Quantitative prediction model for the depth limit of oil accumulation in the deep carbonate rocks:A case study of Lower Ordovician in Tazhong area of Tarim Basin

With continuous hydrocarbon exploration extending to deeper basins,the deepest industrial oil accumulation was discovered below 8,200 m,revealing a new exploration field.Hence,the extent to which oil exploration can be extended,and the prediction of the depth limit of oil accumulation(DLOA),are issues that have attracted significant attention in petroleum geology.Since it is difficult to characterize the evolution of the physical properties of the marine carbonate reservoir with burial depth,and the deepest drilling still cannot reach the DLOA.Hence,the DLOA cannot be predicted by directly establishing the relationship between the ratio of drilling to the dry layer and the depth.In this study,by establishing the relationships between the porosity and the depth and dry layer ratio of the carbonate reservoir,the relationships between the depth and dry layer ratio were obtained collectively.The depth corresponding to a dry layer ratio of 100%is the DLOA.Based on this,a quantitative prediction model for the DLOA was finally built.The results indicate that the porosity of the carbonate reservoir,Lower Ordovician in Tazhong area of Tarim Basin,tends to decrease with burial depth,and manifests as an overall low porosity reservoir in deep layer.The critical porosity of the DLOA was 1.8%,which is the critical geological condition corresponding to a 100%dry layer ratio encountered in the reservoir.The depth of the DLOA was 9,000 m.This study provides a new method for DLOA prediction that is beneficial for a deeper understanding of oil accumulation,and is of great importance for scientific guidance on deep oil drilling.

The molecular mechanism of WRINKLED1 transcription factor regulating oil accumulation in developing seeds of castor bean

The transcription factor WRINKLED1(WRI1),a member of AP2 gene family that contain typical AP2 domains,has been considered as a master regulator regulating oil biosynthesis in oilseeds.However,the regulatory mechanism of RcWRI1 in regulating oil accumulation during seed development has not been clearly addressed.Castor bean(Ricinus communis)is one of the most important non-edible oil crops and its seed oils are rich in hydroxy fatty acids,widely applied in industry.In this study,based on castor bean reference genome,three RcWRIs genes(RcWRI1,RcWRI2 and RcWRI3)were identified and the expressed association of RcWRI1 with oil accumulation were determined.Heterologous transformation of RcWRI1 significantly increased oil content in tobacco leaf,confirming that RcWRI1 activate lipid biosynthesis pathway.Using DNA Affinity Purification sequencing(DAP-seq)technology,we confirmed RcWRI1 binding with Transcription Start Site of genes and identified 7961 WRI1-binding candidate genes.Functionally,these identified genes were mainly involved in diverse metabolism pathways(including lipid biosynthesis).Three cis-elements AW-box([CnTnG](n)7[CG])and AW-boxes like([GnAnC](n)6[GC]/[GnAnC](n)7[G])bound with RcWRI1 were identified.Co-expression network analysis of RcWRI1 further found that RcWRI1 might be widely involved in biosynthesis of storage materials during seed development.In particular,yeast one hybrid experiments found that both AP2 domains within RcWRI1 were required in binding targeted genes.These results not only provide new evidence to understand the regulatory mechanism of RcWRI1 in regulation of oil accumulation during castor bean seed development,but also give candidate gene resource for subsequent genetic improvement toward increasing oil content in oilseed crops.

Effects of Heat Stress during Seed Filling Stage on Brassica napus Seed Oil Accumulation and Chlorophyll Fluorescence Characteristics

