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Formation and destruction processes of upper Sinian oil-gas pools in the Dingshan-Lintanchang structural belt, southeast Sichuan Basin, China

Formation and destruction processes of upper Sinian oil-gas pools in the Dingshan-Lintanchang... Lintanchang structure in the southeast Sichuan Basin were of medium-good quality with two excellent hydrocarbon-generating centers developed in the periphery areas, with a possibility of forming a medium to large-sized oil-gas fi eld. Good reservoir rocks were the upper Sinian (Dengying Formation) dolomites. The mudstone in the lower Cambrian Niutitang Formation with a good sealing capacity was the cap rock. The widely occurring bitumen in the Dengying Formation indicates that a paleo oil pool was once formed in the study area. The fi rst stage of paleo oil pool formation was maturation of the lower Cambrian source rocks during the late Ordovician. Hydrocarbon generation from the lower Cambrian source rocks stopped due to the Devonian-Carboniferous uplifting. The lower Cambrian source rocks then restarted generation of large quantities of hydrocarbons after deposition of the middle Permian sediments. This was the second stage of the paleo oil pool formation. The oil in the paleo oil pool began to crack during the late Triassic and a paleo gas pool was formed. This paleo gas pool was destroyed during the Yanshan-Himalayan folding, uplifting and denudation. Bitumen can be widely seen in the Dengying Formation in wells and outcrops in the Sichuan Basin and its periphery areas. This provides strong evidence that the Dengying Formation in the Sichuan Basin and its periphery areas was once an ultra-large structural-lithologic oil-gas fi eld, which was damaged during the Yanshan-Himalayan period. Sinian, oil-gas pool, Dingshan-Lintanchang structural belt, Sichuan Basin Key words: Sinian are not yet fully understood. 1 Introduction The southeastern area of the Sichuan Basin is part of The Sichuan Basin is one of the most important oil-gas the eastern steep structural belt and the southern gentle bearing superimposed basins in south China. The oldest structural belt (Fig. 1). Currently, this area is ideal for sedimentary strata are upper Sinian (Dengying Formation). research on accumulation and formation of upper Sinian oil The Dengying Formation is one of the most signifi cant high- and gas pools in the Sichuan Basin due to several factors: (1) quality reservoirs and gas-bearing horizons in the Sichuan Abundant information is available as Sinopec has conducted Basin. The Weiyuan gas field, discovered in 1964, was the extensive exploration and seismic prospecting in this area first large and medium-sized natural gas field found in the in recent years. The upper Sinian was the target stratum in Sichuan Basin with proven natural gas reserves of 408×10 the Dingshan 1 Well and Lin 1 Well, but no economic gas m . The major pay zone consisted primarily of Dengying accumulation was found; (2) Core is available from wells Formation carbonate rocks (Sun et al, 2009). Although drilled through the Dengying Formation; (3) The outcrops of exploration and research have been carried out for over the upper Sinian in the area are suitable for comprehensive 50 years in the Dengying Formation in the Sichuan Basin, study of the surface and subsurface geology. no other similar-sized natural gas fields have been found The study area is located at the southeastern edge of in this formation. It appears that the controlling factors of the Sichuan Basin and at the northern slope of the Central preservation and distribution of gas and oil in the upper Guizhou Uplift. Outcropping strata in the study area are mainly Palaeozoic, while those in the Sichuan Basin are mainly Cretaceous and Jurassic (Fig. 1). The study of the *Corresponding author. email: lsg@cdut.edu.cn formation of oil and gas pools in the upper Sinian in this Received September 7, 2009 Shimian Fault SW. Sichuan FTB ST IC Xianshuihe fault Triassic suture zone Micang Mountain EQTS Triassic suture zone Middle Paleozoic (Shangdan fault zone) Daba N QT suture zone 290 Pet.Sci.(2010)7:289-301 stretching to the Nanchuan-Zunyi major fault and western area is of great importance for petroleum exploration in area stretching to the gentle fold area in the southern Sichuan the Sichuan Basin and evaluation of the upper Sinian oil Basin, limited by the Xingwen-Gulin hidden fault. The and gas exploration potential of the central Guizhou area. Dingshan and Lintanchang structures are two secondary Based on comprehensive research on geology, geochemistry, anticlines on the western flank of the Sangmuchang geophysics, and surface and subsurface geology, we not composite anticline (Fig. 1). Sedimentary strata outcropping only present the petroleum accumulation conditions and the in this area are from the upper Sinian to the Cretaceous, with processes of formation and destruction of the upper Sinian the upper Silurian, Devonian and Carboniferous absent due petroleum pool in the Dingshan–Lintanchang structural to the Caledonian movement. However, the upper Sinian belt in the southeast Sichuan Basin, but also we discuss the strata can be found only on the axis of the Sangmuchang characteristics of the upper Sinian paleo oil-gas reservoirs in composite anticline (in the Heba-Runnan region) in an area the Sichuan Basin. of about 10 km (Fig. 1). Petroleum exploration revealed that the Dengying Formation is about 1,200 m thick (Dingshan 2 Regional background 1 Well) with the top at a depth of 2,500-3,500 m. The Sinian The Dingshan–Lintanchang northeast structural belt is unconformably overlain by lower Cambrian because of belongs to the transitional zone between the southeastern the Tongwan movement. Two major fault systems, namely, edge of the Sichuan Basin and the western edge of the northeast-trending Sangmuchang and nearly north-south Xuefeng Mountain Uplift. This structural belt is the front of trending Nanchuan-Zunyi faults are developed. In addition, a the Loushanguan trough-like fold belt, with its eastern area few northwest-trending faults can be found (Fig. 1). 102° 104° 108° 112° E 106° 110° 34°N West Qinling FD Dingshan 1 MO Ningshan Fault Simianshan BT HD Wudang DT3 (J sn) Datong Songpan-Ganzi 2 East Terrain Terrain Qinling FTB 32° DSA (P l Jiudianya (from Dingshan 1 Well) SD Guandu Sichuan Basin 30° Yueping ATF QB NCB Qinling ADX4(Js 2 ) QT DT6 (J sn) Tanlu Fault 2 LT SGT ACQ1(Js 2 ) 28° DT4 (J zl) SCB Wenshui Dabie Mountain 1-2 100km DT DT2 (T xj) ALC14 (P q) ACQ1 (J s) 2 Houtan Xishui Runnan Lin 1 Tudiba Z PS Heba Sangmu Tongzhi K J T P P-T S O ∈ Z Geochemical section County Town Cretaceous Jurassic Triassic Permian Permian-Triassic Silurian Ordovician Cambrian Sinian Apatite sample Well Fig. 1 Simplifi ed geological map of the study area NCB: North China block, SCB: South China block, SGT: Songpan-Ganzi terrain, QB: Qaidam Basin, QT: Qiangtang terrain, LT: Lhasa terrain, DT: Southeast Asia terrain, ICT: Indochina terrain, ST: Simao terrain, ATF: Altyn Tagh Fault, NQT: North Qinling terrain, EQTS: East Qinling thrust system, BT: Bikou terrain, FD: Foping dome, SD: Shennongjia dome, HD: Hannan dome, FTB: fold-thrust belt, MO: Mianlüe ophiolite, AO: Anyemaqen ophiolite. Modifi ed from Meng et al (2005) 2003). 3 Source, reservoir and cap rocks GC-MS analysis showed that there is adamantane in the upper Sinian bitumen with minor characteristics of water- 3.1 Lower Cambrian source rocks washing oxidation, which indicates that the reservoir bitumen Abundant bitumen is present in the Dengying Formation in the Dengying Formation was formed by the pyrolysis of in the Sichuan Basin and its periphery areas. It was sourced crude oil. mainly from the lower Cambrian argillaceous rocks (Dai, The lower Cambrian argillaceous rocks in the Sichuan FTB Mountain Sichuan Basin Longmen Mountain Thrust Belt Sangmuchang Fault Eastern Sichun fold-thrust belt Nanchuan-Zunyi Fault Pet.Sci.(2010)7:289-301 291 Basin are mainly distributed in the Yibin-Luzhou area Sinian cores of the Dingshan 1 Well and Lin 1 Well (Sichuan Province), western Hunan and Hubei Provinces and (Fig. 2(a)). Some of them could represent dissolution Nanjiang-Ningqiang areas, with a thickness of 200-300 m. originated vugs subsequently partly infilled. The Two centers of hydrocarbon generation in the lower Cambrian petrological characteristics of the fillings (e.g. crusty source rocks exist in the Yibin-Luzhou and west Hunan-west structure developing at the edge) and the geochemical Hubei areas with a hydrocarbon generation capacity of (150- characteristics of carbon, oxygen and strontium isotopes 8 3 2 200)×10 m /km . In the study area, the medium-good quality showed that the formation mechanism was freshwater source rocks in the lower Cambrian Niutitang Formation are karstification. The density of vugs and fractures in the 0-20 m thick (Liang et al, 2008). In the Dingshan 1 Well, Dingshan 1 Well was 0.15-8 per meter and 0.21 per the black carbonaceous mudstone at the bottom of the lower meter respectively, with most of the vugs being medium Cambrian Niutitang Formation is 13 m thick. The TOC in 29 and small in size. The dissolved vugs and fractures in samples was 0.3%-3.9%, with 25% of the samples having the study area did not develop as much as those in the TOC higher than 0.5% and the average is 0.7%. Kerogen palaeo-uplift areas in the central Sichuan Basin (Liu et is of type II , and is a medium quality hydrocarbon source al, 2008a). (Qin et al, 2005; 2006; 2009). In the Lin 1 Well, the dark (2) Pores and vugs dissolved by acid fl uids mudstone of the Niutitang Formation is 158 m thick, with Carbonate rocks in the study area have been strongly a kerogen type of I-II . The mudstone is dark gray except corroded by acidic fluids (Barth and Bjørlykke, 1993), for the grayish-black areas at the bottom. The TOC of eight indicated by dissolution along stylolite seams (Fig. 2(b)). samples was 0.07%-0.95%, and that of two samples between Dissolution along fractures and algal laminae (Fig. 2(c)) as 2,462.13 m and 2,468.43 m was only 0.07% and 0.08% well as pores were found in cores from the Lin 1 Well, with respectively. Thus, it is evident that the upper part of the a residual plane porosity of 3% (Fig. 2(d)). Thin-section Niutitang Formation was a fairly poor source rock and had observation of the rocks showed corrosion and fi lling of late almost no hydrocarbon generating capacity, but the lower part acid silica along stylolites in the upper Sinian rocks. was relatively good hydrocarbon source rock, with its organic (3) Vugs dissolved by hydrothermal fl uids carbon content higher than 0.55%. Silica rocks, zebra dolomite (Fig. 2(e), (f) and (g)), The lower Cambrian source rocks in the southeast Sichuan lead zinc ore, fluorite and barite are present in the upper Basin were of medium-good quality and comprised beds Sinian of the Dingshan–Lintanchang structural belt in the only 13-20 m thick. However, two high-quality hydrocarbon southeast Sichuan Basin. Their presence indicates that the generation centers developed in the periphery area. Being an Dengying Formation has been altered by hydrothermal fl uids, important area for oil and gas migration and accumulation, and the porosity is up to 20% (Fig. 2(e)), especially at the this study area contained petroleum sources needed for the anticline axis area of Sangmu Town. At the well cores near formation of a large or medium-sized oil-gas fi eld (Liu et al, the unconformity