Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Differential interformational velocity analysis as an effective direct hydrocarbon indicator under velocity reversal conditions, an example from the anomalously high temperature and over-pressured DF1-1 Gas Field in the Yinggehai Basin, South China Sea

Differential interformational velocity analysis as an effective direct hydrocarbon indicator... Pet.Sci.(2009)6:339-346 339 339 DOI 10.1007/s12182-009-0053-5 Differential interformational velocity analysis as an effective direct hydrocarbon indicator under velocity reversal conditions, an example from the anomalously high temperature and over-pressured DF1-1 Gas Field in the Yinggehai Basin, South China Sea 1, 2 2 3 1 1 Zhu Hongtao , Liu Keyu , Chen Kaiyuan , Li Min and Huang Shengbing Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China CSIRO Petroleum, P. O. Box 1130, Bentley, WA 6102, Australia School of Energy Resources, China University of Geosciences, Beijing 100083, China Abstract: In the DF1-1 Gas Field in the Yinggehai Basin, South China Sea, the velocity-depth plot and velocity spectra show signifi cant variations from a linear trend, exhibiting a distinct reversal phenomenon. Velocity parameters derived from velocity spectral analysis of the seismic data and sonic logs indicate that the interval velocity reverses below 2,100 m (2.2 s two-way time (TWT)) in the DF1-1 Gas Field. Some direct hydrocarbon indicators (DHIs) models developed for the shallow strata cannot be simply applied to the moderately to deeply buried strata for direct exploration target recognition because the velocity reversal has caused the middle-deep gas reservoirs to exhibit a moderate or weak seismic amplitude. The hydrocarbon indicator method of “Differential Interformational Velocity Analysis (DIVA)” with the aid of other hydrocarbon indicating techniques was employed to effectively identify DHIs in the middle-deep strata under velocity inversion. The result has shown that the DIVA technique can be effectively used as a DHI in both the shallow and the middle-deep strata in the study area with the shallow strata characterized by Type I DIVA anomaly and the middle-deep strata characterized by the Type II DIVA anomaly. Key words: DF1-1 Gas Field, velocity reversal, direct hydrocarbon indicator, differential interformational velocity analysis, South China Sea a linear velocity-depth trend and concluded that ‘‘no general 1 Introduction velocity-depth function can be used when performing more Seismic velocity is an important parameter in petroleum accurate analyses like depth conversion of seismic data, exploration and can be used to derive Direct Hydrocarbon pore-pressure prediction, or basin modeling’’. Japsen (2006), Indicators (DHIs). Velocity data from well logs become a however, believed that there existed a general velocity-depth vital tool to (1) differentiate lithologies, (2) determine facies function that could be used for various analyses. Therefore, it and reservoir attributes, (3) deduce fluid types and detect has been a subject of heated debate on whether one can come overpressure regimes, (4) obtain information about the burial up with a general velocity-depth function for sedimentary and thermal history, and (5) generate synthetic sections for rocks. So far there is no general consensus reached on this the effective interpretation of seismic data. issue. Normally, the relationship between the acoustic velocity The bright spot technique, an important seismically of sedimentary rocks and their burial depth is linear. Such a derived DHI method, has been successfully applied in relationship is often used to estimate sediment composition, delineating hydrocarbon reservoirs seismically in some porosities, pore-pressures, burial history, and compaction parts of the world (Yin et al, 2003; Young and Tatham, processes (He et al, 2002). However, Storvoll et al (2005) 2007). However, this method appears to be only effective for used log data from 60 wells on the Norwegian Shelf to identifying the shallow gas reservoirs (above 2,100 m), in the investigate the velocity-depth trends in sedimentary rocks. Yinggehai Basin, South China Sea (Li, 2000). Below 2,100 They found that most of the velocity variations did not follow m, the bright spot technique becomes ineffective. In fact in the Yinggehai Basin, some bright spots do not appear to indicate oil or gas accumulations in the middle-deep strata at *Corresponding author. email: zhuht_oscar@yahoo.com.cn all (Li, 2000). It is thus necessary to investigate whether this Received January 12, 2009 340 Pet.Sci.(2009)6:339-346 technique can be applied in the region. Because the bright depth trend in the Yinggehai Basin is non-linear; instead, the spots are mainly controlled by the velocity parameters, it interval velocity exhibits reversal below 2.2 s (or 2,100 m). is essential to understand how the velocity changes in the This study also aims to test and evaluate the hydrocarbon Yinggehai Basin under the special geological settings of indicator method of DIVA that may be applied effectively high temperature, high pressure, and moderately to deeply under velocity reversal conditions, so as to help reduce the buried strata. The sensitivity and limitations of the bright risks associated with future exploration in this area. spot technique in deeper strata in general need to be further 2 Geological setting evaluated. The effects of seismic velocity characteristics on exploration target delineation and the corresponding The Yinggehai Basin, which is located southeast of measures require vigorous investigation in order to provide Hainan Island and has a NW-SE orientation, is a relatively insight for future exploration of the middle-deep oil and gas young, gas-rich, Cenozoic extensional and super-pressured reservoirs in this basin. basin in the South China Sea (Fig. 1). This basin is fi lled with The Differential Interformational Velocity Analysis up to 18 km of Tertiary clastic sediments on the Paleozoic and (DIVA) technique using ultra refined seismic velocity Mesozoic basement rocks (Huang et al, 2003). The complete analysis has been developed to identify and delineate low stratigraphic sequence has not been drilled through (Yuan et velocity anomalies in the subsurface. Porous reservoir rock al, 2009; Zhou and Yao, 2009). The structural evolution of formations can be distinguished by their reduced seismic the Yinggehai Basin can be divided into two major stages: an velocities regardless of whether hydrocarbons are present. Eocene-Oligocene syn-rift sequence and a post-rift thermal Where gas is present, the velocity reduction can be signifi cant subsidence sequence. which makes the DIVA anomaly an excellent indicator with The Yinggehai Basin has recently been studied extensively much visual impact compared to the bright spot technique. because it is the major hydrocarbon producing basin in the The DIVA technique has not only been proven by drilling South China Sea, and has many unique geological features but also been used to define unconventional exploration including: 1) high subsidence and deposition rates with the plays which cannot be detected using conventional seismic maximum sedimentation rate up to 1.2 mm/yr (Huang et al, parameters (Cook et al, 1983). 2005; Zhang et al, 2008; Yan et al, 2009), 2) high geothermal Although velocity reversal has been recognized in the gradients varying from 3.1 to 4.56 °C/100m (He et al, 2002; Norwegian Shelf by Storvoll et al (2005), its impacts on Huang et al, 2003), 3) high heat fl ow ranging from 69 to 86 the DHI have not been discussed in any detail. The aim of 2 2 mW/m with a mean value of 79±7 mW/m (He et al, 2002), this study is to understand the velocity-depth relationship 4) abnormally high overpressure with the pressure coeffi cient of the study area based on the velocity data from sonic logs up to 2.3 (Hao et al, 2000; Huang et al, 2003), and 5) the and seismic data. Published data showed that the velocity- Pet.Sci.