific data for hydrocarbon exploration Basin is a multi-cycle superimposed basin, and during of the Silurian in the Tarim Basin. the geological periods, it underwent tectonic 2. Distribution characteristics of Silurian oil destruction for multiple times and thus its strata and gas in the Tazhong area distributions and tectonic characteristics are extremely complicated. In the Tazhong area, 57 exploratory wells have At present, the discovered oil and gas fields are been targeted at the Silurian, and there are 8 with mainly located in the areas of cratonic basin (the commercial oil, 6 with low-production oil, 19 with Tazhong area in central Tarim and the Tabei area in oil and gas shows, and 14 with asphalt shows. The northern Tarim). The Tarim Basin has several sets of five discovered oil pools are mainly of faulted petroliferous horizons, one of which is the Silurian. anticline type, and secondarily of lithologic type The Silurian strata are distributed in the North Uplift, controlled by the structural background. The Silurian Manjiaer-Awati Sags and the Central Uplift, and the petroliferous region in the northern slope of the distribution area is ca. 24x 10 km'. In recent years, the Tazhong area covers an area of 6,000 km". The Silurian is the focus of exploration, and a breakthrough geology of hydrocarbon pool formation in the of exploration has been made in the Tazhong area and Silurian in the Tazhong area is characterized by large five commercial oil pools, including Tazhong 11, distribution area, low hydrocarbon abundance and Tazhong 47, Tazhong 12, Tazhong 50 and Tazhong structural control. These Silurian oils and gases are 15-16, have been found, which demonstrates that the mainly distributed in favorable traps in TZ lOWell Silurian of the Tarim possesses a large petroleum structural belt, being apparently controlled by the potential. The Silurian reservoirs are characterized by faults (Fig. 2). Vertically, these oils and gases mainly multi-stage hydrocarbon migration, accumulation and occur in the 1st and 3rd sub-members of the upper pool formation, resulting in a variety of models of pool member of the Kepingtage Formation, being formation and coexistence of different forms of apparently controlled by the regional cap rocks of the hydrocarbon (asphalt, heavy oil, conventional oil, light lower member of the Tataaiertage Formation and the oil and gas). Influenced by Caledonian movement, the 2nd sub-member of the upper member of the Tarim Basin uplifted and the Silurian strata in the Kepingtage Formation (Table 1). The vertical reservoir Tazhong and Tabei areas were weathered and denuded, heterogeneity is serious, and oil and gas shows are 2007 2 Petroleum Science Imi")I [?j] I~"\I l._. -JL·· [Q] "-...G ~...-' Range ofthe Range of source Range of source Range of source Division line of Silurian asphaltic rock ofthe later rock of the later rock of the structural units sandstone Hercynian Yanshan-Himalayan Caledonian [Z] I~I Silurian movable Oil migration Oil migration Hydrocarbon migration oil pool direction ofthe direction of the direction of the later Caledonian later Hercynian Yanshan-Himalayan a-Tazhong 47 oil pool; b-Tazhong II oil pool; c-Tazhong 15-50 oil pool; d-Tazhong 16 oil pool Fig. 1 Distribution of hydrocarbon resources of Silurian in the Tarim Basin mainly distributed in the reservoir intervals with good destroyed oil pools which were formed by the expelled physical properties. The better the physical properties of hydrocarbons generated from the Cambrian source a reservoir, the more common are oil and gas shows. rocks of the Awati Sag during the Hercynian. The The horizontal stratigraphic correlations indicated that source kitchens of different stages of pool formation in the Silurian oil and gas shows in the western part of the the Tarim Basin are shown in Fig. 1. In the Tarim Basin, northern slope are mainly within the 3rd sub-member of two sets of commercial marine source rocks, the the upper member of Kepingtage Formation, and lower-middle Cambrian and the upper-middle secondarily within the 1st sub-member of the upper Ordovician, with a thickness of 5-7 km, can be member of the Formation, whereas in the higher part of identified from the marine Cambrian and Ordovician the northern slope (towards east), the Silurian oil and series. The correlation between oil and source rock gas shows are mainly located in the 1st sub-member, demonstrated that the preserved commercial and secondarily in the 3rd sub-member (Fig. 3). hydrocarbons of marine origin in the Basin were mainly from the marls of the middle-upper Ordovician (Liang, 3. Source rocks of the Silurian hydrocarbons et aI., 2000). By analyzing a large number of GC-MS in the Tazhong and Tabei areas and GC-MS-MS data, Li, et al. concluded that the source rocks and oils of the Cambrian-lower Ordovician It is commonly acknowledged that the Silurian possess the following characteristics: high contents of asphaltic sandstones in the Tazhong and Tabei areas are dinosteranes, 4-methyl-steranes, C , 24-norcholestanes, the remnants of destroyed oil pools which were formed 26 C , 24-nordiacholestanes, C steranes and by the expelled hydrocarbons generated from the 26 28-regular gammaceranes, but a low content of diasteranes, while Cambrian source rocks of Majiaer Sag during the late Caledonian, and similarly the Silurian asphaltic the source rocks and oils generated from the sandstones around the Awati Sag are the remnants of middle-upper Ordovician are just the opposite (Li, et al., VolA NoA Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin 1999; Li, 2000) (Table 2). According to these data, Li, middle-upper Ordovician, but the oils of a few oil pools et al. (1999) proposed the parameters of source rock came from the Cambrian-lower Ordovician source type division and oil derivation, and also made sure that rocks. the main manne source rocks discovered are the Table 1 Strata division of the Silurian in the Tarim Basin Strata Reservoir-cap Oil rocks Lithological column Lithologic description shows System Series Formation Member combination .