Basic principles of the whole petroleum system

This paper expounds the basic principles and structures of the whole petroleum system to reveal the pattern of conventional oil/gas-tight oil/gas-shale oil/gas sequential accumulation and the hydrocarbon accumulation models and mechanisms of the whole petroleum system.It delineates the geological model,flow model,and production mechanism of shale and tight reservoirs,and proposes future research orientations.The main structure of the whole petroleum system includes three fluid dynamic fields,three types of oil and gas reservoirs/resources,and two types of reservoir-forming processes.Conventional oil/gas,tight oil/gas,and shale oil/gas are orderly in generation time and spatial distribution,and sequentially rational in genetic mechanism,showing the pattern of sequential accumulation.The whole petroleum system involves two categories of hydrocarbon accumulation models:hydrocarbon accumulation in the detrital basin and hydrocarbon accumulation in the carbonate basin/formation.The accumulation of unconventional oil/gas is self-containment,which is microscopically driven by the intermolecular force(van der Waals force).The unconventional oil/gas production has proved that the geological model,flow model,and production mechanism of shale and tight reservoirs represent a new and complex field that needs further study.Shale oil/gas must be the most important resource replacement for oil and gas resources of China.Future research efforts include:(1)the characteristics of the whole petroleum system in carbonate basins and the source-reservoir coupling patterns in the evolution of composite basins;(2)flow mechanisms in migration,accumulation,and production of shale oil/gas and tight oil/gas;(3)geological characteristics and enrichment of deep and ultra-deep shale oil/gas,tight oil/gas and coalbed methane;(4)resource evaluation and new generation of basin simulation technology of the whole petroleum system;(5)research on earth system-earth organic rock and fossil fuel system-whole petroleum system.

Petroleum Accumulation Associated with Volcanic Activity in the Tarim Basin —— Taking Tazhong-47 Oilfield as an Example

The discovery of the Tazhong-47 oilfield confirmed that it is realistic to explore oil and gas fields around igneous rocks in the Tarim basin. The favorable conditions for petroleum accumulation due to volcanic activity and igneous rocks formed during the activity show in two aspects. A) The contact surface of igneous rocks and the surrounding sedimentary rocks, like a vertical unconformity surface, formed the conduit of petroleum migration. Petroleum would accumulate once it encountered a trap in which the reservoir had fine porosity and permeability. B) It formed a trap barriered by igneous rocks, or changed or cut the original trap. In addition, volcanic rocks are a kind of potential reservoir, there are many such examples in the world, and oil also shows in the Permian igneous rocks in well Zhong-1 on Tazhong uplift. Petroleum accumulation associated with volcanic activity will be a new exploring field in the Tarim basin.

Effects of Overpressured Fluid Flow on Petroleum Accumulation in the Yinggehai Basin

The Yinggehai Basin is a strongly overpressured Cenozoic basin developed in the northern continental shelf of the South China Sea. The flow of overpressured fluids in this basin has given rise to strong effects on petroleum accumulation. (1) The overpressured fluid flow has enhanced the maturation of shallow-buried source rocks, which has caused the source rocks that would have remained immature under the conduction background to be mature for hydrocarbon generation. As a result, the overpressured fluid flow has increased the volume and interval of mature source rocks. (2) The overpressured fluid flow has strong extraction effects on the immature or low-mature source rocks in the shallow parts. This has increased, to some extent, the expulsion efficiency of the source rocks. More importantly, the extraction effects have strongly limited the effectiveness of biomarker parameters from oil and condensate in reflecting the source and maturity of the oil and gas. (3) The flow has caused the sandstones in the shallow parts to get into the late diagenesis stage, and significantly reduced the porosity and permeability of the sandstones. This study confirms that even in sedimentary basins in which no topography-driven groundwater flow systems have ever developed, the cross-formation migration of overpressured fluids and the resultant energy conduction and material exchange can significantly affect the thermal regime, source rock maturation and sandstone diagenesis. As a result, the effects of overpressured fluid flow must be taken into account in analyzing the mechanism of petroleum accumulation.

