Whole petroleum system and ordered distribution pattern of conventional and unconventional oil and gas reservoirs

The classical source-to-trap petroleum system concept only considers the migration and accumulation of conventional oil and gas in traps driven dominantly by buoyance in a basin,although revised and improved,even some new concepts as composite petroleum system,total petroleum system,total composite petroleum system,were proposed,but they do not account for the vast unconventional oil and gas reservoirs within the system,which is not formed and distributed in traps dominantly by buoyancedriven.Therefore,the petroleum system concept is no longer adequate in dealing with all the oil and gas accumulations in a basin where significant amount of the unconventional oil and gas resources are present in addition to the conventional oil and gas accumulations.This paper looked into and analyzed the distribution characteristics of conventional and unconventional oil/gas reservoirs and their differences and correlations in petroliferous basins in China and North America,and then proposed whole petroleum system(WPS)concept,the WPS is defined as a natural system that encompasses all the conventional and unconventional oil and gas,reservoirs and resources originated from organic matter in source rocks,the geological elements and processes involving the formation,evolution,and distribution of these oil and gas,reservoirs and resources.It is found in the WPS that there are three kinds of hydrocarbons dynamic fields,three kinds of original hydrocarbons,three kinds of reservoir rocks,and the coupling of these three essential elements lead to the basic ordered distribution model of shale oil/gas reservoirs contacting or interbeded with tight oil/gas reservoirs and separated conventional oil/gas reservoirs from source rocks upward,which is expressed as“S\T-C”.Abnormal conditions lead to other three special ordered distribution models:The first is that with shale oil/gas reservoirs separated from tight oil/gas reservoirs.The second is that with two direction ordered distributions from source upward and downward.The third is with lateral distribution from source outside.

Upper Paleozoic total petroleum system and geological model of natural gas enrichment in Ordos Basin, NW China

Based on the analysis of Upper Paleozoic source rocks, source-reservoir-caprock assemblage, and gas accumulation characteristics in the Ordos Basin, the gas accumulation geological model of total petroleum system is determined. Then, taking the Carboniferous Benxi Formation and the Permian Taiyuan Formation and Shanxi Formation as examples, the main controlling factors of gas accumulation and enrichment are discussed, and the gas enrichment models of total petroleum system are established. The results show that the source rocks, faults and tight reservoirs and their mutual coupling relations control the distribution and enrichment of gas. Specifically, the distribution and hydrocarbon generation capacity of source rocks control the enrichment degree and distribution range of retained shale gas and tight gas in the source. The coupling between the hydrocarbon generation capacity of source rocks and the physical properties of tight reservoirs controls the distribution and sweet spot development of near-source tight gas in the basin center. The far-source tight gas in the basin margin is mainly controlled by the distribution of faults, and the distribution of inner-source, near-source and far-source gas is adjusted and reformed by faults. Generally, the Upper Paleozoic gas in the Ordos Basin is recognized in four enrichment models: inner-source coalbed gas and shale gas, inner-source tight sandstone gas, near-source tight gas, and far-source fault-transported gas. In the Ordos Basin, inner-source tight gas and near-source tight gas are the current focuses of exploration, and inner-source coalbed gas and shale gas and far-source gas will be important potential targets in the future.

Whole petroleum system and hydrocarbon accumulation model in shallow and medium strata in northern Songliao Basin,NE China

Based on the oil and gas exploration practice in the Songliao Basin,combined with the latest exploration and development data such as seismic,well logging and geochemistry,the basic geological conditions,oil and gas types and distribution characteristics,reservoir-forming dynamics,source-reservoir relationship and hydrocarbon accumulation model of the whole petroleum system in shallow and medium strata in the northern part of Songliao Basin are systematically studied.The shallow-medium strata in northern Songliao Basin have the conditions for the formation of whole petroleum system,with sufficient oil and gas sources,diverse reservoir types and well-developed transport system,forming a whole petroleum system centered on the source rocks of the Cretaceous Qingshankou Formation.Different types of oil and gas resources in the whole petroleum system are correlated with each other in terms of depositional system,lithologic association and physical property changes,and they,to a certain extent,have created the spatial framework with orderly symbiosis of shallow-medium conventional oil reservoirs,tight oil reservoirs and shale oil reservoirs in northern Songliao Basin.Vertically,the resources are endowed as conventional oil above source,shale oil/tight oil within source,and tight oil below source.Horizontally,conventional oil,tight oil,interlayer-type shale oil,and pure shale-type shale oil are developed in an orderly way,from the margin of the basin to the center of the depression.Three hydrocarbon accumulation models are recognized for the whole petroleum system in northern Songliao Basin,namely,buoyancy-driven charging of conventional oil above source,retention of shale oil within source,and pressure differential-driven charging of tight oil below source.