- '" c: ::: ILM of ... 8~ GR ILO Mainly brown-red and green mudstones and silty Yimugantawu Cap rocks Middle mudstones, interbedded with Formation thin-layer gray and brown siltstones and fine sands c: Mainly light gray and brown­ .~ Upper red fine sands, interbedded with brown silty mudstones and Reservoir rocks E member ::; - mudstones t: <l) .~ r- Lower Mainly middle-thick Cap rocks member layer brown mudstones ~: Silurian Lower 1st Light gray siltite and fine Reservoir --.:i 5200 ~~ ... : sub­ sands, interbedded with silty rocks ..r - member mudstones and mudstones ~ ...... * 2nd Mainly gray and graygreen Cap rocks sub­ mudstones 2 member 5300 - Mainly light gray siltite and fine sand interbedded with Reservoir gray -green silty mudstones rocks and mudstones Gray and green-gray mudstones, interbedded with green-gray thin layer argillaceous siltstones 55001-:-· .. 'u .~ Upper "E o Petroleum Science 4 2007 TZ49\i> II» TZ 33 TZ32 @ OTZ9 III Well with Well with Well with low-prodution hydrocarbon industrial oil shows oil Dry well Well absent Well with of Silurian asphalt Pinchout line Igneous rock Fault of Silurian [§] Oil migraton directon Oil migraton directon oflater Caledonian ~of y anshan-Him~aYan 1-I4-methyl 1,8-/2,4-dimethyl 1,8-/Nex's Trimethyl carbazole(Al/(Cl carbazole carbazole dimethyl carbazole Fig.2 Distribution ofthe Silurian hydrocarbon pools of the Tazhong area in the Tarim Basin Elevation, m TZI5 TZl6 TZJ61 Elevation, m TZ47 well -2700,----t-------+----;------, -3850 -r-,---=r-IT:::::::=:::=-;----, -3900 ·2900 -4000 ·3 J00 j::::-......;;;t::::::;;;I' ·3300 -4100 A B Elevation. m TZJ2 TZ50 Elevation, mTZlITZl17 TZlll TZl12 -3200r------:;;::F:=---- --;----~ -3200 -3300 -3400 -3500 ·3350 c D CDLower member of the Tataaiertage Formation; ®Upper member of the Kepingtage Formation; ®the Yimugantawu Formation; ®the Tataaiertage Formation; @lst sub-member of the upper member of the Kepingtage Formation; ®Upper member ofthe Tataaiertage Formation; (1)2nd sub-member of the upper member of the Kepingtage Formation; @3rd sub-member of the upper member ofthe Kepingtage Formation Fig. 3 Sections of hydrocarbon reservoirs of the Silurian in the Tazhong area in the Tarim Basin Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 5 Table 2 Characteristics of biomarkers of oils from different source rocks in the Tarim Basin (Li, et aI., 1999) Strata Cambrian-lower Ordovician source rocks Middle-upper Ordovician source rocks Higher in dinosteranes, Dino/(Dino+3-M) ~ 0.2 Lower in dinosteranes, Dino/(Dino+3-M)< 0.15 Higher in 4-methyl-steranes, 4-M/(4-M+3-M) ~ 0.35 Lower in 4-methyl-steranes, 4-M/(4-M+3-M) < 0.25 '" .~ Higher in C 24-/(24-+27-) > 0.3 Lower in C 24-norcholestanes, 24-/(24-+27-) < 0.2 .~ 26,24-norcholestanes, 26, ... Higher in C 24- nordiacholestanes, diaC Lower in C 24- nordiacholestanes, diaC diaC 26, 24-/(diaC 24-+ 24-/( 24-+ 26, ..<:: o diaC > 0.25 diaC < 0.2 27-) 27-) ... Q) ~ Higher in gammaceranes, gammaceranes/Cj, H> 0.1 Lower in gammaceranes, gammaceranes/Cj-H < 0.05 OS Higher in C steranes, Lower in C steranes, 28-regular 2s-regular C28/(C27+C28+C29) > 20% C28/(C27+C28+C29) < 20% Lower in diasteranes, diaC < I Higher in diasteranes, diaC > 2 27/regC27 27/regC 27 In this study, all the Silurian movable oil also generated from the Cambrian-lower Ordovician. samples of the Tazhong area were analyzed by By comparison with the above-mentioned oils, the GC-MS, and some of them by GC-MS-MS. The Silurian oil samples from TZ 16 Well (4,125-4,129 results demonstrated that some of the Silurian oils m), TZ 30 Well (4,244.5-4,260 m) and TZ 50 Well originated from the Cambrian-lower Ordovician (4,378-4,385 m), which contain relatively low source rocks, some of them from the middle-upper contents of dinosteranes and 4-methyl-steranes, are Ordovician source rocks, and some from both. Four thought to be from both of the Cambrian-lower Silurian oil samples, which were collected from the Ordovician and middle-upper Ordovician source Tazhong area, TZ 11 Well (4,301-4,307 m), TZ 16 rocks (mixed oils). The oil at the depth of Well (4,125-4,129 m), TZ 30 Well (4,244.5-4,260 m) 4,417-4,435 m of TZ 11 Well are typically and TZ 50 Well (4,378-4,385 m) respectively, are characterized by very low contents of dinosterane and 4-methyl-sterane, and it was generated characterized by higher gammaceranes, higher C steranes, and lower diasteranes (Table 3), obviously from the middle-upper Ordovician source 28-regular rocks. The biomarkers of different oils are shown in and these oils are considered to originate from the Cambrian-lower Ordovician source rocks. These Fig. 4. four Silurian oils, together with an Ordovician oil 4. Stages and characteristics of the formation sample from TZ 16 Well (4,248-4,268 m) and a Silurian oil from TZ 11 Well (4,417-4,435 m), were of Silurian hydrocarbon pools in the basinal also analyzed by GC-MS-MS, and the results facies areas of the Tarim Basin showed that the Silurian oil from TZ 11 Well (4,301-4,307 m) possesses the following Some scholars have done much work on formation characteristics: high content of dinosteranes mechanism of asphaltic sandstones (Evans, et al., 1971; [Dino/(Dino+3-M)=0.29]; high content of 4-methyl­ Bailey, et al., 1973; Lafargue and Barker, 1988; Price, steranes [4-M/(4-M+3-M)=OAO]; and high content 1980). Also, much work on the formation stages of of C26,24-norcholestanes [24-/(24-+27 -)=0.69]. asphalts, heavy oils and normal oils, which widely Based on these characteristics, it is certain that the occur in the Silurian in the Tarim Basin, has been oil at the depth of 4,301-4,307 m of TZ 11 Well, carried out by Chinese scholars (Huang and Liang, 1995; which is light oil formed in the later stage, was Xiao, et al., 1997; Lu, et al., 1996; Lu, 1997; Liang, et al., 1998; Liu, et al., 2000a; 2000b; 2001; Li, et al., generated from the Cambrian-lower Ordovician 2002). In this study, the following methods have been source rocks. The Ordovician oil at the depth of 4,248-4,268 m of TZ 16 Well contains a large used to determine the formation stages of the Silurian amount of dinosterane and 4-methyl-sterane and was hydrocarbon pools. Petroleum Science 6 2007 Oil sources TZII(S,4301-4307m) TZ 16(0,4248-4268m) Oil sources £--0, TZ 16(S,4l25-4129m) Oil sources £~O, and 0'+1 Oil sources £-0, and 0,.1 TZ30(S,4244.5-4260m) Oil sources £-0, and 0'+1 Oil sources TZll(S,4417-4435m) o., I =4u,23,24-trimethylcholestane 3=3a-methyl-24-ethylcholestane 4=4u-methyl-24-ethylcholestane 2=413-methyl-24-ethylcholestane Fig.4 GC-MS-MS analytic results of the Silurian oils of the Tazhong area Table 3 Biomarker parameters of the Silurian oils of the Tazhong area Depth Gammaceranes Diasteranes Regular steranes Determination of oil Well No. m IC diaC C28/(C27+C2S+C29) source rock 30H 27/regC27 TZll 4301-4307 0.25 0.15 23.5% TZ16 4125-4129 0.17 0.48 59.6% TZ50 4378-4385 0.19 0.55 20.3% TZ30 4244.5-4260 0.27 0.73 23.4% 4248-4268 TZ16 (Ordovician oil) TZll 4417-4435 Notes: C C diaC "F C diasteranes; regC regular steranes 30H= 30Hopane; 2 27 27=C27 Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 7 4.1 Determination of the formation stages of the (Table 4), and these inclusions are distributed within Silurian movable oil pool by use of the fluorescence quartz-dissolved fissures and secondary overgrowth spectrum parameters and homogenization margins of quartz. While the fluorescence of temperature of hydrocarbon inclusion in hydrocarbon inclusions at other depths is in yellow tone combination with sedimentary burial history (Amax=600-640 nm) and they occur in the later stage There are two kinds of fluorescence spectra for the pores of quartz grains. In light of the fluorescence hydrocarbon inclusions in TZ 117 Well, i.e. green-dark spectrum characteristics and the hydrocarbon inclusion green and light yellow-yellow-bright yellow, with occurrences, we conclude that there were at least two yellow being the major one. The hydrocarbon inclusions stages of hydrocarbon migration and charging occurring fluorescence at the depth of 4,417.80 m and 4,436.10 m in this area, which took place mainly in the later stage of TZ 117 Well is in green tone (Amax=520-540 nm) with normal oils. Table 4 Fluorescence spectrum parameters of the Silurian hydrocarbon inclusions in TZ 47 Well and TZ 117 Well Depth, m Q6501500 Tone Spectrum characteristics Types Well No. Amax,nm Yellow Double peaks Gas + Liquid 4892.23 600 1.54 TZ47 4992.83 630 1.32 Yellow Double peaks Gas + Liquid 4415.20 640 1.77 Yellow Double peaks Liquid 0.72 Green Sharp single peak Liquid 4417.80 540 4421.25 590 1.96 Yellow Double peaks Liquid Light yellow Liquid 4431.64 640 1.78 Multiple peaks TZ117 4431.64 620 1048 Bright yellow Multiple peaks Liquid 4436.10 520 0.39 Deep Green Sharp single peak Liquid 4441.42 Yellow Double peaks Liquid 630 1.34 4453.00 640 1.68 Yellow Double peaks Liquid group, more than 150°C, is actually non The analysis results of the hydrocarbon inclusions of homogenization temperatures, and it is necessary to the Silurian asphaltic sandstones at the depth of 4,419.2-4,447.5 m of TZ 11 Well indicate that the point out that at the end of the Silurian the hydrocarbon asphaltic sandstones and the oil sands contain rich charging (the first hydrocarbon migration) could not hydrocarbon inclusions, and the homogenization form any hydrocarbon inclusions because the burial temperatures of these inclusions can be divided into three depth was shallow. groups. The first group ranges from 62 to 74°C and its Table 5 Hydrocarbon inclusion homogenization average is 67°C. temperatures of the Silurian asphaltic sands and oil sands in Based on the assumption that the temperature of the TZ11 Well of the Tarim Basin paleo-surface of the Tazhong area is 15°C and its Depth Hydrocarbon inclusion homogenization average paleogeothermal gradient is 3 °C /100m, the temperature, ·C depths of reservoir beds when the second hydrocarbon 4419.2 62,64,65,63.5 migration took place should range from 1,566 to 4423.7 72,74, 103 1,966m then, which correspond to the burial depth of the upper member of the Silurian Kepingtage Formation 4428.6 62,66,68 from the Permian to the end of the Triassic. At that time, 4433.0 125,135,155,188,>200 the migrating hydrocarbon was mainly normal oil. The 4447.5 67,69,155,>150 second group ranges from 100 to 135°C and it corresponds to the present formation temperatures of the upper member of the Kepingtage Formation, which The Silurian hydrocarbon inclusions of TZ 47 Well, represents the temperatures of the third-stage with yellow fluorescence (Table 4) and red/green color hydrocarbon migration of mainly light oil. The third ratio (Q6soIS00) more than 1, are normal mature oil, and Petroleum Science 8 2007 activities, which was possibly related to the Permian they indicate that at least one episode of hydrocarbon migration took place in this