Migration and accumulation mechanisms and main controlling factors of tight oil enrichment in a continental lake basin

Based on the typical dissection of various onshore tight oil fields in China,the tight oil migration and accumulation mechanism and enrichment-controlling factors in continental lake basins are analyzed through nuclear magnetic resonance(NMR)displacement physical simulation and Lattice Boltzmann numerical simulation by using the samples of source rock,reservoir rock and crude oil.In continental lake basins,the dynamic forces driving hydrocarbon generation and expulsion of high-quality source rocks are the foundational power that determines the charging efficiency and accumulation effect of tight oil,the oil migration resistance is a key element that influences the charging efficiency and accumulation effect of tight oil,and the coupling of charging force with pore-throat resistance in tight reservoir controls the tight oil accumulation and sweet spot enrichment.The degree of tight oil enrichment in continental lake basins is controlled by four factors:source rock,reservoir pore-throat size,anisotropy of reservoir structure,and fractures.The high-quality source rocks control the near-source distribution of tight oil,reservoir physical properties and pore-throat size are positively correlated with the degree of tight oil enrichment,the anisotropy of reservoir structure reveals that the parallel migration rate is the highest,and intralayer fractures can improve the migration and accumulation efficiency and the oil saturation.

Evolution of Hydrodynamic Field, Oil-Gas Migration and Accumulation in Songliao Basin, China

The oil-gas migration and accumulation in the Songliao Basin were analyzed in the view of fluid dynamics by the authors. The key point of fluid dynamics is hydrodynamics. Oil-gas migration and accumulation are related closely with formation and evolution of hydrodynamic field. Based on abundant data, initial formation pressure and other parameters, such as water head were studied. They can be used to understand the present distribution of hydrodynamic field and its hydrochemical features. Generally, the hydrodynamic field in the basin is obviously asymmetrical. In its north and east part, there are the areas of centripetal flow caused by topographic relief when meteoric water permeate downwards. Its south part is an evaporation-concentration area. The central depression is an area of centrifugal flow driven by sediment compaction and its cross-formational flow area. Only at the basin margin and in the local uplifted and denudated area are the meteoric water permeating downwards areas. The centrifugal flow driven by sediment compaction is the main dynamic factor that induces oil-gas migration and accumulation and its formation period corresponding to the main stage of oil-gas migration and accumulation. Moreover, the evolution of hydrodynamic field has the cyclic property, which results in phased oil-gas migration by stages, and further dominates the terraced annular oil and gas distribution, concentric with their corresponding sags.

Petroleum Retention,Intraformational Migration and Segmented Accumulation within the Organic-rich Shale in the Cretaceous Qingshankou Formation of the Gulong Sag,Songliao Basin,Northeast China

In this study,organic geochemical and petrological analyses were conducted on 111 shale samples from a well to understand the retention,intraformational migration and segmented accumulation(shale oil enrichment in different intervals is unconnected)features of shale oil within the organic-rich shale in the Qingshankou Formation of the Gulong Sag.Our study shows that retained petroleum characteristics in the investigated succession are mainly influenced by three factors:organic richness,intraformational migration and segmented accumulation.Organic matter richness primarily controls the amount of retained petroleum,especially the‘live’component indicated by the S_(2)value rather than the total organic carbon(TOC)figure alone.The negative expulsion efficiencies determined by mass-balance calculations of hydrocarbons reveal that petroleum from adjacent organic-rich intervals migrates into the interval of about 2386-2408 m,which is characterized by high free hydrocarbon(S_(1)),OSI and saturated hydrocarbons content,along with a greater difference inδ^(13)C values between polar compounds(including resins and asphaltenes)and saturated hydrocarbons.The depth-dependent heterogeneity of carbon isotope ratios(δ^(13)C)of mud methane gas,δ^(13)C of extracts gross composition(SARA),δ^(13)C of kerogen and SARA content of extracts suggest that the studied succession can be subdivided into four intervals.The shale oil sealing enrichment character in each interval is further corroborated by the distinctδ^(13)C values of mud methane gas in different intervals.Due to the migration of petroleum into the 2386-2408 m interval,the S_(1),OSI and saturated hydrocarbons content of the interval show higher relative values.The maturity of organic matter in the 2471-2500 m interval is at the highest with the smaller size molecular components of the retained petroleum.Thus,favorable‘sweet spots’may be found in the 2386-2408 m interval and the 2471-2500 m interval,according to the experiment results in this study.