As global temperature rise,the threat of heat stress to rapeseed production is becoming more obvious.Exploring the response characteristics of two important biological pathways,oil accumulation and photosynthesis,to heat stress during B.napus seed filling is helpful in the genetic improvement of heat-tolerant rapeseed.The effects of heat stress on seed oil accumulation and chlorophyll fluorescence characteristics of 29 B.napus germplasms with different oil content and environmental sensitivity,including 6 rapeseed varieties which exhibited environmentsensitive/insensitive and with high,medium or low oil content,were tested by whole plant heat stress or the in vitro silique culture system.Both assay exhibited similar trend on oil content of the rapeseed germplasms.The heat effect on the chlorophyll fluorescence kinetic parameters F_(v)/F_(m),ETR and Y(Ⅱ)were also consistent.Heat stress significantly decreased oil content,although there was abundant genetic variation on heat tolerance among the genotypes.Correlation analysis showed that the decrease rate of F_(v)/F_(m) of silique heat-stressed B.napus developing seed was positive correlative to the decrease rate of mature seed oil content of the whole plant heat-stressed rapeseed(R=0.9214,P-value<0.01).Overall,the results indicated that heat stress inhibited oil accumulation and photosynthesis in B.napus developing seed.The decrease rate of chlorophyll fluorescence parameter F_(v)/F_(m) of heat-stressed developing seed could be used as the index of heat tolerant rapeseed identification.Further,two heat insensitive rapeseed varieties with high oil content were identified.

Antagonistic MADS-box transcription factors SEEDSTICK and SEPALLATA3 form a transcriptional regulatory network that regulates seed oil accumulation

Transcriptional regulation is essential for balancing multiple metabolic pathways that influence oil accumulation in seeds.Thus far,the transcriptional regulatory mechanisms that govern seed oil accumulation remain largely unknown.Here,we identified the transcriptional regulatory network composed of MADS-box transcription factors SEEDSTICK(STK)and SEPALLATA3(SEP3),which bridges several key genes to regulate oil accumulation in seeds.We found that STK,highly expressed in the developing embryo,positively regulates seed oil accumulation in Arabidopsis(Arabidopsis thaliana).Furthermore,we discovered that SEP3 physically interacts with STK in vivo and in vitro.Seed oil content is increased by the SEP3 mutation,while it is decreased by SEP3 overexpression.The chromatin immunoprecipitation,electrophoretic mobility shift assay,and transient dual-luciferase reporter assays showed that STK positively regulates seed oil accumulation by directly repressing the expression of MYB5,SEP3,and SEED FATTY ACID REDUCER 4(SFAR4).Moreover,genetic and molecular analyses demonstrated that STK and SEP3 antagonistically regulate seed oil production and that SEP3 weakens the binding ability of STK to MYB5,SEP3,and SFAR4.Additionally,we demonstrated that TRANSPARENT TESTA 8(TT8)and ACYL-ACYL CARRIER PROTEIN DESATURASE 3(AAD3)are direct targets of MYB5 during seed oil accumulation in Arabidopsis.Together,our findings provide the transcriptional regulatory network antagonistically orchestrated by STK and SEP3,which fine tunes oil accumulation in seeds.

BnWRI1 coordinates fatty acid biosynthesis and photosynthesis pathways during oil accumulation in rapeseed

Photosynthesis in“green”seeds, such as rapeseed, soybean, and Arabidopsis, plays a substantial role in the improved efficiency of oil accumulation. However, the molecular mecha-nism underpinning the coordinated expression of fatty acid （FA） biosynthesis-and photosynthesis-related genes in such develop-ing seeds remains to be elucidated. Here, we found that seed-specific overexpression of BnWRI1, a WRI1 homolog from rapeseed （Brassica napus cv. ZGY2）, results in enhanced chlorophyl content in developing seeds and increased oil content and seed mass in matured seeds. BnWRI1 was co-expressed with BnBCCP and BnCAB, two marker genes of FA biosynthesis and photosynthesis during seed development, respectively. Over-expression of BnWRI1 increased expression of both marker genes. Further, the nuclear-localized BnWRI1 protein was found to act as a transcription activator. It could bind to the GT1-element and/or GCC-box, which are widespread in the upstream regions of genes involved in FA biosynthesis and photosynthesis pathways. Accordingly, BnWRI1 could interact with promoters of BCCP2 and LHB1B2 in vivo. These results suggested that BnWRI1 may coordinate FA biosynthesis and photosynthesis pathways in developing seeds via directly stimulating expression of GT1-element and/or GCC-box containing genes.

