Quantitative effect of kerogen type on the hydrocarbon generation potential of Paleogene lacustrine source rocks,Liaohe Western Depression,China

Kerogen types exert a decisive effect on the onset and capacity of hydrocarbon generation of source rocks.Lacustrine source rocks in the Liaohe Western Depression are characterized by thick deposition,high total organic carbon(TOC)content,various kerogen types,and a wide range of thermal maturity.Consequently,their hydrocarbon generation potential and resource estimation can be misinterpreted.In this study,geochemical tests,numerical analysis,hydrocarbon generation kinetics,and basin modeling were integrated to investigate the differential effects of kerogen types on the hydrocarbon generation potential of lacustrine source rocks.Optimized hydrocarbon generation and expulsion(HGE)models of different kerogen types were established quantitatively upon abundant Rock-Eval/TOC/vitrinite reflectance(R_(o))datasets.Three sets of good-excellent source rocks deposited in the fourth(Es4),third(Es3),and first(Es1)members of Paleogene Shahejie Formation,are predominantly types I-II_(1),II_(1)-II_(2),and II-III,respectively.The activation energy of types I-II_(2)kerogen is concentrated(180-230 kcal/mol),whereas that of type III kerogen is widely distributed(150-280 kcal/mol).The original hydrocarbon generation potentials of types I,II_(1),II_(2),and III kerogens are 790,510,270,and 85 mg/g TOC,respectively.The Ro values of the hydrocarbon generation threshold for type I-III source rocks gradually increase from 0.42%to 0.74%,and Ro values of the hydrocarbon expulsion threshold increase from 0.49%to 0.87%.Types I and II_(1)source rocks are characterized by earlier hydrocarbon generation,more rapid hydrocarbon expulsion,and narrower hydrocarbon generation windows than types II_(2)and III source rocks.The kerogen types also affect the HGE history and resource potential.Three types(conventional,tight,and shale oil/gas)and three levels(realistic,expected,and prospective)of hydrocarbon resources of different members in the Liaohe Western Depression are evaluated.Findings suggest that the Es3 member has considerable conventional and unconventional hydrocarbon resources.This study can quantitatively characterize the hydrocarbon generation potential of source rocks with different kerogen types,and facilitate a quick and accurate assessment of hydrocarbon resources,providing strategies for future oil and gas exploration.

Sedimentary Characteristics and Formation Mechanisms of Lacustrine Beach-Bars in the Fourth Member of the Shahejie Formation in the Shubei Area, Western Depression of the Liaohe Oilfield, China

Beach-bars are well developed in the fourth member of the Shahejie Formation in the Shubei area, western depression of the Liaohe oilfield. The fourth member is composed of two three-order sequences, including a lower Sequence 1（SQ1） and an upper Sequence 2（SQ2）. SQ2, which is the focus of this study, comprises a lowstand systems tract（LST） and a transgressive system tract（TST）. Three types of beach-bars have been recognized： shallow-littoral lacustrine sandy beach-bars, semi-deep lacustrine storm-related sandy beach-bars and shallow-littoral lacustrine carbonate beach-bars. Paleo-environments during the deposition of SQ2, including the paleo-geomorphology, paleo-water depth and paleo-source have been reconstructed to determine the formation mechanisms of the different types of beach-bars. The shallow-littoral lacustrine sandy beach-bars formed mainly by waves and currents and were mostly deposited in the LST, where water depths ranged from 3 to 9 m and the terrestrial clast supply is sufficient. The storm-related sandy beach-bars developed in the TST in the southern part of the study area near previously massively deposited sand bodies where there was sufficient water depth to preserve them. The carbonate beach-bars formed primarily in the TST in the Shubei low buried hill belt where there was a lack of terrestrial clast supply and complex, uneven geomorphology that easily gave rise to a lagoonal environment.

Genesis of the low-permeability reservoir bed of upper Triassic Xujiahe Formation in Xinchang gas field,western Sichuan Depression

The genesis of a reservoir is a result of the combined action of deposition, diagenesis, tectonic reworking, and interaction of rock and fluid and the evolutionary environment. We discuss the genetic and evolution mechanism of a low-permeability reservoir bed of the Xujiahe Formation in the western Sichuan Depression on the basis of the study of diagenesis, diagenetic reservoir facies and the diagenetic evolution sequence. The research indicated that this reservoir bed can be divided into five types of diagenetic reservoir facies, namely strong dissolution, chlorite-lined intergranular pores, compaction and pressure solution, carbonate cementation and secondary quartz increase. There are, however, just two diagenetic reservoir facies which provide low-permeability reservoir beds, namely strong dissolution and chlorite-lined intergranular pores. We also analyzed their diagenetic evolution sequences and the origin of the low-permeability reservoir bed. Besides, it was also indicated that the composition and structure of sandstones, types of sedimentary microfacies, diagenesis history as well as the tectonic reworking in later periods are the main factors controlling the formation of the low-permeability reservoir bed. The above- mentioned factors establish the foundation for the forecasting the distribution of high quality reservoir beds.