(2009)6:339-346 341 presence of regional diapir activities (Xia et al, 2006). et al, 1998), and 3) velocity data of normally compacted The DF1-1 Gas Field, fi rst discovered in 1992, is located shales from 32 wells in the North Sea and Norwegian Sea in the northern part of the Yinggehai Basin. The fi eld contains (Hansen, 1996). 8 3 estimated gas resources of over 1,000×10 m consisting Interval “A” (Fig. 2) shows a good linear velocity- mainly of hydrocarbon gas, N , and CO (Hao et al, 2000). depth trend when excluding the data from the gas-bearing 2 2 The main source rocks are in the Miocene Huangliu, sandstones. However, the slope of the trend line shows a Meishan, and Sanya formations comprising calcareous higher velocity-depth gradient compared with the published shales, grey sandy limestones and grey biogenic limestones data listed above. Interval “A” consists mainly of the interbedded with grey and dark grey mudstones, light grey Yinggehai Formation, which is made up of argillaceous siltstones and fine sandstones. The overlying Huangliu- silty sandstone, fine sandstone, siltstone, and mudstone. Yinggehai formations and the Quaternary sediments consist Furthermore, there are distinct variations between the gas- of sandstones and shales, forming several sets of reservoir- bearing sandstones and the surrounding rocks from 1,250 seal pairs. Shale diapir structures have developed in the to 1,500 m in the shallow stratigraphic section. Combined central depression of the Yinggehai Basin (Fig. 1). with drilling and seismic data, it is demonstrated that the seismic compressional wave (P) velocity of the gas-bearing 3 Velocity characteristics sandstones above 2,000 m is 30% or more lower than that of surrounding mudstone rocks (Li, 2000). The sonic velocity data from the wells in the DF1-1 Gas Interval “B” shows the velocity data from the Meishan Field are shown in Fig. 2. These are plotted separately for to Yinggehai formations. The sediments are similar to those different lithologies and gas-bearing intervals so as to better of interval “A” except for the presence of argillaceous fine illustrate the variations. Compared with the estimated trend siltstone. The velocity data of interval “B” do not show a line of the published data (red dash-line), the velocity-depth classical linear velocity-depth trend, instead exhibiting a plot of the field exhibits two intervals with different trends reversal compared with the published data below 2,100 m. (“A” and “B” in Fig. 2). Detailed information of the published Moreover, the difference between the gas-bearing sandstones sonic velocity data in Fig. 2 include: 1) the sonic velocity of and the surrounding rocks below 2,100 m in the middle-deep nine shaly units of Oligocene to early Cretaceous age from 81 stratigraphic sections is not apparent. wells located in the North Sea (Teige et al, 1999), 2) velocity Fig. 3 shows the velocity spectral data derived from the data of the Garn sandstone and the Not and Ror shales from seismic data in the study area including interval velocity (a) the further 28 wells located offshore mid-Norway (Hermanrud and a single velocity spectrum at common depth point (CDP) Velocity, m/s 1500 2000 2500 3000 3500 Argillaceous siltstone Siltstone Fine sandstone Mudstone Poor gas-bearing layer Good gas-bearing layer =Trend line of published data Sonic velocity data derived from well logs above the Meishan Formation, DF1-1 Gas Field. The velocities of different lithofacies are plotted Fig. 2 in different colors and legends to better illustrate the variations. The dashed line shows the classical linear trend; solid lines show the real trends Depth, m 342 Pet.Sci.(2009)6:339-346 2,400 (b). In the interval velocity section (Fig. 3(a)), the (the left part of Fig. 3(b)), the stacked velocity still shows measured seismic velocity of each CDP increases gradually a linear velocity-depth trend until 2.25 s. It shows a sub- until about 2.25 s. Below this depth, it exhibits a distinct vertical velocity-depth trend down to 2,520 m/s. The interval velocity reversal as indicated by the velocity map (Fig. 3(a)). velocities of CDP 2,400 (the right part of Fig. 3(b)) are Different CDPs appear to have different velocity reversal similar to the velocity data from wells, with a linear velocity- depths. On the single velocity spectrum of CDP 2,400 depth trend until 2.25 s but exhibiting reversal below 2.25 s. Fig. 3 Interval velocity characteristic sections derived from velocity spectral analysis of a representative seismic line in the DF1-1 Gas Field. The left (a) is the interval velocity section (see Fig. 1 for the location of line), and the right (b) is the single velocity spectrum at CDP 2,400 with position shown as red line in the left (a) Fig. 4 displays the density variations with depths from 4) porosity, 5) effective pressure (both pore pressure and well logs above the Meishan Formation in the DF1-1 Gas overburden pressure), 6) rock composition (mainly shale Field. They are similar to the trends of their velocity-depth content), 7) granularity, 8) cementation, 9) temperature, and plot with a normal linear density-depth trend until 2,100 m 10) age/depth (Storvoll and Bjørlykke, 2004). We believe below which they exhibit reversal. that the dominant factors that influence velocities need to be determined for each individual area under different conditions. We do not intend to discuss the controlling factors that cause the velocity anomalies in details, but concentrate on how the velocity anomalies affecting DHIs. The Yinggehai Basin is presently under high temperature and abnormally high pressure conditions. For example, the DF1-1-11 well with a total depth (TD) of 3,508 m has a maximum borehole temperature of 167.85 °C compared with a borehole temperature of 107.84 °C at 2,100 m, has a maximum pressure of more than 70 MPa, and has a pressure coeffi cient of 2.18. Fig. 5 shows the measured and estimated pressure coefficient distributions, in which the pressure coefficient increases abruptly below 2,100 m. Although pressure estimations from mud weights give a rough estimate of maximum pore pressure in the well, they still can be thought of as a reference to indicate underground pore Fig. 4 Log-derived density-depth plots above the Meishan pressure. Through analyzing the measured velocity data of Formation, DF1-1 Gas Field. The data are plotted in different sandstones and mudstones of the DF1-1-11 well, the starting symbols to better illustrate the variations depth of velocity reversal is determined at approximately 2,100 m, which corresponds to the overpressure depth. 4 Factors affecting velocity anomalies Detailed analysis of the lithostratigraphic column of DF1-1- The main factors affecting velocity are believed to 11 well indicates that the velocity reversal does not correlate be: 1) fluid density, 2) matrix density, 3) water saturation, with any signifi cant lithological changes. Pet.Sci.(2009)6:339-346 343 low velocity. The phenomenon appears in both under- Pressure coefficient, g/cc compacted mudstones and sandstone reservoirs because of 0 0.5 1 1.5 2 2.5 the presence of abundant natural gas (Fig. 2). 