Source and accumulation process of Jurassic biodegraded oil in the Eastern Junggar Basin,NW China

Biodegradation usually obscures or even radically alters the original characteristics of oil biomarkers.The mixing of oil from multiple sources makes each source difficult to trace.Identifying the source of biodegraded oil from multiple sources has always been a hard nut to crack.Rising to this challenge,in this study-we carried out a comprehensive investigation of biodegradation impacts,oil-source correlation,and oil charging history to trace the source and reveal the mixing process of biodegraded oil in the Toutunhe Formation(J_(2)t)in the eastern Junggar Basin,NW China.The oil of this area was biodegraded to different extent,consequently,many commonly used biomarker parameters(e.g.Pr/Ph,Pr/n C_(17))became less powerful for oil-source correlation.To address this problem,the resistance of many biomarkers to biodegradation was analyzed,and those of high bio resistance were selected to generate a more reliable oil-source correlation.The results revealed that biodegraded oil was a mixture of oil sourced from Lucaogou Formation(P_(2)l)and Xiaoquangou Formation(T_(2-3)xq).Core sample observation,microscopic fluorescent analysis and fluid inclusion analysis were combined to analyze comprehensively oil charging history.The analysis of accumulation process exhibited that the existing oil in J_(2)t was a mixture originated from the P_(2)l and T_(2-3)xq source rocks in two separate charging stages when it underwent a complicated process of charging,biodegradation,recharging and mixing.

Characteristics of Oil and Gas Accumulation in Yong'an-Meitai Area of the Fushan Depression,Beibuwan Basin,South China Sea

The Yong＇an-Meitai area is the focus of the present exploration in the Fushan Depression, Beibuwan Basin, South China Sea. All oils from this area are geochemically characterized by higher Pr/Ph ratio, higher proportion of heavy molecular weight hydrocarbons, and higher proportion of C29 regular steranes, which indicate that the organic matter of source rocks might have been deposited in an oxidizing palaeoenvironment and be dominated by higher plant organic matter input. The oil from E3w2 （the second member of Weizhou Fro. of the Oligocene） has a much higher density, relatively higher Pr/nC17 and Ph/nC18 ratios, and a ＂UCM--unresolved complex mixture＂ on gas chromatograms, which indicate that it has been slightly biodegraded. CPI and other terpane and sterane isomer ratios suggest they are all mature oils. The timing of oil charging in E3w2 and E2I1 （the first member of the Liushagang Fro. of the Eocene） determined by the homogenization temperatures of fluid inclusions and thermal evolution history are from 9-3 Ma and 8-3 Ma, respectively. Thus, the interpretation of E3w2 as a secondary reservoir is unlikely. The timing of oil charging is later than that of hydrocarbon generating and expulsion of Liushagang Fin. source rocks and trap formation, which is favorable for oil accumulation in this area. All molecular parameters that are used for tracing oil filling direction decrease with shallower burial depth, which suggests vertical oil migration. The widely occurring faults that penetrate through the source rocks of the Liushagang Fro. may serve as a fine oil charging conduit.

Oil and gas source and accumulation of Zhongqiu 1 trap in Qiulitage structural belt, Tarim Basin, NW China

Well Zhongqiu 1 obtained highly productive oil-gas stream in the footwall of Zhongqiu structure, marking the strategic breakthrough of Qiulitag structural belt in the Tarim Basin. However, the oil and gas sources in Zhongqiu structural belt and the reservoir formation process in Zhongqiu 1 trap remain unclear, so study on these issues may provide important basis for the next step of oil and gas exploration and deployment in Qiulitage structural belt. In this study, a systematic correlation of oil and gas source in Well Zhongqiu 1 has been carried out. The oil in Well Zhongqiu 1 is derived from Triassic lacustrine mudstone, while the gas is a typical coal-derived gas and mainly from Jurassic coal measures. The oil charging in Well Zhongqiu 1 mainly took place during the sedimentary period from Jidike Formation to Kangcun Formation in Neogene, and the oil was mainly contributed by Triassic source rock;large-scale natural gas charging occurred in the sedimentary period of Kuqa Formation in Neogene, and the coal-derived gas generated in the late Jurassic caused large-scale gas invasion to the early Triassic crude oil reservoirs. The Zhongqiu 1 trap was formed earlier than or at the same period as the hydrocarbon generation and expulsion period of Triassic-Jurassic source rocks. Active faults provided paths for hydrocarbon migration. The source rocks-faults-traps matched well in time and space. Traps in the footwall of the Zhongqiu structural fault have similar reservoir-forming conditions with the Zhongqiu 1 trap, so they are favorable targets in the next step of exploration.