Dynamic Systems of Petroleum Accumulation in the Nanpu Depression,Bohai Bay Basin,China

It is significant to distinguish the dynamic systems of petroleum accumulation （DSPA） for the understanding of petroleum accumulation and distribution. According to the formation pressure framework, genetic types of petroleum and characteristics of conduit systems, three dynamic systems of petroleum accumulation were identified in the vertical profile in the Nanpu depression, Bohai Bay basin. The deeper DSPA （including formations Es3 to Es2） is a sealed system with high-overpressure and high-mature self-sourced oil. Most of the crude oil in the system accumulated in the periods of late Oligocene （23.5 Ma） and late Pliocene （2.4 Ma）. The middle DSPA （including formations Es~ to Edl） is an overpressured half-sealed system with mature or lower-mature self-sourced oil. The accumulation of oil in the system also occurred in the late Oligocene （23.5 Ma） and late Pliocene （2.4 Ma）. The shallower DSPA （including formations Ed2 to Q） is a hydrostatic system with lower-mature aliensourced oil from the middle system. Oil within this system accumulated only in the late Pliocene period. The oil in the shallower system migrated vertically along the faults from the formerly accumulated oil in the middle system by lateral migration along the sandbodies, whereas petroleum accumulation in the deeper system was mainly derived from the system itself by lateral migration along the sandbodies and rarely migrated out of the system. In this case, it seems that the deeper system is a more potential exploration prospect in addition to the other two proved favorable systems.

Mechanism of petroleum migration and accumulation in western China's superposed basins

In western China, most petroliferous basins are superposed due to their multi-periodic tectonic evolution, and the mechanisms of petroleum migration and accumulation are so complex that much more sophis- ticated methodologies are necessary for depiction of these mechanisms and identification of petroleum occurrences. For this purpose, in this article, a new methodology was formulated which includes： （I） ver- tical identification of petroleum migration and accumulation fluid dynamic systems in the superposed basins; （2） analysis of the effect of large scale regional faults and fault combinations on the fluids exchange between the vertically identified different systems; （3） analysis of petroleum migration and accumulation in each vertically identified system, and establishment of appropriate geological model of petroleum migration and accumulation for each vertically identified system. Using this methodology, the satisfactory results obtained in the Lunnan Uplift of Tarim Basin and Ludong Uplift of Jungar Basin case studies are： （1） existence of different vertical fluid dynamic systems in western China＇s superposed basins which are very necessary for understanding the mechanism of petroleum migration and accumu- lation; （2） in deep system, long-distance lateral petroleum migration and accumulation mainly take place along the long time exposed unconformity with weathered, fractured or karst reservoir rocks; （3） regio- nal faults are the main conducts for fluids migration from deep system up to middle and/or upper sys- tems. As to middle and/or upper systems, regional faults play a role of ＂petroleum source＂. Small faults within middle and/or upper systems conduct petroleum to carrier beds with less impeding force; （4） petroleum migrated from deep system vertically up to middle and/or upper systems will migrate lat- erally in carrier beds of these systems and accumulate to form nools near or far from faults.

Oderly accumulation theory of shale system petroleum resource and its prospecting significance-A case study of Chang 7 Member of Yanchang Formation in Ordos Basin

1 Introduction Shale formations bear abundant mineral resource and*unconventional petroleum resource,and the unconventional petroleum resource that contain in the shale formation should be integrated and researched.

Decipher hydrocarbon generation and accumulation based on fluid inclusion and chronology:A case study from the Upper Paleozoic buried-hills in Huanghua Depression,Bohai Bay Basin