5 Effects of velocity characteristics on exploration target delineation Velocity is an important parameter that has been routinely used to derive DHIs. The velocity reversal in the middle- deep strata in the DF1-1 Gas Field of the Yinggehai Basin has caused much unsatisfactory interpretation or complete failure of the velocity-derived DHI techniques. In the shallow section of the Yinggehai Basin, the reflection of the shallow gas reservoirs is characterized by strong amplitude anomalies (bright spots) on seismic profi les (Fig. 6) because of the pronounced acoustic impedance difference between the gas-bearing strata and the overlying or underlying strata, and is characterized by Type III AVO Observed pressure (Amplitude Versus Offset) anomaly whose amplitude Mud weight increases with offset on CDP gathers (Fig. 6) (Loizou et al, 2008). The acoustic impedance is controlled by the velocity and density of the strata. Because the velocity and density Observed pressure and mud weight curves of the DF1-1-11 well Fig. 5 of gas-bearing strata in the study area are both signifi cantly lower than those of the surrounding rocks (Figs. 2 and The velocity reversal phenomenon appears to be 5), there is a strong acoustic impedance difference with controlled mainly by the factors of high temperature and high pronounced bright spots shown on the shallow seismic pressure. Because of the high subsidence and sedimentation profi les in the study area. However, in the middle-deep strata, rates of the basin, sediments were generally under-compacted the gas reservoirs appear to have a different refl ection mode with large amounts of fl uids trapped in the sediments, which and usually display a moderate or weak amplitude. This does led to super-pressure that decreased velocity and caused not allow the development of strong acoustic impedance velocity reversal. Such a phenomenon is more obvious if oil difference (bright spots) because the reversal of both the and gas, especially natural gas, is present. Rocks bearing a velocity and density causes the ambiguity between the gas- large amount of un-discharged fl uids may cause abnormally bearing sandstones and the surrounding rocks. Fig. 6 Refl ective characteristics of gas-bearing sandstones, showing high acoustic impedance difference and high amplitude and bright spots refl ection between gas-bearing sandstones and the surrounding rocks in the shallow strata of the study area Velocity reversal can cause the bright spot technique to can cause “fake bright spots” on the deep seismic sections. be ineffective in the middle-deep strata. It often shows faded The “fake bright spots” mentioned above may be the latter spots or dark spots on the seismic profi le in the middle-deep two types of Li (2000). The “true” gas-bearing bright spots strata of the Yinggehai Basin. Therefore, the exploration become dark spots or fl at spots due to the changes of special model developed from the shallow gas reservoirs cannot physical and geological conditions, such as high temperature, be directly applied to the deep reservoirs. Three types of overpressure, and the degree of rock consolidation. It can bright spots were previously recognized by Li (2000) in the be misleading and lead to exploration failures if we only Yinggehai Basin, including 1) low-velocity type, 2) density rely on the bright spots as the DHI techniques for exploring type, and 3) calcium type. When using the bright spot hydrocarbon reservoirs in deeper strata. technique, it must be noticed that non-gas-bearing sandstones It is therefore important to detect some bright spots or Depth, m 344 Pet.Sci.(2009)6:339-346 other characteristic hydrocarbon indicators on the seismic velocity curve to assemble the next (deep) curve sequentially profi les in the middle or deep strata of the Yinggehai Basin. until the last curve; use the second shallow velocity curve If the bright spot technique is to be used, one should pay to repeat above-mentioned step until the second last to fi nal great attention to searching for the “phase reversal” and curves with the shallow curves marked as solid lines and the “dark spots”. Alternatively one may use other hydrocarbon deep ones as broken lines; indicating techniques to detect hydrocarbons from the 3) Arrange the DIVA sections by using the DIVA curves characteristics of the seismic profiles, such as 1) Jason assembled in step 2) to generate the DIVA sections according inversion using seismic and well-log data; 2) AVO attribute to the order of sequence from shallow to deep; parameter cross-plot including Gradient Versus Intercept, 4) Rank and interpret the DIVA sections. Gradient Versus Fluid Factor, and Gradient Versus Poisson When the deep velocity is lower than the shallow one, Ratio; 3) DIVA; and 4) high-order statistics to detect other it would show a velocity abnormal area that can be used to hydrocarbon indicators of the seismic expression in middle evaluate and ascertain the potential gas-bearing zones based or deep strata. Some of the techniques have been put into on the reliability of velocity anomalies shown on the DIVA practice in stratigraphic gas exploration in the Yinggehai sections. The DIVA abnormal classes are usually marked with Basin, resulting in successive discoveries of several medium Roman numerals on the DIVA sections with class I being to small gas fi elds. the best potential zone while class IV being the worst. Each Roman number has a numerical footnote representing the 6 Application of the DIVA technique serial number grades of abnormal zones with the smaller ones being the better in the same class. The DIVA, a useful seismic technique, which makes use of ultra refi ned seismic velocity analysis, has been developed to 6.2 Experimental results identify and localize low velocity anomalies in the subsurface For the evaluation and application of the DIVA technique (Cook et al, 1983). The DIVA technique has been shown to be in the middle-deep strata of the line in the DF1-1 Gas Field, an effective hydrocarbon indicating technique and has been one seismic section (Fig. 1) was selected. We compared it applied successfully in some parts of the world, especially in with the DIVA sections derived from the shallow, middle, and some terrigenous basins (Zhu et al, 2004). However, there are deep strata to evaluate the effectiveness of DIVA as a DHI in no publications that directly address the DIVA method with the deep strata of the Yinggehai Basin. In this test, the shallow regard to the velocity reversal issue. Therefore, it is essential objective is named as “a” segment, the middle objective to evaluate whether this technique can be successfully applied as “b”, the deep objective as “c”, each of which comprises in the Yinggehai Basin, South China Sea. about 10 refl ecting interfaces beginning from numeral 1 from shallow to deep strata. 