The accumulation characteristics and exploration potential of oil and gas in the back-arc basin of Japan under the background of high heat flow

The Sea of Japan is located in the southeast margin of Eurasia, in the triangle area of the western Pacific Ocean. Due to the interaction of the Pacific plate, Eurasian plate and Philippine plate, its tectonic environment is complex, forming a typical trench-arc-basin system. At present, 148 oil and gas fields have been discovered in Japan, with an oil and gas resource of 255.78×10^(6) t, showing a good prospect for oil and gas exploration. Based on the previous research and the recently collected geological and geophysical data, the characteristics of tectonic-sedimentary evolution and geothermal field in the basins around the Sea of Japan are analyzed. The results show that the tectonic evolution of the basin is mainly controlled by plate subduction and back-arc oceanic crust expansion, and it mainly undergone four tectonic-sedimentary evolution stages: Subduction period, basin development period, subsidence period and compression deformation period. The overall heat flow value of Japan Sea is high, and it is distributed annularly along Yamato Ridge. The geothermal heat flow value is about 50–130 MW/m^(2), and the average heat flow is75.9±19.8 MW/m^(2), which has a typical “hot basin ”. The high heat flow background provides unique thermal evolution conditions for hydrocarbon generation, which leads to the high temperature and rapid evolution. The authors summarized as “early hydrocarbon generation, rapid maturity and shallow and narrow hydrocarbon generation window”. The type of oil and gas is mainly natural gas, and it mainly distributed in Neogene oil and gas reservoirs. The trap types are mainly structural traps, lithologic traps and composite traps. In addition, the pre-Neogene bedrock oil and gas reservoirs also show a good exploration prospect. The resource prospecting indicates that Niigata Basin, Ulleung Basin and kitakami Basin are the main target areas for future exploration and development.

A Study of the Migration and Accumulation Efficiency and the Genesis of Hydrocarbon Natural Gas in the Xujiaweizi Fault Depression

In order to investigate the migration and accumulation efficiency of hydrocarbon natural gas in the Xujiaweizi fault depression, and to provide new evidence for the classification of its genesis, a source rock pyrolysis experiment in a closed system was designed and carried out. Based on this, kinetic models for describing gas generation from organic matter and carbon isotope fractionation during this process were established, calibrated and then extrapolated to geologic conditions by combining the thermal history data of the Xushen-1 Well. The results indicate that the coal measures in the Xujiaweizi fault depression are typical ＂high-efficiency gas sources＂, the natural gas generated from them has a high migration and accumulation efficiency, and consequently a large-scale natural gas accumulation occurred in the area. The highly/over matured coal measures in the Xujiaweizi fault depression generate coaliferous gas with a high δ^13C1 value （〉 -20‰） at the late stage, making the carbon isotope composition of organic alkane gases abnormally heavy. In addition, the mixing and dissipation through the caprock of natural gas can result in the negative carbon isotope sequence （δ^13C1 〉δ^13C2 〉δ^13C3 〉δ^13C4） of organic alkane gases, and the dissipation can also lead to the abnormally heavy carbon isotope composition of organic alkane gases. As for the discovery of inorganic nonhydrocarbon gas reservoirs, it can only serve as an accessorial evidence rather than a direct evidence that the hydrocarbon gas is inorganic. As a result, it needs stronger evidence to classify the hydrocarbon natural gas in the Xujiaweizi fault depression as inorganic gas.

Migration and accumulation characteristics of natural gas hydrates in the uplifts and their slope zones in the Qiongdongnan Basin,China

Various factors controlling the accumulation of natural gas hydrates(NGHs)form various enrichment and accumulation modes through organic combination.This study mainly analyzes the geological and geophysical characteristics of the NGHs occurrence in the uplifts and their slope zones within the deep-water area in the Qiongdongnan(QDN)Basin(also referred to as the study area).Furthermore,it investigates the dominant governing factors and models of NGHs migration and accumulation in the study area.The results are as follows.(1)The uplifts and their slope zones in the study area lie in the dominant pressure-relief direction of fluids in central hydrocarbon-rich sags in the area,which provide sufficient gas sources for the NGHs accumulation and enrichment through pathways such as gas chimneys and faults.(2)The top and flanks of gas chimneys below the bottom simulating reflectors(BSRs)show high-amplitude seismic reflections and pronounced transverse charging of free gas,indicating the occurrence of a large amount of gas accumulation at the heights of the uplifts.(3)Chimneys,faults,and high-porosity and high-permeability strata,which connect the gas hydrate temperature-pressure stability zones(GHSZs)with thermogenic gas and biogenic gas,form the main hydrate migration system.(4)The reservoir system in the study area comprises sedimentary interlayers consisting of mass transport deposits(MTDs)and turbidites.In addition,the reservoir system has developed fissure-and pore-filling types of hydrates in the pathways.The above well-matched controlling factors of hydrate accumulation enable the uplifts and their slope zones in the study area to become the favorable targets of NGHs exploration.