Control Factors and Diversities of Phase State of Oil and Gas Pools in the Kuqa Petroleum System

Based on the analysis of the hydrocarbon geochemical characteristics in the Kuqa petroleum system of the Tarim Basin, this study discusses the causes and controlling factors of the phase diversities and their differences in geochemical features. According to the characteristics and differences in oil and gas phase, the petroleum system can be divided into five categories： oil reservoir, wet gas reservoir, condensate gas-rich reservoir, condensate gas-poor reservoir and dry gas reservoir. The causes for the diversities in oil and gas phases include diversities of the sources of parent material, maturity of natural gas and the process of hydrocarbon accumulation of different hydrocarbon phases. On the whole, the Jurassic and Triassic terrestrial source rocks are the main sources for the hydrocarbon in the Kuqa Depression. The small differences in parent material may cause diversities in oil and gas amount, but the impact is small. The differences in oil and gas phase are mainly affected by maturity and the accumulation process, which closely relates with each other. Oil and gas at different thermal evolution stage can be captured in different accumulation process.

Petroleum System of the Sufyan Depression at the Eastern Margin of a Huge Strike-slip Fault Zone in Central Africa

The present paper mainly studies the petroleum system of the Sufyan Depression in the Muglad Basin of central Africa and analyzes its control of hydrocarbon accumulation. On the basis of comprehensive analysis of effective source rock, reservoir bed types and source-reservoir-seal assemblages, petroleum system theory has been used to classify the petroleum system of the Sufyan Depression. Vertically, the Sufyan Depression consists of two subsystems. One is an Abu Gabra subsystem as a self generating, accumulating and sealing assemblage. The other subsystem is composed of an Abu Gabra source rock, Bentiu channel sandstone reservoir and Darfur group shale seal, which is a prolific assemblage in this area. Laterally, the Sufyan Depression is divided into eastern and western parts with separate hydrocarbon generation centers more than 10 000 m deep. The potential of the petroleum system is tremendous. Recently, there has been a great breakthrough in exploration. The Sufyan C-1 well drilled in the central structural belt obtained high-yielding oil flow exceeding 100 tons per day and controlled geologic reserves of tens of millions of tons. The total resource potential of the Sufyan Depression is considerable. The central structural belt is most favorable as an exploration and development prospect.

Characteristics of Triassic Petroleum Systems in the Longmenshan Foreland Basin,Sichuan Province,China

The Triassic in the Longmengshan foreland basin is rich in oil and gas resources. Its reservoirs feature low-porosity, low-permeability, small pore throat, high water saturation, and strong heterogeneity. The existence of abnormally high pressure and various reservoir-cap combinations developed at different times provide favorable conditions for trapping oil and gas. Taking the theory of petroleum systems as a guide, and beginning with research on tectonics, sedimentary history, distribution and evolution of source rocks, reservoir evolution, hydraulic force distribution, and hydrocarbon migration, analysis and study of static factors like source rocks, reservoirs and cap rocks, and dynamic factors such as hydrocarbon generation, migration, and accumulation revealed the characteristics of the Upper Triassic petroleum system in western Sichuan province. The deepbasin gas in the central hydrocarbon kitchen of the Upper Triassic, structural-lithological combination traps on the surrounding slopes, and the structural traps of the Indo-Chinese-Yangshan paleohighs, are potential plays. The relatively well- developed fault zones in the southern segment of the Longmengshan foothill belt are favorable Jurassic gas plays. Pengshan-Xinjin, Qiongxi, and Dayi are recent exploration targets for Jurassic oil/gas reservoirs.