6.1 Experimental method Table 1 lists the velocity analysis interface parameters The DIVA procedure consists of four steps: illustrated in three DIVA sections on the seismic line (Figs. 1) Plot the velocity data by using the interval velocity data 7-9), where the “a” segment between T and T (see Fig. 27 29 from the pre-stack seismic data with the velocity as y-axis and 1 for location) reflecting interfaces is the main gas-bearing CDPs as x-axis; strata, made up of 10 refl ecting interfaces in the DF1-1 Gas 2) Assemble the DIVA curves by using the first shallow Field. Velocity analysis interface parameters of target intervals using DIVA in the DF1-1 Gas Field Table 1 Target zone TWT, s CDP range Layer range Layer attribute Remarks a -a 1.2-1.5 1984-3078 T -T Gas-bearing layers: a , a , a Unused layers: a , a , a 1 10 27 29 2 4 6 3 5 7 b -b 1.8-2.0 1874-3047 T -T Potential gas-bearing: b , b , b Unused layers: b , b , b , b , b 1 11 29 30 5 7 9 2 4 6 8 10 c -c 2.1-2.4 1923-3015 T -T Potential gas-bearing: c , c , c Unused layers: c , c , c , c 1 11 30 40 3 5 7 2 4 6 8 Based on the DIVA interpretation sections of both the strata. Fig. 8 shows a class II and a class III abnormal DIVA shallow and middle-deep strata, it can be clearly demonstrated zones. Fig. 9 shows two class II abnormal DIVA zones. One that the class I abnormal DIVA zones are normally in the occurs at the depth about T , located on CDPs from 2,420 shallow strata, whereas the class II abnormal DIVA zones are to 3,000, deeper than the other with a depth around T -T , 30 40 usually in the middle-deep strata. located on CDPs from 1,920 to 2,250. The former appears to Fig. 7 shows two class I abnormal DIVA zones located on be more obvious than the latter in terms of abnormal degrees. CDPs of 2,000-2,200 and 2,630-2,900, separated by one class The abnormal DIVA zones discussed above are closely II abnormal DIVA zone in-between. related to the abnormal velocity distribution. The velocity In the middle-deep strata a strong abnormal DIVA zone is of gas-bearing sandstones is normally lower than that of shown, but their classes are lower than those in the shallow the surrounding rocks displaying primarily class I DIVA Pet.Sci.(2009)6:339-346 345 346 346 Pet.Sci.(2009)6:339-346 He L, Xiong L and Wang J. Heat flow and thermal modeling of the characteristics in the corresponding zones in the shallow Yinggehai Basin, South China Sea. Tectonophysics. 2002. 351(3): strata. In the middle-deep strata, the class I abnormally 245-253 changes into class II as shown on the DIVA sections because Her manrud C, Wensaas L, Teige G M G, et al. Shale porosities from well of the reduction of the velocity declination degree in the gas- logs on Haltenbanken (offshore mid-Norway) show no infl uence of bearing layers, showing velocity reversal and poor seismic overpressuring. In: Law B E, Ulmishek G F and Slavin V I (Eds.), quality. Abnormal pressures in hydrocarbon environments. AAPG Memoir. 1998. 70: 65-85 7 Conclusions Hua ng B J, Xiao X M and Li X X. Geochemistry and origins of natural gases in the Yinggehai and Qiongdongnan basins, offshore South Velocity analyses from well logs and seismic data of the China Sea. Organic Geochemistry. 2003. 34(7): 1009-1025 DF1-1 Gas Field in the Yinggehai Basin, South China Sea Hua ng B J, Xiao X M, Hu Z L, et al. Geochemistry and episodic indicate that the velocity-depth trend is linear excluding accumulation of natural gases from the Ledong Gas Field in the the gas-bearing sandstones until the depth of 2,100 m (2.25 Yinggehai Basin, offshore South China Sea. Organic Geochemistry. s TWT) in the shallow strata. The slope of the trend line 2005. 36(12): 1689-1702 indicates a higher velocity-depth gradient in the shallow strata Jap sen P. Velocity-depth trends in Mesozoic and Cenozoic sediments compared with published data elsewhere. In contrast in the from the Norwegian Shelf: Discussion. AAPG Bulletin. 2006. 90(7): middle-deep strata below 2,100 m (2.25 s TWT), the velocity- 1141-1143 Li X X. Seismic recognition techniques of shallow gas reservoirs in the depth relationship does not show a normal linear trend, but Yinggehai Basin. China Offshore Oil and Gas. 2000. 14(3): 193-199 exhibits distinct velocity reversal. The velocity between (in Chinese) the gas-bearing sandstones and the surrounding rocks also Loi zou N, Liu E and Chapman M. AVO analyses and spectral exhibits distinct variations in the shallow strata, but becomes decomposition of seismic data from four wells west of Shetland, UK. indistinguishable in the middle-deep strata. The velocity Petroleum Geoscience. 2008. 14(4): 355-368 reversal is believed to be mainly caused by high temperature Sto rvoll V and Bjørlykke K. Sonic velocity and grain contact properties and abnormally high pressure as the velocity anomaly and in reservoir sandstones. Petroleum Geoscience. 2004. 10(3): 215-226 pressure anomaly occur at the same depth. Sto rvoll V, Bjørlykke K and Mondol N H. Velocity-depth trends in The DIVA method has been found to be an effective DHI Mesozoic and Cenozoic sediments from the Norwegian Shelf. AAPG both in the shallow and the middle-deep strata in the study Bulletin. 2005. 89(3): 359-381 area. Two DIVA DHI anomalies have been identifi ed on the Tei ge G M G, Hermanrud C, Wensaas L, et al. The lack of relationship between overpressure and porosity in North Sea and Haltenbanken seismic profi les with class I abnormal DIVA zones associated shales: Overpressure research. Marine and Petroleum Geology. 1999. with the shallow strata, and class II abnormal DIVA zones 16(4): 321-335 associated with the middle-deep strata. Xia B, Zhang Y, Cui X J, et al. Understanding of the geological and geodynamic controls on the formation of the South China Sea: A Acknowledgements numerical modelling approach. Journal of Geodynamics. 2006. 42(1/3): 63-84 This study was supported by the National Natural Science Yan Y, Hu X Q, Lin G, et al. Denudation history of South China block Foundation of China (No.40702024) and partly funded by and sediment supply to northern margin of the South China Sea. AAPG Grant-in-Aid to the fi rst author. Also, the Project was Earth Science—Journal of China University of Geosciences. 2009. sponsored by the Scientific Research Foundation for the 20(1): 57-65 Returned Overseas Chinese Scholars, Ministry of Education Yin P, Berne S, Vagner P, et al. Mud volcanoes at the shelf margin of the of China (No.2009022014) and Open Research Foundation of East China Sea. Marine Geology. 2003. 194(3/4): 135-149 Key Laboratory of Tectonics and Petroleum Resources (China You ng K T and Tatham R H. Fluid discrimination of poststack “bright University of Geosciences), Ministry of Education (No.TPR- spots” in the Columbus Basin, offshore Trinidad. The Leading Edge. 2009-33). The authors would like to thank the Zhanjiang 2007. 26(12): 1508-1515 Branch of CNOOC for providing well and seismic data. Yua n S Q, Yao G S, Lü F L, et al. Features of late Cenozoic deepwater sedimentation in southern Qiongdongnan Basin, northwestern References South China Sea. Earth Science—Journal of China University of Geosciences. 2009. 20(1): 172-179 Coo k E E, Neidell N S and Beard J H. Precision measurements of Zha ng Y F, Sun Z, Zhou D, et al. Stretching characteristics and its interval velocity differences from seismic data. AAPG Annual dynamic significance of the northern continental margin of South Convention. 1983. 67(3): 442 China Sea. Science in China Series D: Earth Sciences. 2008. 51(3): Han sen S. Quantification of net uplift and erosion on the Norwegian 422-430 Shelf south of 66 degrees N from sonic transit times of shale. Norsk Zho u D and Yao B C. Tectonics and sedimentary basins of the South Geologisk Tidsskrift. 1996. 76: 245-252 China Sea: Challenges and progresses. Earth Science—Journal of Hao F, Li S T, Gong Z S, et al. Thermal regime, inter-reservoir China University of Geosciences. 2009. 20(1): 1-12 compositional heterogeneities, and reservoir-filling history of the Zhu H T, Chen K Y and Zhu P M. Application of DIVA technique in Dongfang Gas Field, Yinggehai Basin, South China Sea: Evidence Yinggehai Basin. Oil Geophysical Prospecting. 2004. 39(3): 319- for episodic fl uid injections in overpressured basins. AAPG Bulletin. 321 (in Chinese) 2000. 84(5): 607-626 (Edited by Hao Jie) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

Differential interformational velocity analysis as an effective direct hydrocarbon indicator under velocity reversal conditions, an example from the anomalously high temperature and over-pressured DF1-1 Gas Field in the Yinggehai Basin, South China Sea

Download PDF
8 pages

Loading next page...