SIMULATION AND APPLICATION OF THREE_DIMENSIONAL MIGRATION_ACCUMULATION OF OIL RESOURCES

Numerical simulation of oil migration and accumulation is to describe the history of oil migration and accumulation in basin evolution. It is of great value in the exploration of oil resources and their rational evaluation. In this paper, from such actual conditions as the effects of mechanics of fluids in porous media and 3-dimensional geology characteristics, a kind of modified method of second order splitting-up implicit interactive scheme is pur forward. For the famous hydraulic experiment of secondary migration-accumulation, the numerical simulation test has been done, and both the computational and experimental results are basically identical. For the actual problem of Dongying hollow of Shengli Petroleum Oil Field, the numerical simulation test and the actual conditions are basically coincident. Thus the well-known problem has been solved.

Formation,evolution,reconstruction of black shales and their influence on shale oil and gas resource

Black shales are important products of material cycling and energy exchange among the lithosphere,atmosphere,hydrosphere,and biosphere.They are widely distributed throughout geological history and provide essential energy and mineral resources for the development of human society.They also record the evolution process of the earth and improve the understanding of the earth.This review focuses on the diagenesis and formation mechanisms of black shales sedimentation,composition,evolution,and reconstruction,which have had a significant impact on the formation and enrichment of shale oil and gas.In terms of sedimentary environment,black shales can be classified into three types:Marine,terrestrial,and marine-terrestrial transitional facies.The formation processes include mechanisms such as eolian input,hypopycnal flow,gravity-driven and offshore bottom currents.From a geological perspective,the formation of black shales is often closely related to global or regional major geological events.The enrichment of organic matter is generally the result of the interaction and coupling of several factors such as primary productivity,water redox condition,and sedimentation rate.In terms of evolution,black shales have undergone diagenetic evolution of inorganic minerals,thermal evolution of organic matter and hydrocarbon generation,interactions between organic matter and inorganic minerals,and pore evolution.In terms of reconstruction,the effects of fold deformation,uplift and erosion,and fracturing have changed the stress state of black shale reservoirs,thereby having a significant impact on the pore structure.Fluid activity promotes the formation of veins,and have changed the material composition,stress structure,and reservoir properties of black shales.Regarding resource effects,the deposition of black shales is fundamental for shale oil and gas resources,the evolution of black shales promotes the shale oil and gas formation and storage,and the reconstruction of black shales would have caused the heterogeneous distribution of oil and gas in shales.Exploring the formation mechanisms and interactions of black shales at different scales is a key to in-depth research on shale formation and evolution,as well as the key to revealing the mechanism controlling shale oil and gas accumulation.The present records can reveal how these processes worked in geological history,and improve our understanding of the coupling mechanisms among regional geological events,black shales evolution,and shale oil and gas formation and enrichment.

Tight sandstone gas accumulation mechanisms and sweet spot prediction, Triassic Xujiahe Formation, Sichuan Basin, China

The prediction of continental tight sandstone gas sweet spots is an obstacle during tight sandstone gas exploration. In this work, the classic physical fluid charging experimental equipment is improved, the combination of the gas migration and accumulation process with the pore network numerical simulation method is investigated, and application of the permeability/porosity ratio is proposed to predict the gas saturation and sweet spots of continental formations. The results show that (1) as the charging pressure increases, the permeability of the reservoir increases because more narrow pore throats are displaced in the percolation process;and (2) based on pore network numerical simulation and theoretical analysis, the natural gas migration and accumulation mechanisms are revealed. The gas saturation of tight sandstone rock is controlled by the gas charging pressure and dynamic percolation characteristics. (3) The ratio of permeability/porosity and fluid charging pressure is proposed to predict the gas saturation of the formation. The ratio is verified in a pilot and proven to be applicable and practical. This work highlights the tight sandstone gas migration and accumulation mechanisms and narrows the gap among microscale physical experiments, numerical simulation research, and field applications.