Important Roles of Transcription Factors in Regulating Seed Oil Biosynthesis to Increase Plant Storage Lipid Content

In this article, the biosynthetic pathways of storage oil accumulation in oilseed plants were briefly introduced, and the transcription factors, such as B3 do- main supeffamily genes, lecl gene, wril gene etc., and their important role in oil accumulation regulation was mainly elucidated. Overexpession of transcription factors as feasible ways of genetic manipulation to increase oJl content in oilseed crops are promising in a long-term perspective.

A Study of the Accompanying Relationships between Uranium and Oil

It is not occasional that uranium deposits and oil accumulation occur in the same depression in the Erlian basin, Inner Mongolia. Some evidences show certain relations between uranium and oil in origin. This paper discusses and analyses the evidence for the relations between uranium deposits and oil and gas accumulation in terms of spatial distribution, geology, hydrochemistry and geochemistry. The paper also deals with the mechanism of the formation of uranium deposits and points out that it is of significance to use uranium as an indicator to search for oil and vice versa.

Evolution of Ordovician YJ1X ultra-deep oil reservoir in the Yuecan oilfield, Tarim Basin, NW China

Based on a combined in-situ calcite U-Pb dating, molecular geochemical correlations of reservoir oil and extract from reservoir rocks, and fluid inclusion analysis, the charge and evolution history of the YJ1X ultra-deep oil reservoir of the Ordovician Yijianfang Formation in the southwestern part of the Tabei Uplift has been determined systematically.(1) The reservoir oil, free oil and inclusion oil have similar geochemical characteristics and are presumably derived from marine source rock deposited in similar sedimentary environment.(2) The reservoir oil, free oil and inclusion oil have similar maturities with calculated equivalent vitrinite reflectance values in the range of 0.80%-0.96%.(3) Two types(Group Ⅰ and Ⅱ) of oil inclusion assemblages(OIAs) have been identified in the reservoir, of which Group Ⅰ represents the original gas-saturated oil entering the trap during the initial oil charge, whereas Group Ⅱ represents undersaturated residual oil retained in the reservoir after minor leakage of light hydrocarbon.(4) The reservoir experienced oil charge only once during the Early Devonian around 425 Ma and has been well preserved after the minor light hydrocarbon leakage in the Middle Devonian. The study shows that there may be old oil and gas accumulations in ultra-deep strata of petroliferous basins with well-developed caprock and stable tectonic background.

Main Controls on Hydrocarbon Accumulation in the Paleozoic in Central Saudi Arabia

Sandi Arabia is renown for its rich oil and gas Mesozoic. However, the discovery of Paleozoic fields in resources with the bulk of the reserves reservo/red in the the late 1980s has encouraged further exploration in the Paleozoic. This paper reviews the salient features of the Paleozoic petroleum geology in central Saudi Arabia and discusses the main factors controlling hydrocarbon accumulation in the Paleozoic. The Lower Silurian Qusaiba hot shale is the principal source rock for the hydrocarbons discovered in the Ordovician to Permian reservoirs. Of them, the Permo- Carboniferous Unayzah and Upper Ordovician Sarah Formations have the best exploration potential. The key factors controlling hydrocarbon accumulation in the Unayzah Formation are migration pathways and reservoir petrophysics. The key factors controlling hydrocarbon accumulation in the Sarah Formation are reservoir petrophysics and the development of structural traps.

Oil source and migration process in oblique transfer zone of Fushan Sag,northern South China Sea

The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the E_2l_1^(L+M) and the E_2l_3~U. 46 oil and rock samples reveal that the oil in the transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The E_2l_3~U oil, characterized by high maturity, Pr/Ph ratio and oleanane/C_(30)-hopane ratio, shows a close genetic affinity with the E_2l_3~b source rocks, while the E_2l_1^(L+M) oil, characterized by lower maturity, Pr/Ph ratio and oleanane/C_(30)-hopane ratio, is suggested to be derived from the E_2l_(1+2)~b source rocks. The homogenization temperatures of aqueous fluid inclusions, taking the burial history of the reservoirs into account, reflect that the oil charge mainly occurred from mid-Miocene to Pliocene in the oblique transfer zone. The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Of these, the faults are the most complicated and significant. The faults differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge stage, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults during the vertical migration in the west area. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone. Eventually, an oil migration and accumulation model is built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.

