Petroleum geology features and research developments of hydrocarbon accumulation in deep petroliferous basins

As petroleum exploration advances and as most of the oil–gas reservoirs in shallow layers have been explored, petroleum exploration starts to move toward deep basins, which has become an inevitable choice. In this paper, the petroleum geology features and research progress on oil–gas reservoirs in deep petroliferous basins across the world are characterized by using the latest results of worldwide deep petroleum exploration. Research has demonstrated that the deep petroleum shows ten major geological features.(1) While oil–gas reservoirs have been discovered in many different types of deep petroliferous basins, most have been discovered in low heat flux deep basins.(2) Many types of petroliferous traps are developed in deep basins, and tight oil–gas reservoirs in deep basin traps are arousing increasing attention.(3) Deep petroleum normally has more natural gas than liquid oil, and the natural gas ratio increases with the burial depth.(4) The residual organic matter in deep source rocks reduces but the hydrocarbon expulsion rate and efficiency increase withthe burial depth.(5) There are many types of rocks in deep hydrocarbon reservoirs, and most are clastic rocks and carbonates.(6) The age of deep hydrocarbon reservoirs is widely different, but those recently discovered are predominantly Paleogene and Upper Paleozoic.(7) The porosity and permeability of deep hydrocarbon reservoirs differ widely, but they vary in a regular way with lithology and burial depth.(8) The temperatures of deep oil–gas reservoirs are widely different, but they typically vary with the burial depth and basin geothermal gradient.(9) The pressures of deep oil–gas reservoirs differ significantly, but they typically vary with burial depth, genesis, and evolution period.(10) Deep oil–gas reservoirs may exist with or without a cap, and those without a cap are typically of unconventional genesis. Over the past decade, six major steps have been made in the understanding of deep hydrocarbon reservoir formation.(1) Deep petroleum in petroliferous basins has multiple sources and many different genetic mechanisms.(2) There are high-porosity,high-permeability reservoirs in deep basins, the formation of which is associated with tectonic events and subsurface fluid movement.(3) Capillary pressure differences inside and outside the target reservoir are the principal driving force of hydrocarbon enrichment in deep basins.(4) There are three dynamic boundaries for deep oil–gas reservoirs; a buoyancy-controlled threshold, hydrocarbon accumulation limits, and the upper limit of hydrocarbon generation.(5)The formation and distribution of deep hydrocarbon reservoirs are controlled by free, limited, and bound fluid dynamic fields. And(6) tight conventional, tight deep, tight superimposed, and related reconstructed hydrocarbon reservoirs formed in deep-limited fluid dynamic fields have great resource potential and vast scope for exploration.Compared with middle–shallow strata, the petroleum geology and accumulation in deep basins are morecomplex, which overlap the feature of basin evolution in different stages. We recommend that further study should pay more attention to four aspects:(1) identification of deep petroleum sources and evaluation of their relative contributions;(2) preservation conditions and genetic mechanisms of deep high-quality reservoirs with high permeability and high porosity;(3) facies feature and transformation of deep petroleum and their potential distribution; and(4) economic feasibility evaluation of deep tight petroleum exploration and development.     

Fluid migration paths in the marine strata of typical structures in the western Hubei– eastern Chongqing area, China

The western Hubei-eastern Chongqing area is an important prospective zone for oil and gas exploration in the central Yangtze area. Three representative structures, the Xinchang structure, Longjuba gas-bearing structure and the Jiannan gas field, were selected to analyze biomarker parameters in marine strata and to examine various types of natural gas and hydrocarbon sources. Fluid inclusions; carbon, oxygen, and strontium isotopic characteristics; organic geochemical analysis and simulation of hydrocarbon generation and expulsion history of source rocks were used for tracing fluid migration paths in marine strata of the study area. The Carboniferous-Triassic reservoirs in three typical structures all experienced at least two stages of fluid accumulation. All marine strata above the early Permian were shown to have fluids originating in the Permian rocks, which differed from the late stage fluids. The fluids accumulated in the late Permian reservoirs of the Xinchang structure were Cambrian fluids, while those in the late Carboniferous reservoirs were sourced from a combination of Silurian and Cambrian fluids. A long-distance and large-scale cross-formational flow of fluids destroyed the preservation conditions of earlier accumulated hydrocarbons. A short-distance cross-formational accumulation of Silurian fluids was shown in the late Permian reservoirs of the Longjuba structure with favorable hydrocarbon preservation conditions. The fluid accumulation in the Carboniferous reservoirs of the Jiannan structure mainly originated from neighboring Silurian strata with a small amount from the Cambrian strata. As a result, the Jiannan structure was determined to have the best preservation conditions of the three. Comparative analysis of fluid migration paths in the three structures revealed that the zone with a weaker late tectonism and no superimposition and modification of the Upper and Lower Paleozoic fluids or the Upper Paleozoic zone with the fluid charging from the Lower Paleozoic in the western Hubei-eastern Chongqing area are important target areas for future exploration.     

