When high-resolution sequence stratigraphy represented by T. A. Cross Genetic Stratigraphy Group of Colorado School of Mines was introduced to the pubic,it immediately drew the attention of academic circles.The practical effect in application is remarkable.This marks a new development in sequence stratigraphy.The high-resolution sequence stratigraphy introduced in this paper focuses on more accurate prediction of sedimentary mineral deposit,especially concrete objective of oil and gas exploration.The high-resolution sequence stratigraphy takes the stratigraphic and sedimentary process-response dynamic principles,including the principles of stratigraphic base level,volumetric partition,factes differentiation and rule for chronostratigraphic cycle correlation as its theoretical bases.The practice of hydrocarbon exploration and development has proved that the high-resolution sequence stratigraphy has significant importance in factes analysis,3-D prediction of reservoir and the establishment of petrophysical model for fluid-flow numerical simulation.
A passage system means the total amount of passages within a petroliferous system and its relative country rocks.The passage system could be divided into passage type, fracture type, fault type and unconformity type.A petroliferous system could have a passage system of several types.Different passage systems may result in different oil and gas migration and accumulation patterns,and in turn,the migration and accumulation patterns could reform the old passage system or even form new ones.Since the passage systems are relatively independent,spacio temporal characterisics and complicated,their 3-D space comfigurations and transporting capacities should be studied.
Tarim Basin is a large composite and superimposed sedimentary basin developed on the basement of pre-Sinian continental crust. It has undergone three mega-cycles of extension and convergence that separately occurred in Sinian-Middle Devonian, Late Devonian-Triassic and Jurassic-Quaternary. During Sinian to Middle Devonian, i.e. the period of paleo-Asian ocean or proto-Tethyan ocean, it experienced the developmental cycle of intracontinental rift-passive epicontinental basin-foreland basin. From Late Devonian to Triassic, the period of paleo-Tethyan ocean ocean, the southwestern margin of Tarim experienced the developmental cycle of intracontinental rift/passive epicontinental basin-back-arc extensional basin-back-arc foreland basin. From Jurassic to Quaternary, the period of neo-Tethyan ocean, it experienced the developmental cycle of intracontinental rift/depression-compressional adjustment-late foreland basin. There were three secondary cycles during the intracontinental rift/depression-compressional adjustment period. Stratigraphic sequences of the prototype basin in the extensional stage are much more continuous, while those in the convergence stage are characterized by large lateral variation. The complex and superimposed geologic configuration of the basin had been determined by the geodynamic process and pattern of the basin evolution, as well as by the tectonic framework transformation, which would control the basic characteristics of hydrocarbon accumulation and distribution.
Based on the study of cores, well cuttings, and outcrop sections, the facies of volcanic rocks can be divided into 4 types and 12 sub-facies. Pores and fissures are the two major types of reservoir spaces in volcanic rocks in Xujiaweizi fault-depression. The types of reservoir space in volcanic reservoirs have been found to be controlled by the facies. The volcanic reservoirs found in various facies belts have quite different pores and fissures and their assemblages. Gas cavities, fissures in rhyolitic lamination, and joint fissures have been identified mainly in eruption-overflow facies. The main pore-type identified in explosive facies is intergranular pore; and inter-breccia pore and primary fracture are dominant in extrusive facies.Joint fissures and various primary fractures are mainly developed in volcanic conduit facies.It is found that “unconsolidated formation” often distributes at the base of each cooling unit in explosive explosive facies,which might be resulted from fast covering and conservation of the weathered crusts of the original basement rocks or unclinkered drifting clasts. A large “unconsolidated bodies in rock-dome” can often be found in the inner belt sub-facies of extrusive facies, which are actually of large accumulation bodies of perlite balls. Special attention should be paid to looking for these two-type reservoirs in the intermediate-acidic volcanic rock area. The major tectoclase in this area is a set of conjugated, high-angle (fault plane with dip angle of 50°～60°), NNE and NNW strike, and shear or compressive-shear fractures, which constitute the main migration pathway of oil & gas.