Prediction of Apparent Equivalent Thickness Using the Spontaneous Potential Method and Its Application to Oilfield Development

The upper spontaneous potential produced by oil and gas accumulation is of a stable potential field, and its intensity is directly proportional to the content of the source and inversely proportional to the radius apart from the source. Theoretical research and practical results show that anomalies of spontaneous potential can indicate oil-bearing sandstone bodies and locate the areas of oil and gas accumulation. In oil areas which have fewer reservoir beds, the petroleum reservoir thickness can be predicted by determining the linear relationship between potential intensity and apparent equivalent thickness. In the Weixing （卫星） oilfield, which is devoid of sufficient reservoir beds, its apparent equivalent thickness can be predicted by the linear equation h= -0.19x＋0.74. On the basis of geological research, we use the spontaneous potential method to predict the equivalent thickness, helping in the selection of the most appropriate drill sites to enhance the probability of successful well boring so as to serve the next round development of the oilfield.

Differences and controlling factors of composite hydrocarbon accumulations in the Tazhong uplift, Tarim Basin, NW China

Based on three-dimensional seismic interpretation, structural and sedimentary feature analysis, and examination of fluid properties and production dynamics, the regularity and main controlling factors of hydrocarbon accumulation in the Tazhong uplift, Tarim Basin are investigated. The results show that the oil and gas in the Tazhong uplift has the characteristics of complex accumulation mainly controlled by faults, and more than 80% of the oil and gas reserves are enriched along fault zones. There are large thrust and strike-slip faults in the Tazhong uplift, and the coupling relationship between the formation and evolution of the faults and accumulation determine the difference in complex oil and gas accumulations. The active scale and stage of faults determine the fullness of the traps and the balance of the phase, that is, the blocking of the transport system, the insufficient filling of oil and gas, and the unsteady state of fluid accumulation are dependent on the faults. The multi-period tectonic sedimentary evolution controls the differences of trap conditions in the fault zones, and the multi-phase hydrocarbon migration and accumulation causes the differences of fluid distribution in the fault zones. The theory of differential oil and gas accumulation controlled by fault is the key to the overall evaluation, three-dimensional development and discovery of new reserves in the Tazhong uplift.

Petroleum exploration breakthrough and geological significance in Cretaceous Yingcheng and Denglouku formations of Shuangcheng area, northern Songliao Basin, NE China

Based on drilling geological,geochemical,geophysical and production test data,the characteristics of source rocks,reservoir rocks and caprocks,as well as the process of hydrocarbon generation,trap evolution and oil accumulation of the oil-bearing assemblage composed of the Cretaceous Yingcheng Formation(K1yc)and Denglouku Formation(K1d)in the Shuangcheng area,northern Songliao Basin,NE China,were analyzed by using the research methods for petroleum systems.The source rocks mainly exist in K1yc,with the organic matters mainly originated from lower aquatic organisms and algae,and reaching the grade of high-quality source rock in terms of organic abundance.The crude oil,which is characterized by low density,high freezing point and high wax content,is believed to have generated by the K1yc source rocks.The reservoir rocks include K1d sandstones and K1yc glutenites.The mudstone in the fourth member of K1d serves as the direct caprock of the oil reservoir.The oil was generated during the period between Yaojia Formation and Nenjiang Formation,and then accumulated during the periods of Nenjiang Formation and Paleogene–Neogene.The traps evolved in three stages:the late Yingcheng Formation,the late Quantou Formation and the late Nenjiang Formation,forming structural and structural-lithologic reservoirs.It is concluded that good source-reservoir-caprock assemblage,late hydrocarbon charging,short migration distance and stable tectonic setting are favorable factors for the formation of oil reservoirs.