Classification and characteristics of tight oil plays

Based on the latest conventional–unconventional oil and gas databases and relevant reports,the distribution features of global tight oil were analyzed.A classification scheme of tight oil plays is proposed based on developed tight oil fields.Effective tight oil plays are defined by considering the exploiting practices of the past few years.Currently,potential tight oil areas are mainly distributed in 137 sets of shale strata in 84 basins,especially South America,North America,Russia,and North Africa.Foreland,craton,and continental rift basins dominate.In craton basins,tight oil mainly occurs in Paleozoic strata,while in continental rift basins,tight oil occurs in Paleozoic–Cenozoic strata.Tight oil mainly accumulates in the Cretaceous,Early Jurassic,Late Devonian,and Miocene,which correspond very well to six sets of globaldeveloped source rocks.Based on source–reservoir relationship,core data,and well-logging data,tight oil plays can be classified into eight types,above-source play,below-source play,beside-source play,in-source play,between-source play,in-source mud-dominated play,insource mud-subordinated play,and interbedded-source play.Specifically,between-source,interbedded-source,and in-source mud-subordinated plays are major targets for global tight oil development with high production.Incontrast,in-source mud-dominated and in-source plays are less satisfactory.     

Further Recognition of Petroleum Exploration Potential of Marine Carbonates in Western Tarim Basin

A series of significant discoveries in marine carbonate rocks show great petroleum exploration potential in the Tarim Basin. However, the oil and gas fields discovered in the carbonate rocks are mainly distributed around the Manjiaer Sag in the eastern Tarim Basin. Some explorations occurred and no oil or gas field was discovered around the Awati Sag in the western Tarim Basin. Information from wells and outcrops reveals that there are excellent oil and gas source rock conditions around the Awati Sag. Transformed reef-shoal reservoirs could be formed in the Ordovician carbonate rocks with paleo-geographic background and hydrothermal conditions. Therefore, it is necessary to make a systematical study and overall evaluation of the potential of the periphery of the Awati Sag in terms of source rock evolution, resource potential, high-grade reservoir formation and distribution, and main factors controlling hydrocarbon migration and accumulation.     

Trial of Natural Gas Injection Made Success in China

The trial of oil displacement driven by natural gas injection in China has been successfully accomplished recently for the first time in Zhongyuan Oilfield, Henan Province. This technology is ready for oil? eld application in July.According to introduction, one-third of the reserves in Zhongyuan Oil? eld, being well known for its multiple reservoir types, deep burial depth and complex structures, are contained in reservoirs with low and ultra-low permeability. As these reserves are unrecove…     

Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments,central Bohai Sea,China

The characteristics and formation mechanisms of the mixed siliciclastic–carbonate reservoirs of the Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and scanning electron microscopy observations, X-ray diffractometry, carbon and oxygen stable isotope geochemistry,and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type Ⅰ and Type Ⅱ mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and intrafossil pores, feldspar dissolution pores mainly developed in Type Ⅱ sediments. Type Ⅰ mixed sediments are characterized by precipitation of early pore-lining dolomite, relatively weak mechanical compaction during deep burial, and the occurrence of abundant oil inclusions in high-quality reservoirs. Microfacies played a critical role in the formation of the mixed reservoirs, and high-quality reservoirs are commonly found in high-energy environments, such as fan delta underwater distributary channels,mouth bars, and submarine uplift beach bars. Abundant intrafossil pores were formed by bioclastic decay, and secondary pores due to feldspar dissolution further enhance reservoir porosity. Mechanical compaction was inhibited by the precipitation of pore-lining dolomite formed duringearly stage, and oil emplacement has further led to the preservation of good reservoir quality.     