area. The size of these igneous rock intrusions. In this study, the formation inclusions varies considerably and their shapes mainly temperatures of the analyzed hydrocarbon inclusions contain rice-grain, circle, ellipse, polygon and irregular from TZ 47 Well are 100°C, 110°C, 125°C, 130°C and forms. Most of these inclusions occur in quartz, calcite, 140°C, respectively, which accord with the above two gypsum and cement, along the suture lines, in beaded groups of temperatures of salt-water inclusions of TZ 47 style, which demonstrates that the micro pathways of Well (l00-120°C and l20-140°C), and these data the hydrocarbon migration are microfractures. The indicate that in the two main temperature intervals large major homogenization temperatures of the salt-water scale hydrocarbon migration occurred. Based on the inclusions of TZ 47 Well contain three groups, data above and combined with the sedimentary burial 100-120°C, 120-l40°C and 160-210°C, among which history, one can conclude that the formation of Silurian the group with the highest temperature (l60-210°C) hydrocarbon pools of TZ 47 Well had two stages: the reflects that the Silurian reservoirs in TZ 47 Well were late Hercynian-IndoChinese stage and the Himalayan stage (Fig. 5). affected by the deep-seated hydrothermal liquid O"'r\·· ..·..·..A 60"C\········· \ v"' .. , ~~ 80·C\··.... \ '::":':=-~: ~.,,~~-=>, 100"C .... 120"C .. 140·C\ .... ·........···,,>==-c.... ... E 4000 160'C 180"C ..c OJ a 6000 600 500 400 300 200 100 o Age, Ma Fig. 5 Characteristics of thermal evolution and determination of hydrocarbon pool formation stage in the Silurian ofTZ 47 Well in the Tazhong area 4.2 Determination of the formation stages of the 117 Well at the depths of 4,300.13 m and 4,408.62 m, Silurian movable oil pool by use of GC data of respectively, for the study of the Silurian hydrocarbon different hydrocarbon components in the Silurian pool formation. The oil at the depth of 4,300.13 m, reservoirs charged in the first-stage (hydrocarbon inclusion and Pan, et al. (2000) carried out a study on the formation bound hydrocarbon), is normal oil, with maximal GC stages of hydrocarbon pools, hydrocarbon sources and peak being nC , content of high carbon number alkanes charging stages based on GC data of different comparatively high (nC and the w/nC22+=0.74-0.66), hydrocarbon components of the reservoirs, and achieved similar distribution characteristics and rich contents of good results. Using the same method, we analyzed the terpane and sterane (Fig. 6). The carbon numbers with reservoir samples from the Tazhong Silurian sandstones. highest abundance' of hydrocarbons charged in the The biomarkers of different hydrocarbon components, second-stage (free state) are nC , and the content of I 5 WnC the fluid inclusion homogenization temperatures of the lower carbon number alkane in this oil is comparatively reservoirs and the fluorescence spectrum parameters of high (nC Compared with the first-stage 21-/nC 22+=8.5l). organic inclusions in the analyzed 16 samples of the hydrocarbons, the distribution characteristics and Silurian oil pools indicate that the formation of Silurian contents of terpane and sterane of the second-stage have pools in the Tazhong area has mainly two stages, and a distinct difference. The characteristics of the oil sample only a minority of pools showed one stage of at the depth of 4,408.62 m are similar to those of the hydrocarbon charging (Table 6). sample at 4,300.13 m, and the former also shows two Two sandstone samples were collected from stages of hydrocarbon charging. The characteristics of oil-production intervals of the Silurian reservoir of TZ alkane, terpane and sterane of each stage of hydrocarbon Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 9 Table 6 Determination of the stage of hydrocarbon charge into the Silurian reservoirs of the Tazhong area by use of GC data Well No. Strata* Depth, m Oil properties Hydrocarbon charging 3770.1 Heavy oil Single stage TZ4 3819.55 Normal oil TZ6 Two stage 4303.18 Light oil Two stage rzn 4447.70 Heavy oil Two stage TZ12 4409.31 Heavy oil Single stage 4126.1 Heavy oil Two stage TZ16 ® TZ20 4699 Heavy oil Two stage TZ30 4257.5 Heavy oil Two stage TZ31 4594.5 Heavy oil Single stage TZ32 3792.80 Heavy oil Two stage TZ35 4945.17 Heavy oil Two stage Carboniferous 4389 Light oil Two stage TZ47 4980040 Light oil Two stage 4300.13 Light oil Two stage TZll7 4408.62 Heavy oil Two stage TZ161 4225.7 Heavy oil Two stage Notes: ® 1st sub-member of the upper member ofKepingtage Formation; ® 3rd sub-member of the upper member ofKepingtage Formation charging for the sample of 4,408.62m are similar to those 5.1 Migration direction of hydrocarbons in the for the sample of 4,300. 13m. This result of two-stage Silurian in the Tazhong area charging is in agreement with the conclusions made from the above-mentioned analysis based on hydrocarbon (1) Hydrocarbon migration direction during the inclusion fluorescence. first-stage pool formation at the end of Silurian in the Tazhong area 5. Migration directions of hydrocarbons in the During the formation of Silurian paleo-pools in the Silurian reservoirs Tazhong Uplift at the end of the Silurian, the There are three formation stages of Silurian hydrocarbons were mainly generated from the hydrocarbon pools in the Tarim Basin. The widely middle-lower Cambrian source rocks in the Manjiaer distributed asphaltic sandstones in the Tazhong and Sag, and migrated towards the southwest-south, Tabei areas are the results of destruction of hydrocarbon entering the Tazhong Silurian reservoirs first. Then, pools formed in the first-stage, and the