between the upper Sinian and the lower 2006). Cambrian in the Lin 1 Well, isolated pores developed inside the white coarse-grain dolomites with the vug porosity up 3.2 Sinian reservoir rocks to 3%. Moreover, intercrystalline pores also developed in saddle dolomites which now contain bitumen 5% of the rock Reservoir rocks in the Dengying Formation are mainly (Fig. 2(h)), which indicates that the Dengying Formation had algal boundstones, and doloarenite (Ao et al, 2006). Porous suffi cient porosity during the oil-gas charging process. dolomites developed near the unconformities between the Therefore, deep-burial acid fluid dissolution and upper Sinian and the Cambrian. The samples from the Lin hydrothermal dissolution formed a good-quality reservoir on 1 Well, Dingshan 1 Well and the outcrops showed that the the basis of early freshwater karstic pores in the Dengying Dengying Formation reservoir rocks were tight with the Formation. porosity of 1%-2% (Liu et al, 2007; 2008b). The reservoir space is mainly intercrystalline (dissolved) pores, vugs and/or 3.3 Cap rocks fractures. (1) Pores and vugs formed by fresh water dissolution Apart from the mudstones in the lower Cambrian 13 18 The δ C and δ O isotopic values of carbonate rocks Niutitang Formation, as direct cap rocks for the Dengying under the unconformity were suddenly lowered by the Formation, gypsum, gypsum dolomite, or limestone in the interaction between the fresh water and rocks, leading to middle-lower Cambrian had high sealing capacity. abrupt change of carbon and oxygen isotopes (Cander, (1) Argillaceous cap rocks 1995; Dickson and Saller, 1995; Wagner et al, 1995). Distributed over the whole study area, lower Cambrian The carbon and oxygen isotopes of carbonate rocks and argillaceous cap rocks such as shale and sandy mudstone fillings of the pores and vugs below the unconformity are interlayered with limestone. For example, the lower between the upper Sinian and the lower Cambrian in Cambrian in the profi les at Houtan, Xishui County, has a total the Dingshan 1 Well showed a significant decline and thickness of 423 m, among which argillaceous rocks are 213 negative bias, indicating palaeo-karstification. The m thick, accounting for 50% of the strata. The total thickness karstification reached a depth of around 150 m below of argillaceous limestone and argillaceous dolomite is 32 m, the unconformity surface. A large quantity of dissolution accounting for 7.6%. Therefore, the total thickness of the cap pores, vugs and karst breccia can be found in the upper rocks reached up to 245 m, accounting for 58% of the strata. 292 Pet.Sci.(2010)7:289-301 a b c d e f g h Reservoir rock photographs of the upper Sinian Dengying Formation in the study area Fig. 2 (a) Dissolution pores by meteoric water (arrow). The pores are filled with botryoidal crust and fine-crystalline dolomite in supergene stage, and coarse crystalline dolomite, quartz, and bitumen in the burial stage. Residual porosity is about 5%. Upper Sinian Dengying Formation, Dingshan 1 Well. (b) Burial dissolution along the stylolite. Residual porosity is about 3%. Upper Sinian Dengying Formation, Lin 1 Well. (c) Algae doloarenite. Dissolution pores developed in the algae-rich laminae by deep burial dissolution. Residual porosity was about 5%. Upper Sinian Dengying Formation. Lin 1 Well (2,625.5 m). (d) Corrosion and fi lling-up of fracture. Residual porosity is about 3%. Upper Sinian Dengying Formation, Lin 1 Well (2,652.2 m). (e) Pores by hydrothermal dissolution. Residual porosity was about 20%. Upper Sinian Dengying Formation, Heba, Xishui (f) Zebra dolomite at the top of the Sinian. Pores by hydrothermal dissolution developed inside. Residual porosity is about 8%-10%. Heba, Xishui. (g) Zebra dolomite and pores formed by hydrothermal dissolution. Residual porosity is about 3%. Upper Sinian Dengying Formation, Lin 1 Well. (h) Bitumen in intercrystalline pores of saddle dolomite. Bitumen content is 5%. Upper Sinian Dengying Formation, Lin 1 Well. Pet.Sci.(2010)7:289-301 293 The maximum thickness of a single layer of argillaceous The pure gypsum rocks (white and gray-white) are 29 m cap rocks in the lower Cambrian Niutitang Formation in thick, and gray-white gypsiferous dolomite (limestone) is 10 the Dingshan 1 Well and Lin 1 Well is 28.5 m. The general m thick in the well. The gypsum in the Lin 1 Well is mainly thickness of a single layer is 6.0 m and the average total located at the top of the Qingxudong Formation, with a thickness is 146 m. The breakthrough pressure of these rocks thickness of 13 m. Gypsum is overlain and underlain by gray- is 77-87 MPa. white gypsiferous dolomites. Gypsum rock in the middle Therefore, it can be concluded that the lower Cambrian Cambrian Shilengshui Formation is 27 m thick, and in the argillaceous rocks can be classifi ed as high-quality regional Qingxudong Formation is 17 m thick. Thus, the middle-lower cap rocks for the upper Sinian. Cambrian gypsum cap rocks provide excellent sealing for the (2) Gypsum cap rocks underlying Dengying Formation. Gypsum which is considered to be an ideal cap rock is 4 Characteristics of structural traps widely developed during the middle-lower Cambrian. The Cambrian gypsum-salt strata mainly occur in the middle The Dingshan-Lintanchang structural traps were primarily Cambrian Shilengshui Formation and lower Cambrian formed during Cambrian sedimentation, with inheritance at Qingxudong Formation, with a regional thickness of 2-46 m. later stages. However, the sizes and widths of traps changed Basically, gypsum rocks in the Dingshan 1 Well are evenly greatly in different periods and the structural high had a developed in the Qingxudong and Shilengshui formations. degree of migration as well (Table 1, Fig. 3). Table 1 Contrast of trap elements at the top surface of the Sinian between the Dingshan Structure and the Lintanchang Structure Items Dingshan Structure Lintanchang Structure Closed area, Closed height, Closed height, m Structural high of traps Closed area, km Structural high of traps Periods 2 km m Cambrian 130 100-<200 SE Dingshan 1 Well 220 200 SE Lin 1 Well Defl ecting toward Ordovician 160 200 310 400 SW Lin 1 Well Dingshan 1 Well Defl ecting toward Silurian 120 400 250 400 SW Lin 1 Well Dingshan 1 Well Structural high Devonian-Carboniferous 155 400 205 400 SW Lin 1 Well maintained Structural high Permian–Late Cretaceous 120 500 220 400 SW Lin 1 Well maintained Late Cretaceous-Present 130 700 SE Dingshan 1 Well 190 600 Lin 1 Well The subsurface depth of the Dengying Formation in the 5 Scale and formation process of the Sinian Lin 1 Well is 2,585 m to 2,910 m with a thickness of only paleo oil pool 325 m (without reaching the bottom of the formation). The average content of bitumen in the thin sections from the upper 5.1 Scale of the Sinian paleo pool and its oil-cracking Sinian cuttings is 2.2%. Microscopically, the bitumen content gas in the thin sections from the cores is between 0.1%-8%. 50% of all thin sections from cores of the Dengying Formation Microscopically, the bitumen content in the cores of the in the Lin 1 Well contain bitumen. The bitumen distribution Dengying Formation in the Dingshan 1 Well is 0.5%-5.6% is discontinuous, most is found within 100 m under the top (Fig. 4). On average the thin sections contained 3% bitumen of the unconformity. Below the depth of 2,827 m, no large with only a few reaching up to 9% bitumen. The bitumen quantities of bitumen have been found. Therefore, the depth content of thin sections from near the unconformity between of 2,827 m can be identified as the oil-water interface and the Sinian and the Cambrian is relatively high and gradually the oil column height of the Sinian paleo oil pool in the Lin 1 declines below the unconformity. However, the bitumen Well was at least 242 m (Fig. 4). content increased in some upper Sinian intervals in the The peak of hydrocarbon generation from the lower Dingshan 1 Well. The average bitumen content at the depth of Cambrian Niutitang Formation source rocks was during the 3,573.9 to 3,604.2 m is 0.17%. However, the bitumen content Permian, which was the main period for the formation of is up to 0.5% at the deeper level of 3,650.61 m in the well (Sun paleo oil pools in the study area. At the end of the Permian, et al, 2010). The drilling depth of the Dengying Formation in the Lin 1 Well was actually not the structural high of the the Dingshan 1 Well was 3,492-4,610 m (without reaching Lintanchang structural trap, but a local high point on the the bottom of the formation). Bitumen occurred continuously slope (Fig. 3(e)). With the palaeo-structure reconstruction of from the top surface to a depth of 4,070 m, but below this the top surface of the upper Sinian after the middle Permian depth no bitumen could be found. Therefore, this may sedimentation, it could be inferred that the paleo oil column indicate that the oil-water interface in the upper Sinian paleo height in the Lintanchang structural trap was about 350 m, oil pool was at a depth of 4,070 m and its oil column height with an oil-bearing area of 220 km (Fig. 3(e)). was about 500 m (Fig. 4). 294 Pet.Sci.(2010)7:289-301 Evolution of the Dingshan-Lintanchang structural belt in the southeast Sichuan Basin Fig. 3 (a) Structural map of the top surface of the Sinian after the Lower-Cambrian sedimentation. (b) Structural map of the top surface of the Sinian after the Ordovician sedimentation. (c) Structural map of the top surface of the Sinian after the Lower-Silurian sedimentation. (d) Structural map of the top surface of the Sinian after the Silurian sedimentation. (e) Structural map of the top surface of the Sinian after the Middle- Permian sedimentation. (f) Structural map of top surface of the Sinian at present. Based on the content of the reservoir bitumen in the Lintanchang and Dingshan structural paleo-traps could be 8 8 Dengying Formation in the two exploration wells and the calculated as 6.7×10 t and 1.9×10 t respectively using the characteristics of the paleo-structural traps, oil reserves in the equation: Pet.Sci.(2010)7:289-301 295 Bitumen in cuttings Bitumen in cuttings 0 10 Depth, 0 10 Depth, Lithology Bitumen distribution Lithology Bitumen distribution Bitumen in cores Bitumen in cores 0 10 0 10 -2550 -3450 -3500 -2600 Hydrothermal dolomite is Bitumen was mainly very common in this interval, present in the bitumen is mainly present in intercrystal pores of the intercrystalline pores of dolomite, some present dolomite in the fracture and intraclasts Bitumen was mainly present in microfracture -2700 -3600 caused by dissolution Bitumen is mainly or tectonic movement, present in the some present in the intercrystalline intercrystal pores of pores in dolomite dolomite in fractures Paleo oil-water -2800 -3700 interface -2827 No bitumen in the pores H≥242m below -2827m, so this depth may be the paleo oil-water interface. Paleo oil column height may exceed 242m because of drilling did not reach the bottom of Sinian -2900 -3800 Can not find any bitumen in the cuttings Fig. 4 Vertical bitumen content profiles in the upper Sinian Dengying Formation in Dingshan 1 well and Lin 1 Well (Left: Dingshan 1 Well; Right: Lin 1 Well) -3900 m (Barker, 1990; Lu et al, 2002). The amount of gas from the cracked crude oil in the Lintanchang and Dingshan structures 3 8 3 can be calculated as G =Q ×620 m /t=6.7×10 t×620 m /t 1 1 8 3 3 8 3 =4154×10 m and G =Q ×620 m /t=1.9×10 t×620 m /t 2 2 8 3 =1178×10 m respectively. Therefore, the amount of gas from cracked paleo oil pool in the Dingshan-Lintanchang structural 8 3 belt was G =G +G =5332×10 m , which indicated presence 1 2 Bitumen was mainly of a rich source rock for the formation of paleo gas pool. -4000 present in the intercrystalline pores in dolomite Parameters for reserves calculation of Sinian paleo-traps in the Table 2 Lintanchang and Dingshan structures No bitumen in pores below -4070m, Parameter so this depth may be 2 3 8 the paleo oil-water -4070 A , km H, m H , m Ф, % S , % Boi ρ, t/m Q, 10 t o o o interface. The paleo oil Structure H≈500m column height was about 500m. Lintanchang 220 350 120 5 70 1.24 0.9 6.7 Dingshan 120 500 42 7.5 70 1.24 0.9 1.9 Q =A ×H × ×S ×ρ /Boi (Wu, 2005) (See Table 2 for A : Oil-bearing area; H: Oil column height; H : Effective reservoir thickness; o o o Ф: Effective porosity; S : Oil saturation; Boi: Oil volume factor; ρ: Oil parameter determination) o density; Q: Paleo-reserves ( S , Boi and ρ are from Sun et al (2007)) Therefore, in the Dingshan-Lintanchang structural belt, there was a paleo oil pool with the petroleum reserve of 5.2 Formation of the Sinian paleo oil (gas) pool 8 8 8 8.6×10 t (6.7×10 t+1.9×10 t). After the lower Cambrian sedimentation, the buried depth The average gas cracked from one ton of oil is about 620 Lower Upper Sinian Series Cambrian Forma- Dengying Niutitang tion Lower Series Upper Sinian Cambrian Forma- Dengying Niutitang tion 296 Pet.Sci.