Two highly efficient accumulation models of large gas fields in China

Based on reserve abundance,large gas fields in China can be divided into two types：type one of high abundance large gas fields,dominated by structural gas reservoirs; type two of low abundance large gas fields,dominated by stratigraphic and lithologic gas reservoirs.The formation of these two types of large gas fields is related to the highly efficient accumulation of natural gas.The accumulation of high abundance gas fields is dependent on the rapid maturation of the source kitchen and huge residual pressure difference between the gas source kitchen and reservoir,which is the strong driving force for natural gas migration to traps.Whereas the accumulation of low abundance gas fields is more complicated,involving both volume flow charge during the burial stage and diffusion flow charge during the uplift stage,which results in large area accumulation and preservation of natural gas in low porosity and low permeability reservoirs.This conclusion should assist gas exploration in different geological settings.     

Characteristics and accumulation model of the late Quaternary shallow biogenic gas in the modern Changjiang delta area, eastern China

The Changjiang(Yangtze)is one of the largest rivers in the world.It formed a huge incised valley at its mouth during the Last Glacial Maximum;the incised-valley fill,approximately 80–110 m thick,supplies an important foundation for the generation of shallow biogenic-gas reservoirs.Two cores and 13 cone penetration tests were used to elaborate the characteristics,formation mechanism,and distribution of the shallow biogenic-gas reservoirs in the study area.The natural gas is mainly composed of CH_4(generally[95%)with a δ~(13)C_(CH4) and δ~(13)C_(CO2) of-75.8 to-67.7% and -34.5 to-6.6%,respectively,and a δD_(CH4) of-215 to-185%,indicating a biogenic origin by the carbon dioxide reduction pathway.Commercial biogenic gas occurs primarily in the sand bodies of fluvial-channel,floodplain,and paleo-estuary facies with a burial depth of 50–80 m.Gas sources as well as cap beds are gray to yellowish-gray mud of floodplain,paleoestuary,and offshore shallow marine facies.The organic matter in gas sources is dominated by immature type III kerogen(gas prone).The difference in permeability(about4–6 orders of magnitude)between cap beds and reservoirs makes the cap beds effectively prevent the upward escape of gas in the reservoirs.This formation mechanism is consistent with that for the shallow biogenic gas in the late Quaternary Qiantang River incised valley to the south.Therefore,this study should provide further insight into understanding the formation and distribution of shallow biogenic gas in other similar postglacial incised-valley systems.     

Characteristics of Oil-gas Resources in Sichuan Basin

Sichuan Basin, a large and old oil-bearing superimposed basin in western China, has an acreage of 190 thousand square kilometers. Its superimposed strata are composed of 7000-12000 meters of sedimentary rock from the Sinian to the Quaternary, in which the strata are mainly marine carbonate below the Middle Triassic and terrestrial clastic rock above the Upper Triassic. For more than 50 years, CNPC has been working in the basin for oil-gas exploration and development and has discovered so far 106 gas fields and 14 oil fields with proven gas reserves achieving 840 billion cubic meters （bcm）and annual gas and crude production reaching 12 bcm and 145 thousand tons respectively. Oil/gas fields in Sichuan Province is currently China＇s largest gas-producing region. Recent exploration and development practices show that gas reserves and production of the basin are still in the upsoaring stage and an understanding of oilgas distribution in the basin is of vital importance for a sustainable development of natural gas in the basin.     

Tight sandstone gas accumulation mechanism and development models

Tight sandstone gas serves as an important unconventional hydrocarbon resource, and outstanding results have been obtained through its discovery both in China and abroad given its great resource potential.However, heated debates and gaps still remain regarding classification standards of tight sandstone gas, and critical controlling factors, accumulation mechanisms, and development modes of tight sandstone reservoirs are not determined. Tight sandstone gas reservoirs in China are generally characterized by tight strata, widespread distribution areas, coal strata supplying gas, complex gas–water relations, and abnormally low gas reservoir pressure. Water and gas reversal patterns have been detected via glass tube and quartz sand modeling, and the presence of critical geological conditions without buoyancy-driven mechanisms can thus be assumed. According to the timing of gas charging and reservoir tightening phases, the following three tight sandstone gas reservoir types have beenidentified:(a) ‘‘accumulation–densification'(AD), or the conventional tight type,(b) ‘‘densification–accumulation'(DA), or the deep tight type, and(c) the composite tight type. For the AD type, gas charging occurs prior to reservoir densification, accumulating in higher positions under buoyancy-controlled mechanisms with critical controlling factors such as source kitchens(S), regional overlaying cap rocks(C), gas reservoirs,(D) and low fluid potential areas(P). For the DA type, reservoir densification prior to the gas charging period(GCP) leads to accumulation in depressions and slopes largely due to hydrocarbon expansive forces without buoyancy, and critical controlling factors are effective source rocks(S), widely distributed reservoirs(D), stable tectonic settings(W) and universal densification of reservoirs(L). The composite type includes features of the AD type and DA type, and before and after reservoir densification period(RDP), gas charging and accumulation is controlled by early buoyancy and later molecular expansive force respectively. It is widely distributed in anticlinal zones, deep sag areas and slopes, and is controlled by source kitchens(S), reservoirs(D), cap rocks(C), stable tectonic settings(W), low fluid potential areas(P), and universal reservoir densification(L). Tight gas resources with great resource potential are widely distributed worldwide, and tight gas in China that presents advantageous reservoir-forming conditions is primarily found in the Ordos, Sichuan, Tarim, Junggar, and TurpanHami basins of central-western China. Tight gas has served as the primary impetus for global unconventional natural gas exploration and production under existing technical conditions.     