asphaltic controlled by the fault distribution and structural shapes, sandstones around the Awati Sag were formed in the the hydrocarbons further migrated along the high second-stage. The hydrocarbon mig ration structural positions of the Tazhong No.1 Faulted characteristics reflected by the residual dry asphalts Structural Belt, the TZ 10 Well Structural Belt and the could represent the migration characteristics of Central Faulted Horst Belt, along unconformity surfaces hydrocarbons in the Silurian paleo-pools, while the or within the Silurian reservoir beds from northwest to present movable oil in the Silurian reservoirs is related southeast towards the Silurian up-dip direction in the to the later-stage (the third-stage) hydrocarbon Tazhong area, namely, from TZ 23 Well, TZ 37 Well to accumulation. TZ 4 Well, from TZ 47 Well, TZ 11 Well to TZ 15 Well, Petroleum Science 10 2007 GC graphs of saturates of different compositions Free state nC" Mass chromatograms of terpanes of different compositions 29HC,,,H Free state Bound state Hydrocarbon inclusion Mass chromatograms of steranes C pregnane of different compositions Free state C pregnane C pregnane Hydrocarbon inclusion Fig.6 GC of saturates and mass chromatograms of terpanes and steranes in the Silurian oil at the depth of 4300. 13m of Tazhong 117 Well in the Tazhong area and from TZ 67 Well, TZ 31 Well, TZ 30 Well to Well Manjiaer Sag to the Tazhong area (Fig. 1). However, TZ 44. In addition, hydrocarbon migration occurred probably, a few hydrocarbons, generated from the also from the Manjiaer Sag to the Silurian pinchout Cambrian source rocks in the Tazhong area, migrated boundary in the Tazhong area in the SWS direction, vertically through faults up to the Silurian reservoirs in namely from TZ 32 Well to TZ 44 Well (Fig. 2). a short distance. The above understandings of Generally speaking, horizontally, the oil and gas hydrocarbon migration directions were obtained from migration is characterized by a long distance from the the distribution characteristics of nitrogen compounds Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 11 extracted from asphaltic sandstones. The ratios of benzocarbazole [a] and alkyl carbazole migrated faster trimethyl carbazole (A)/(C), benzocarbazole [a]/[c] and than trimethyl carbazole (C), benzocarbazole [c] and alkyl carbazole/(alkyl carbazole +benzocarbazole) of benzocarbazole, respectively (Liu and Xu, 1996; Liu the Silurian asphaltic sandstone extracts of the Tazhong and Mao, 1996; Liu, et al.., 1997; 1998; Liu and Kang, area are comparatively high, and they are 2.9-34.7, 1999; Liu, 1997; 1998; Li, et al., 2000; 2001; Chen, et 0.6-5.3 (average 1.6) and 0.4-1.0 (0.85 for most al., 2004). Therefore, these three indices increase with samples), respectively, and these figures are much increasing migrating distance (Fig. 7). Also, in light of higher than those ratios of the oils from Putaohua oil the total contents of carbazole-type compounds, it could layer and Heidimiao oil layer in Xinzhan Oilfield of the be clearly shown that the hydrocarbons of the Silurian Songliao Basin, northeast China where the paleo-pools of the Tazhong area migrated from hydrocarbons had a short migration distance [trimethyl northwest to southeast, because the amounts of the carbazole (A)/(C), benzocarbazole [a]/[c] being 1.3-1.7, carbazole-type compounds, which have larger polarity et al., 2000)]. Because of than saturated and aromatic hydrocarbons, decrease 0.95-1.2 (average 1.1) (Wang, with increasing migrating distance (Fig. 8). the differences in molecular polarity, shape and size, during hydrocarbon migration, trimethyl carbazole (A), 8 30 TZ23 TZ37 TZ4 TZ47 TZllTZ15 lZ67 TZ31 TZ30 TZ44 TZ32 Fig. 7 Average values of3-methylcarbazoles (A)/3-methylcarbazoles (C) of the Silurian dry asphalts in different wells of the Tazhong area 2: o 14 u.J ~ 12 ::i. if 10 0- ~ 8 "0 ~ 6 0; 4 .... 2 ~O+-L,-----r--I"".---r-"'''''''''L..,--L.r--''''''''''---.---.........,..-L., ;: TZ23 TZ27 TZ4 TZ47 TZII TZ15 TZ67 TZ31 TZ30 TZ44 TZ32 Fig. 8 Average values of total carbazole-type compounds contents of the Silurian dry asphalts in different wells of the Tazhong area (2) Hydrocarbon migration direction during the contents of the movable oil pools which were formed during the third-stage hydrocarbon charging, it can be third-stage pool formation in Yanshan-Himalayan stage concluded that the hydrocarbon migration directions in the Tazhong area and pathways of the present movable oil pools are Based on the study of carbazole-type compound Petroleum Science 12 2007 basically the same as the ones of the first-stage pool amount of hydrocarbons. At the end of the Permian, formation in the Tazhong area. Former researches when the Silurian traps were charged and the showed that the content parameter of carbazole-type paleo-pools in the marginal areas around the Awati Sag compounds and the indices of trimethyl carbazole were formed, hydrocarbons were mainly generated and (A)/(C), 1-/4-methyl carbazole, 1,8-/2,4-dimethyl came from the lower-middle Cambrian source rocks in carbazole and 1,8-INEX's-dimethyl carbazole are useful the Awati Sag. Investigation of nitrogen compounds indicators for determining migration direction. indicates that the hydrocarbons from the source center Generally, the content of carbazole-type compounds migrated along the favorable pathways towards the decreases with the increase of migration distance, marginal areas of the Awati Sag in all directions within whereas the latter four ratios increase with the increase the Silurian reservoir beds, and accumulated to form of migration distance. The results of this study