The Middle-Late Ordovician carbonate platform margin in Tarim basin can be divided into 4 types, including low energy,relatively low energy,relatively high energy,and high energy types.Different organic reef-bank complexes have been developed in different platform margin types.1)The low energy type carbonate platform margin can be represented by lime mud mound-muddy bank complex in Tazhong I faulted zone which have been developed during the early-stage deposition of the Upper Ordovician Lianglitage Formation.2)The relatively low energy type carbonate platform margin can be represented by the barrier reef-lime mud bank complex in Bachu outcrop area which have been developed during the deposition of Middle Ordovician Yijianfang Formation(Dawangou Formation).3)The relatively high energy type carbonate platform margin can be represented by the barrier reef-bank complex in Lunnan area which have been developed during the deposition of Middle Ordovician Yijianfang Formation.4)The high energy type carbonate platform margin can be represented by the framework reef-bank complex in Tazhong I faulted zone which have been developed during the latestage deposition of the Upper Ordovician Lianglitage Formation.The framework reef-bank complexes in the high energy carbonate platform margin are the most favorable exploration targets.Sea level ascending,tectonic uplifting and descending and carbonate yield are the main factors that control the migration of carbonate platform margins and their types,and they further control the development of organic reefs and banks on carbonate platform margins.
Rock cores testing indicate that space of carbonate rock reservoirs in Tahe oilfield are mainly interparticle pores, fissures and cavities; Their matrix almost do not have reserving capability. Reservoirs in cavity type, weathering fissure type and structural fissure type are commonly developed in Tahe region. Fissures in the surface eluvial belts-cavity reservoirs are largely spread near the unconformable contacts of C1 and O1. Reservoirs in dissolution porosity- type in particle limestone in the platform edge of bank facies are commonly spread along the area of the lower Ordovician in Southern Sha well No.76.It is conjectured by analyzing relations between extension of the fissure series and the diagenetic fabrics that the cam shaft part in Akekuler would be favorable well-developed area for reservoirs in cavity type. New results from well drilling indicate that the area along well S69-S76 in southern margin of Tahe oilfield will be a very important one for the exploration of reservoirs in dissolution porosity-type in particle limestone in the platform edge of bank facies.
A classical concept of foreland basin can be defined as follows: Formed between the contracted orogen and its neighboring craton,an asymmetrical thrust flexure basin parallels to the orogen and extends in a long and narrow type. This definition includes the main characteristics of distribution location,crustal feature, dynamic mechanism developing stage, planar distribution and profile structures along with corresponding sedimentary formation. However, the basic structures and the location of main units of the foreland basin are variable in various stages of its evolution and also with difference both in its pattern and oil and gas bearing features. The foreland basin, on the basis of its geodynamic environment, can be subdivided into three types of peripheral foreland basin, retro-arc foreland basin and broken foreland basin. The former two types are the fundamental subdivisions. Many compressive basins around orogens in mid-west China generally have the structural features, deformation and deposition of foreland basins. According to dynamic sources, developing stages, and main characters during the development of the foreland basin, the basin could be named in a new type of foreland basin namely, intra-continent foreland basin that can be discriminated from the peripheral foreland basin and retro-arc foreland basin. Therefore they are corresponding to the three different featured convergent stages of ocean subduction, continental collision and intra-continent contraction in the structural evolution of plates. The basis for the definition and the contents for the research of the foreland basin are as follows:(1) tectonic attribute in which kinds the foreland basin belongs to;(2)developing states;(3) basin and orogen coupling;(4) regional bacground;(5) prototype basin topography;(6)evolutional process;(7) sedimentary reaction;(8) vertical distribution. Attention should be paid to the generality of the foreland basin while defining it; Importance should be attached to the individuality of the foreland basin when studying it.
Recently,the black sandstone in the 2nd member of Wengxiang group(S1-2wn2)in Majiang,Duyun,Dangzhai and Kaili regions in eastern Gui-zhou was studied by detailed field survey and a lot of laboratory tests, and was discoverde no other than bituminous sandstone. The bitumen, with which the pore spaces between asnd grains are filled, is a variation of petroleum.
Although accounting for a small proportion of the cap rocks all over the world,the saline deposit is of vital importance for accumulation and preservation of hydrocarbons,for it serves as seal to most of the discovered hydrocarbon reserves and is also the major cap rock in many huge and giant oil and gas fields.The close relationship between the Middle-Lower Triassic saline deposit cap rock and the distribution of natural gas in Sichuan basin suggests that distribution of saline deposit is crucial to the understanding and evaluation of the gas preserve conditions in southern China.Saline deposit forms mostly during the middle and late stages of basin development.In southern China,saline deposit cap rock are mainly developed in Middle-Lower Cambrian,Car-boniferous,Middle-Lower Triassic,and Cretaceous-Paleogene,but they are unevenly distributed and have different sealing importance.Under the influence of deposition,tectonic settings,and late destruction,the saline-deposit rock of Middle-Lower Cambrian mainly distribute in the eastern and southern parts of Sichuan basin and the northwestern part of Guizhou province;the Carboniferous saline deposit mainly distributed in the central of Hunan province,and the Cretaceous-Paleogene saline deposit mainly occurs in Jianghan basin.The Middle-Lower Triassic Saline-deposit,with the widest distribution and the biggest thickness,plays an important role in the sealing of natural gas in Sichuan basin,northern Guizhou Province,Chuxiong basin,and parts of the Middle-Lower Yangtze.