(2010)7:289-301 of the top Sinian surface (the bottom of the lower Cambrian 5.3 Processes which destroyed the Sinian paleo gas Niutitang Formation source rocks) was about 800 m in the pool Dingshan 1 Well and 700 m in the Lin 1 Well, so the lower 5.3.1 Folding, uplifting and denudation during Yanshan- Cambrian Niutitang Formation source rocks (mudstone) were Himalayan period not yet mature (Fig. 5). (1) Trough-like fold formation After the Ordovician sedimentation, the burial depth of Since the early Cretaceous, the southeast Sichuan Basin the Sinian top surface reached about 2,000 m in the Dingshan had gone through important folding, uplifting and denudation 1 Well and 1,900 m in the Lin 1 Well. During the late periods (Lü and Xia, 2005; Hu et al, 2009). Infl uenced by the Ordovician, the lower Cambrian source rocks became mature, major stress fi eld from southeast of the region, the strata were but the main hydrocarbon generating center was located in the folded. The age of folded strata decreased from southeast to Dingshan structure. The fi rst stage paleo oil pool was located northwest, accompanied by gradually decreasing intensity in the Lintanchang and Dingshan structures. of deformation. During this period, the multiple directions After the Silurian sedimentation, the burial depth of of N-E, E-W and S-N structures were superimposed in the the Sinian top surface reached 2,600 m in the Dingshan 1 southeast Sichuan Basin and the basin was fi nally shaped into Well and 2,400 m in the Lin 1 Well. During the Devonian- a N-E trough-like structure, with the wide-gentle anticlines Carboniferous, the source rocks in the study area went and tight-closed synclines (Fig. 1). through an intermission in hydrocarbon generation. The (2) Intense uplifting and formation of fracture systems paleo oil pool in the Dingshan structure was damaged by the Fission track analysis was used as a quantitative Caledonian movement, whereas the pool in the Lintanchang simulation for the samples from the Dingshan-Lintanchang structure was well preserved. region, to reconstruct their thermal and tectonic uplift After the Permian sedimentation, the buried depth of the histories. The process of tectonic uplift and subsidence in top of the Sinian reached 2,800 m in the Dingshan 1 Well and the above area was divided into two major phases. Before 2,600 m in the Lin 1 Well, with the early structural framework the late Cretaceous, subsidence was dominant, but tectonic being preserved. However, the hydrocarbon generation center uplift (resulting in denudation) has been dominant since migrated toward the Lin 1 Well. The Lintanchang-Dingshan the late Cretaceous. The key period of tectonic transition structural belt was high at the west and low at the east. At that from subsidence to uplift (denudation) was around 80 Ma. time, the lower Cambrian source rocks intensely generated A geothermal gradient 30°C/100m and surface temperature hydrocarbon and the Lin 1 Well located on the high point of 20°C were used for the simulation (Zhang, 1997). Middle or the structure captured a large quantity of oil. The size of the high-speed uplift occurred during 65-82 Ma, at a rate of 93 two phases superimposed paleo oil pool reached up to 6.7×10 m/Ma, with an uplift amplitude of 1,030 m. Low-speed uplift t in the Lin 1 Well. The Dingshan 1 Well is located at the occurred during 25-65 Ma, at a rate of 14.8 m/Ma, with an top of the Dingshan structure and the size of paleo oil pool uplift amplitude of 720 m. Moderate-speed uplift began to reached 1.9×10 t (Fig. 5). occur in the 25Ma period until present at a rate of 59.8 m/Ma Afterwards, the buried depth, temperature, and with an uplift amplitude of 1,360 m. Therefore, since the late pressure at the Sinian top surface increased due to Cretaceous, at least 3,000 m of the strata have been eroded continuous subsidence. In the late Triassic, the buried in this region. As a result of the uplift, the strata outcropping depth of the Sinian top surface reached 4,800 m and the in this area are relatively old (Fig. 1) and many steep faults geotemperature was 170°C. Large amounts of oil trapped and fracture systems were formed, which greatly changed in the Sinian began to crack, the pressure in the oil-gas previous good reservoir conditions in the area. Because of reservoir increased further, and reached a peak during tectonic movements, reservoirs were destroyed and the gas the late early Cretaceous (74 Ma). Fission track analysis generated from the cracked crude oil escaped, so the paleo of the apatite showed that the buried depth of the Sinian gas pool was totally destroyed. top surface in the Dingshan 1 Well once reached 7,060 5.3.2 Sinian preservation conditions m, with temperature up to 220°C. Thus a large paleo The isotopic composition of strontium in seawater is gas pool was formed from a large amount of cracked considered to be globally consistent (McArthur et al, 1992). hydrocarbons. The main cap rocks (including gypsum) in Thus, strontium isotope measurements can be employed the area were very good, and gypsum was a good sealing to trace and contrast fluid sources and interpret oil and gas for gas diffusion. The sealing was destroyed after folding, preservation conditions. The following discussion on fluid uplifting and denudation during the Yanshan-Himalayan sources and preservation conditions is based on analyses of period. Therefore, there was a paleo gas pool because of oil samples from the Lin 1 Well. cracking before the sealing was destroyed. The Lintanchang (1) Isotopic geochemical characteristics and paleo-fluid 8 3 structure had a paleo gas pool of 4,154×10 m and that sources 8 3 of the Dingshan structure was 1,178×10 m . Therefore, Through contrasts of the strontium isotope ratio in the the size of the whole paleo gas pool in the Lintanchang- host rocks and vugs, or fracture fi llings of different horizons 8 3 Dingshan structural belt reached about 5,300×10 m in the Lin 1 Well, the lower assemblage (Sinian to lower (Fig. 3(e), Fig. 5). Palaeozoic) is characterized as follows (Fig. 6): (1) The Pet.Sci.(2010)7:289-301 297 Lin 1 Well Sangmu anticline Xianfang Luocun SE Depth a (m) -1000 Destroyed completely -3000 -5000 Lin 1 Well Second stage bitumen -1000 in Permian? Residual gas pool -3000 -5000 Lin 1 Well First stage bitumen Jura-Cretaceous in Permian? -1000 Triassic -3000 Paleo-gas pool -5000 Gas dissolved in water Lin 1 Well Permian -1000 Second stage paleo-oil pool -3000 superimposed on the first oil pool Lin 1 Well Silurian -1000 -3000 Lin 1 Well Silurian -1000 -3000 Ordovician Cambrian -1000 First stage paleo-oil pool Dengying -1000 Dengying Fig. 5 Formation and destruction processes of Lintanchang paleo oil and paleo gas pools in the southeast Sichuan Basin (Based on seismic line TTBli-05-608 balanced section) (a) Present; (b) Himalayan; (c) Before Himalayan; (d) After the Permian sedimentation; (e) Before the Permian sedimentation; (f) After the Silurian sedimentation; (g) After the Ordovician sedimentation; (h) After the Sinian sedimentation Subside Subside Uplift Subside Subside Uplift Present 298 Pet.Sci.(2010)7:289-301 strontium isotope ratio of host rocks and fillings is higher Palaeozoic strata generally had the same fl uid sources as the than that of contemporary normal seawater. (2) The strontium upper Sinian, which revealed that there was cross-formation isotope ratio of fi llings is higher than that of host carbonate fluid migration between the upper Sinian and the lower rocks, but is lower than that of host clastic rocks. (3) Carbon Palaeozoic. This also indicated that the overall preservation and oxygen isotope ratios of fi llings are greatly different from conditions of the upper Sinian in this structure were relatively those of host rocks. (4) Strontium isotope ratios of adjacent poor. For instance, in the Dingshan-Yueping profi le, the lower horizons vary greatly. All these characteristics indicate that Cambrian cap rocks had good sealing capacity before the oil there were four sets of different fluid systems in the lower started to crack when Yanshan-Himalayan uplifting began. assemblage (Fig. 6). Fluids in the lower Silurian Shiniulan The upper Sinian and the overlying lower Palaeozoic had Formation and the first stage of the upper Sinian had the mutually independent fluid systems. The abnormal pressure same source (system C ), whereas source rocks in the lower in the reservoir strata in the Dengying Formation reached Silurian Longmaxi and lower Cambrian Niutitang Formation up to 78-86 MPa during oil cracking (data obtained from belonged to two other independent fluid systems (system simulation pressure tests of fluid inclusions), approaching A and system B). The upper Sinian fluids were extremely the breakthrough pressure of the Niutitang Formation. complex with three subsystems (system C , C , C ) (Fig. 6). Furthermore, high angle faults and fracture systems were 1 2 3 Therefore, there were three major sources of fl uids in the Lin formed. The loss of sealing by the overlying argillaceous 1 Well: (1) the fl uid modifi ed by re-dissolution from the lower rocks and gypsum led to cross-formation fluid migration Silurian Longmaxi Formation itself (system A); (2) the re- among the upper Sinian, the Ordovician and the Silurian. At dissolution fl uid of the lower Cambrian Niutitang Formation the same time, the cracked gas pool in the upper Sinian was (system B); (3) the two phases of strontium-enriched fl uid in destroyed. the Dengying Formation (system C). It can be concluded that the Cambrian gypsum and (2) Loss of preservation conditions and destruction of gas mudstone in the Dingshan and Lintanchang structural belt pools had a relatively good sealing capacity before and at the Fig. 6 is a trace diagram showing geochemical strontium beginning of the oil cracking. Nevertheless, the study area isotope ratios in the Lin 1 Well, Dingshan 1 Well and the had been strongly tectonically folded, uplifted and denuded sections of the structural belt. Fig. 6 shows that the upper since the late Cretaceous, with the upper Sinian to the lower