Deciphering hydrocarbon generation and accumulation stage is of significance to understand oil and gas evolution and seek exploration targets.Taking the Upper Paleozoic buried-hills in the Huanghua Depression,Bohai Bay Basin,as a case study,hydrocarbon generation environment and detailed accumulation process are revealed by fluid inclusions observations,Laser Raman spectroscopy,Fourier Infrared spectroscopy,and K-Ar isotope measurements.The results show that both oil and gas inclusion were captured in the quartz overgrowth,dissolved feldspar and calcite microfractures,showing blue to dark brown fluoresce.The grains containing oil inclusions index(GOI)of oil,oil&gas and gas being 25%,65%,and 10%and the inclusions with abundant methyl groups and short chains,both indicate high thermal maturity.One series of fluids inclusion is generally observed,evidenced by the concentrated homogenization temperature of 135-145℃ and salinity of 3%-15 w.t.%NaCl equiv,indicating one primary charging stage.The gas and gas&liquid inclusions mainly contain CH_(4),with also peaks indicating CO_(2) and N_(2.)The Carboniferous and Permian biomarkers show reducing environment with brackish water,with organic matter sources both from marine and continental.The relative content ofααα20RC_(27),ααα20RC_(28),andααα20RC_(29) exhibit source contributions both from algae and higher plants,and mainly of II2 to III kerogen.Both coal derived gas and oil associated hydrocarbons are identified from most of the buried-hills.Combining the fluid homogenization temperature and salinity,as well as the thermal evolution history,the hydrocarbon generated from the Upper Paleozoic was concentrated at the end of the Eocene(40 Ma±),while the beginning of charging is 60 Ma±.The Wumaying Buried-hill is of only coal derived gas and has potential for inner coal measure natural gas exploration.The results provide a detailed understanding of hydrocarbon accumulations in the study area,which can also be reference for improving petroleum exploration efficiency in similar basins.

Comparative Analysis of Sequence Characteristics among Different Superimposed Stages of the Chelif Basin,Algeria

Superimposed basins were investigated with respect to tectonic evolution, sediment deposition and petroleum characteristics within a single superposition stage generally. The comparative study was seldom seen. Sequence characteristics were compared for two different superimposed stages - an expanding rifting stage and a depression-foreland transition stage - in the Chelif Basin during the Miocene in this paper. A model and mechanism for sequence evolution of superimposed basins in different dynamic situations are discussed with respect to sequence similarities and differences. The compared characters include sequence thickness, sequence boundaries and system tracts, as well as sediment deposition within sequences and sequence development patterns. Finally, some typical features of sequence development concomitant with changes of superimposed stages in the Chelif Basin are discussed.

Main Controlling Factors and Accumulation Model of Chang 9 Reservoir in Northwest Ordos Basin, China

In northwestern Ordos Basin, the Triassic reservoir Chang 9 has favorable reservoir forming conditions, extensive reservoir development, and huge potential for oil exploration and exploitation. Studying the main controlling factors and accumulation model of Chang 9 reservoir in this area can provide a basis for the production targets, and assist in formulating reasonable development technology policy. In this paper, to explore and summarize the hydrocarbon accumulation model, the Chang 9 reservoir were analyzed from the aspects of oil source, fracture, oil migration, structure, lithology and reservoir physical properties for the main controlling factors in this area. Organic geochemical and geological comprehensive analysis that the oil-source of the Chang 9 reservoir in the northwest of Ordos Basin is derived from Chang 7 hydrocarbon source rocks. The fractures provide a sound channel for the "vertical multi-point filling" of the oil source from Chang 7 to Chang 9. The crude oil migrates vertically from Chang 7 to Chang 9, then expands horizontally to form a reservoir. Structures play an important role in controlling the distribution of reservoirs, the control by sand in small layer and physical property is also obvious. This paper creatively establishes the reservoir accumulation model of Chang 9 in northwest of Ordos Basin, which is characterized by Vertical multi-point filling, horizontal expansion becomes oil pool. It reveals the genetic mechanism of the development of Chang 9 multi-reservoir in the study area, which provides guidance for exploration and evaluation deployment.

Development of Petroleum Geology in China:Discussion on Continuous Petroleum Accumulation