hydrocarbon pools in the structural highs or favorable demonstrated that the migration of the Silurian movable traps (Fig. 1). These hydrocarbon pools rose up to the oils in the Tazhong area is mainly in the lateral direction. surface and the present widely distributed dry asphaltic By making use of the above-mentioned indices, it can sandstones were formed due to the late Hercynian be seen that the later-stage (the third-stage) movement. hydrocarbons migrated from northwest to southeast, 6. Models of hydrocarbon pool formation and mainly from Tazhong 47, Tazhong 35, Tazhong 111, prediction of favorable oil-gas accumulation Tazhong 30 to Tazhong 16 Wells (Fig. 2). However, a areas of the Silurian distinct vertical migration occurred for the Silurian On the basis of the theory and methods of effective movable oil in the Tazhong area. That the indices of source rock, the amounts of hydrocarbon generated and trimethyl carbazole (A)/(C), 1-I4-methyl carbazole, expelled from the marine source rocks in the Tarim 1,8-12,4-dimethyl carbazole and 1,8-INEX's dimethyl basinal areas were calculated in this project. The result carbazole increase with decreasing sampling depth tells that during the whole geological ages, the reflects the vertical migration of hydrocarbon. cumulative amount of hydrocarbons expelled from the 5.2 Migration direction of hydrocarbons during the Cambrian-Ordovician source rocks is 4,301 x 10 t, first-stage pool formation at the end of the Silurian 8 8 x10 xI0 among which oil is 1,670 t and gas is 2,631 in the Tabei area tons oil equivalent. In the total hydrocarbon expulsion In the Tabei area, during the first-stage pool amount, contributions of the lower-middle Cambrian formation, the hydrocarbons also experienced a and middle-upper Ordovician source rocks are 81% and long-distance migration. Generated in the middle-lower 19%, respectively, and the expulsion amount of the Cambrian source rocks in Manjiaer Sag, the Cambrian is further more than that of the Ordovician, hydrocarbons migrated towards the northwest into the which is consistent with the characteristics of large area, Tabei Silurian reservoir beds first, then continued to huge thickness and high organic abundance of the migrate up-dip towards the northwest within the Cambrian source rocks. From Fig. 9, it can be seen that Silurian reservoir beds (or along the unconformity by the amounts of hydrocarbon expulsion in different surfaces) and finally entered the traps (Fig. 1). As in the geological stages, there exist three peak stages of Tazhong area, the carbazole-type contents and the other expulsion in the basinal areas, i.e. middle-late ratios could well indicate that hydrocarbons of the Caledonian (02+3-S), late Hercynian-Indo-Chinese, and Silurian paleo-pools in the Tabei area, originated from Yanshan-Himalayan stages. the source areas of the Manjiaer Sag, migrated In summary, combining tectonic cycles with northwest, and it is unnecessary to go into details here. sedimentary characteristics and source rock thermal 5.3 Migration direction of hydrocarbons during the evolution history of the basin, we believe that there are second-stage pool formation in the areas around the three stages of Silurian hydrocarbon pool formation in Awati Sag the Tarim Basin. In the first- and the third-stages, there During the second-stage hydrocarbon pool were plenty of hydrocarbons charging into the Silurian, formation of the basin, the Manjiaer Sag continuously and the late stage (the third-stage) is the major phase for rose up, while the Awati Sag continuously subsided. The forming today's Silurian oil and gas pools (Fig. 10). latter's major development stages were in the The first-stage is in the late Caledonian period (at Carboniferous-Permian, during which a huge thickness the end of the Silurian) and the Cambrian-lower of sediments were deposited and they provided Ordovician source rocks (especially in Manjiaer Sag) favorable conditions for the generation of a large started to expel numerous hydrocarbons. These Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin VolA NoA 13 t: t: 0 0 t: Basin -€ '" ~§ t> ee .S (,).- ::l evolution g ~ r.n Age £1+2 HEA, lOSt £ +0 HEA, lOSt 0'+3 HEA, 10"1 2$ ~ stages ~::l0J) -5OE~ 2 c, .£ 8.<8 ~ £~~ 0 500 1000 1500 0 100 200 300 0 500 1000 1500 Complex a ~:::: foreland _0 ~~ ,,~ basin ,<:- ~ " t:- ,,~ '" ",OJ) OJ) " '" t: a '" "'~- (1).- 0 ~'" "'~ .... a 0 ~:r: .... '" ~,Q, Faulted ,,~ ,co .... basin ....1~ '" Foreland "0 Q, basin oj '" '" '" Cratonic e-V :t ",OJ) " ::c marginal '0 ,,;9 ... depression and - '" 'co " " ::g~ a .- t: ....1 intracratonic ir: ::E~ rift Peripheral t: <: D foreland 1;; '" basin ~~ '" ~" §1?'o "'OJ) CIl~ § 0; " S ....,,~ '" .~ 0._ lZl .- '" U Cratonic ... ~~ .;~g8 maginal "'-l~ t;; " aulacogen ....1 ~~~= <:" ".- .o~o Fig. 9 Division of stages of the Silurian hydrocarbon pool formation of the Tarim Basin Hydrocarbons migrated towards the southwest to the formation of asphalts and heavy oils, and the Tazhong Uplift, and towards the northwest to the Tabei bio-degradation and water washing grade of this stage Uplift, and were entrapped in the favorable structures of is much lower than that of the late stage of the the Ordovician and Silurian in the Tazhong and Tabei Caledonian. In the Carboniferous-Permian, the Tarim areas, to form large-sized oil/gas pools. Influenced by Basin was in the stage of Cratonic marginal depression the late Caledonian movement, the basin rose up and the and intracratonic rift. During the late stage of earlier strata in the Tazhong and Tabei Uplifts suffered Permian period, with collision activity of the south weathering and erosion, where