Sinian internal fluid was the most complex fluid with 2-3 Palaeozoic strata subaerially exposed. Thus the earlier good subsystems no matter whether it was underground (in drilled preservation conditions were lost and the cracked gas pools in wells) or at the surface profi le. However, the overlying lower the Dengying Formation were destroyed (Fig. 5). Observed geochemical sections and well name Age Lithology Dingshan-Yueping Wenshui-Chengjiashan Lin 1 well Dingshan 1 well Notes 87 86 87 86 87 86 87 86 Sr/ Sr Fluid conductivity Sr/ Sr Fluid conductivity Sr/ Sr Fluid conductivity Sr/ Sr Fluid conductivity fluid system D Limestone 0.7113 0.70-0.7076 Not interconnected Seal between fluid systems 0.7123 0.7118-0.7120 fluid system C 0.7131 fluid system C 0.7114-0.7116 0.7115 Cross-formation 0.7170-0.7180 fluid system A Fluid migration Interconnected 0.7149 Cross-formation 0.7119 Late period Fluid migration fluid system C 0.7142 Cross-formation Interconnected interconnected Fluid migration with fluid Interconnected Not interconnected system D 0.7131 between fluid systems fluid system A 0.7160 0.7109 0.7116 Cross-formation Fluid system C Fluid migration Cross-formation Cross-formation Interconnected cross-formation Fluid migration migration and Fluid migration 0.7115 Interconnected Interconnected forming mixed fluid Dolomite 0.7131 0.7092 Cross-formation Fluid migration 0.7186-0.7187 fluid system B Interconnected 0.7157 0.7119-0.7121 fluid system B 0.7119-0.7121 0.7119-0.7121 fluid system C fluid system C 0.7112-0.7114 1 0.7136-0.7138 3 0.7112-0.7114 0.7129-0.7132 fluid system C fluid system C 0.7120-0.7121 0.7107-0.7109 fluid system C fluid system C 0.7107-0.7109 3 0.7102-0.7110 1 Fig. 6 Diagram showing cross-formation fl uid migration from Sinian to lower Palaeozoic in the Dingshan-Lintanchang structure, southeast Sichuan Basin Sichuan Basin was the highest at the axis of the Caledonian 6 Discussion Leshan-Longnüsi paleo-uplift (also called the central Sichuan uplift). The bitumen content in the Nüji Well reached 8.1% 6.1 Distribution of oil saturation in the Dengying (Fig. 7). The bitumen content declined from the uplift to Formation controlled by paleo-uplifts the depressions (e.g. Woshen 1 Well 0.4%, Zishen 1 Well The bitumen content in the Dengying Formation in the 1.1%) (Wang et al, 2002). This indicated that the Caledonian Sinian Cambrian Ordovician Silurian Permian Superpressure fluid released during Yanshan-Himalayan uplifting, which caused fluid cross-formation migration and interconnected. Preservation conditions were better at early stage and poor at late stage. Pet.Sci.(2010)7:289-301 299 Guanyinqiao Guanyinqiao Dingshan 1 Dingshan 1 3.16 0.83 50 25.4 25.4 Hanjiadian Hanjiadian Guandu Guandu Wenshui Wenshui 0.5 0.5 1.5 2.0 1.0 3.0 Xishui Xishui Lin 1 Lin 1 0.5 0.5 Jiuba Jiuba 5.5-10 5.5-10 1.37 2.87 Sangmu Sangmu 47.7 47.7 Well City Town Sinian outcrop Well City Town Sinian outcrop Taipingdu Taipingdu Percentage of thin-sections containing Percentage of thin-sections containing bitumen from paleo-oil pool bitumen from paleo-oil pool Average content of bitumen in the thin-sections Average content of bitumen in all the thin-sections containing bitumen Average content of bitumen in all the Average content of bitumen in the thin-sections containing bitumen from paleo-oil pool thin-sections from paleo-oil pool 0.5 0.5 5.5-10 Percentage of thin-sections 5.5-10 Percentage of thin-sections containing bitumen from paleo-oil pool containing bitumen from paleo-oil pool Zi 1 Wei 117 Zishen 1 Gongshen 1 Dingshan 1 Zi 1 Wei 117 Zishen 1 Gongshen 1 Dingshan 1 Depth, m Depth,m Chengdu Zunyi Chengdu Zunyi -500 -500 TODF>1160m POCH:500m -1000 -1000 TODF:560m POCH:123m -1500 -1500 Paleo-structure of top surface of Sinian before Triassic Paleo-structure of top surface -2000 -2000 of Sinian before Permian TODF:613m -2500 -2500 POCH:272m -3000 -3000 No drilling information, supposed paleo-oil pool and similar oil column TODF:1208m -3500 -3500 POCH:350m -4000 -4000 TODF:Thickness of Dengying Formation POCH:Paleo-oil Column Height -4500 -4500 No drilling information, supposed paleo-oil : Well pool and oil column height: 350-500m -5000 -5000 Slope of Central Slope of Central Central Sichuan Central Sichuan Slope Depression Guizhou Slope Depression Guizhou b Paleouplift Paleouplift Paleouplift Paleouplift Fig. 7 Characteristics of paleo oil pools of the upper Sinian in the Sichuan Basin and its peripheral areas (a) Plan distribution of the Dengying Formation bitumen in the paleo oil pool in the Dingshan-Lintanchang structural belt; (b) Profi le of the paleo oil fi eld from the central Sichuan paleo-uplift to the slope of central Guizhou paleo-uplift (See the well location in Fig. 8) paleotectonic framework controlled the distribution of oil in of Jinsha Yankong, Kaiyang Yangshui (Yongshaba) and the Dengying Formation, and that the paleo-uplift was the Weng’an Baidoushan (Qianchang) in the central Guizhou primary area of oil migration and accumulation. paleo-uplift indicate that there was an upper Sinian large The southeast Sichuan Basin is located in the slope- paleo oil fi eld similar to that in the Leshan-Longnüsi paleo- depression area of the central Guizhou paleo-uplift with the uplift (Tian et al, 2006; Zhou and Liang, 2006; Yang et al, Woshen-Gongshen depression between this region and the 2008). Therefore, bitumen can be observed today in any of Leshan-Longnüsi paleo-uplift. The Permian-Triassic was the the Dengying Formation outcrops and well cores. There was most important period for the generation of hydrocarbons in probably an ultra-large, structure related oil-gas field in the the study area. Before the Triassic and the Permian, the study Dengying Formation in the Sichuan Basin and its peripheral area was located at the northern slope of the central Guizhou areas (Fig. 8). paleo-uplift. During the peak of oil fi lling, the Lintanchang- 6.2 Positive correlation between oil column heights Dingshan belt was a local structural high on the slope of of paleo oil pools and residual thickness of the central Guizhou paleo-uplift with large quantities of oil Dengying Formation accumulating (Fig. 7). The bitumen content in the Dengying Formation in Lin 1 Fig. 7(b) is the profi le of the upper Sinian paleo oil fi eld well and Dingshan 1 well was lower than that at the Leshan- from the central Sichuan paleo-uplift (Leshan-Longnüsi Longnüsi paleo-uplift (Fig. 7). The percentage of the thin paleo-uplift) to the Lintanchang-Dingshan structural belt in sections containing bitumen in Lin 1 well and Dingshan 1 the southeast Sichuan Basin. It can be seen that the Dengying well is similar with that of the paleo-uplift. For instance, the Formation in Zi 1 Well, Wei 117 Well (uplift area) and percentage of thin sections containing bitumen inside paleo Gongshen 1 Well (depression area) had a residual thickness oil column in the Lin 1 Well (at least 242 m in height) was of 560 m, 613 m and 1,208 m, with an oil column height 47% (25% in the Dingshan 1 Well) (Fig. 7(a)), close to that of 123 m, 272 m and 350 m, respectively. Also located in a in the Woshen 1 Well located in the depression, but is higher depression area of the northern slope of the central Guizhou than that in the Wei 117 Well (39%) located on the slope. paleo-uplift, the Dengying Formation in the Dingshan 1 well The characteristics of the upper Sinian paleo oil pools was of at least 1,160 m thick with 500 m oil column height. Legend Legend 300 Pet.Sci.(2010)7:289-301 Yangba,Nanjiang 1 N Ningqiang Shuimoba,Wangcang Qiang 1 Zhengyuan,Wangcang Zeng 1 Yanjinghe,Wangcang Qingchuan 12 0 40 80km Ganhe,Wangcang 5 2 Pingwu Guangyuan Hui 1 Shixihe,Chengkou Nanjiang Mukuihe,Chengkou Sangmuchang,Xishui Jinhekou,Ebian Chengkou Tianquan Lingguangzhen,Baoxing Guanzhuang,Qingchuan Maoxian No data Chengdu Li 1 Nji Anping 1 Zhi 1 Zhi 2 Gaoke 1 Tianquan Yaan Shizhu Zhougong 1 Emeishan Wei117 Leshan Pan 1 Chongqing Zishen 1 Legend Laolong 1 Ebian Sinian outcrop Woshen 1 City Mabian Gongshen 1 Dingshan 1 Well, reach Sinian, have bitumen Well, reach Sinian, lack information Xishui Location, have bitumen Ning 2 8 in Sinian Lin 1 Ning 1 Supposed paleo-oil field Fig. 8 Diagram showing the distribution of the ultra-large structure-lithologic oil-gas fi eld in the upper Sinian in the Sichuan Basin and its peripheral areas Hence, it could be concluded that bitumen content in the fi rst stage of paleo oil pool formation was maturation of the uplift area was high, whereas the paleo oil column height was lower Cambrian source rocks during the late Ordovician. relatively low. The hydrocarbon generation from the lower Cambrian source In conclusion, the high content of bitumen of the rocks stopped during the Devonian to the Carboniferous Dengying Formation in the paleo-uplift areas suggested tectonic uplifting. The lower Cambrian source rocks then that the paleo-uplifts were the preferential areas of oil-gas generated large quantities of hydrocarbons during the second migration and accumulation. The low paleo oil columns in stage of paleo oil pool formation, after the middle Permian these areas indirectly indicated that there was no unified deposition. The oil began to crack and the paleo gas pool oil-water interface in the upper Sinian paleo oil field from was formed during the late Triassic. This paleo gas pool paleo-uplifts to slope areas. The Dengying Formation was damaged and reservoirs were destroyed as a result is characterized by petroleum pool formation and local of extensive Yanshan-Himalayan folding, uplifting and hydrocarbon enrichment in the Sichuan Basin and its denudation. periphery areas. 4) Residual bitumen can be found in the Dengying Formation inside the Sichuan Basin as well as in its periphery 7 Conclusions areas. It is suggested that there was an ultra-large oil-gas fi eld formed within the Dengying Formation, with petroleum 1) The lower Cambrian source rocks in the Dingshan- pools formed widely from local hydrocarbon enrichment in Lintanchang structural belt in the southeast Sichuan the Sichuan Basin and its periphery areas. Consequently the Basin were of medium-good quality with two excellent paleo gas pools were formed from thermal cracking of crude hydrocarbon-generating centers developed in the periphery oil, with the pools destroyed during the Yanshan-Himalayan areas. The upper Sinian dolomite (Dengying Formation) folding, uplifting and denudation. formed good reservoir rocks. The overlying cap rocks of the lower Cambrian Niutitang Formation mudstone provided Acknowledgements satisfactory sealing capacity. Therefore, there were excellent original geologic conditions for hydrocarbon accumulation in This study was supported by the National Basic Research the Dengying Formation. Program of China (No. 2005CB422106). We are thankful to 2) The Dingshan-Lintanchang structural belt had a paleo Luba Jansa for the assistance during fi eld studies. oil pool with 8.6×10 t oil and then a paleo gas pool with 8 3 References 5,300×10 m gas from cracked oil. 3) The formation and destruction processes of the Ao M C, Lü T Z, Hu N F, et al. Lithological features of the Dengying Dengying Formation oil-gas pool in the Dingshan- Formation of the Sinian in Well Dingshan 1 in south-eastern part Lintanchang structural belt occurred in several stages. 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Formation and destruction processes of upper Sinian oil-gas pools in the Dingshan-Lintanchang structural belt, southeast Sichuan Basin, China