 
/lp/springer-journals/differential-interformational-velocity-analysis-as-an-effective-direct-vazHVsHvUg

References (27)

Publisher
Springer Journals
Copyright
Copyright © 2009 by China University of Petroleum (Beijing) and Springer Berlin Heidelberg
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
ISSN
1672-5107
eISSN
1995-8226
DOI
10.1007/s12182-009-0053-5
Publisher site
See Article on Publisher Site

Abstract

Pet.Sci.(2009)6:339-346 339 339 DOI 10.1007/s12182-009-0053-5 Differential interformational velocity analysis as an effective direct hydrocarbon indicator under velocity reversal conditions, an example from the anomalously high temperature and over-pressured DF1-1 Gas Field in the Yinggehai Basin, South China Sea 1, 2 2 3 1 1 Zhu Hongtao , Liu Keyu , Chen Kaiyuan , Li Min and Huang Shengbing Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China CSIRO Petroleum, P. O. Box 1130, Bentley, WA 6102, Australia School of Energy Resources, China University of Geosciences, Beijing 100083, China Abstract: In the DF1-1 Gas Field in the Yinggehai Basin, South China Sea, the velocity-depth plot and velocity spectra show signifi cant variations from a linear trend, exhibiting a distinct reversal phenomenon. Velocity parameters derived from velocity spectral analysis of the seismic data and sonic logs indicate that the interval velocity reverses below 2,100 m (2.2 s two-way time (TWT)) in the DF1-1 Gas Field. Some direct hydrocarbon indicators (DHIs) models developed for the shallow strata cannot be simply applied to the moderately to deeply buried strata for direct exploration target recognition because the velocity reversal has caused the middle-deep gas reservoirs to exhibit a moderate or weak seismic amplitude. The hydrocarbon indicator method of “Differential Interformational Velocity Analysis (DIVA)” with the aid of other hydrocarbon indicating techniques was employed to effectively identify DHIs in the middle-deep strata under velocity inversion. The result has shown that the DIVA technique can be effectively used as a DHI in both the shallow and the middle-deep strata in the study area with the shallow strata characterized by Type I DIVA anomaly and the middle-deep strata characterized by the Type II DIVA anomaly. Key words: DF1-1 Gas Field, velocity reversal, direct hydrocarbon indicator, differential interformational velocity analysis, South China Sea a linear velocity-depth trend and concluded that ‘‘no general 1 Introduction velocity-depth function can be used when performing more Seismic velocity is an important parameter in petroleum accurate analyses like depth conversion of seismic data, exploration and can be used to derive Direct Hydrocarbon pore-pressure prediction, or basin modeling’’. Japsen (2006), Indicators (DHIs). Velocity data from well logs become a however, believed that there existed a general velocity-depth vital tool to (1) differentiate lithologies, (2) determine facies function that could be used for various analyses. Therefore, it and reservoir attributes, (3) deduce fluid types and detect has been a subject of heated debate on whether one can come overpressure regimes, (4) obtain information about the burial up with a general velocity-depth function for sedimentary and thermal history, and (5) generate synthetic sections for rocks. So far there is no general consensus reached on this the effective interpretation of seismic data. issue. Normally, the relationship between the acoustic velocity The bright spot technique, an important seismically of sedimentary rocks and their burial depth is linear. Such a derived DHI method, has been successfully applied in relationship is often used to estimate sediment composition, delineating hydrocarbon reservoirs seismically in some porosities, pore-pressures, burial history, and compaction parts of the world (Yin et al, 2003; Young and Tatham, processes (He et al, 2002). However, Storvoll et al (2005) 2007). However, this method appears to be only effective for used log data from 60 wells on the Norwegian Shelf to identifying the shallow gas reservoirs (above 2,100 m), in the investigate the velocity-depth trends in sedimentary rocks. Yinggehai Basin, South China Sea (Li, 2000). Below 2,100 They found that most of the velocity variations did not follow m, the bright spot technique becomes ineffective. In fact in the Yinggehai Basin, some bright spots do not appear to indicate oil or gas accumulations in the middle-deep strata at *Corresponding author. email: zhuht_oscar@yahoo.com.cn all (Li, 2000). It is thus necessary to investigate whether this Received January 12, 2009 340 Pet.Sci.(2009)6:339-346 technique can be applied in the region. Because the bright depth trend in the Yinggehai Basin is non-linear; instead, the spots are mainly controlled by the velocity parameters, it interval velocity exhibits reversal below 2.2 s (or 2,100 m). is essential to understand how the velocity changes in the This study also aims to test and evaluate the hydrocarbon Yinggehai Basin under the special geological settings of indicator method of DIVA that may be applied effectively high temperature, high pressure, and moderately to deeply under velocity reversal conditions, so as to help reduce the buried strata. The sensitivity and limitations of the bright risks associated with future exploration in this area. spot technique in deeper strata in general need to be further 2 Geological setting evaluated. The effects of seismic velocity characteristics on exploration target delineation and the corresponding The Yinggehai Basin, which is located southeast of measures require vigorous investigation in order to provide Hainan Island and has a NW-SE orientation, is a relatively insight for future exploration of the middle-deep oil and gas young, gas-rich, Cenozoic extensional and super-pressured reservoirs in this basin. basin in the South China Sea (Fig. 1). This basin is fi lled with The Differential Interformational Velocity Analysis up to 18 km of Tertiary clastic sediments on the Paleozoic and (DIVA) technique using ultra refined seismic velocity Mesozoic basement rocks (Huang et al, 2003). The complete analysis has been developed to identify and delineate low stratigraphic sequence has not been drilled through (Yuan et velocity anomalies in the subsurface. Porous reservoir rock al, 2009; Zhou and Yao, 2009). The structural evolution of formations can be distinguished by their reduced seismic the Yinggehai Basin can be divided into two major stages: an velocities regardless of whether hydrocarbons are present. Eocene-Oligocene syn-rift sequence and a post-rift thermal Where gas is present, the velocity reduction can be signifi cant subsidence sequence. which makes the DIVA anomaly an excellent indicator with The Yinggehai Basin has recently been studied extensively much visual impact compared to the bright spot technique. because it is the major hydrocarbon producing basin in the The DIVA technique has not only been proven by drilling South China Sea, and has many unique geological features but also been used to define unconventional exploration including: 1) high subsidence and deposition rates with the plays which cannot be detected using conventional seismic maximum sedimentation rate up to 1.2 mm/yr (Huang et al, parameters (Cook et al, 1983). 