the shallow parts of the margin of the Tarim plate and closing of the south hydrocarbon pools were exposed to air and underwent Tianshan Mountain, the eruption of intermediate oxidation and bio-degradation forming asphalts, while volcanic rocks and intrusion of diabase took place in the deep parts experienced bio-degradation, water the Tazhong Uplift, which resulted in strong tectonic washing and oil-deasphalting forming asphalts and activity playing a certain role in destructing pools, and heavy oils. simultaneously also provided the migration pathways The second-stage is in the late Hercynian period. for late hydrocarbon accumulation. In the Awati Sag, The source rocks of the Cambrian-lower Ordovician in the Cambrian-lower Ordovician source rocks were on the Manjiaer Sag were in the late-mature stage, and the large-scale hydrocarbon expulsion stage, and they they still expelled a few hydrocarbons; the source supplied hydrocarbons for forming oil/gas pools rocks of the middle-upper Ordovician were in the around the Awati Sag. mature stage with maximum expulsion of hydrocarbon. The third-stage is in the Yanshan-Himalayan period These expelled hydrocarbons migrated towards the (mainly in the Tertiary). The source rocks of middle­ favorable structures of the Tazhong Uplift, and upper Ordovician were at the peak of hydrocarbon accumulated to form oil-gas pools. The surface water expulsion, and there was also a gas expulsion peak with permeated the Silurian oil pools through the the Cambrian-lower Ordovician source rocks. These Carboniferous and Permian strata and bio-degradation hydrocarbons migrated towards the favorable structures and water washing took place, resulting in the through faults and sandstone carrier beds and then Petroleum Science 14 2007 C: Yanshan-Himalayan period Tazhong Uplift Tabei Upift TZ4 MXI H04 THI LN44 LNI N Elevation, m K, -----~ -10000 B: Late Hercynian period C--O 02+3 ~ £ -0, <::LI:::t=CO ---""::::::::::::=:===zz.===:::====== A: Late Caledonian period EJ Hydrocarbon kitchen Migration direction of hydrocarbon Oil pools Fig. 10 Evolution model of the Silurian hydrocarbon reservoirs in the Tarim Basin the present hydrocarbon pools of the Silurian were and stratigraphic-overlap reservoirs might be discovered formed. Owing to the increase of gas content, near the pinchout line of the main horst area in the deasphalting occurred in the pools, which increased the Tazhong area. (2) In the Tabei area, oil-sealing amount of oil and thus condensate gas reservoirs were conditions for the Silurian are poor, so that the Silurian formed. In the preservation process of the pools, if a stratigraphic-overlap and lithological pinchout pools large amount of light hydrocarbons were charged into formed during the Yanshan-Himalayan might be found the pools in the late stage, evaporative fractionation in the areas where the Carboniferous mudstones overlay would take place, and then condensate gas and asphalt directly the Silurian sandstones. (3) In the slopes around mattress would be formed. the Awati Sag, fault-sealing or anticlinal reservoirs Considering the research achievements above and formed in the Hercynian might be discovered. (4) The based on the formation stages of oil pools and the areas from the Tazhong northern slope to Gucheng assemblage relationship of pool formation, the nose-uplift are favorable for exploration of residual following areas are suggested to be favorable for paleo-pools formed during the late Caledonian. Silurian oil pool formation: (l) Structural reservoirs 7. Discussion and conclusions formed in the Yanshan-Himalayan period and lithological reservoirs formed in the late Caledonian I) In the Tazhong area of the Tarim Basin, the period could be discovered mainly in the Tazhong area, discovered Silurian oil pools are mainly faulted VolA NoA Mechanism of Silurian Hydrocarbon Pool Formation in the Tarim Basin 15 anticlinal pools, and lithological pools controlled by the Source Rocks and Oil Sources in the Tarim Basin. Research structural background are less common. The Silurian report of the state key project of the Ninth Five-Year Plan (in hydrocarbon shows of the basin are mainly distributed Chinese) in favorable traps of the TZ lOWell Structural Belts, Liang D. G, Zhang S. C., Zhang B. M., et al. (2000) being apparently controlled by faults. Horizontally, the Understanding on marine oil generation in China basined on Silurian oil and gas are mainly located in the 1st and 3rd the Tarim basin. Earth Science Frontiers (China University of sub-members of the upper member of Kepingtage Geoscience, Beijing), 7(4),534-547 (in Chinese) Formation, being apparently controlled by the regional Li M. W. (2000) Quantification of petroleum secondary cap rocks of the lower member of Tataaiertage migration distances: Fundaments and case histories. Formation and the 2nd sub-member of the upper Petroleum Exploration and Development, 27 (4),"11-17 (in member of Kepingtage Formation. Vertically, the Chinese) oil-gas shows are located in the Silurian intervals with Li M. w., Wang P. R., Xiao Z. Y., et al. (1999) Age Specific good physical properties (porosity and permeability). Biomarker Profile in NW China and Determination of Major The horizontal stratigraphic correlations indicate that Marine Petroleum Source Rocks in the Tarim Basin. Internal the Silurian oil and gas shows in the west part of the report to PetroChina Tarim Oil, CNPC (in Chinese) Li S. M., Liu L. F. and Wang T. G (2000) A comparison study on northern slope of the Tazhong area are mainly in the 3rd sub-member of the upper member of Kepingtage the effectiveness of using biomarker and