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Springer Journals
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Copyright © 2010 by China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg
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
DOI
10.1007/s12182-010-0071-3
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Abstract

Lintanchang structure in the southeast Sichuan Basin were of medium-good quality with two excellent hydrocarbon-generating centers developed in the periphery areas, with a possibility of forming a medium to large-sized oil-gas fi eld. Good reservoir rocks were the upper Sinian (Dengying Formation) dolomites. The mudstone in the lower Cambrian Niutitang Formation with a good sealing capacity was the cap rock. The widely occurring bitumen in the Dengying Formation indicates that a paleo oil pool was once formed in the study area. The fi rst stage of paleo oil pool formation was maturation of the lower Cambrian source rocks during the late Ordovician. Hydrocarbon generation from the lower Cambrian source rocks stopped due to the Devonian-Carboniferous uplifting. The lower Cambrian source rocks then restarted generation of large quantities of hydrocarbons after deposition of the middle Permian sediments. This was the second stage of the paleo oil pool formation. The oil in the paleo oil pool began to crack during the late Triassic and a paleo gas pool was formed. This paleo gas pool was destroyed during the Yanshan-Himalayan folding, uplifting and denudation. Bitumen can be widely seen in the Dengying Formation in wells and outcrops in the Sichuan Basin and its periphery areas. This provides strong evidence that the Dengying Formation in the Sichuan Basin and its periphery areas was once an ultra-large structural-lithologic oil-gas fi eld, which was damaged during the Yanshan-Himalayan period. Sinian, oil-gas pool, Dingshan-Lintanchang structural belt, Sichuan Basin Key words: Sinian are not yet fully understood. 1 Introduction The southeastern area of the Sichuan Basin is part of The Sichuan Basin is one of the most important oil-gas the eastern steep structural belt and the southern gentle bearing superimposed basins in south China. The oldest structural belt (Fig. 1). Currently, this area is ideal for sedimentary strata are upper Sinian (Dengying Formation). research on accumulation and formation of upper Sinian oil The Dengying Formation is one of the most signifi cant high- and gas pools in the Sichuan Basin due to several factors: (1) quality reservoirs and gas-bearing horizons in the Sichuan Abundant information is available as Sinopec has conducted Basin. The Weiyuan gas field, discovered in 1964, was the extensive exploration and seismic prospecting in this area first large and medium-sized natural gas field found in the in recent years. The upper Sinian was the target stratum in Sichuan Basin with proven natural gas reserves of 408×10 the Dingshan 1 Well and Lin 1 Well, but no economic gas m . The major pay zone consisted primarily of Dengying accumulation was found; (2) Core is available from wells Formation carbonate rocks (Sun et al, 2009). Although drilled through the Dengying Formation; (3) The outcrops of exploration and research have been carried out for over the upper Sinian in the area are suitable for comprehensive 50 years in the Dengying Formation in the Sichuan Basin, study of the surface and subsurface geology. no other similar-sized natural gas fields have been found The study area is located at the southeastern edge of in this formation. It appears that the controlling factors of the Sichuan Basin and at the northern slope of the Central preservation and distribution of gas and oil in the upper Guizhou Uplift. Outcropping strata in the study area are mainly Palaeozoic, while those in the Sichuan Basin are mainly Cretaceous and Jurassic (Fig. 1). The study of the *Corresponding author. email: lsg@cdut.edu.cn formation of oil and gas pools in the upper Sinian in this Received September 7, 2009 Shimian Fault SW. Sichuan FTB ST IC Xianshuihe fault Triassic suture zone Micang Mountain EQTS Triassic suture zone Middle Paleozoic (Shangdan fault zone) Daba N QT suture zone 290 Pet.Sci.(2010)7:289-301 stretching to the Nanchuan-Zunyi major fault and western area is of great importance for petroleum exploration in area stretching to the gentle fold area in the southern Sichuan the Sichuan Basin and evaluation of the upper Sinian oil Basin, limited by the Xingwen-Gulin hidden fault. The and gas exploration potential of the central Guizhou area. Dingshan and Lintanchang structures are two secondary Based on comprehensive research on geology, geochemistry, anticlines on the western flank of the Sangmuchang geophysics, and surface and subsurface geology, we not composite anticline (Fig. 1). Sedimentary strata outcropping only present the petroleum accumulation conditions and the in this area are from the upper Sinian to the Cretaceous, with processes of formation and destruction of the upper Sinian the upper Silurian, Devonian and Carboniferous absent due petroleum pool in the Dingshan–Lintanchang structural to the Caledonian movement. However, the upper Sinian belt in the southeast Sichuan Basin, but also we discuss the strata can be found only on the axis of the Sangmuchang characteristics of the upper Sinian paleo oil-gas reservoirs in composite anticline (in the Heba-Runnan region) in an area the Sichuan Basin. of about 10 km (Fig. 1). Petroleum exploration revealed that the Dengying Formation is about 1,200 m thick (Dingshan 2 Regional background 1 Well) with the top at a depth of 2,500-3,500 m. The Sinian The Dingshan–Lintanchang northeast structural belt is unconformably overlain by lower Cambrian because of belongs to the transitional zone between the southeastern the Tongwan movement. Two major fault systems, namely, edge of the Sichuan Basin and the western edge of the northeast-trending Sangmuchang and nearly north-south Xuefeng Mountain Uplift. This structural belt is the front of trending Nanchuan-Zunyi faults are developed. In addition, a the Loushanguan trough-like fold belt, with its eastern area few northwest-trending faults can be found (Fig. 1). 102° 104° 108° 112° E 106° 110° 34°N West Qinling FD Dingshan 1 MO Ningshan Fault Simianshan BT HD Wudang DT3 (J sn) Datong Songpan-Ganzi 2 East Terrain Terrain Qinling FTB 32° DSA (P l Jiudianya (from Dingshan 1 Well) SD Guandu Sichuan Basin 30° Yueping ATF QB NCB Qinling ADX4(Js 2 ) QT DT6 (J sn) Tanlu Fault 2 LT SGT ACQ1(Js 2 ) 28° DT4 (J zl) SCB Wenshui Dabie Mountain 1-2 100km DT DT2 (T xj) ALC14 (P q) ACQ1 (J s) 2 Houtan Xishui Runnan Lin 1 Tudiba Z PS Heba Sangmu Tongzhi K J T P P-T S O ∈ Z Geochemical section County Town Cretaceous Jurassic Triassic Permian Permian-Triassic Silurian Ordovician Cambrian Sinian Apatite sample Well Fig. 1 Simplifi ed geological map of the study area NCB: North China block, SCB: South China block, SGT: Songpan-Ganzi terrain, QB: Qaidam Basin, QT: Qiangtang terrain, LT: Lhasa terrain, DT: Southeast Asia terrain, ICT: Indochina terrain, ST: Simao terrain, ATF: Altyn Tagh Fault, NQT: North Qinling terrain, EQTS: East Qinling thrust system, BT: Bikou terrain, FD: Foping dome, SD: Shennongjia dome, HD: Hannan dome, FTB: fold-thrust belt, MO: Mianlüe ophiolite, AO: Anyemaqen ophiolite. Modifi ed from Meng et al (2005) 2003). 3 Source, reservoir and cap rocks GC-MS analysis showed that there is adamantane in the upper Sinian bitumen with minor characteristics of water- 3.1 Lower Cambrian source rocks washing oxidation, which indicates that the reservoir bitumen Abundant bitumen is present in the Dengying Formation in the Dengying Formation was formed by the pyrolysis of in the Sichuan Basin and its periphery areas. It was sourced crude oil. mainly from the lower Cambrian argillaceous rocks (Dai, The lower Cambrian argillaceous rocks in the Sichuan FTB Mountain Sichuan Basin Longmen Mountain Thrust Belt Sangmuchang Fault Eastern Sichun fold-thrust belt Nanchuan-Zunyi Fault Pet.Sci.(2010)7:289-301 291 Basin are mainly distributed in the Yibin-Luzhou area Sinian cores of the Dingshan 1 Well and Lin 1 Well (Sichuan Province), western Hunan and Hubei Provinces and (Fig. 2(a)). Some of them could represent dissolution Nanjiang-Ningqiang areas, with a thickness of 200-300 m. originated vugs subsequently partly infilled. The Two centers of hydrocarbon generation in the lower Cambrian petrological characteristics of the fillings (e.g. crusty source rocks exist in the Yibin-Luzhou and west Hunan-west structure developing at the edge) and the geochemical Hubei areas with a hydrocarbon generation capacity of (150- characteristics of carbon, oxygen and strontium isotopes 8 3 2 200)×10 m /km . In the study area, the medium-good quality showed that the formation mechanism was freshwater source rocks in the lower Cambrian Niutitang Formation are karstification. The density of vugs and fractures in the 0-20 m thick (Liang et al, 2008). In the Dingshan 1 Well, Dingshan 1 Well was 0.15-8 per meter and 0.21 per the black carbonaceous mudstone at the bottom of the lower meter respectively, with most of the vugs being medium Cambrian Niutitang Formation is 13 m thick. The TOC in 29 and small in size. The dissolved vugs and fractures in samples was 0.3%-3.9%, with 25% of the samples having the study area did not develop as much as those in the TOC higher than 0.5% and the average is 0.7%. Kerogen palaeo-uplift areas in the central Sichuan Basin (Liu et is of type II , and is a medium quality hydrocarbon source al, 2008a). (Qin et al, 2005; 2006; 2009). In the Lin 1 Well, the dark (2) Pores and vugs dissolved by acid fl uids mudstone of the Niutitang Formation is 158 m thick, with Carbonate rocks in the study area have been strongly a kerogen type of I-II . The mudstone is dark gray except corroded by acidic fluids (Barth and Bjørlykke, 1993), for the grayish-black areas at the bottom. The TOC of eight indicated by dissolution along stylolite seams (Fig. 2(b)). samples was 0.07%-0.95%, and that of two samples between Dissolution along fractures and algal laminae (Fig. 2(c)) as 2,462.13 m and 2,468.43 m was only 0.07% and 0.08% well as pores were found in cores from the Lin 1 Well, with respectively. Thus, it is evident that the upper part of the a residual plane porosity of 3% (Fig. 2(d)). Thin-section Niutitang Formation was a fairly poor source rock and had observation of the rocks showed corrosion and fi lling of late almost no hydrocarbon generating capacity, but the lower part acid silica along stylolites in the upper Sinian rocks. was relatively good hydrocarbon source rock, with its organic (3) Vugs dissolved by hydrothermal fl uids carbon content higher than 0.55%. Silica rocks, zebra dolomite (Fig. 2(e), (f) and (g)), The lower Cambrian source rocks in the southeast Sichuan lead zinc ore, fluorite and barite are present in the upper Basin were of medium-good quality and comprised beds Sinian of the Dingshan–Lintanchang structural belt in the only 13-20 m thick. However, two high-quality hydrocarbon southeast Sichuan Basin. Their presence indicates that the generation centers developed in the periphery area. Being an Dengying Formation has been altered by hydrothermal fl uids, important area for oil and gas migration and accumulation, and the porosity is up to 20% (Fig. 2(e)), especially at the this study area contained petroleum sources needed for the anticline axis area of Sangmu Town. At the well cores near formation of a large or medium-sized