2005; Zhang et al, 2008; Yan et al, 2009), 2) high geothermal Although velocity reversal has been recognized in the gradients varying from 3.1 to 4.56 °C/100m (He et al, 2002; Norwegian Shelf by Storvoll et al (2005), its impacts on Huang et al, 2003), 3) high heat fl ow ranging from 69 to 86 the DHI have not been discussed in any detail. The aim of 2 2 mW/m with a mean value of 79±7 mW/m (He et al, 2002), this study is to understand the velocity-depth relationship 4) abnormally high overpressure with the pressure coeffi cient of the study area based on the velocity data from sonic logs up to 2.3 (Hao et al, 2000; Huang et al, 2003), and 5) the and seismic data. Published data showed that the velocity- Pet.Sci.(2009)6:339-346 341 presence of regional diapir activities (Xia et al, 2006). et al, 1998), and 3) velocity data of normally compacted The DF1-1 Gas Field, fi rst discovered in 1992, is located shales from 32 wells in the North Sea and Norwegian Sea in the northern part of the Yinggehai Basin. The fi eld contains (Hansen, 1996). 8 3 estimated gas resources of over 1,000×10 m consisting Interval “A” (Fig. 2) shows a good linear velocity- mainly of hydrocarbon gas, N , and CO (Hao et al, 2000). depth trend when excluding the data from the gas-bearing 2 2 The main source rocks are in the Miocene Huangliu, sandstones. However, the slope of the trend line shows a Meishan, and Sanya formations comprising calcareous higher velocity-depth gradient compared with the published shales, grey sandy limestones and grey biogenic limestones data listed above. Interval “A” consists mainly of the interbedded with grey and dark grey mudstones, light grey Yinggehai Formation, which is made up of argillaceous siltstones and fine sandstones. The overlying Huangliu- silty sandstone, fine sandstone, siltstone, and mudstone. Yinggehai formations and the Quaternary sediments consist Furthermore, there are distinct variations between the gas- of sandstones and shales, forming several sets of reservoir- bearing sandstones and the surrounding rocks from 1,250 seal pairs. Shale diapir structures have developed in the to 1,500 m in the shallow stratigraphic section. Combined central depression of the Yinggehai Basin (Fig. 1). with drilling and seismic data, it is demonstrated that the seismic compressional wave (P) velocity of the gas-bearing 3 Velocity characteristics sandstones above 2,000 m is 30% or more lower than that of surrounding mudstone rocks (Li, 2000). The sonic velocity data from the wells in the DF1-1 Gas Interval “B” shows the velocity data from the Meishan Field are shown in Fig. 2. These are plotted separately for to Yinggehai formations. The sediments are similar to those different lithologies and gas-bearing intervals so as to better of interval “A” except for the presence of argillaceous fine illustrate the variations. Compared with the estimated trend siltstone. The velocity data of interval “B” do not show a line of the published data (red dash-line), the velocity-depth classical linear velocity-depth trend, instead exhibiting a plot of the field exhibits two intervals with different trends reversal compared with the published data below 2,100 m. (“A” and “B” in Fig. 2). Detailed information of the published Moreover, the difference between the gas-bearing sandstones sonic velocity data in Fig. 2 include: 1) the sonic velocity of and the surrounding rocks below 2,100 m in the middle-deep nine shaly units of Oligocene to early Cretaceous age from 81 stratigraphic sections is not apparent. wells located in the North Sea (Teige et al, 1999), 2) velocity Fig. 3 shows the velocity spectral data derived from the data of the Garn sandstone and the Not and Ror shales from seismic data in the study area including interval velocity (a) the further 28 wells located offshore mid-Norway (Hermanrud and a single velocity spectrum at common depth point (CDP) Velocity, m/s 1500 2000 2500 3000 3500 Argillaceous siltstone Siltstone Fine sandstone Mudstone Poor gas-bearing layer Good gas-bearing layer =Trend line of published data Sonic velocity data derived from well logs above the Meishan Formation, DF1-1 Gas Field. The velocities of different lithofacies are plotted Fig. 2 in different colors and legends to better illustrate the variations. The dashed line shows the classical linear trend; solid lines show the real trends Depth, m 342 Pet.Sci.(2009)6:339-346 2,400 (b). In the interval velocity section (Fig. 3(a)), the (the left part of Fig. 3(b)), the stacked velocity still shows measured seismic velocity of each CDP increases gradually a linear velocity-depth trend until 2.25 s. It shows a sub- until about 2.25 s. Below this depth, it exhibits a distinct vertical velocity-depth trend down to 2,520 m/s. The interval velocity reversal as indicated by the velocity map (Fig. 3(a)). velocities of CDP 2,400 (the right part of Fig. 3(b)) are Different CDPs appear to have different velocity reversal similar to the velocity data from wells, with a linear velocity- depths. On the single velocity spectrum of CDP 2,400 depth trend until 2.25 s but exhibiting reversal below 2.25 s. Fig. 3 Interval velocity characteristic sections derived from velocity spectral analysis of a representative seismic line in the DF1-1 Gas Field. The left (a) is the interval velocity section (see Fig. 1 for the location of line), and the right (b) is the single velocity spectrum at CDP 2,400 with position shown as red line in the left (a) Fig. 4 displays the density variations with depths from 4) porosity, 5) effective pressure (both pore pressure and well logs above the Meishan Formation in the DF1-1 Gas overburden pressure), 6) rock composition (mainly shale Field. They are similar to the trends of their velocity-depth content), 7) granularity, 8) cementation, 9) temperature, and plot with a normal linear density-depth trend until 2,100 m 10) age/depth (Storvoll and Bjørlykke, 2004). We believe below which they exhibit reversal. that the dominant factors that influence velocities need to be determined for each individual area under different conditions. We do not intend to discuss the controlling factors that cause the velocity anomalies in details, but concentrate on how the velocity anomalies affecting DHIs. The Yinggehai Basin is presently under high temperature and abnormally high pressure conditions. For example, the DF1-1-11 well with a total depth (TD) of 3,508 m has a maximum borehole temperature of 167.85 °C compared with a borehole temperature of 107.84 °C at 2,100 m, has a maximum pressure of more than 70 MPa, and has a pressure coeffi cient of 2.18. Fig. 5 shows the measured and estimated pressure coefficient distributions, in which the pressure coefficient increases abruptly below 2,100 m. Although pressure estimations from mud weights give a rough estimate of maximum pore pressure in the well, they still can be thought of as a reference to indicate underground pore Fig. 4 Log-derived density-depth plots above the Meishan pressure. Through analyzing the measured velocity data of Formation, DF1-1 Gas Field. The data are plotted in different sandstones and mudstones of the DF1-1-11 well, the starting symbols to better illustrate the variations depth of velocity reversal is determined at approximately 2,100 m, which corresponds to the overpressure depth. 4 Factors affecting velocity anomalies Detailed analysis of the lithostratigraphic column of DF1-1- The main factors affecting velocity are believed to 11 well indicates that the velocity reversal does not correlate be: 1) fluid density, 2) matrix density, 3) water saturation, with any signifi cant lithological changes. Pet.Sci.(2009)6:339-346 343 low velocity. The phenomenon appears in both under- Pressure coefficient, g/cc compacted mudstones and sandstone reservoirs because of 0 0.5 1 1.5 2 2.5 the presence of abundant natural gas (Fig. 2). 