Methods of Studying Petroleum Systems in Residual Basins-A Case Study of Mesozoic and Palaeozoic MarineBasins in Southern China

Through an integrated study of Mesozoic and Palaeozoic petroleum geology insouthern China and a summing-up of the results of exploration, the authors tentatively put forward aset of methods of studying petroleum systems in modified residual basins or superposed basins. Itscore idea is to put emphasis on the study of the dynamic evolution of petroleum systems. Thetempo-spatial evolution, hydrocarbon-generating processes and hydrocarbon-generating intensities andamounts of resources in different geological stages of chief source rocks are mainly deducedbackward by 3-D basin modelling. The regularities of formation and destruction of oil and gasaccumulations are summarized by analyzing the fossil and existing oil and gas accumulations, thedirection of migration is studied by palaeo-structural analysis, and the dynamic evolution ofPalaeozoic and Mesozoic petroleum systems in southern China is studied according to stages of majortectonic movements. The authors suggest that the realistic exploration targets of Palaeozoic andMesozoic petroleum systems in southern China are secondary and hydrocarbon-regeneration petroleumsystems, while the existing primary petroleum systems are rare. They propose that the favourableareas for exploration of Palaeozoic and Mesozoic petroleum systems in southern China are the frontarea of the Daba Mountains and the steep anticlinal zone on the western side of the Shizhusynclinorium in northeastern Sichuan, the Funin-Yancheng-Hai'an-Xinghua-Baoying area in the northernJiangsu basin, the Qianjiang-Xiantao-Paizhou-Chacan 1 well area in the southern part of the Chenhuarea of the Jianghan basin, the South Poyang basin in Jiangxi and the North subbasin of the Chuxiongbasin. This view has been supported by the discovery of the Zhujiadun gas field in the Yanchengsubbasin of the northern Jiangsu basin and the Kaixiantaixi oil-bearing structure in the southernpart of the Chenhu area of the Jianghan basin.

Tectonic Evolution and Petroleum Systems in the Junggar Basin

The Junggar basin is located in the northern part of Xinjiang of China. It is part of the Kazakstan plate, surrounded by the Paleozoic folded mountains: the Halaart, Zayier and Chepaizi Mountains in the northwest, the Qingelidi and Karamaili Mountains in the northeast, and the Tianshan Mountains in the south. In different evolution stages, the basin's types are different, and the stratigraphy and deposition are also different. From the Carboniferous to Tertiary the basin has in turn gone through rift basin, collision foreland basin, intraplate depression basin and regenerated foreland basin. Based on an analysis of thermal evolution history and buried history of the source rocks, three major periods of oil generation are found in the basin. According to the characteristics of source rock distribution, evolution, oil-source correlation, structure and multi-phase and mixed pools, the Junggar basin could be divided into 4 composite petroleum systems. Due to the variation in sedimentary facies, difference in structural patterns and development histories, the petroleum pool-forming conditions in different areas and horizons are greatly different, so are the petroleum pool types, the accumulation mechanisms in different areas and horizons.

Evaluation of Petroleum System in Xihu Depression, East China Sea Shelf Basin

According to the theory of petroleum system and the characteristics of petroleum geology, the Xihu depression in the East China Sea shelf basin is divided into four petroleum systems: the Pinghu Formation as source rock and the Huagang Formation as reservoir rock, the Huagang Formation as source rock and reservoir rock, the Paleocene as source rock and the burial-hill as reservoir rock, and the Miocene as source rock and reservoir rock. The system with the Pinghu Formation as source rock and the Huagang Formation as reservoir rock is the most important one in the depression, which has high hydrocarbon generation and accumulation efficiency and is the most important object to hydrocarbon exploration at present.