oil-gas fi eld (Liu et al, the unconformity between the upper Sinian and the lower 2006). Cambrian in the Lin 1 Well, isolated pores developed inside the white coarse-grain dolomites with the vug porosity up 3.2 Sinian reservoir rocks to 3%. Moreover, intercrystalline pores also developed in saddle dolomites which now contain bitumen 5% of the rock Reservoir rocks in the Dengying Formation are mainly (Fig. 2(h)), which indicates that the Dengying Formation had algal boundstones, and doloarenite (Ao et al, 2006). Porous suffi cient porosity during the oil-gas charging process. dolomites developed near the unconformities between the Therefore, deep-burial acid fluid dissolution and upper Sinian and the Cambrian. The samples from the Lin hydrothermal dissolution formed a good-quality reservoir on 1 Well, Dingshan 1 Well and the outcrops showed that the the basis of early freshwater karstic pores in the Dengying Dengying Formation reservoir rocks were tight with the Formation. porosity of 1%-2% (Liu et al, 2007; 2008b). The reservoir space is mainly intercrystalline (dissolved) pores, vugs and/or 3.3 Cap rocks fractures. (1) Pores and vugs formed by fresh water dissolution Apart from the mudstones in the lower Cambrian 13 18 The δ C and δ O isotopic values of carbonate rocks Niutitang Formation, as direct cap rocks for the Dengying under the unconformity were suddenly lowered by the Formation, gypsum, gypsum dolomite, or limestone in the interaction between the fresh water and rocks, leading to middle-lower Cambrian had high sealing capacity. abrupt change of carbon and oxygen isotopes (Cander, (1) Argillaceous cap rocks 1995; Dickson and Saller, 1995; Wagner et al, 1995). Distributed over the whole study area, lower Cambrian The carbon and oxygen isotopes of carbonate rocks and argillaceous cap rocks such as shale and sandy mudstone fillings of the pores and vugs below the unconformity are interlayered with limestone. For example, the lower between the upper Sinian and the lower Cambrian in Cambrian in the profi les at Houtan, Xishui County, has a total the Dingshan 1 Well showed a significant decline and thickness of 423 m, among which argillaceous rocks are 213 negative bias, indicating palaeo-karstification. The m thick, accounting for 50% of the strata. The total thickness karstification reached a depth of around 150 m below of argillaceous limestone and argillaceous dolomite is 32 m, the unconformity surface. A large quantity of dissolution accounting for 7.6%. Therefore, the total thickness of the cap pores, vugs and karst breccia can be found in the upper rocks reached up to 245 m, accounting for 58% of the strata. 292 Pet.Sci.(2010)7:289-301 a b c d e f g h Reservoir rock photographs of the upper Sinian Dengying Formation in the study area Fig. 2 (a) Dissolution pores by meteoric water (arrow). The pores are filled with botryoidal crust and fine-crystalline dolomite in supergene stage, and coarse crystalline dolomite, quartz, and bitumen in the burial stage. Residual porosity is about 5%. Upper Sinian Dengying Formation, Dingshan 1 Well. (b) Burial dissolution along the stylolite. Residual porosity is about 3%. Upper Sinian Dengying Formation, Lin 1 Well. (c) Algae doloarenite. Dissolution pores developed in the algae-rich laminae by deep burial dissolution. Residual porosity was about 5%. Upper Sinian Dengying Formation. Lin 1 Well (2,625.5 m). (d) Corrosion and fi lling-up of fracture. Residual porosity is about 3%. Upper Sinian Dengying Formation, Lin 1 Well (2,652.2 m). (e) Pores by hydrothermal dissolution. Residual porosity was about 20%. Upper Sinian Dengying Formation, Heba, Xishui (f) Zebra dolomite at the top of the Sinian. Pores by hydrothermal dissolution developed inside. Residual porosity is about 8%-10%. Heba, Xishui. (g) Zebra dolomite and pores formed by hydrothermal dissolution. Residual porosity is about 3%. Upper Sinian Dengying Formation, Lin 1 Well. (h) Bitumen in intercrystalline pores of saddle dolomite. Bitumen content is 5%. Upper Sinian Dengying Formation, Lin 1 Well. Pet.Sci.(2010)7:289-301 293 The maximum thickness of a single layer of argillaceous The pure gypsum rocks (white and gray-white) are 29 m cap rocks in the lower Cambrian Niutitang Formation in thick, and gray-white gypsiferous dolomite (limestone) is 10 the Dingshan 1 Well and Lin 1 Well is 28.5 m. The general m thick in the well. The gypsum in the Lin 1 Well is mainly thickness of a single layer is 6.0 m and the average total located at the top of the Qingxudong Formation, with a thickness is 146 m. The breakthrough pressure of these rocks thickness of 13 m. Gypsum is overlain and underlain by gray- is 77-87 MPa. white gypsiferous dolomites. Gypsum rock in the middle Therefore, it can be concluded that the lower Cambrian Cambrian Shilengshui Formation is 27 m thick, and in the argillaceous rocks can be classifi ed as high-quality regional Qingxudong Formation is 17 m thick. Thus, the middle-lower cap rocks for the upper Sinian. Cambrian gypsum cap rocks provide excellent sealing for the (2) Gypsum cap rocks underlying Dengying Formation. Gypsum which is considered to be an ideal cap rock is 4 Characteristics of structural traps widely developed during the middle-lower Cambrian. The Cambrian gypsum-salt strata mainly occur in the middle The Dingshan-Lintanchang structural traps were primarily Cambrian Shilengshui Formation and lower Cambrian formed during Cambrian sedimentation, with inheritance at Qingxudong Formation, with a regional thickness of 2-46 m. later stages. However, the sizes and widths of traps changed Basically, gypsum rocks in the Dingshan 1 Well are evenly greatly in different periods and the structural high had a developed in the Qingxudong and Shilengshui formations. degree of migration as well (Table 1, Fig. 3). Table 1 Contrast of trap elements at the top surface of the Sinian between the Dingshan Structure and the Lintanchang Structure Items Dingshan Structure Lintanchang Structure Closed area, Closed height, Closed height, m Structural high of traps Closed area, km Structural high of traps Periods 2 km m Cambrian 130 100-<200 SE Dingshan 1 Well 220 200 SE Lin 1 Well Defl ecting toward Ordovician 160 200 310 400 SW Lin 1 Well Dingshan 1 Well Defl ecting toward Silurian 120 400 250 400 SW Lin 1 Well Dingshan 1 Well Structural high Devonian-Carboniferous 155 400 205 400 SW Lin 1 Well maintained Structural high Permian–Late Cretaceous 120 500 220 400 SW Lin 1 Well maintained Late Cretaceous-Present 130 700 SE Dingshan 1 Well 190 600 Lin 1 Well The subsurface depth of the Dengying Formation in the 5 Scale and formation process of the Sinian Lin 1 Well is 2,585 m to 2,910 m with a thickness of only paleo oil pool 325 m (without reaching the bottom of the formation). The average content of bitumen in the thin sections from the upper 5.1 Scale of the Sinian paleo pool and its oil-cracking Sinian cuttings is 2.2%. Microscopically, the bitumen content gas in the thin sections from the cores is between 0.1%-8%. 50% of all thin sections from cores of the Dengying Formation Microscopically, the bitumen content in the cores of the in the Lin 1 Well contain bitumen. The bitumen distribution Dengying Formation in the Dingshan 1 Well is 0.5%-5.6% is discontinuous, most is found within 100 m under the top (Fig. 4). On average the thin sections contained 3% bitumen of the unconformity. Below the depth of 2,827 m, no large with only a few reaching up to 9% bitumen. The bitumen quantities of bitumen have been found. Therefore, the depth content of thin sections from near the unconformity between of 2,827 m can be identified as the oil-water interface and the Sinian and the Cambrian is relatively high and gradually the oil column height of the Sinian paleo oil pool in the Lin 1 declines below the unconformity. However, the bitumen Well was at least 242 m (Fig. 4). content increased in some upper Sinian intervals in the The peak of hydrocarbon generation from the lower Dingshan 1 Well. The average bitumen content at the depth of Cambrian Niutitang Formation source rocks was during the 3,573.9 to 3,604.2 m is 0.17%. However, the bitumen content Permian, which was the main period for the formation of is up to 0.5% at the deeper level of 3,650.61 m in the well (Sun paleo oil pools in the study area. At the end of the Permian, et al, 2010). The drilling depth of the Dengying Formation in the Lin 1 Well was actually not the structural high of the the Dingshan 1 Well was 3,492-4,610 m (without reaching Lintanchang structural trap, but a local high point on the the bottom of the formation). Bitumen occurred continuously slope (Fig. 3(e)). With the palaeo-structure reconstruction of from the top surface to a depth of 4,070 m, but below this the top surface of the upper Sinian after the middle Permian depth no bitumen could be found. Therefore, this may sedimentation, it could be inferred that the paleo oil column indicate that the oil-water interface in the upper Sinian paleo height in the Lintanchang structural trap was about 350 m, oil pool was at a depth of 4,070 m and its oil column height with an oil-bearing area of 220 km (Fig. 3(e)). was about 500 m (Fig. 4). 294 Pet.Sci.(2010)7:289-301 Evolution of the Dingshan-Lintanchang structural belt in the southeast Sichuan Basin Fig. 3 (a) Structural map of the top surface of the Sinian after the Lower-Cambrian sedimentation. (b) Structural map of the top surface of the Sinian after the Ordovician sedimentation. (c) Structural map of the top surface of the Sinian after the Lower-Silurian sedimentation. (d) Structural map of the top surface of the Sinian after the Silurian sedimentation. (e) Structural map of the top surface of the Sinian after the Middle- Permian sedimentation. (f) Structural map of top surface of the Sinian at present. Based on the content of the reservoir bitumen in the Lintanchang and Dingshan structural paleo-traps could be 8 8 Dengying Formation in the two exploration wells and the calculated as 6.7×10 t and 1.9×10 t respectively using the characteristics of the paleo-structural traps, oil reserves in the equation: Pet.Sci.