Petroleum exploration targets are extending gradually from the single conventional trap reservoirs to the large-scale unconventional continuous accumulations. Oil and gas reservoirs have been divided into two types based on the trapping mechanism and distribution of oil and gas： conven- tional single-trap reservoirs, such as the Daqing oil field in Songliao Basin and the Kela-2 gas field in Tarim Basin; and unconventional continuous petroleum accumulation, such as Upper Paleozoic tight gas and Mesozoic tight oil in Ordos Basin, and Upper Triassic tight gas in Sichuan Basin. Two typical geologic characteristics of continuous petroleum accumulation involve： （1） coexisting source and reser- voir, petroleum pervasive throughout a large area tight reservoirs, and no obvious traps or well-defined water-oil and gas contracts; （2） non-buoyancy accumulation, continuous petroleum charge, and no sig- nificant influence by buoyancy. Continuous petroleum accumulation generally have nm-scale pore throats, and the diameters range of 10-500 nm. The geometry and connectivity of these pore throats has significant impact on the migration and distribution of oil and gas in continuous petroleum accu- mulation. China has numerous continuous petroleum accumulation containing various petroleum de- posits, and the exploration of continuous resources is very promising. Unconventional petroleum geol- ogy will become an important new subject in petroleum geology in future, and the nano-technology will function greatly on research, exploration and development of the hydrocarbon accumulation in nano-pore-throats.

Complex Petroleum Migration and Accumulation in Central Region of Southern Junggar Basin,Northwest China

The central region of the southern Junggar basin （Northwest China） is a key exploration target in this petroliferous basin. As there are four sets of potential source rocks （e.g., Permian, Jurassic, Cretaceous and Paleogene sequences）, petroleum migration and accumulation are likely complex. This study represents an attempt to understand this complexity in order to provide fundamental information for future regional petroleum exploration and geological studies. Based on petroleum geology and geochemistry, it is implied that there are mainly three types of hydrocarbons, including Cretaceous- and Paleogene-sourced oils （with the former being dominant） and Jurassic-sourced gas. The petroleum migration and accumulation mainly cover three stages. The first stage is the late period of the Early Pleistocene, in which the Cretaceous-sourced oils migrate and accumulate. Then, in the second stage （from the late period of the Middle Pleistocene to the early period of the Late Pleistocene）, the Cretaceous- sourced oils, together with the Paleogene-sourced oils, participate in the migration and accumulation. At last, in the end of the Late Pleistocene, large quantities of oils remigrate and accumulate, with gas （especially Jurassic- sourced gas） migrating along faults to accumulate. Thus, petroleum charge events in the area are complex, reflecting the control of complex tectonic evolution on petroleum migration and accumulation.

Fluid Mobility Evaluation of Tight Sandstones in Chang 7 Member of Yanchang Formation, Ordos Basin

Fluid mobility has been important topic for unconventional reservoir evaluation.The tight sandstones in Chang 7 Member of the Ordos Basin has been selected to investigate the fluid mobility based on the application of core flooding-NMR combined method and core centrifugation-NMR combined method,and the porous structure is studied using optical microscope,field emission scanning electron microscope(FE-SEM),CT and mercury injection.Our results include:(i)Feldsparrock fragments dissolution pores,calcite dissolution pores,clay mineral dissolution pores,intergranular dissolution expansion pores,inter-granular pores,intra-kaolinite pores,and intra-illite/smectite mixed layer pores are developed in Chang 7 tight sandstones;3D CT pore structure shows that the pore connectivity is positively related to physical properties,and the overall storage space is connected by the throat with diameter between 0.2 and 0.3μm.The percentage of storage space connected by throats with diameter less than 100 nm can reach more than 35%.(ii)Movable fluid saturation of Chang 7 tight sandstones is between 10%and 70%,and movable oil saturation is between 10%and 50%.Movable fluid saturation may cause misunderstanding when used to evaluate fluid mobility,so it is recommended to use movable fluid porosity in the evaluation of fluid mobility.The porosity ranging from 5%to 8%is the inflection point of the fluidity and pore structure.For samples with porosity less than 8%,the movable fluid porosity is generally less than 5%.Moreover,the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.1 to 1μm.For samples with porosity greater than 8%,the porosity of the movable fluid is more than 5%,and the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.2 to 2μm.(iii)The movable fluid saturation measured by core flooding-NMR combined method is generally higher than that measured by core centrifugation-NMR combined method.The former can evaluate the mobility of the oil-water two-phase fluid in samples,while the latter can better reflect the pore structure and directly evaluate the movable fluid in the pore system controlled by different throat diameters.All these results will provide valuable reference for fluid mobility evaluation in tight reservoirs.