Paleozoic Composite Petroleum System of North Africa:Hydrocarbon Distribution and Main Controlling Factors

North Africa, which is one of the main oil and gas producing regions in the world, is best known for its subsalt Paleozoic-Triassic reservoirs and Paleozoic source rocks. Hydrocarbon abundance varies greatly from one structural domain to another areally and from one stratigraphic interval to another vertically. Analyses of the essential elements and geological processes of the Paleozoic petroleum system indicate that the distribution of the Lower Silurian shale source rocks, the development of a thick Mesozoic overburden, the presence of the Upper Triassic-Lower Jurassic evaporite seal are the most important factors goveming the distribution of the Paleozoic-sourced hydrocarbons in North Africa. The Mesozoic sequence plays a critical role for hydrocarbons to accumulate by enabling the maturation of the Paleozoic source rocks during the Mesozoic-Paleogene times and preserving the accumulated hydrocarbons. Basins and surrounding uplifts, particularly the latter, with a thick Mesozoic sequence and a regional evaporite seal generally have abundant hydrocarbons. Basins where only a thin Mesozoic overburden was developed tend to have a very poor to moderate hydrocarbon prospectivity.

Petroleum systems of the Taoudeni Basin,West Africa

The Taoudeni Basin is a typical steady intracratonic basin. Based on the distribution of effective source rocks in the Taoudeni Basin, combined with the structure characteristics of the basin and the distribution characteristics of reservoir beds, two petroleum systems are recognized in the basin： the infra-Cambrian petroleum system and the Silurian petroleum system. Structural uplift and timing of petroleum generation controlled the timing of petroleum charging and preservation of hydrocarbon accumulations. Maturity, evolution history, and distribution of effective source rocks controlled hydrocarbon richness. The geological key factors and geological processes controlled the type of hydrocarbon accumulations.

Burial and thermal maturity modeling of the Middle Cretaceous-Early Miocene petroleum system,Iranian sector of the Persian Gulf

The Cretaceous Kazhdumi and Gurpi forma- tions, Ahmadi Member of the Sarvak Formation, and Paleogene Pabdeh Formation are important source rock candidates of the Middle Cretaceous-Early Miocene pet- roleum system in the Persian Gulf. This study characterizes generation potential, type of organic matter, and thermal maturity of 262 cutting samples （marls and argillaceous limestones） from these rock units taken from 16 fields in the Iranian sector of the Persian Gulf. In addition, the burial and thermal histories of these source rocks were analyzed by one-dimensional basin modeling. Based on the total organic carbon and genetic potential values, fair hydro- carbon generation potential is suggested for the studied samples. Based on Tma~ and vitrinite reflectance values, the studied samples are thermally immature to mature for hydrocarbon generation. The generated models indicate that studied source rocks are immature in central wells. The Gurpi and Pabdeh formations are immature and the Ahmadi Member and Kazhdumi Formation are early mature in the western wells. The Pabdeh Formation is within the main oil window and other source rocks are at the late oil window in the eastern wells. The hydrocarbon expulsion from the source rocks began after deposition of related caprocks which ensures entrapment and preserva- tion of migrated hydrocarbon.

Subvolcanic Rock Petroleum System Potential in the South Malang Region, East Java, Indonesia

The South Malang Region is located in the south-eastern part of the Southern Mountain Volcanic Arc;it presents different opportunities for hydrocarbon exploration. The stratigraphy of the study area from old to young consists of Oligocene Volcanic rocks (Mandalika, Watupatok, and Arjosari Formation), Early Miocene Carbonates (Campurdarat and Jaten Formation), Middle Miocene Volcanic (Wuni Formation), Late Miocene-Pliocene Carbonates (Nampol, Oyo, and Wonosari Formation), and Holocene alluvial deposits (Kalipucang Formation). The dominance of volcanic rocks makes this area considered an area without hydrocarbon play prospects. Petroleum system potential is revealed by evaluating and analyzing potential source rock and reservoir rock outcrop samples. The study shows that the Nampol Formation can be considered as a gas-prone source rock, with type III kerogen, total organic content ranging from 3.48 - 26.18 wt%, and possess the potential to produce good to very good hydrocarbons and a hydrogen index ranging from 43 to 86 mgHC/g TOC. Furthermore, rock core analysis and petrographic studies were carried out on the Nampol sandstone where the rock samples showed good reservoir properties. However, the Nampol and Wonosari limestone that was considered as the secondary target for reservoir possesses a low quality of reservoir properties. This study shows that there is a potential for petroleum system existence in the Southern Mountain subvolcanic arc, which is indicated by the presence of source rock and potential reservoir rock as one of the various elements and processes present in a petroleum system.