(2010)7:289-301 295 Bitumen in cuttings Bitumen in cuttings 0 10 Depth, 0 10 Depth, Lithology Bitumen distribution Lithology Bitumen distribution Bitumen in cores Bitumen in cores 0 10 0 10 -2550 -3450 -3500 -2600 Hydrothermal dolomite is Bitumen was mainly very common in this interval, present in the bitumen is mainly present in intercrystal pores of the intercrystalline pores of dolomite, some present dolomite in the fracture and intraclasts Bitumen was mainly present in microfracture -2700 -3600 caused by dissolution Bitumen is mainly or tectonic movement, present in the some present in the intercrystalline intercrystal pores of pores in dolomite dolomite in fractures Paleo oil-water -2800 -3700 interface -2827 No bitumen in the pores H≥242m below -2827m, so this depth may be the paleo oil-water interface. Paleo oil column height may exceed 242m because of drilling did not reach the bottom of Sinian -2900 -3800 Can not find any bitumen in the cuttings Fig. 4 Vertical bitumen content profiles in the upper Sinian Dengying Formation in Dingshan 1 well and Lin 1 Well (Left: Dingshan 1 Well; Right: Lin 1 Well) -3900 m (Barker, 1990; Lu et al, 2002). The amount of gas from the cracked crude oil in the Lintanchang and Dingshan structures 3 8 3 can be calculated as G =Q ×620 m /t=6.7×10 t×620 m /t 1 1 8 3 3 8 3 =4154×10 m and G =Q ×620 m /t=1.9×10 t×620 m /t 2 2 8 3 =1178×10 m respectively. Therefore, the amount of gas from cracked paleo oil pool in the Dingshan-Lintanchang structural 8 3 belt was G =G +G =5332×10 m , which indicated presence 1 2 Bitumen was mainly of a rich source rock for the formation of paleo gas pool. -4000 present in the intercrystalline pores in dolomite Parameters for reserves calculation of Sinian paleo-traps in the Table 2 Lintanchang and Dingshan structures No bitumen in pores below -4070m, Parameter so this depth may be 2 3 8 the paleo oil-water -4070 A , km H, m H , m Ф, % S , % Boi ρ, t/m Q, 10 t o o o interface. The paleo oil Structure H≈500m column height was about 500m. Lintanchang 220 350 120 5 70 1.24 0.9 6.7 Dingshan 120 500 42 7.5 70 1.24 0.9 1.9 Q =A ×H × ×S ×ρ /Boi (Wu, 2005) (See Table 2 for A : Oil-bearing area; H: Oil column height; H : Effective reservoir thickness; o o o Ф: Effective porosity; S : Oil saturation; Boi: Oil volume factor; ρ: Oil parameter determination) o density; Q: Paleo-reserves ( S , Boi and ρ are from Sun et al (2007)) Therefore, in the Dingshan-Lintanchang structural belt, there was a paleo oil pool with the petroleum reserve of 5.2 Formation of the Sinian paleo oil (gas) pool 8 8 8 8.6×10 t (6.7×10 t+1.9×10 t). After the lower Cambrian sedimentation, the buried depth The average gas cracked from one ton of oil is about 620 Lower Upper Sinian Series Cambrian Forma- Dengying Niutitang tion Lower Series Upper Sinian Cambrian Forma- Dengying Niutitang tion 296 Pet.Sci.(2010)7:289-301 of the top Sinian surface (the bottom of the lower Cambrian 5.3 Processes which destroyed the Sinian paleo gas Niutitang Formation source rocks) was about 800 m in the pool Dingshan 1 Well and 700 m in the Lin 1 Well, so the lower 5.3.1 Folding, uplifting and denudation during Yanshan- Cambrian Niutitang Formation source rocks (mudstone) were Himalayan period not yet mature (Fig. 5). (1) Trough-like fold formation After the Ordovician sedimentation, the burial depth of Since the early Cretaceous, the southeast Sichuan Basin the Sinian top surface reached about 2,000 m in the Dingshan had gone through important folding, uplifting and denudation 1 Well and 1,900 m in the Lin 1 Well. During the late periods (Lü and Xia, 2005; Hu et al, 2009). Infl uenced by the Ordovician, the lower Cambrian source rocks became mature, major stress fi eld from southeast of the region, the strata were but the main hydrocarbon generating center was located in the folded. The age of folded strata decreased from southeast to Dingshan structure. The fi rst stage paleo oil pool was located northwest, accompanied by gradually decreasing intensity in the Lintanchang and Dingshan structures. of deformation. During this period, the multiple directions After the Silurian sedimentation, the burial depth of of N-E, E-W and S-N structures were superimposed in the the Sinian top surface reached 2,600 m in the Dingshan 1 southeast Sichuan Basin and the basin was fi nally shaped into Well and 2,400 m in the Lin 1 Well. During the Devonian- a N-E trough-like structure, with the wide-gentle anticlines Carboniferous, the source rocks in the study area went and tight-closed synclines (Fig. 1). through an intermission in hydrocarbon generation. The (2) Intense uplifting and formation of fracture systems paleo oil pool in the Dingshan structure was damaged by the Fission track analysis was used as a quantitative Caledonian movement, whereas the pool in the Lintanchang simulation for the samples from the Dingshan-Lintanchang structure was well preserved. region, to reconstruct their thermal and tectonic uplift After the Permian sedimentation, the buried depth of the histories. The process of tectonic uplift and subsidence in top of the Sinian reached 2,800 m in the Dingshan 1 Well and the above area was divided into two major phases. Before 2,600 m in the Lin 1 Well, with the early structural framework the late Cretaceous, subsidence was dominant, but tectonic being preserved. However, the hydrocarbon generation center uplift (resulting in denudation) has been dominant since migrated toward the Lin 1 Well. The Lintanchang-Dingshan the late Cretaceous. The key period of tectonic transition structural belt was high at the west and low at the east. At that from subsidence to uplift (denudation) was around 80 Ma. time, the lower Cambrian source rocks intensely generated A geothermal gradient 30°C/100m and surface temperature hydrocarbon and the Lin 1 Well located on the high point of 20°C were used for the simulation (Zhang, 1997). Middle or the structure captured a large quantity of oil. The size of the high-speed uplift occurred during 65-82 Ma, at a rate of 93 two phases superimposed paleo oil pool reached up to 6.7×10 m/Ma, with an uplift amplitude of 1,030 m. Low-speed uplift t in the Lin 1 Well. The Dingshan 1 Well is located at the occurred during 25-65 Ma, at a rate of 14.8 m/Ma, with an top of the Dingshan structure and the size of paleo oil pool uplift amplitude of 720 m. Moderate-speed uplift began to reached 1.9×10 t (Fig. 5). occur in the 25Ma period until present at a rate of 59.8 m/Ma Afterwards, the buried depth, temperature, and with an uplift amplitude of 1,360 m. Therefore, since the late pressure at the Sinian top surface increased due to Cretaceous, at least 3,000 m of the strata have been eroded continuous subsidence. In the late Triassic, the buried in this region. As a result of the uplift, the strata outcropping depth of the Sinian top surface reached 4,800 m and the in this area are relatively old (Fig. 1) and many steep faults geotemperature was 170°C. Large amounts of oil trapped and fracture systems were formed, which greatly changed in the Sinian began to crack, the pressure in the oil-gas previous good reservoir conditions in the area. Because of reservoir increased further, and reached a peak during tectonic movements, reservoirs were destroyed and the gas the late early Cretaceous (74 Ma). Fission track analysis generated from the cracked crude oil escaped, so the paleo of the apatite showed that the buried depth of the Sinian gas pool was totally destroyed. top surface in the Dingshan 1 Well once reached 7,060 5.3.2 Sinian preservation conditions m, with temperature up to 220°C. Thus a large paleo The isotopic composition of strontium in seawater is gas pool was formed from a large amount of cracked considered to be globally consistent (McArthur et al, 1992). hydrocarbons. The main cap rocks (including gypsum) in Thus, strontium isotope measurements can be employed the area were very good, and gypsum was a good sealing to trace and contrast fluid sources and interpret oil and gas for gas diffusion. The sealing was destroyed after folding, preservation conditions. The following discussion on fluid uplifting and denudation during the Yanshan-Himalayan sources and preservation conditions is based on analyses of period. Therefore, there was a paleo gas pool because of oil samples from the Lin 1 Well. cracking before the sealing was destroyed. The Lintanchang (1) Isotopic geochemical characteristics and paleo-fluid 8 3 structure had a paleo gas pool of 4,154×10 m and that sources 8 3 of the Dingshan structure was 1,178×10 m . Therefore, Through contrasts of the strontium isotope ratio in the the size of the whole paleo gas pool in the Lintanchang- host rocks and vugs, or fracture fi llings of different horizons 8 3 Dingshan structural belt reached about 5,300×10 m in the Lin 1 Well, the lower assemblage (Sinian to lower (Fig. 3(e), Fig. 5). Palaeozoic) is characterized as follows (Fig. 6): (1) The Pet.Sci.(2010)7:289-301 297 Lin 1 Well Sangmu anticline Xianfang Luocun SE Depth a (m) -1000 Destroyed completely -3000 -5000 Lin 1 Well Second stage bitumen -1000 in Permian? Residual gas pool -3000 -5000 Lin 1 Well First stage bitumen Jura-Cretaceous in Permian? -1000 Triassic -3000 Paleo-gas pool -5000 Gas dissolved in water Lin 1 Well Permian -1000 Second stage paleo-oil pool -3000 superimposed on the first oil pool Lin 1 Well Silurian -1000 -3000 Lin 1 Well Silurian -1000 -3000 Ordovician Cambrian -1000 First stage paleo-oil pool Dengying -1000 Dengying Fig. 5 Formation and destruction processes of Lintanchang paleo oil and paleo gas pools in the southeast Sichuan Basin (Based on seismic line TTBli-05-608 balanced section) (a) Present; (b) Himalayan; (c) Before Himalayan; (d) After the Permian sedimentation; (e) Before the Permian sedimentation; (f) After the Silurian sedimentation; (g) After the Ordovician sedimentation; (h) After the Sinian sedimentation Subside Subside Uplift Subside Subside Uplift Present 298 Pet.Sci.(2010)7:289-301 strontium isotope ratio of host rocks and fillings is higher Palaeozoic strata generally had the same fl uid sources as the than that of contemporary normal seawater. (2) The strontium upper Sinian, which revealed that there was cross-formation isotope ratio of fi llings is higher than that of host carbonate fluid migration between the upper Sinian and the lower rocks, but is lower than that of host clastic rocks. (3) Carbon Palaeozoic. This also indicated that the overall preservation and oxygen isotope ratios of fi llings are greatly different from conditions of the upper Sinian in this structure were relatively those of host rocks. (4) Strontium isotope ratios of adjacent poor. For instance, in the Dingshan-Yueping profi le, the lower horizons vary greatly. All these characteristics indicate that Cambrian cap rocks had good sealing capacity before the oil there were four sets of different fluid systems in the lower started to crack when Yanshan-Himalayan uplifting began. assemblage (Fig. 6). Fluids in the lower Silurian Shiniulan The upper Sinian and the overlying lower Palaeozoic had Formation and the first stage of the upper Sinian had the mutually independent fluid systems. The abnormal pressure same source (system C ), whereas source rocks in the lower in the reservoir strata in the Dengying Formation reached Silurian Longmaxi and lower Cambrian Niutitang Formation up to 78-86 MPa during oil cracking (data obtained from belonged to two other independent fluid systems (system simulation pressure tests of fluid inclusions), approaching A and system B). The upper Sinian fluids were extremely the breakthrough pressure of the Niutitang Formation. complex with three subsystems (system C , C , C ) (Fig. 6). Furthermore, high angle faults and fracture systems were 1 2 3 Therefore, there were three major sources of fl uids in the Lin formed. The loss of sealing by the overlying argillaceous 1 Well: (1) the fl uid modifi ed by re-dissolution from the lower rocks and gypsum led to cross-formation fluid migration Silurian Longmaxi Formation itself (system A); (2) the re- among the upper Sinian, the Ordovician and the Silurian. At dissolution fl uid of the lower Cambrian Niutitang Formation the same time, the cracked gas pool in the upper Sinian was (system B); (3) the two phases of strontium-enriched fl uid in destroyed. the Dengying Formation (system C). It can be concluded that the Cambrian gypsum and (2) Loss of preservation conditions and destruction of gas mudstone in the Dingshan and Lintanchang structural belt pools had a relatively good sealing capacity before and at the Fig. 6 is a trace diagram showing geochemical strontium beginning of the oil cracking. Nevertheless, the study area isotope ratios in the Lin 1 Well, Dingshan 1 Well and the had been strongly tectonically folded, uplifted and denuded sections of the structural belt. Fig. 6 shows that the upper since the late