Reservoir characteristics, formation mechanisms and petroleum exploration potential of volcanic rocks in China

Characterized by complex lithology and strong heterogeneity, volcanic reservoirs in China developed three reservoir space types: primary pores, secondary pores and fractures. The formation of reservoir space went through the cooling and solidification stage(including blast fragmentation, crystallization differentiation and solidification)and the epidiagenesis stage(including metasomatism, filling, weathering and leaching, formation fluid dissolution and tectonism). Primary pores were formed at the solidification stage, which laid the foundation for the development and transformation of effective reservoirs. Secondary pores were formed at the epidiagenesis stage, with key factors as weathering and leaching, formation fluid dissolution and tectonism. In China, Mesozoic–Cenozoicvolcanic rocks developed in the Songliao Basin and Bohai Bay Basin in the east and Late Paleozoic volcanic rocks developed in the Junggar Basin, Santanghu Basin and Tarim Basin in the west. There are primary volcanic reservoirs and secondary volcanic reservoirs in these volcanic rocks, which have good accumulation conditions and great exploration potential.     

Control of hydrocarbon accumulation by Lower Paleozoic cap rocks in the Tazhong Low Rise, Central Uplift, Tarim Basin, West China

Despite the absence of regional cap rocks in the Lower Paleozoic for the entire Tazhong Low Rise,several sets of effective local cap rocks are well preserved on the Northern Slope.Of these the best is the Ordovician mudstone of the Sangtamu Formation; the second is the Silurian Red Mudstone Member of the Tatairtag Formation and the marl of the Ordovician Lianglitag Formation; and the third is the gray mudstone of the Silurian Kepingtag Formation.The dense limestone of the Ordovician Yingshan Formation and the gypsum of the Middle Cambrian have shown initial sealing capacity.These effective cap rocks are closely related to the distribution of Lower Palaeozoic hydrocarbons in the Tazhong Low Rise.With well-preserved Sangtamu Formation mudstone and its location close to migration pathways,rich Lower Paleozoic hydrocarbon accumulation can be found on the Northem Slope.Vertically,most of the reserves are distributed below the Sangtamu Formation mudstone; areally,hydrocarbons are mainly found in the areas with well-developed Sangtamu Formation mudstone and Lianglitag Formation marl.Burial history and hydrocarbon charging history show that the evolution of Lower Palaeozoic cap rocks controlled the accumulation of hydrocarbon in the Tazhong Low Rise.Take the Red Mudstone Member of the Tatairtag Formation and Sangtamu Formation mudstone for examples：1） In the hydrocarbon charging time of the Late Caledonian-Early Hercynian,with top surfaces at burial depths of over 1,100 m,the cap rocks were able to seal oil and gas; 2） During the intense uplifting of the Devonian,the cap rocks with top surfaces at burial depths of 200-800 m and 500-1,100 m respectively were denuded in local areas,thus hydrocarbons trapped in earlier time were degraded to widespread bitumen; 3） In the hydrocarbon charging time of the Late Hercynian and Himalayan,the top surfaces of the cap rocks were at burial depths of over 2,000 m without intense uplifting and denudation thereafter,so trapped hydrocarbons were preserved.Based on cap rocks,the Ordovician Penglaiba Formation and Lower Cambrian dolomite could be potential targets for exploration on the Tazhong Northern Slope,and combined with hydrocarbon migration,less risk would be involved.     