Quantitative Assessment of Hydrocarbon Expulsion of Petroleum Systems in the Niuzhuang Sag, Bohai Bay Basin, East China

Based on a detailed survey of the distribution and organic geochemical characteristics of potential source rocks in the South Slope of the Niuzhuang Sag, Bohai Bay Basin, eastern China, a new approach to assess the amount of hydrocarbons generated and expelled has been developed. The approach is applicable to evaluate hydrocarbons with different genetic mechanisms. The results show that the models for hydrocarbon generation and expulsion vary with potential source rocks, depending on thermal maturity, types of organic matter and paleoenvironment. Hydrocarbons are mostly generated and expelled from source rocks within the normal oil window. It was calculated that the special interval (algal-rich shales of the ES4 member formed in brackish environments) in the South Slope of the Niuzhuang Sag has a much higher potential of immature oil generation than the other intervals in the area. This suggests that hydrocarbons can definitely be generated in early diagenesis, especially under certain special geological settings. The proportion of hydrocarbons generated and expelled from the ES4 shales in the early diagenetic stage is up to 26.75% and 17.36%, respectively. It was also observed that laminated shales have a much higher expulsion efficiency than massive mudstones. In contrast, the special interval of the ES4 shales proposed from previous studies is probably not the whole rock for oil in the South Slope of the Niuzhuang Sag because of the small proportion of the gross volume and corresponding low percentage of hydrocarbons generated and expelled. A much lower expulsion efficiency of the source rock during the early stage relative to that within the normal oil window has been calculated. Our results indicate that the ES4 mudstones rather than the shales deposited in the Niuzhuang and Guangli Sag are the main source rocks for the oil discovered.

The petroleum system:a new classification scheme based on reservoir qualities

A new classification of petroleum systems(PSs) based on reservoir qualities is proposed. We classify PSs into the following three basic types:(1) source-rock petroleum system(SPS);(2) tight-reservoir or tight petroleum system(TPS);and(3) conventional-reservoir or conventional petroleum system(CPS). The CPS is a PS in which hydrocarbons accumulate in conventional reservoirs, and all the essential elements and processes are significant and indispensable. Oil and gas accumulations are geographically discrete and therefore exist as discontinuous accumulations. The TPS is a PS where hydrocarbons accumulate in tight reservoirs and the source rock, reservoir, seal, migration, and trap are also indispensable,but the traps are mostly non-anticlinal and the accumulations are primarily quasi-continuous and secondarily discontinuous. The SPS is a PS where both hydrocarbon generation and accumulation occurred in source rocks and traps and migration are unnecessary or inconsequential; the hydrocarbon distribution is extensive and continuous and has no distinct boundaries. The aforementioned three PSs can be derived from a common hydrocarbon source kitchen and are closely linked in terms of their formation and distribution. Therefore, to maximize the exploration efficiency, a comprehensive study and different strategies are needed by considering the SPS, TPS, and CPS as parts of a greater whole.

Controls of Tectonics on both Sedimentary Sequences and Petroleum Systems in Tarim Basin, Northwest China

Various orders of sequences were recognized in the Tarim Basin from unconformities. Three mega-sequence groups, six mega-sequences, sixteen super-sequences and forty-two sequences were determined from the Sinian to the Quaternary. The mega-sequences and super-sequences were in accordance with the locally tectonic events occurring in both the north and the south margins of the Tarim plate. The global sea level changes only worked to control formations in the tectonically stable periods or in the low order sequences. The sequences had close relationship to the source rocks, reservoirs and cap rocks, and the tectonic events determined the migration, accumulation, and preservation of the hydrocarbon. The three mega-sequence group cycles, including the early cycle-the Sinian-middle Devonian, the middle cycle-the upper Devonian-Triassic, and the late cycle-the Jurassic-Quaternary, corresponded to three reservoir formation cycles. So, it can be concluded that the local tectonic events controlled both the sequences and the distribution of oil and gas in the Tarim Basin.