Cretaceous, with the upper Sinian to the lower Sinian internal fluid was the most complex fluid with 2-3 Palaeozoic strata subaerially exposed. Thus the earlier good subsystems no matter whether it was underground (in drilled preservation conditions were lost and the cracked gas pools in wells) or at the surface profi le. However, the overlying lower the Dengying Formation were destroyed (Fig. 5). Observed geochemical sections and well name Age Lithology Dingshan-Yueping Wenshui-Chengjiashan Lin 1 well Dingshan 1 well Notes 87 86 87 86 87 86 87 86 Sr/ Sr Fluid conductivity Sr/ Sr Fluid conductivity Sr/ Sr Fluid conductivity Sr/ Sr Fluid conductivity fluid system D Limestone 0.7113 0.70-0.7076 Not interconnected Seal between fluid systems 0.7123 0.7118-0.7120 fluid system C 0.7131 fluid system C 0.7114-0.7116 0.7115 Cross-formation 0.7170-0.7180 fluid system A Fluid migration Interconnected 0.7149 Cross-formation 0.7119 Late period Fluid migration fluid system C 0.7142 Cross-formation Interconnected interconnected Fluid migration with fluid Interconnected Not interconnected system D 0.7131 between fluid systems fluid system A 0.7160 0.7109 0.7116 Cross-formation Fluid system C Fluid migration Cross-formation Cross-formation Interconnected cross-formation Fluid migration migration and Fluid migration 0.7115 Interconnected Interconnected forming mixed fluid Dolomite 0.7131 0.7092 Cross-formation Fluid migration 0.7186-0.7187 fluid system B Interconnected 0.7157 0.7119-0.7121 fluid system B 0.7119-0.7121 0.7119-0.7121 fluid system C fluid system C 0.7112-0.7114 1 0.7136-0.7138 3 0.7112-0.7114 0.7129-0.7132 fluid system C fluid system C 0.7120-0.7121 0.7107-0.7109 fluid system C fluid system C 0.7107-0.7109 3 0.7102-0.7110 1 Fig. 6 Diagram showing cross-formation fl uid migration from Sinian to lower Palaeozoic in the Dingshan-Lintanchang structure, southeast Sichuan Basin Sichuan Basin was the highest at the axis of the Caledonian 6 Discussion Leshan-Longnüsi paleo-uplift (also called the central Sichuan uplift). The bitumen content in the Nüji Well reached 8.1% 6.1 Distribution of oil saturation in the Dengying (Fig. 7). The bitumen content declined from the uplift to Formation controlled by paleo-uplifts the depressions (e.g. Woshen 1 Well 0.4%, Zishen 1 Well The bitumen content in the Dengying Formation in the 1.1%) (Wang et al, 2002). This indicated that the Caledonian Sinian Cambrian Ordovician Silurian Permian Superpressure fluid released during Yanshan-Himalayan uplifting, which caused fluid cross-formation migration and interconnected. Preservation conditions were better at early stage and poor at late stage. Pet.Sci.(2010)7:289-301 299 Guanyinqiao Guanyinqiao Dingshan 1 Dingshan 1 3.16 0.83 50 25.4 25.4 Hanjiadian Hanjiadian Guandu Guandu Wenshui Wenshui 0.5 0.5 1.5 2.0 1.0 3.0 Xishui Xishui Lin 1 Lin 1 0.5 0.5 Jiuba Jiuba 5.5-10 5.5-10 1.37 2.87 Sangmu Sangmu 47.7 47.7 Well City Town Sinian outcrop Well City Town Sinian outcrop Taipingdu Taipingdu Percentage of thin-sections containing Percentage of thin-sections containing bitumen from paleo-oil pool bitumen from paleo-oil pool Average content of bitumen in the thin-sections Average content of bitumen in all the thin-sections containing bitumen Average content of bitumen in all the Average content of bitumen in the thin-sections containing bitumen from paleo-oil pool thin-sections from paleo-oil pool 0.5 0.5 5.5-10 Percentage of thin-sections 5.5-10 Percentage of thin-sections containing bitumen from paleo-oil pool containing bitumen from paleo-oil pool Zi 1 Wei 117 Zishen 1 Gongshen 1 Dingshan 1 Zi 1 Wei 117 Zishen 1 Gongshen 1 Dingshan 1 Depth, m Depth,m Chengdu Zunyi Chengdu Zunyi -500 -500 TODF>1160m POCH:500m -1000 -1000 TODF:560m POCH:123m -1500 -1500 Paleo-structure of top surface of Sinian before Triassic Paleo-structure of top surface -2000 -2000 of Sinian before Permian TODF:613m -2500 -2500 POCH:272m -3000 -3000 No drilling information, supposed paleo-oil pool and similar oil column TODF:1208m -3500 -3500 POCH:350m -4000 -4000 TODF:Thickness of Dengying Formation POCH:Paleo-oil Column Height -4500 -4500 No drilling information, supposed paleo-oil : Well pool and oil column height: 350-500m -5000 -5000 Slope of Central Slope of Central Central Sichuan Central Sichuan Slope Depression Guizhou Slope Depression Guizhou b Paleouplift Paleouplift Paleouplift Paleouplift Fig. 7 Characteristics of paleo oil pools of the upper Sinian in the Sichuan Basin and its peripheral areas (a) Plan distribution of the Dengying Formation bitumen in the paleo oil pool in the Dingshan-Lintanchang structural belt; (b) Profi le of the paleo oil fi eld from the central Sichuan paleo-uplift to the slope of central Guizhou paleo-uplift (See the well location in Fig. 8) paleotectonic framework controlled the distribution of oil in of Jinsha Yankong, Kaiyang Yangshui (Yongshaba) and the Dengying Formation, and that the paleo-uplift was the Weng’an Baidoushan (Qianchang) in the central Guizhou primary area of oil migration and accumulation. paleo-uplift indicate that there was an upper Sinian large The southeast Sichuan Basin is located in the slope- paleo oil fi eld similar to that in the Leshan-Longnüsi paleo- depression area of the central Guizhou paleo-uplift with the uplift (Tian et al, 2006; Zhou and Liang, 2006; Yang et al, Woshen-Gongshen depression between this region and the 2008). Therefore, bitumen can be observed today in any of Leshan-Longnüsi paleo-uplift. The Permian-Triassic was the the Dengying Formation outcrops and well cores. There was most important period for the generation of hydrocarbons in probably an ultra-large, structure related oil-gas field in the the study area. Before the Triassic and the Permian, the study Dengying Formation in the Sichuan Basin and its peripheral area was located at the northern slope of the central Guizhou areas (Fig. 8). paleo-uplift. During the peak of oil fi lling, the Lintanchang- 6.2 Positive correlation between oil column heights Dingshan belt was a local structural high on the slope of of paleo oil pools and residual thickness of the central Guizhou paleo-uplift with large quantities of oil Dengying Formation accumulating (Fig. 7). The bitumen content in the Dengying Formation in Lin 1 Fig. 7(b) is the profi le of the upper Sinian paleo oil fi eld well and Dingshan 1 well was lower than that at the Leshan- from the central Sichuan paleo-uplift (Leshan-Longnüsi Longnüsi paleo-uplift (Fig. 7). The percentage of the thin paleo-uplift) to the Lintanchang-Dingshan structural belt in sections containing bitumen in Lin 1 well and Dingshan 1 the southeast Sichuan Basin. It can be seen that the Dengying well is similar with that of the paleo-uplift. For instance, the Formation in Zi 1 Well, Wei 117 Well (uplift area) and percentage of thin sections containing bitumen inside paleo Gongshen 1 Well (depression area) had a residual thickness oil column in the Lin 1 Well (at least 242 m in height) was of 560 m, 613 m and 1,208 m, with an oil column height 47% (25% in the Dingshan 1 Well) (Fig. 7(a)), close to that of 123 m, 272 m and 350 m, respectively. Also located in a in the Woshen 1 Well located in the depression, but is higher depression area of the northern slope of the central Guizhou than that in the Wei 117 Well (39%) located on the slope. paleo-uplift, the Dengying Formation in the Dingshan 1 well The characteristics of the upper Sinian paleo oil pools was of at least 1,160 m thick with 500 m oil column height. Legend Legend 300 Pet.Sci.(2010)7:289-301 Yangba,Nanjiang 1 N Ningqiang Shuimoba,Wangcang Qiang 1 Zhengyuan,Wangcang Zeng 1 Yanjinghe,Wangcang Qingchuan 12 0 40 80km Ganhe,Wangcang 5 2 Pingwu Guangyuan Hui 1 Shixihe,Chengkou Nanjiang Mukuihe,Chengkou Sangmuchang,Xishui Jinhekou,Ebian Chengkou Tianquan Lingguangzhen,Baoxing Guanzhuang,Qingchuan Maoxian No data Chengdu Li 1 Nji Anping 1 Zhi 1 Zhi 2 Gaoke 1 Tianquan Yaan Shizhu Zhougong 1 Emeishan Wei117 Leshan Pan 1 Chongqing Zishen 1 Legend Laolong 1 Ebian Sinian outcrop Woshen 1 City Mabian Gongshen 1 Dingshan 1 Well, reach Sinian, have bitumen Well, reach Sinian, lack information Xishui Location, have bitumen Ning 2 8 in Sinian Lin 1 Ning 1 Supposed paleo-oil field Fig. 8 Diagram showing the distribution of the ultra-large structure-lithologic oil-gas fi eld in the upper Sinian in the Sichuan Basin and its peripheral areas Hence, it could be concluded that bitumen content in the fi rst stage of paleo oil pool formation was maturation of the uplift area was high, whereas the paleo oil column height was lower Cambrian source rocks during the late Ordovician. relatively low. The hydrocarbon generation from the lower Cambrian source In conclusion, the high content of bitumen of the rocks stopped during the Devonian to the Carboniferous Dengying Formation in the paleo-uplift areas suggested tectonic uplifting. The lower Cambrian source rocks then that the paleo-uplifts were the preferential areas of oil-gas generated large quantities of hydrocarbons during the second migration and accumulation. The low paleo oil columns in stage of paleo oil pool formation, after the middle Permian these areas indirectly indicated that there was no unified deposition. The oil began to crack and the paleo gas pool oil-water interface in the upper Sinian paleo oil field from was formed during the late Triassic. This paleo gas pool paleo-uplifts to slope areas. The Dengying Formation was damaged and reservoirs were destroyed as a result is characterized by petroleum pool formation and local of extensive Yanshan-Himalayan folding, uplifting and hydrocarbon enrichment in the Sichuan Basin and its denudation. periphery areas. 4) Residual bitumen can be found in the Dengying Formation inside the Sichuan Basin as well as in its periphery 7 Conclusions areas. It is suggested that there was an ultra-large oil-gas fi eld formed within the Dengying Formation, with petroleum 1) The lower Cambrian source rocks in the Dingshan- pools formed widely from local hydrocarbon enrichment in Lintanchang structural belt in the southeast Sichuan the Sichuan Basin and its periphery areas. Consequently the Basin were of medium-good quality with two excellent paleo gas pools were formed from thermal cracking of crude hydrocarbon-generating centers developed in the periphery oil, with the pools destroyed during the Yanshan-Himalayan areas. The upper Sinian dolomite (Dengying Formation) folding, uplifting and denudation. formed good reservoir rocks. The overlying cap rocks of the lower Cambrian Niutitang Formation mudstone provided Acknowledgements satisfactory sealing capacity. Therefore, there were excellent original geologic conditions for hydrocarbon accumulation in This study was supported by the National Basic Research the Dengying Formation. Program of China (No. 2005CB422106). We are thankful to 2) The Dingshan-Lintanchang structural belt had a paleo Luba Jansa for the assistance during fi eld studies. oil pool with 8.6×10 t oil and then a paleo gas pool with 8 3 References 5,300×10 m gas from cracked oil. 3) The formation and destruction processes of the Ao M C, Lü T Z, Hu N F, et al. Lithological features of the Dengying Dengying Formation oil-gas pool in the Dingshan- Formation of the Sinian in Well Dingshan 1 in south-eastern part Lintanchang structural belt occurred in several stages. 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Petroleum Science – Springer Journals

Published: Aug 3, 2010

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