This paper is concerned with a study of about 100 crude oil samples fromvarious major basins in China,which was made through“the whole hydro-carbon chromatography”.In combination with the occurrence of crude oil,the distribution of isoprenoid alkanes and its relation with the geologic environmentare discussed here.The authors consider that the conspicuous difference in total concentration of isoprenoids and pristane-phytane ratios(Pr/Ph)in crude oil may indicatethe variability of the continental lake-basin depositional envisonments of sourcerocks.According to the range of the Pr/Ph ratios,the crude oil can be sta-tistically classified into the following three types:phytane-predominant type(0.20—0.80),pristane-phytane balanced type(0.8—2.80)and pristane-pre-dominant type(2.80—4.00).The source rocks of these three types of oil weredeposited under strong reduction,reduction and weak oxidation-reduction environments,respectively.Being mainly dependant on the depositonal envi-ronments of source rocks,the pristane-phytane ratio can serve as good in-dicator for oil-to-oil and oil-to-source rock correlation. The increase in terrestrial phyto-constituents of primary organic matter andthe rise in maturity of source rocks may lead to an increase in Pr/Ph ra-tios for source rocks and crude oils in various degrees.It seems,however,that the influence of these two factors is too small to change thoroughly thepristane-phytane ratios determined by related depositional environments ofsource rocks.Nevertheless,in the case that the crude oil was subjected tosecondary oxidation,the bacterial degradation would lead to considerable chan-ges in the primary distribution of the isoprenoid alkanes.
China marine shale gas field is characterized by wide distribution and richness in resources.But compared with North America, because of suffering multi-stages tectonic reworking, China marine shale is older and more mature, as a result, it is unique in preservation condition and buried depth.Based on these features, the theory of ‘three-element enrichment’, whose cores include the conditions of hydrocarbon generation, reservoir conditions and shale gas preservation conditions, is put forward.On this basis, shale gas exploration is focused on Sichuan Basin and its peripheral regions, and the Lower Silurian of Southeast region of Sichuan Basin is chosen to be the preferred breakthrough field for shale gas exploration by China Petroleum & Chemical Corporation.Shale gas zone and the target evaluation method is set up, and Jiaoshiba Block of Fuling is as the target area.And in 2012, well Jiaoye 1 was drilled, and discovered the first large shale gas field in China, i.e.marine shale gas field of Longmaxi Formation in Jiaoshiba Block.Drilling technologies and multi-stage fracturing technologies were developed for the marine shale with shallow to moderate burial depths.The rapid and highly efficient development of the Jiaohsiba shale gas field was realized through effective management under the guidance of integrated exploration and development.
By the end of 1999,there are 171 discovered hydrocarbon gas fields in China(not including Taiwan Province;the same below).Their total geological reserves amount to 20 635×108m3,recoverable reserves 13 049×108m3,the remaining recoverable reserves 10 213×108m3.Ever since 1981 natural gas reserves have increased by 7.9 times.due to the enhancement of exploration and researches,as well as the application of the theory of coal measure gas.Large or medium size gas fields with proven geological reserves over 100×108m3 are 47.Their total proven geological reserves are up to 15 171×108m3,and covers 74% of total proven geological reserves in China.Within 6 largest gas fields with proved geological reseves over 500×108m3,the first fields 4 are coal-formed gasfields,indicating the importance of coal-formed gas field in China's gas reserves.Newly estimated conventional gas resources in China are 50.6×1012m3,with 13.3×1012m3 recoverable reserves.By the end of 1999,only 9.8% of the recoverable resources are proved.If the proved rate can be as high as that of Canada,the proved rate could be 41.9%,proved recoverable reserves would be 5.58×1012m3.It is estimated that annual gas production will reach 500×108m3 in the year 2005 and 1 000×108m3 in the year 2015 in China.The ratio of reserve to production of Romania,ltaly,Mexico,and German were only 14～27 when its production reached 120×108m3,and this production kept for 30 year or even more.Compared with them,Tarim Basin can insure 30 years of stable gas production of 120×108m3 per year with its 3 397×108m3 remaining recoverable gas reserves.(the reserve-production ratio is 28).So the gas reserves in Tarim Basin is ready for the "West-East Gas Pipeline Project".