Construction of a Normalized Full-Length cDNA Library of Sesame Developing Seed by DSN and SMART^(TM)

Sesame （Sesamue indicum L.） is one of the most important oilseed crops with high oil yield. Here, we described a simple and efficient method for constructing a normalized cDNA library from a high oil content cultivar of sesame Zhongzhi 14, during its oil accumulation stages. It combined switching mechanism at 5？end of RNA transcript （SMART） technique and duplex-specific nuclease （DSN） normalization methods. Double-stranded cDNAs were synthesized from mRNAs, processed by normalization and Sfi I restriction endonuclease, and finally the cDNAs were ligated to pDNR-LIB vector. The ligation mixture was transformed into Escherichia coli DH10B by electroporation. The capacity of the library was 1.0？06 clones in this library. Gel electrophoresis results indicated the fragments ranged from 700 to 2 000 bp, with the average size of 1 800 bp. Random picking clones showed that the recombination rate was 100%. The results showed that the cDNA library constructed successfully was a full-length library with high quality, and could be used to screen the genes related to development of oil synthesis.

Comparative transcriptome analysis on candidate genes involved in lipid biosynthesis of developing kernels for three walnut cultivars in Xinjiang

Walnut(Juglans regia L.)is a good source of lipids and polyunsaturated fatty acids(PUFAs).In order to explore the biosynthesis molecular mechanism of oil accumulation and fatty acid(FA)synthesis in walnut,the samples at different development periods of three walnut cultivars,’Zhipi’(ZP),’Xinwu 417’(W417)and’Xinwen 81’(W81)were collected for transcriptomic analysis.The analysis of oil accumulation and FA profiles showed that the oil content in mature walnut kernel was nearly 70%,and over 90%of FAs were PUFAs.We identified 126 candidate genes including 64 genes for FA de novo synthesis,45 genes for triacylglycerol assembly,and 17 genes for oil bodies involved in lipid biosynthesis by RNA-sequencing.Ten key enzymes including ACCase,LACS6,LACS8,SAD,FAD2,FAD3,LPAAT1,DGAT2,PDAT2,and PLC encoded by 19 genes were highly associated with lipid biosynthesis.Quantitative PCR analysis further validated 9 important genes,and the results were well consistent with our transcriptomic data.Finally,5 important transcription factors including WRI1,ABI3,FUS3,PKL and VAL1 were identified,and their main regulatory genes might contain ACCase,KASII,LACS,FAD3 and LPAAT which were determined through correlation analysis of expression levels for 27 walnut samples.These findings will provide a comprehensive understanding and valuable information on the genetic engineering and molecular breeding in walnut.

Formation and accumulation of oil and gas in marine carbonate sequences in Chinese sedimentary basins

Advances in studies of formation and accumulation mechanisms of oil and gas in marine carbonate sequences have led to continuing breakthroughs of petroleum exploration in marine carbonate sequences in Chinese sedimentary basins in recent years. The recently discovered giant Tahe Oil Field and Puguang Gas Field have provided geological entities for further studies of the formation and accumulation of oil and gas in marine carbonate sequences. Marine carbonate sequences in China are characterized by old age, multiple structural deformation, differential thermal evolution of source rocks, various reservoir types (i.e. reef-bank complex and paleo-weathered crust karst reservoir), uneven development of caprocks, especially gypsum seal, and multi-episodes of hydrocarbon accumulation and readjustment. As a result, the formation of hydrocarbon accumulations in the Chinese marine carbonate sequences has the following features: (i) the high-quality marine source rocks of shale and calcareous mudstone are often associated with siliceous rocks or calcareous rocks and were deposited in slope environments. They are rich in organic matter, have a higher hydrocarbon generation potential, but experienced variable thermal evolutions in different basins or different areas of the same basin. (ii) High quality reservoirs are controlled by both primary depositional environments and later modifications including diagenetic modifications, structural deformations, and fluid effects. (iii) Development of high-quality caprocks, especially gypsum seals, is the key to the formation of large-and medium-sized oil and gas fields in marine carbonate sequences. Gypsum often constitutes the caprock for most of large sized gas fields. Given that Chinese marine carbonate sequences are of old age and subject to multiple episodes of structural deformation and superposition, oil and gas tend to accumulate in the slopes and structural hinge zones, since the slopes favor the development of effective assemblage of source-reservoir-caprock, high quality source rocks, good reservoirs such as reef-bank complex, and various caprocks. As the structural hinge zones lay in the focus area of petroleum migration and experienced little structural deformation, they are also favorable places for hydrocarbon accumulation and preservation.