5 Effects of velocity characteristics on exploration target delineation Velocity is an important parameter that has been routinely used to derive DHIs. The velocity reversal in the middle- deep strata in the DF1-1 Gas Field of the Yinggehai Basin has caused much unsatisfactory interpretation or complete failure of the velocity-derived DHI techniques. In the shallow section of the Yinggehai Basin, the reflection of the shallow gas reservoirs is characterized by strong amplitude anomalies (bright spots) on seismic profi les (Fig. 6) because of the pronounced acoustic impedance difference between the gas-bearing strata and the overlying or underlying strata, and is characterized by Type III AVO Observed pressure (Amplitude Versus Offset) anomaly whose amplitude Mud weight increases with offset on CDP gathers (Fig. 6) (Loizou et al, 2008). The acoustic impedance is controlled by the velocity and density of the strata. Because the velocity and density Observed pressure and mud weight curves of the DF1-1-11 well Fig. 5 of gas-bearing strata in the study area are both signifi cantly lower than those of the surrounding rocks (Figs. 2 and The velocity reversal phenomenon appears to be 5), there is a strong acoustic impedance difference with controlled mainly by the factors of high temperature and high pronounced bright spots shown on the shallow seismic pressure. Because of the high subsidence and sedimentation profi les in the study area. However, in the middle-deep strata, rates of the basin, sediments were generally under-compacted the gas reservoirs appear to have a different refl ection mode with large amounts of fl uids trapped in the sediments, which and usually display a moderate or weak amplitude. This does led to super-pressure that decreased velocity and caused not allow the development of strong acoustic impedance velocity reversal. Such a phenomenon is more obvious if oil difference (bright spots) because the reversal of both the and gas, especially natural gas, is present. Rocks bearing a velocity and density causes the ambiguity between the gas- large amount of un-discharged fl uids may cause abnormally bearing sandstones and the surrounding rocks. Fig. 6 Refl ective characteristics of gas-bearing sandstones, showing high acoustic impedance difference and high amplitude and bright spots refl ection between gas-bearing sandstones and the surrounding rocks in the shallow strata of the study area Velocity reversal can cause the bright spot technique to can cause “fake bright spots” on the deep seismic sections. be ineffective in the middle-deep strata. It often shows faded The “fake bright spots” mentioned above may be the latter spots or dark spots on the seismic profi le in the middle-deep two types of Li (2000). The “true” gas-bearing bright spots strata of the Yinggehai Basin. Therefore, the exploration become dark spots or fl at spots due to the changes of special model developed from the shallow gas reservoirs cannot physical and geological conditions, such as high temperature, be directly applied to the deep reservoirs. Three types of overpressure, and the degree of rock consolidation. It can bright spots were previously recognized by Li (2000) in the be misleading and lead to exploration failures if we only Yinggehai Basin, including 1) low-velocity type, 2) density rely on the bright spots as the DHI techniques for exploring type, and 3) calcium type. When using the bright spot hydrocarbon reservoirs in deeper strata. technique, it must be noticed that non-gas-bearing sandstones It is therefore important to detect some bright spots or Depth, m 344 Pet.Sci.(2009)6:339-346 other characteristic hydrocarbon indicators on the seismic velocity curve to assemble the next (deep) curve sequentially profi les in the middle or deep strata of the Yinggehai Basin. until the last curve; use the second shallow velocity curve If the bright spot technique is to be used, one should pay to repeat above-mentioned step until the second last to fi nal great attention to searching for the “phase reversal” and curves with the shallow curves marked as solid lines and the “dark spots”. Alternatively one may use other hydrocarbon deep ones as broken lines; indicating techniques to detect hydrocarbons from the 3) Arrange the DIVA sections by using the DIVA curves characteristics of the seismic profiles, such as 1) Jason assembled in step 2) to generate the DIVA sections according inversion using seismic and well-log data; 2) AVO attribute to the order of sequence from shallow to deep; parameter cross-plot including Gradient Versus Intercept, 4) Rank and interpret the DIVA sections. Gradient Versus Fluid Factor, and Gradient Versus Poisson When the deep velocity is lower than the shallow one, Ratio; 3) DIVA; and 4) high-order statistics to detect other it would show a velocity abnormal area that can be used to hydrocarbon indicators of the seismic expression in middle evaluate and ascertain the potential gas-bearing zones based or deep strata. Some of the techniques have been put into on the reliability of velocity anomalies shown on the DIVA practice in stratigraphic gas exploration in the Yinggehai sections. The DIVA abnormal classes are usually marked with Basin, resulting in successive discoveries of several medium Roman numerals on the DIVA sections with class I being to small gas fi elds. the best potential zone while class IV being the worst. Each Roman number has a numerical footnote representing the 6 Application of the DIVA technique serial number grades of abnormal zones with the smaller ones being the better in the same class. The DIVA, a useful seismic technique, which makes use of ultra refi ned seismic velocity analysis, has been developed to 6.2 Experimental results identify and localize low velocity anomalies in the subsurface For the evaluation and application of the DIVA technique (Cook et al, 1983). The DIVA technique has been shown to be in the middle-deep strata of the line in the DF1-1 Gas Field, an effective hydrocarbon indicating technique and has been one seismic section (Fig. 1) was selected. We compared it applied successfully in some parts of the world, especially in with the DIVA sections derived from the shallow, middle, and some terrigenous basins (Zhu et al, 2004). However, there are deep strata to evaluate the effectiveness of DIVA as a DHI in no publications that directly address the DIVA method with the deep strata of the Yinggehai Basin. In this test, the shallow regard to the velocity reversal issue. Therefore, it is essential objective is named as “a” segment, the middle objective to evaluate whether this technique can be successfully applied as “b”, the deep objective as “c”, each of which comprises in the Yinggehai Basin, South China Sea. about 10 refl ecting interfaces beginning from numeral 1 from shallow to deep strata. 