The variation of sorting,grain size and clay content is controlled by hy-drodynamic conditions.Each environment follows its own special law in hy-drodynamic properties and evolution.They may control sand distribution,sequence and log shape,with the last one reflecting the first two macro-scopic sedimentary characters(in this paper,the log refers mainly to SPcurve).Consequently,the combinations of log shapes in various environments are different.Synthesizing the characteristics of log such as amplitude,sha-pe,upper and lower contact,smooth or serrated degree,combination of logshape,and combining them with other geologic logging,we can reach a cor-rect conclusion on sedimentary environment.In eastern China,Tertiary faulted lake basins contain many kinds of en-vironments such as alluvial fans,braided and meandering streams,deltas,littoral beaches and bars,subaqueous alluvial fans,sebaqueous channels ofdensity current and turbidity current(deep Water).This paper summarizesthe characters of logs for the environments mentioned above,including thoseof single serration and combinations of log shapes reflecting the sequenceand their relations with geologic background.In the stage of exploration,a comprehensive interpretation of the charactersof these logs is helpful to geologists in explaining sedimentary environmentsand predicting distribution of sandbodies.So it is an useful method.
Based on the principle of base level,three scales of stratigraphic cycles(short term, intermediate term and long term)could be identified through the analysis of cores,logs and seismic data.The stratigraphic cycles correspond respectively to different order base level cycles. Different types of facies successions occurred in different positions of the base level cycles. In light of the identification of multi order base level cycles,2 D high resolution sequence stratigraphic framework is established.Finally,a new proposal about favorable reservoir formations and hydrocarbon generation conditions in Qianliyuan Depression have been proposed.
In 1945, Mr. Huang Jiqing suggested using the term Himalayan movement to describe the Cenozoic orogenic movement in China. This concept has widely been accepted by the geoscientists. However, disagreements still exist regarding the episodes and geodynamic setting of tectonic movements. Based on analysis of basic concepts suggested previously, and in combination with various data, including geology, geomorphology, magmatic activities and structural deformation, it is proposed that Himalayan movement can be divided into early, middle and late episodes, which are corresponding respectively to tectonic activities in late Eocene, between Paleogene and Neogene, and between Noegene and Quaternary. Collision of Indian and Eurasia plates and continuous compression led to strong compression, shortening and uplifting of Qinghai Tibet Plateau including its southeastern and northern margins. East ward compression of East Asia continent, uplifting of deep lithosphere and back arc spreading resulted in extensional rifting in eastern China and its peri Pacific zone.
The Upper Paleozoic of north Ordos Basin is an independent gas system with a complete source reservoir cap rock assemblage.Gas generation centre along Wushen Flag is characterized by large distribution area,big gas generation intensity and abundant gas resources.Sandbodies deposited in rvier channels,fan deltas and tidal flat are favourable for the formation of larg and mid scale traps;the source-reservoir-cap rock assemblage within the same stratum is favourable to form gasfield;large and mid scale sandbodies at the higher part of the gas generation centre are suitable for gas migration and accumulation.Trap types of the study area are mainly lithologic and structural-lithologic traps.It is suggested that Xinjie-Wushen Flag-Hongshiqiao in the east section of Yishaan Slope,the south part of Hangjing Flag fault terrace and Tianhuan Depression are the targets for gas exploration.
The characteristics of the reformed basins in South China determined the complexity and variety of the basins' hydrocarbon pool forming. According to the pool forming basis,the key and end result,it is suggested that:1.the localization of present hydrocarbon pools is determined by late stage reformation, the reservoir spaces controlled by redistributed hydrocarbon source but not proto type basins; 2.the principal pool forming models shoul be early stage accumulation late stage pool forming and late stage accumulation late stage pool forming two kinds; 3.the property of hydrocarbon high evolution grade and late pool forming has determined that gas exploration should be taken as the main target in South China; 4.the quality of late stage reformed and entire sealing system is the key of pool forming and preservation; 5.the exploration of natural gas in South China must insist on the main target horizons and give consideration to other possible gas bearing beds;must insist on the exploration thinking of "secondary hydrocarbon generation, multi bed reservoiring and entire sealing" due to its property of multiple reservoir beds and multiple reservoir types.