Anatomy of Eastern Niger Rift Basin with Specific References of Its Petroleum Systems

An attempt is made in this paper to present the dynamics of the Eastern Niger Rift Basin (ENRB) with references to the key features and processes of petroleum systems based on published information. The Eastern Niger Basin is a superimposed rift basin with sedimentary structures emplaced during two rifts episodes. The Cretaceous episode is characterized by large, tilted normally faulted blocks trending NW-SE, that were reactivated in the Paleogene, while the Paleogene episode is characterized by normal faulted blocks that trend NNW-SSE. The rifting resulted in different basin structures with the north section dominated by asymmetric half-grabens while the south section is dominated by full-grabens. Three source rocks each belonging to three different play fairways exist: 1) The Paleogene Sokor-1 Member source belongs to second cycle syn-rift play associated with fluvial/deltaic facies;2) Cretaceous Yogou and Donga sources from first cycle post-rift play associated with alluvial/fluvial/deltaic and marine clastic and carbonate facies;and 3) Cretaceous Yogou source from first cycle transitional play associated with mudstone and shale of transitional facies. The ENRB comprises two source-reservoir-seal assemblages: a lower assemblage of Upper Cretaceous and an Upper assemblage of the Paleogene. Except for the Yogou source which possesses a self-contained petroleum system, the rest of the source rocks release their oils into the Paleogene Sokor-1 Member reservoir sealed regionally by the Oligocene Sokor-2 Member. The Paleogene assemblage is charged from the Upper Cretaceous Yogou Formation through fractures emplaced during the rifting episodes.

Petroleum systems in the Damintun Depression, Liaohe Oilfield

There are two different types of oils—high-wax oil and normal oil—found in the Damintun Depression of Liaohe Oilfield after several years of exploration and development, but their distributions and origins had confused the explorers in the oilfield. The introduction of petroleum-system concept shifts the view of geoscientists from geology and geophysics to oil, gas and their related source rocks. After detailed study, two petroleum systems have been identified in the Damintun Depression: (1) the ES42-Ar buried hill petroleum system (called the high-wax oil petroleum system) and (2) the ES41+ES34-ES4 and ES3 petroleum system (called the normal oil petroleum system). Based on the detailed analysis of the basic components, and all the geological processes required to create these elements of the two petroleum systems, it is put forward that targets for future exploration should include the area near Dongshenpu-Xinglongpu and the area near the Anfutun Sag. This provides scientific basis and has theoretical and practical meaning for the exploration and development.

Petroleum system analysis-conjoined 3D-static reservoir modeling in a 3-way and 4-way dip closure setting: Insights into petroleum geology of fluvio-marine deposits at BED-2 Field (Western Desert, Egypt)

Imperfect determination of petroleum system processes coincidence,entrapment and charge timing,along with reservoir heterogeneity can considerably cause high risks throughout exploration and development phases of petroleum.Therefore,a complete subsurface visualization of the petroleum system nature,elements and processes,is badly required.To this end,we corroborated,in this study,static reservoir modeling with petroleum system analysis workflows,to better characterize the Cenomanian fluvio-marine reservoir,sandstones of Bahariya Formation,at Bed-2 Field,Abu Gharadig Basin(Western Desert,Egypt).We used dataset of 2D seismic profiles and well logs of eight wells.The geometry and property of the reservoir were acquired performing static reservoir geocellular modeling approach.The geohistory,timing of charge,migration pathways,and accumulation sites were identified performing 1D and 2D basin modeling approaches.Combining both approaches was aimed at identifying new petroleum prospect areas,and estimating the hydrocarbon volumes,that are the need for such poorly-defined petroleum systems area.As indicated by the constructed,robust,reservoir and 1D-2D basin models,an additional hydrocarbon prospect,to the north central part of Bed-2 Field,is proposed to be drilled during the further oilfield development phases of the area.This prospect has all features that adequately lead to reliable inferences regarding the ultimate petroleum potential of the area.