The Relationship between Fractures and Tectonic Stress Field in the Extra Low-Permeability Sandstone Reservoir at the South of Western Sichuan Depression

The formation and distribution of fractures are controlled by paleotectonic stress field, and their preservative status and effects on development are dominated by the modern stress field. Since Triassic, it has experienced four tectonic movements and developed four sets of tectonic fractures in the extra low-permeability sandstone reservoir at the south of western Sichuan depression. The strikes of fractures are in the S-N, NE-SW, E-W, and NW-SE directions respectively. At the end of Triassic, under the horizontal compression tectonic stress field, for which the maximum principal stress direction was NW.SE, the fractures were well developed near the S-N faults and at the end of NE-SW faults, because of their stress concentration. At the end of Cretaceous, in the horizontal compression stress fields of the NE-SW direction, the stress was obviously lower near the NE-SW faults, thus, fractures mainly developed near the S-N faults. At the end of Neogene-Early Pleistocene, under the horizontal compression tectonic stress fields of E-W direction, stress concentrated near the NE-SW faults and fractures developed at these places, especially at the end of the NE-SE faults, the cross positions of NE-SW, and S-N faults. Therefore, fractures developed mostly near S-N faults and NE-SW faults. At the cross positions of the above two sets of faults, the degree of development of the fractures was the highest. Under the modern stress field of the NW-SE direction, the NW-SE fractures were mainly the seepage ones with tensional state, the best connectivity, the widest aperture, the highest permeability, and the minimum opening pressure.

Geochemical characteristics of noble gases in natural gas and their application in tracing natural gas migration in the middle part of the western Sichuan Depression, China

Noble gases in natural gas, from Xiaoquan, Xinchang, Hexingchang and Fenggu gas reservoirs in the middle part of the western Sichuan Depression, China, were analysed. Results show that the volume content of crustal noble gases accounts for 97.9% to 99.7% of the total noble gas content, indicating that the noble gases in the study area are very largely derived from the crust. Moreover, the 40Ar time-accumulating effect of source rocks is used to determine the complex relationship between gases and source rocks in this area, and the results agree well with that from analysis of source rock light hydrocarbons. Due to the short migration distance, the separation of 4He and 40Ar is not significant in Xujiahe natural gas and Lower and Middle Jurassic natural gas, so it is difficult to trace natural gas migration. However, this separation characteristic of 4He and 40Ar in Middle and Upper Jurassic natural gas is significant, which indicates that natural gas migration was from the Middle Jurassic to Upper Jurassic formations. In addition, the variation trends of 3He/4He ratio and δ13C1 value indicates that natural gas migration is from the Xujiahe formation to the Jurassic layer in the study area.

Hydrocarbon Generation and Expulsion of the Upper Triassic T3x5Source Rocks in the Western Sichuan Depression: Assessment for Unconventional Natural Gas

Tight-sand gas in the Jurassic and shale gas within the fifth member of Xujiahe Formation （T3xs） in the Western Sichuan Basin （WSD） are currently regarded as the most prolific emerging unconventional gas plays in China. This study conducted a conventional evaluation of T3x5 source rocks in the WSD, and investigated their hydrocarbon generation and expulsion characteristics, including intensity, efficiency and amount. The results show that, the T3x5 source rocks are thick （generally 〉200 m）, and have a high total organic content （TOC）, ranging from 2.5 to 4.5 wt%. It is thus indivative of a great hydrocarbon generation potential when they underwent high thermal evolution （Ro〉1.2%） in the area. In addition, an improved method of hydrocarbon generation potential is applied, indicating that the source rocks reached a hydrocarbon expulsion threshold with vitrinite reflectance （Ro） reaching 1.06%. and that the comprehensive hydrocarbon expulsion efficiency is about 60%. The amount of hydrocarbon generation and expulsion from Tax5 source rocks is 3.14x10^10 t and 1.86x10^10 t, respectively, with a residual amount of 1.28x10^10t within them. Continuous-type tight-sand gas is predicted to have developed in the Jurassic in the Chengdu Sag of the WSD because of the good source-reservoir configuration; the Jurassic sandstone reservoirs are tight, and the gas expelled from the T3xs source rocks migrates for very short distances vertically and horizontally. The amount of gas accumulation in the Jurassic reservoirs derived from T3x5 source rocks is up to 9.3x10s t. Geological resources of shale gas are up to 1.05x10TM t. Small differences between the amounts calculated by the volumetric method and those obtained by hydrocarbon generation potential method may be due to other gas accumulations present within interbedded sands associated with gas shales.