Palaeo oil reservoirs are distributed in a narrow belt, 100 km long in E W direction and 20 km wide in S N direction, in Bilongco Angdarco area of Qiangnan depression in the southern part of the central uplift zone of Qiangtang basin.They are exactly a group of oil seepages.The outcropped rocks are the Middle Jurassic Buqu Fm.,dark brown and brownish gray dolomite.They are deposited mainly as biohermal complex.Correlation shows that hydrocarbons in the palaeo oil reservoirs have been sourced mainly from Lower Jurassic.Modelling of hydrocarbon generation process indicates that there have been two hydrocarbon generation and expulsion processes,one in 140-150 Ma and another since 20 Ma. The discovery of the palaeo oil reservoir belt further verifies that Qiangtang basin has good petroleum prospect.
Two independent but interrelated Cenozoic structural systems—extensional structure and strike-slip structure developed in the Liaodongwan-Xialiaohe Basin.The extensional structure was the main structure styled in half graben-half horst assemblage that con-trolled the formation and evolution of the basin.The strike-slip structure was distributed in the east margin of the basin,reconstructed the extentianal structure and consisted of ligh-angle or nearly vertical basement strike-slip faults and verious associated structures including linar graben and horst,echelon folds,volcanic and mudstone diapir structures.The strike-slip structure with a dextral character may be related with activity of Tanlu deep fault since Oligocene.
According to basin's evolutional history and structural features,the sedimentary basins in China can be divided into monotype and superimposed type basins.The prototypes of the monotype basins can further be divided into craton basins,extensional basins and compressional basins.Superposition of two or three kinds of monotype basins can result in various kinds of superimposed basins.Research of hydrocarbon accumulation in superimposed basins should focus on analyzing the dynamic process of basin formation-hydrocarbon generation-hydrocarbon accumulation.In respect of basin formation,basin-and-range coupling during the key tectonic movements and formation and reformation of basin structures are the bases of research.In respect of hydrocarbon generation,study of sedimentary environment of Paleozoic marine carbonates and good source rocks,and(mecha-)nisms and efficiencies of hydrocarbon generation and expulsion should be strengthened.In respect of hydrocarbon accumulation,emphasis must be placed on study of hydrocarbon accumulation and dissipation mechanisms,as well as evolution of petroleum systems.In view of the multicycle nature of geotectonic evolution in China,wave analysis is one of the effective methods that can be used to study superimposed basins.Whereas superimposed basins have the characteristics of multiple sources,multi-stage hydrocarbon migration and accumulation,multi-stage reservoiring and adjustment,the ideology of "element-structure-function" system is applied to propose the concept of reservoiring system,and the "source-location" classification scheme is used to establish 6 structural types of reservoiring system.
The Neogene in Huizhou Depression were deposited in neritic shelf environment and is gentle in dip.However,a comprehensive study of seismic,drilling,log data indicates that three types of slope-breaks (i.e.deposition breaks,fault breaks and flexural breaks) occur in the area.On the plane,these breaks combine with one another in four patterns,including multistage,diagonal,comb-shaped,and ridge-parallel.Their evolution presents such evident features as inheritance,periodicity,scale grading,and migration cyclicity.Slope-breaks in the north margin and south of the basin control the superimposing pattern of the sequences and the development and spatial distribution of depositional systems.
Several technologies, including GC,GC-MS,GC-MS-MS,match experiment and ruthenium ion catalytic oxidation of asphaltenes,are used to study the sources of heavy oil in Lunnan and Tahe oilfields, Tarim basin. The heavy oil in Lunnan and Tahe oilfields contain 25-norhopane, while the distribution of normal paraffin hydrocarbons is complete and chromatographic base lines are uplifted in varying degrees,showing that the accumulations have been formed in two stages. The heavy oil are characterized by high content of tricyclic terpane, low content of C24 tetracyclic terpane, gammacerane, C24 sterane, triaromatic sterane, 4-methyl cholestane-24-ethyl cholestane, aromized sterane,and 24-norsterane. Oil and source rock correlation indicates that the oil in Lunnan and Tahe oilfields come from the source rocks in Middle and Upper Ordovician. Match experiments show that the oil would have also the characteristics of Cambrian source rock if it is mixed with 25% of TD2well's oil generated from Cambrian source rock,indicating that the hydrocarbons generated from Cambrian source rocks have not extensively migrated into Lunan and Tahe reservoirs. Ruthenium ion catalytic oxidation of asphaltenes further verifies that the source rocks in Middle and Upper Ordovician are the main source rocks of heavy oil in Lunnan and Tahe oilfields.