Origin and accumulation of high-maturity oil and gas in deep parts of the Baxian Depression, Bohai Bay Basin, China

Great quantities of light oil and gas are produced from deep buried hill reservoirs at depths of 5,641 m to 6,027 m and 190 ℃ to 201 ℃ in the Niudong-1 Well, representing the deepest and hottest commercial hydrocarbons discovered in the Bohai Bay Basin in eastern China. This discovery suggests favorable exploration prospects for the deep parts of the basin. However, the discovery raises questions regarding the genesis and accumulation of hydrocarbons in deep reservoirs. Based on the geochemical features of the hydrocarbons and characteristics of the source rocks as well as thermal simulation experiments of hydrocarbon generation, we conclude that the oil and gas were generated from the highly mature Sha-4 Member （Es4） source rocks instead of thermal cracking of crude oils in earlier accumulations. The source kitchen with abnormal pressures and karsted carbonate reservoirs control the formation of high-maturity hydrocarbon accumulations in the buried hills （i.e., Niudong-1） in conjunction with several structural-lithologic traps in the ES4 reservoirs since the deposition of the upper Minghuazhen Formation. This means the oil and gas exploration potential in the deep parts of the Baxian Depression is probably high.

Features of the fault system and its relationship with migration and accumulation of hydrocarbon in Liaodong Bay

The fault system of Liaodong Bay developed extensively under the control of the Tanlu Fault. The fault system can be grouped into strike-slip faults of grade Ⅰ, trunk faults of grade Ⅱand branch faults （induced faults） of grade Ⅲ respectively based on its developmental scale. The faults of grade Ⅰ and Ⅱwere deep, early and large while the faults of grade Ⅲwere shallow, late and small. The formation, evolution and distribution features played a significant role in controlling the migration of oil and gas in both horizontal and vertical directions. The fluid transfer in the fault system occurred in the process of faulting. The strike-slip and trunk faults moved actively forming predominant pathways for oil and gas migration. The branch faults, with weak activity, generally controlled the development of traps and were beneficial for the accumulation and preservation of oil and gas. The faults of grade Ⅰ and Ⅱ formed the major migration pathways for oil and gas, but their fault activity rates appeared to vary along their strikes. The zones with a relatively low fault activity rate might be favorable for oil and gas accumulation. When the activities of strike-slip, trunk, and branch faults came to a halt, the fault seal behavior had a vitally important effect on the accumulation of oil and gas. The controlling role of the fault over fluid distribution was further analyzed by calculating the fault activity quantitatively.

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.

Discussion of the Mode and Mechanism of Oil and Gas Accumulation in the Nanbaxian Pool in the North of the Qaidam Basin

Because of the difference ofoil and gas accumulation condition between the hanging wall and the footwall of a fault, there is a peculiar accumulation mechanism that oil and gas mainly exists in the hanging wall of the basement fault, but in the footwall of the shallow detachment fault in the Nanbaxian pool. The oil and gas of the Nanbaxian pool came from the mature Jurassic hydrocarbon source rock of the Yibei depression located at the south of the Nanbaxian pool. Firstly, the oil and gas accumulated in the traps of the hanging wall of the basement fault by way of the unconformity and the basement faults, and turned into some primary deep pools; and then, the shallow detachment fault that formed in the later tectonic movement broke into the deep primary pools, which caused the oil and gas migration upwards along the basement faults and the shallow detachment faults and the evolvement into some secondary oil and gas pools later. The history of the Nanbaxian oil and gas accumulation can be summarized successively as the syndepositional upheaval controlled by faults; single hydrocarbon source rock; unconformities and faults as migration channels; buoyancy, overpressure and tectonic stress as dynamic forces; multistage migration and accumulation of oil and gas; and finally an overlapped double-floor pattern of oil and gas accumulation. The most important explorative targets in the north of the Qaidam Basin are traps connected with the primary pools in the footwall by shallow detachment faults.

Hydrocarbon Migration and Accumulation in Dongying Basin

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Mesh model building and migration and accumulation simulation of 3D hydrocarbon carrier system

Migration and accumulation simulation of oil and gas in carrier systems has always been a difficult subject in the quantitative study of petroleum geology. In view of the fact that the traditional geological modeling technology can not establish the interrelation of carriers in three dimensional space, we have proposed a hybrid-dimensional mesh modeling technology consisting of body(stratum), surfaces(faults and unconformities), lines and points, which provides an important research method for the description of geometry of sand bodies, faults and unconformities, the 3 D geological modeling of complex tectonic areas, and the simulation of hydrocarbon migration and accumulation. Furthermore, we have advanced a 3 D hydrocarbon migration pathway tracking method based on the hybrid-dimensional mesh of the carrier system. The application of this technology in western Luliang Uplift of Junggar Basin shows that the technology can effectively characterize the transport effect of fault planes, unconformities and sand bodies, indicate the hydrocarbon migration pathways, simulate the process of oil accumulation, reservoir adjustment and secondary reservoir formation, predict the hydrocarbon distribution. It is found through the simulation that the areas around the paleo-oil reservoir and covered by migration pathways are favorable sites for oil and gas distribution.