Applicability of Carbazole Migration Indices in Continental Rift Basins:A Case Study of Western Lujiapu Depression in Kailu Basin,NE China

Kailu Basin in which the Western Lujiapu Depression is located is a typical continental rift basin.Biomarker parameters of the oils indicate that depositional facies and environments vary between the Bao 1 and Bao 14 fault blocks with a higher saline environment in the Bao 1 fault block, but such difference has no significant impact on carbazole abundance and distribution.Maturity and migration distance are the main controls on carbazole abundance and distribution in the Western Lujiapu Depression.The commonly used migration indices,such as ratios of nitrogen shield isomers to nitrogen exposed isomers（1-/4-methylcarbazole ratio,1,8-/2,4-dimethylcarbazole（DMC） ratio and half-shield/exposed-DMC ratio）,absolute concentrations of alkylated carbazoles and BC ratio（=benzo [a]carbazole/（benzo[a]carbazole＋benzo[c]carbazole）） increase at the low mature range and decrease at a higher mature range with increasing maturity.At relatively low maturity stage（Rc〈0.77%）, maturation has reversal effects with migration on the ratios of nitrogen shield isomers to nitrogen exposed isomers,which may cover migration influence and makes these parameters fail to indicate migration effects.Valid migration indicators at this maturity stage are concentrations of alkylated carbazoles and BC ratios,which can provide ideal tools for migration direction assessment even within short migration distance.Maturity effects should be taken into account when carbazole compounds are applied to indicate migration direction,and at different maturity stages,these commonly used parameters have different validity in tracing migration direction.Coupled with our previous study in the Eastern Lujiapu Depression,a conceptual model of the variation of nitrogen migration indices can be established for terrestrial rifted basins,that is,strong fractionation lateral migration model through sandy beds,weak fractionation vertical migration model along faults,and maturity impacts on migration assessment.

A quantitative study on source rocks in the western Leidong depression, northern South China Sea

The Leidong depression is one of the secondary tectonic units and the least explored part of the Beibu Gulf Basin,northern South China Sea.Exploration activities in the basin reveal that the Middle Eocene deposition was controlled by NE-SW-striking faults and the second member of the Eocene Liushagang Formation(LII)consists of high-quality source rocks.The nature and origin of this depression has remained enigmatic due to the complex fault structures as compared to those in other basins.It is important to identify whether LII occurs in the Leidong depression in the context of exploration.Here we analyze available geological,seismic,and drilling data of the depression and the controlling factors among faults,deposition,lithology,and hydrocarbon distribution in the basin.Measures such as stress fracture analysis,fault growth index analysis,and sedimentary strata characterization were used to construct an Eocene sedimentary model controlled by NW-SE striking faults.The model with integrated seismic profile characteristics comparison and velocity analysis results was then used to determine whether the Eocene strata in the west of the depression contain the high-quality source rocks that are widely distributed elsewhere in the basin.Finally,calculations were performed to quantify the intensity of hydrocarbon generation and amount of hydrocarbon generated in the depression based on available data of the source rocks,including their distribution and thickness.The results confirmed the assumption that the Middle Eocene strata in the Leidong depression were controlled by NW-SE strike-slip faults and contain high-quality LII source rocks,thus providing an important foundation for future exploration.

Low organic matter abundance and highly efficient hydrocarbon generation of saline source rock in the Qaidam Basin,NW China

The geochemical analysis and experimental simulation are comprehensively used to systematically study the hydrocarbon generation material,organic matter enrichment and hydrocarbon generation model of Paleogene source rock in the Western Qaidam Depression,Qaidam Basin,NW China.Three main factors result in low TOC values of saline lacustrine source rock of the Qaidam Basin:relatively poor nutrient supply inhibits the algal bloom,too fast deposition rate causes the dilution of organic matter,and high organic matter conversion efficiency causes the low residual organic carbon.For this type of hydrogen-rich organic matter,due to the reduction of organic carbon during hydrocarbon generation,TOC needs to be restored based on maturity before evaluating organic matter abundance.The hydrocarbon generation of saline lacustrine source rocks of the Qaidam Basin is from two parts:soluble organic matter and insoluble organic matter.The soluble organic matter is inherited from organisms and preserved in saline lacustrine basins.It generates hydrocarbons during low-maturity stage,and the formed hydrocarbons are rich in complex compounds such as NOS,and undergo secondary cracking to form light components in the later stage;the hydrocarbon generation model of insoluble organic matter conforms to the traditional“Tissot”model,with an oil generation peak corresponding to Ro of 1.0%.