Paleo morphology resulted from tectonic movement controlled the sequence formation and development of continental basins,especially of rift basins.In the early spreading stage of a rift basin,incised channel fillings and fan deltas developed in front of uplifts;shallow lake bars or bioclastic banks developed on locally uplifted subaqueous platform.In the early middle spreading stage,base level rose,the uplifts turned to be the main provenances,three types of break in slopes developed on the basin margin,which resulted in three types of depositional systems.Nearshore subaqueous fan system resulted from faulted scarp type break in slope;slope fans system developed on faulted terrace type break in slope;fan deltas and sub lake fan system developed on steep slope type break in slope.In base level falling stage,the rift basin was gradually filled by fan deltas and fluvial delta systems.
The minimum trapping pressure ratio of hydrocarbon fluid inclusions can be defined by their isochore single value variations in P-T space projection and physical property of their isochore. Calcite veins, quartz secondary enlarged margins and fluid inclusions in particle cracks in sandstones of Yancheng Formation of YC21-1 structure in Qiongdongnan(southeastern Hainan) basin of the South China Sea indicate in geologic history that four phases of fluid inclusions were developed in that region; the capping bed was once burst during the third mobile phase of thermo-fluid inclusions which would be healed soon after. Therefore, no many free-state natural gas reservoirs could be formed there.
Nanpu Sag is a Meso-Cenozoic petroliferous sag.The sag underwent fault-subsided and depressed stages and possesses episodic sedimentary property.The structural belts within the sag are distributed annularly and could be divided into peripheral uplift-buried-hill structural belt,marginal strutural belt and the central structural belt of the sag.These structural belts are separated into south zone and north zone bounded by Gaoliu Fault.The formation and distribution of traps in the sag were controlled by structural evolution. Traps of anticline and buried-hill types were formed in fault-subsided stage;traps of fault block and fault-antieline types were formed in depressional stage.
Quasi-foreland basins is a kind of complex reformed basins in Western and Central China which are actually superimposition of prototype basins formed in different stage.The basins were effected by different tectonic settings and controlled by stress fields during basin evolution,thus forming different sedimentary systems and sequences.Hence,tectonism and sedimentation were main factors controlling multiphase hydrocarbon generation and multiphase pool-forming of the basins.The development processes of the basins consiste of two stages of hydrocarbon generation and pool-forming conditons,and the sedimentary sequences themselves also possessed source-reservoir-cap rock complex conditions.To explor oil in this kind of basins,one should pay attention to hydrocarbon generation conditions,study the conditions of pool-forming and late preservation of the pools.
The Middle and Lower Ordovician fractured-vuggy reservoirs in the Tahe oilfield experienced multi-stage karstification caused by multi-stage tectonic events from the Middle Caledonian to the Early Hercynian.Based on integration of geological,geophysical and geochemical data,we discuss the development characteristics and main controlling factors of multi-stage karstification of the Ordovician carbonates in the Tahe oilfield.Four main periods of karstification are identified as follows:the 1st Phase,2nd Phase and 3rd Phase of the Middle Caledonian and the Early Hercynian.The origin,evolution and transformation history of the karst reservoirs are also discussed in this paper.According to this study,the major karstification periods are the Early Hercynian,the 1st Phase and the 2nd Phase of the Middle Caledonian,of which the Early Hercynian is the most important period.The main controlling factors of kastification are paleostructures,karst topography and geological conditions of paleo-drainage-system.
This paper introduces the world's shale gas resource potential and the trend of its exploration and exploitation.The author also discusses China's shale gas resource potential and current status of exploration and exploitation.Based on this information,this paper puts forward some strategic concepts of accelerating the survey,exploration and exploitation of shale gas resources in China: 1) to conduct a nationwide survey on shale gas geology and potential,to recognize favorable plays and E&P areas,and to establish pilot areas for shale gas exploration and exploitation;2) to do research on the accumulation and pooling patterns of shale gas,to advance innovation of shale gas exploration and exploitation techniques and to set up a technical system which could support shale gas exploration and exploitation;3) to build talent teams for shale gas survey,exploration and exploitation and to develop a geological data sharing system and a social service system;4) to draw up a plan for shale gas survey,exploration and exploitation,to establish an incentive system for shale gas exploration and production,to perfect the mining rights management system,to reinforce international cooperation and exchange and to establish standards and specifications of shale gas technologies.