nitrogenous Formation, and secondarily in the 1st sub-member of compounds as indexes indicating oil and gas migration. this upper member; whereas in the east part of the Petroleum Exploration and Development, 27(4), 95-98 (in northern slope (higher than the west), the Silurian oil Chinese) and gas shows are mainly in the 1st sub-member, and Li S. M., Liu L. F., Wang T. G, et al. (2001) Application of secondarily in the 3rd sub-member. non-hydrocarbon technique to charging pattern in E31 pool, 2) The hydrocarbons of the Silurian dry asphalts Gasikule oilfield, Qaidam Basin. Earth Science-Journal of originated from the source rocks of the lower-middle China University of Geosciences, 26(5), 621-626 (in Chinese) Cambrian, while the present movable oils were mainly Liu L. F., Zhao J. Z., Zhang S. C., et al. (2000a) Hydrocarbon generated from the source rocks of the middle-upper filling ages and evolution of the Silurian asphalt sandstones in Ordovician. There are three formation stages of Tarim Basin. Acta Sedimentologica Sinica, 18(3),475-479 (in hydrocarbon pools for the Silurian reservoirs of the Chinese) Tarim Basin. In the first and the third stages, there were Liu L. F., Zhao J. Z., Zhang S. C., et al. (2000b) Genetic types plenty of hydrocarbons charging into the Silurian, and and characteristics of the Silurian asphaltic sandstones in the late stage (the third-stage) is the major phase for Tarim basin. Acta Petrolei Sinica, 21(6), 12-17 (in Chinese) forming the today's Silurian oil and gas pools. Liu L. F., Zhao J. Z., Zhang S. c., et al. (2001) The depositional and structural settings and the bituminous sandstone References distribution characters of the Silurian in Tarim basin. Acta Bailey N. J. L., Krouse H: R., Evans C. R., et al. (1973) Petrolei Sinica, 22(6), 11-17 (in Chinese) Alteration of crude oil by water and bacteria-Evidence from Liu L. F. and Xu X. D. (1996a) Nitrogen compounds and study of Geochemical and Isotope studies. AAPG Bulletin, 57 (7), oil migration. Petroleum Explorationist, 1(2), 33-37 (in 1276-1290 Chinese) Chen Y. Z., Liu L. F., Chen L. X., et al. (2004) Hydrocarbon Liu L. F. and Mao D. F. (1996b) New method of oil migration migration of Silurian paleo-pools in the Tazhong and Tabei study. Advance in Earth Sciences, 11(6), 607-610 (in Areas of the Tarim Basin. Earth Science-Journal of China Chinese). University of Geoscience, 29(4), 473-482 (in Chinese) Liu L. F., Xu X. D., Mao D. F., et al. (1997) Application of Evans C. R., Rogers M. A. and Bailey N. J. L. (1971) Evolution carbazole compounds in study of hydrocarbon migration. and alteration of petroleum in western Canada, in Chinese Science Bulletin, 42 (23), 1970-1973 contributions to petroleum geochemistry. Chemical Geology, Liu L. F. (1997) Investigation on pyrrolic nitrogen compounds in 8,147-170 Qun-4 well oil of the Tarim basin. Acta Sedimentologica Huang D. F. and Liang D. G (1995) Generation and Evolution of Sinica, 15(2), 184-187 (in Chinese) Oil and Gas in the Tarim Basin, Research report of the state Liu L. F. and Kang Y. S. (1998) Investigation on secondary key project of the Eighth Five-Year Plan (in Chinese) migration of oils in central Tarim, Tarim Basin using pyrrolic Lafargue E. and Barker C. (1988) Effect of water washing on nitrogen compounds. Geochimica, 27(5), 475-482 (in crude oil compositions. AAPG Bulletin, 72 (3), 263-276 Chinese) Liang D. G, Zhang S. C., Wang F. Y., et al. (1998) Study on Liu L. F. (1998) Distribution and significance of carbazole Petroleum Science 16 2007 compounds in Paleozoic oils from the Tazhong Uplift, depth rule. Chemical geology, 28, 1-30 Tarim Basin. Acta Geologica Sinica (English Edition), 72 Wang T. G, Li S. M., Zhang A. Y, et al. (2000) Oil migration (1),87-93 analysis with pyrrolic nitrogen compounds. Journal of the Liu L. F. and Kang Y. S. (1999) Study on secondary migration of University of Petroleum, China (Edition of Natural Science), hydrocarbons in the Tazhong area of Tarim basin in terms of 24(4),83-86 (in Chinese) carbazole compounds. Chinese Journal of Geochemistry Xiao Z. Y, Zhang S. c., Zhao M. J., et at. (1997) A brief analysis (English Edition), 18 (2), 97-103 on forming periods of Silurian pools in the Tazhong A wei!. Acta Sedimentologica Sinica, 15(2), 150-154 (in Chinese) Li Y. P., Wang Y, Sun Y S., et al. (2002) Two accumulation stages of the Silurian hydrocarbon reservoir in central area of About the first author the Tarim Basin. Chinese Journal of Geology, 37 (Supp!.), 45-50 (in Chinese) Liu Luofu was born in 1958 and Lu X. X., Zhang Y W. and Jin Z. J. (1996) Preliminary study on received his PhD degree in cycling forming pools of Tarim Basin. Chinese Science geochemistry from the University of Bulletin, 41(22), 2064-2066 (in Chinese) Bristol, UK in 1992. He is a Lu X. X. (1997) Preliminary investigation on the formation professor and doctoral student mechanism of Silurian reservoir in the Tazhong low-uplift of adviser in the School of Resources Tarim Basin. Experimental Petroleum Geology, 19(4), and Information, China University 328-331 (in Chinese) of Petroleum (Beijing). His main research interest is in geochemistry. E-mail: Pan C. C., Fu J. M. and Sheng G Y (2000) Continuous extracts of oil and gas-bearing reservoirs and analysis of oil and gas liulf@cup.edu.cn inclusion ingredients in Kuche depression, Tarim basin. (Received September 4, 2006) Chinese Science Bulletin, 45(Supp!.), 2750-2757 (in Chinese) (Edited by Yang Lei) Price L. C. (1980) Crude oil degradation as an explanation on the

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Published: Jun 16, 2010

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