6.1 Experimental method Table 1 lists the velocity analysis interface parameters The DIVA procedure consists of four steps: illustrated in three DIVA sections on the seismic line (Figs. 1) Plot the velocity data by using the interval velocity data 7-9), where the “a” segment between T and T (see Fig. 27 29 from the pre-stack seismic data with the velocity as y-axis and 1 for location) reflecting interfaces is the main gas-bearing CDPs as x-axis; strata, made up of 10 refl ecting interfaces in the DF1-1 Gas 2) Assemble the DIVA curves by using the first shallow Field. Velocity analysis interface parameters of target intervals using DIVA in the DF1-1 Gas Field Table 1 Target zone TWT, s CDP range Layer range Layer attribute Remarks a -a 1.2-1.5 1984-3078 T -T Gas-bearing layers: a , a , a Unused layers: a , a , a 1 10 27 29 2 4 6 3 5 7 b -b 1.8-2.0 1874-3047 T -T Potential gas-bearing: b , b , b Unused layers: b , b , b , b , b 1 11 29 30 5 7 9 2 4 6 8 10 c -c 2.1-2.4 1923-3015 T -T Potential gas-bearing: c , c , c Unused layers: c , c , c , c 1 11 30 40 3 5 7 2 4 6 8 Based on the DIVA interpretation sections of both the strata. Fig. 8 shows a class II and a class III abnormal DIVA shallow and middle-deep strata, it can be clearly demonstrated zones. Fig. 9 shows two class II abnormal DIVA zones. One that the class I abnormal DIVA zones are normally in the occurs at the depth about T , located on CDPs from 2,420 shallow strata, whereas the class II abnormal DIVA zones are to 3,000, deeper than the other with a depth around T -T , 30 40 usually in the middle-deep strata. located on CDPs from 1,920 to 2,250. The former appears to Fig. 7 shows two class I abnormal DIVA zones located on be more obvious than the latter in terms of abnormal degrees. CDPs of 2,000-2,200 and 2,630-2,900, separated by one class The abnormal DIVA zones discussed above are closely II abnormal DIVA zone in-between. related to the abnormal velocity distribution. The velocity In the middle-deep strata a strong abnormal DIVA zone is of gas-bearing sandstones is normally lower than that of shown, but their classes are lower than those in the shallow the surrounding rocks displaying primarily class I DIVA Pet.Sci.(2009)6:339-346 345 346 346 Pet.Sci.(2009)6:339-346 He L, Xiong L and Wang J. Heat flow and thermal modeling of the characteristics in the corresponding zones in the shallow Yinggehai Basin, South China Sea. Tectonophysics. 2002. 351(3): strata. In the middle-deep strata, the class I abnormally 245-253 changes into class II as shown on the DIVA sections because Her manrud C, Wensaas L, Teige G M G, et al. Shale porosities from well of the reduction of the velocity declination degree in the gas- logs on Haltenbanken (offshore mid-Norway) show no infl uence of bearing layers, showing velocity reversal and poor seismic overpressuring. In: Law B E, Ulmishek G F and Slavin V I (Eds.), quality. Abnormal pressures in hydrocarbon environments. AAPG Memoir. 1998. 70: 65-85 7 Conclusions Hua ng B J, Xiao X M and Li X X. Geochemistry and origins of natural gases in the Yinggehai and Qiongdongnan basins, offshore South Velocity analyses from well logs and seismic data of the China Sea. Organic Geochemistry. 2003. 34(7): 1009-1025 DF1-1 Gas Field in the Yinggehai Basin, South China Sea Hua ng B J, Xiao X M, Hu Z L, et al. Geochemistry and episodic indicate that the velocity-depth trend is linear excluding accumulation of natural gases from the Ledong Gas Field in the the gas-bearing sandstones until the depth of 2,100 m (2.25 Yinggehai Basin, offshore South China Sea. Organic Geochemistry. s TWT) in the shallow strata. The slope of the trend line 2005. 36(12): 1689-1702 indicates a higher velocity-depth gradient in the shallow strata Jap sen P. Velocity-depth trends in Mesozoic and Cenozoic sediments compared with published data elsewhere. In contrast in the from the Norwegian Shelf: Discussion. AAPG Bulletin. 2006. 90(7): middle-deep strata below 2,100 m (2.25 s TWT), the velocity- 1141-1143 Li X X. Seismic recognition techniques of shallow gas reservoirs in the depth relationship does not show a normal linear trend, but Yinggehai Basin. China Offshore Oil and Gas. 2000. 14(3): 193-199 exhibits distinct velocity reversal. The velocity between (in Chinese) the gas-bearing sandstones and the surrounding rocks also Loi zou N, Liu E and Chapman M. AVO analyses and spectral exhibits distinct variations in the shallow strata, but becomes decomposition of seismic data from four wells west of Shetland, UK. indistinguishable in the middle-deep strata. The velocity Petroleum Geoscience. 2008. 14(4): 355-368 reversal is believed to be mainly caused by high temperature Sto rvoll V and Bjørlykke K. Sonic velocity and grain contact properties and abnormally high pressure as the velocity anomaly and in reservoir sandstones. Petroleum Geoscience. 2004. 10(3): 215-226 pressure anomaly occur at the same depth. Sto rvoll V, Bjørlykke K and Mondol N H. Velocity-depth trends in The DIVA method has been found to be an effective DHI Mesozoic and Cenozoic sediments from the Norwegian Shelf. AAPG both in the shallow and the middle-deep strata in the study Bulletin. 2005. 89(3): 359-381 area. Two DIVA DHI anomalies have been identifi ed on the Tei ge G M G, Hermanrud C, Wensaas L, et al. The lack of relationship between overpressure and porosity in North Sea and Haltenbanken seismic profi les with class I abnormal DIVA zones associated shales: Overpressure research. Marine and Petroleum Geology. 1999. with the shallow strata, and class II abnormal DIVA zones 16(4): 321-335 associated with the middle-deep strata. Xia B, Zhang Y, Cui X J, et al. Understanding of the geological and geodynamic controls on the formation of the South China Sea: A Acknowledgements numerical modelling approach. Journal of Geodynamics. 2006. 42(1/3): 63-84 This study was supported by the National Natural Science Yan Y, Hu X Q, Lin G, et al. Denudation history of South China block Foundation of China (No.40702024) and partly funded by and sediment supply to northern margin of the South China Sea. AAPG Grant-in-Aid to the fi rst author. Also, the Project was Earth Science—Journal of China University of Geosciences. 2009. sponsored by the Scientific Research Foundation for the 20(1): 57-65 Returned Overseas Chinese Scholars, Ministry of Education Yin P, Berne S, Vagner P, et al. Mud volcanoes at the shelf margin of the of China (No.2009022014) and Open Research Foundation of East China Sea. Marine Geology. 2003. 194(3/4): 135-149 Key Laboratory of Tectonics and Petroleum Resources (China You ng K T and Tatham R H. Fluid discrimination of poststack “bright University of Geosciences), Ministry of Education (No.TPR- spots” in the Columbus Basin, offshore Trinidad. The Leading Edge. 2009-33). The authors would like to thank the Zhanjiang 2007. 26(12): 1508-1515 Branch of CNOOC for providing well and seismic data. Yua n S Q, Yao G S, Lü F L, et al. Features of late Cenozoic deepwater sedimentation in southern Qiongdongnan Basin, northwestern References South China Sea. Earth Science—Journal of China University of Geosciences. 2009. 20(1): 172-179 Coo k E E, Neidell N S and Beard J H. Precision measurements of Zha ng Y F, Sun Z, Zhou D, et al. Stretching characteristics and its interval velocity differences from seismic data. AAPG Annual dynamic significance of the northern continental margin of South Convention. 1983. 67(3): 442 China Sea. Science in China Series D: Earth Sciences. 2008. 51(3): Han sen S. Quantification of net uplift and erosion on the Norwegian 422-430 Shelf south of 66 degrees N from sonic transit times of shale. Norsk Zho u D and Yao B C. Tectonics and sedimentary basins of the South Geologisk Tidsskrift. 1996. 76: 245-252 China Sea: Challenges and progresses. Earth Science—Journal of Hao F, Li S T, Gong Z S, et al. Thermal regime, inter-reservoir China University of Geosciences. 2009. 20(1): 1-12 compositional heterogeneities, and reservoir-filling history of the Zhu H T, Chen K Y and Zhu P M. Application of DIVA technique in Dongfang Gas Field, Yinggehai Basin, South China Sea: Evidence Yinggehai Basin. Oil Geophysical Prospecting. 2004. 39(3): 319- for episodic fl uid injections in overpressured basins. AAPG Bulletin. 321 (in Chinese) 2000. 84(5): 607-626 (Edited by Hao Jie)

Journal

Petroleum Science – Springer Journals

Published: Nov 26, 2009

There are no references for this article.