Reformed basins that are widely and numerously found in China are the main replacing areas for China's oil and gas exploration.Oil and gas generation,accumulation,migration,pool-forming and distribution in the reformed basins were controlled by both the tectonic framework of pre-basin stage and the tectonic movements and structural forms of post-basin stage.So oil and gas pool-forming processes were complicated and oil-gas distribution regularity was varied.The process or thinking for the researches and oil-gas assessment of the reformed basins includes:1.collating the attribution of the basin and analysing its dynamic circumstances;2.reverting to the original basin;3.revealing the reformation process of the basin;4.determining the reformation degree and classifying the basin's retention units;5.approaching the characteristics of oil and gas accumulation and dispersion,assessing resources amount;6.taking multi-source hydrocarbon into account;7.paying more attention to late-stage pool-forming;8.paying attention to differential reformation.
In acidic aqueous media, feldspar minerals can be dissolved to some extents, leading to the formation of secondary pores. Secondary porosity can be quantitatively estimated based on the composition and physicochemical properties of the original and secondary minerals. Secondary porosity formed by dissolution of K-feldspar can be as high as 11.91%, while that of calcium feldspar is the lowest, being only 0.27%, and that of sodium feldspar and andesine range between the former two. Some of the CO2 needed for dissolving feldspar might have been sourced from meteoric water and/or dissolved from the air when water got in touch with atmospheric air, while the others might have been the products of decomposition of organic matter during generation of hydrocarbons.
Formation and development of the coastal basins in West Africa had been related with the opening and later continuous spreading of Atlantic Ocean since Mesozoic.They were continental rift and passive continental margin basins formed as a result of the break-up of Gondwana and the spreading of Atlantic Ocean.Their deve-lopments had been influenced by plate tectonic evolution and had experienced late Paleozoic-Early Mesozoic continental craton stage(pre-rift stage),rift stage and transition stage since Middle-Late Mesozoic,and passive continental margin stage by the end of Mesozoic-Tertiary.Formation of the basins was characterized by early in the south and late in the north,due to the influence of the opening of South Atlantic being early in the south and late in the north.Basins in the central part of the rifts were well developed,followed by basins in the southern part,while those in the northern part were relatively poor.A thick halite was widely developed in this area at the end of rifting.Early development of passive continental margin basins had inherited the sedimentary features in the rifting stage,but their features in late stage(post-Eocene) were contrary to that in the rifting stage,very thick Nigeria deltaic sediments had been deposited in the northern part.The formation of oil and gas reservoirs was obviously later in the northern part than that in the southern part.Petroleum systems in basins to the south of Gabon basin are mainly of those below the halite,while those in Nigeria basin are mainly of Tertiary petroleum systems developed above the halite.The formation of oil and gas reservoirs is closely related to structures formed by halite activities.The oil-and gas-bearing layers get younger toward the ocean.
The giant Daniudi gas field,located in the north of Ordos basin,is of a tight and low-permeability type.Its main gas-bearing horizons from bottom to top are the Taiyuan Formation of the Upper Carboniferous and Shanxi Formation and lower Shihezi Formation of the Lower Permian,forming a complete set of sedimentary sequences of marine tidal flat-paralic-delta-terrestrial braided channel.The sequences,with a thickness of 250 m to 280 m and a stable lateral distribution,are in a shape of a huge box,to which the commercial gas pools are confined due to a dual tight sealing of favorable lithophysics and anomalous pressure of the regional cap in the upper Shihezi Formation.Layered lithologic traps overlap with one another and hydrocarbon reservoiring in them is characterized by near-source and box-like traps.To solve the problems caused by lithologic trap,tight and low-permeability sand reservoirs,and low natural production of the field,core technologies,such as description and evaluation of stacked lithologic traps and seismic prediction of coal bed reservoirs,have been developed during the exploration and development of the Daniudi gas field.And corresponding technologies of quick drilling aiming at protecting gas reservoirs and large-scale fracturing intending to promote gas production of tight reservoirs have also been advanced.