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.

Accumulation Mechanisms and Evolution History of the Giant Puguang Gas Field,Sichuan Basin,China

Solid bitumens were found throughout the carbonate reservoirs in the Puguang gas field, the largest gas field so far found in marine carbonates in China, confirming that the Puguang gas field evolved from a paleo-oil reservoir. The fluid conduit system at the time of intensive oil accumulation in the field was reconstructed, and petroleum migration pathways were modeled using a 3-D model and traced by geochemical parameters. The forward modeling and inversion tracing coincided with each other and both indicated that oils accumulated in the Puguang-Dongyuezhai structure originated from a generative kitchen to the northwest of the Puguang gas field. The deposition of organic-rich Upper Permian source rocks dominated by sapropelic organic matter in the Northeast Sichuan Basin, the development of fluid conduit system that was vertically near-source rock and laterally near-generative kitchen, and the focusing of oils originated from a large area of the generative kitchen, were the three requirements for the formation of the giant paleo-oil reservoir from which the giant Puguang gas field evolved. The Puguang gas field had experienced a three-stage evolution. The post-accumulation processes, especially the organic-inorganic interaction in the hydrocarbon-water-rock system, had not only profoundly altered the composition and characteristics of the petroleum fluids, but also obviously changed the physicochemical conditions in the reservoir and resulted in complicated precipitation and solution of carbonate minerals.

Geochemical Characteristics and Migration Pathways of Ordovician Carbonate Oil Reservoirs in the Tuoputai Area,Tarim Basin,Northwestern China

Exploration potential is huge and the oil resources are rich in the Ordovician reservoirs of the Tarim Basin.However,the mechanism of hydrocarbon accumulation is complex and not yet fully understood.In the Tuoputai area,the hydrocarbon migration pathways and characteristics of deep hydrocarbon accumulation are revealed through analyses of the physical data of rich oil and gas,the geochemical parameters of oil,and fluid inclusions.The results show that the Ordovician oils in the Tuoputai area have the same geochemical characteristics as the mixed oil from the Lower Cambrian source rock and the Middle–Upper Ordovician source rock.The Ordovician reservoirs have been charged three times:in the late Caledonian,late Hercynian,and Himalayan stages.Oil charging occurred in the Hercynian stage,in particular,as it is the main filling period of hydrocarbon.The north-northeast(NNE)-trending TP12 CX major fault,active in in these times and is dominant migration channel of hydrocarbon,but there is segmentation affected by the difference of activities.Oil maturity is higher in the south than in the north and is abnormally high near the major fault.Parameters related to migration indicate that oil migrated northeastward along the NNE-trending TP12 CX major fault and adjusted laterally along the secondary faults and weathering crust,forming the present characteristics of oil and gas distribution.

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.

Microbial Gas in the Mohe Permafrost, Northeast China and its Significance to Gas Hydrate Accumulation in Permafrost across China

The Mohe permafrost in northeast China possesses favorable subsurface ambient temperature, salinity, Eh values and pH levels of groundwater for the formation of microbial gas, and the Mohe Basin contains rich organic matter in the Middle Jurassic dark mudstones. This work conducted gas chromatography and isotope mass spectrometry analyses of nearly 90 core gas samples from the Mk-2 well in the Mohe Basin. The results show that the dryness coefficient(C1/C1–5) of core hydrocarbon gas from approximately 900 m intervals below the surface is larger than 98%, over 70% of the δ13 C values of methane are smaller than-55‰, and almost all δD values of methane are smaller than-250‰, indicative of a microbial origin of the gas from almost 900 m of the upper intervals in the Mohe permafrost. Moreover, the biomarker analyses of 72 mudstone samples from the Mohe area indicate that all of them contain 25-norhopane series compounds, thereby suggesting widely distributed microbial activities in the permafrost. This work has confirmed the prevailing existence of microbial gas in the Mohe area, which may be a potential gas source of gas hydrate formation in the Mohe permafrost. This result is of great significance to gas hydrate accumulation in the permafrost across China.