Reservoir characterization of the Odovician oil and gas pools in the Tahe Oilfield, Tarim Basin, Northwest China

About 88. 1% of the proven reserves in the Tahe Oilfield in the Tarim Basin of Northwest China are trapped in Ordovician carbonate reservoirs. These reservoirs are formed by unconnected and interconnected networks of karstic porosity forming a heterogeneous and complex reservoir system. Oil, water and gas characteristics vary significantly in different portions of the Ordovician reservoir. There is no uniform oil/water contact in the field, adding to its complexity.An acid fracture treatment is beneficial in 76% of the wells, stimulating nonproductive wells and enhancing production in other wells by fracturing into unconnected reservoirs and enhancing flow pathways with acid. Acid fracture treatments should be a standard procedure for developing this and similar oil fields.     

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

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

Geological characteristics and ‘‘sweet area'' evaluation for tight oil

Tight oil has become the focus in exploration and development of unconventional oil in the world especially in North America and China. In North America there has been intensive exploration for tight oil in marine In China, commercial exploration for tight oil in continental sediments is now steadily underway. With the discovery of China's first tight oil field—Xin'anbian Oilfield in the Ordos Basin, tight oil has been integrated officially into the category for reserves evaluation. Geologically tight oil is characterized by distribution in depressions and slopes of basins, extensive, mature, and high-quality source rocks, large-scale reservoir space with micro- and nanopore throat systems, source rocks and reservoirs in close contac and with continuous distribution, and local ‘‘sweet area.'The evaluation of the distribution of tight oil ‘‘sweet area'should focus on relationships between ‘‘six features.'These are source properties, lithology, physical properties brittleness, hydrocarbon potential, and stress anisotropy. In North America, tight oil prospects are distributed in lamellar shale or marl, where natural fractures are frequently present, with TOC 4 %, porosity 7 %, brittle mineral content 50 %, oil saturation of 50 %–80 %API 35°, and pressure coefficient 1.30. In China, tigh oil prospects are distributed in lamellar shale, tight sandstone, or tight carbonate rocks, with TOC 2 %, porosity 8 %, brittle mineral content 40 %, oil saturation of 60 %–90 %, low crude oil viscosity, or high formation pressure. Continental tight oil is pervasive in China and its preliminary estimated technically recoverable resources are about(20–25)×10~8 t.     

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin, southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin.Their occurrence,like other cases worldwide,can be divided into two locations in general,i.e.,platform interior and platform margin locations.Their differences of reservoir features and origins,however,have not been investigated comprehensively due to different exploration degrees.This issue is addressed in this paper,to provide basic data and information for the basin’s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide.We compared the features of these two types of reservoirs in detail,including the depositional and diagenetic features,pore types and petrophysical features.Based on the comparison,the origin of the reservoirs was further discussed.It is shown that the reservoirs in platform interior and platform margin locations differ significantly.The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone,which was weakly compacted and intensely cemented and has undergone meteoric dissolution.Pore types include intragranular dissolution and moldic pores,with low porosities(<6%) and low permeabilities(<0.1 mD).By contrast,the platform margin carbonates were deposited in relatively high energy settings and mainly consisted of dolostones with some limestones.The rocks were strongly compacted but incompletely cemented.As a result,some primary intergranular pores were preserved.Both meteoric solution and burial solution have taken place.There are various types of pore spaces including intergranular and intercrystalline solution pores and residual intergranular pores.This type of reservoir generally has better petrophysical properties(>9% porosity and >0.1 mD permeability) and pore-throat structures than the interior reservoirs.These differences were influenced by both primary depositional features and secondary diagenesis.For the interior carbonate reservoirs,early meteoric dissolution,weak compaction and strong cementation are important controlling factors.By contrast,the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization,preservation of primary pores and burial dissolution.     

Characteristics and accumulation mechanism of tight sandstone gas reservoirs in the Upper Paleozoic, northern Ordos Basin, China

The Ordos Basin is a significant petroliferous basin in the central part of China.The Carboniferous and Permian deposits of transitional and continental facies are the main gas-bearing layers in the north part of the basin.The Carboniferous and Permian natural gas reservoirs in the northern Ordos Basin are mainly tight sandstone reservoirs with low porosity and low permeability,developing lots of ＂sweet spots＂ with comparatively high porosity and permeability.The tight sandstones in the study area are gas-bearing,and the sweet spots are rich in gas.Sweet spots and tight sandstones are connected rather than being separated by an interface seal.Sweet spot sand bodies are vertically and horizontally overlapped,forming a large gas reservoir group.In fact,a reservoir formed by a single sweet spot sand body is an open gas accumulation.In the gentle dipping geological setting and with the source rocks directly beneath the tight reservoirs over a large area,the balance between gas charging into tight reservoirs from source rocks and gas loss from tight reservoirs through caprock is the key of gas accumulation in tight sandstones.Both the non-Darcy flow charging driven by source-reservoir excess pressure difference and the diffusion flow charging driven by source-reservoir gas concentration difference play an important role in gas accumulation.The results of mathematical modeling indicate that the gas accumulation cannot be formed by just one of the above mechanisms.The diffusion of gas from source rocks to reservoirs is a significant mechanism of tight sandstone gas accumulation.     

Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China

Tight sand gas is an important unconventional gas resource occurring widely in different petroleum basins.In coal-bearing formations of the Upper Triassic in the Sichuan Basin and the Carboniferous and Permian in the Ordos Basin,coal measure strata and tight sandstone constitute widely distributed source-reservoir assemblages and form the basic conditions for the formation of large tight sand gas fields.Similar to most tight gas basins in North America,the Sichuan,and Ordos Basins,all experienced overall moderate uplift and denudation in MesoCenozoic after earlier deep burial.Coal seam adsorption principles and actual coal sample simulation experiment results show that in the course of strata uplift,pressure drops and desorption occurs in coal measure strata,resulting in the discharge of substantial free gas.This accounts for 28%—42%of total gas expulsion from source rocks.At the same time,the free gases formerly stored in the pores of coal measure source rocks were also discharged at a large scale due to volumetric expansion resulting from strata uplift and pressure drop.Based on experimental data,the gas totally discharged in the uplift period of Upper Paleozoic in the Ordos Basin,and Upper Triassic Xujiahe Formation in the Sichuan Basin is calculated as(3-6) × 10~8 m~3/km~2.Geological evidence for gas accumulation in the uplift period is found in the gas reservoir analysis of the above two basins.Firstly,natural gas discharged in the uplift period has a lighter carbon isotope ratio and lower maturity than that formed in the burial period,belonging to that generated at the early stage of source rock maturity,and is absorbed and stored in coal measure strata.Secondly,physical simulation experiment results at high-temperature and high-salinity inclusions,and almost actual geologic conditions confirm that substantial gas charging and accumulation occurred in the uplift period of the coal measure strata of the two basins.Diffusive flow is the main mode for gas accumulation in the uplift period,which probably reached 56 × 10~(12)m~3 in the uplift period of the Xujiahe Formation of the Sichuan Basin,compensating for the diffusive loss of gas in the gas reservoirs,and has an important contribution to the formation of large gas fields.The above insight has promoted the gas resource extent and potential of the coal measure tight sand uplift area;therefore,we need to reassess the areas formerly believed unfavorable where the uplift scale is large,so as to get better resource potential and exploration prospects.     

Geological characteristics and accumulation mechanisms of the “continuous” tight gas reservoirs of the Xu2 Member in the middlesouth transition region, Sichuan Basin, China

“Continuous” tight gas reservoirs are those reservoirs which develop in widespread tight sandstones with a continuous distribution of natural gas. In this paper, we summarize the geological features of the source rocks and “continuous” tight gas reservoirs in the Xujiahe Formation of the middlesouth transition region, Sichuan Basin. The source rocks of the Xu1 Member and reservoir rocks of the Xu2 Member are thick (Xu1 Member: 40 m, Xu2 Member: 120 m) and are distributed continuously in this study area. The results of drilled wells show that the widespread sandstone reservoirs of the Xu2 Member are charged with natural gas. Therefore, the natural gas reservoirs of the Xu2 Member in the middle-south transition region are “continuous” tight gas reservoirs. The accumulation of “continuous” tight gas reservoirs is controlled by an adequate driving force of the pressure differences between source rocks and reservoirs, which is demonstrated by a “one-dimensional” physical simulation experiment. In this simulation, the natural gas of “continuous” tight gas reservoirs moves forward with no preferential petroleum migration pathways (PPMP), and the natural gas saturation of “continuous” tight gas reservoirs is higher than that of conventional reservoirs.     

Petroleum Discoveries and Exploration Prospect in China＇s Major Petroliferous Basins

Up to now, there are 29 oil- and gas-bearing basins with recoverable oil and gas reserves in China. However, most of the reserves were mainly discovered in parts of the 29 basins, which played an important role in China＇s petroleum exploration and development. Based on the data statistics and analyses of reserves and resources, the current situation of oil and gas discoveries and the resources potential in these major basins are discussed in the paper. The so-called major oil- and gas-bearing basins here are the basins with cumulative recoverable oil （excluding condensate oil） reserves of more than 50 million tons or cumulative recoverable gas （excluding dissolved gas） reserves of more than 50 billion cubic meters.