The multicycle tectonic evolution of Tarim craton controls the formation of superimposed the Tarim Basin,petroleum-geology conditions as well as the features of composite petroleum system,thus determines hydrocarbon distribution and resources potential in the basin.Dominated by the evolution of Paleo-Asian tectonic domain and Tethyan tectonic domain,the basin experienced three extensional-compressional cycles,which can be divided into six phases,with various prototype basins.Affected by 6 large-scale tectonic movements,including Caledonian,early Hercynian,latest Hercynian,Indosinian,Yanshan,and Himalayan,these prototype basins were reshaped by uplifting,denudation,folding,and faulting.The Tarim Basin have the following geologic characteristics:many sets of source rocks,several hydrocarbon kitchens,various types of reservoirs,multiply of reservoir-cap rock combinations,and multistage hydrocarbon generation and accumulation,as well as multiphase adjustment,reforming,and destruction.According to the distribution of the hydrocarbon kitchens and their relation with re-servoirs containing hydrocarbon generated by them,10 composite petroleum systems can be recognized in the basin.The Palaeozoic petroleum system has the following three critical periods:Late Caledonian-early Hercynian,late Hercynian and Himalayan.The hydrocarbon distribution of the petroleum system in each critical period is controlled by the relationships between the hydrocarbon kitchens and paleohighs,and finalized by the superposition of multi-stage petroleum systems.
It is previously believed that Kaijiang-Liangping area was an ocean trough during Changxing age of Late Permian-Early Indo-Chinese epoch in Early Triassic,and was called "Kaijiang-Liangping ocean trough".The understanding and definition of the sedimentary framework in this area are directly related with exploration deployment in the northeastern part of Sichuan basin.Based on sedimentological study and the new understan-(dings) obtained from drilling and seismic data,it is demonstrated,in respects of tectonic setting and the concept and sedimentation of ocean trough,that Kaijiang-Liangping area was not an ocean trough during Changxing age of Late Permian-early Early Triassic,but a platform-shelf with relatively deep water within a carbonate platform,and the sediment was dominated by micrite with some high energy shoal deposits.
The northwestern edge of Junggar Basin experienced multiphase tectonic movements of different natures, and it appeared as a foreland basin since Cenozoic. Extensive collision, compression and thrusting occurred along with regional tectonic movement, and faulting apparently had the nature of overthrusting, resulting in fold structures to be in parallel with the fracture zone. The overthrust faults were commonly syngenetic and controlled obviously the development of fans. Thrusting occurred continuously, so topographies on the hanging walls were greatly different from those on the footwalls. Clastics denuded from the thrust sheets accumulated on the foot of fault scarps, leading to the formation of various fans (alluvial fan, fan delta and submarine fan, etc.). Step thrust faults led to the formation of multistage fans. Tectonic activities attenuated gradually from Permian to Jurassic and the sizes of fans decreased gradually. Lateral displacement of tectonic activities resulted in the migration of fans. As a whole, the strongest and longest tectonic activities occurred in Baikouquan-Xiazijie area, hence the fans in that area would be relatively large, highly superimposed and largely migrated; while those in Cheguai-Karamay area were relatively weak, so the fans would be small, poorly superimposed and shortly migrated.
Sealing ability of cap rock in large and medium gas fields mainly depends on the thickness of cap rock,displacement pressure,interior energy of gas reservoir(pressure coefficient)and gas property(flowing viscosity).Cap rock sealing index(CSI)of gas reservoir is directly proportional to gas accumulation efficiency.The accumulation efficiency of China's large and medium gas fields can be classified into three categories:(1)High efficiency gas reservoirs have accumulation efficiency of over 100?106 m3/(km2·Ma).They are mainly distributed in Tarim,Qaidam,Yinggehai-Qiongzhou,Bohai Bay and Songliao basins,of which Tarim,Qaidam and Yinggehai-Qiongzhou basins are in the majority.(2)Moderate efficiency gas reservoirs have accumulation efficiency in the range of 10?106-100?106 m3/(km2·Ma).They are mainly distributed in Sichuan,East China Sea,Songliao,Bohai Bay,Tarim and Turpan-Hami basins,and most of them are in Sichuan basin.(3)Low efficiency gas reservoirs have accumulation efficiency of lower than 10?106 m3/(km2·Ma).They are mainly distributed in Ordos and Sichuan basins.For the formation of large and medium gas fields in China,the CSI value of cap rocks should be over 109m/s.For gas fields with relatively higher accumulation efficiency,however,the CSI value of cap rocks should be raised even higher.
Through the study of eight well cores from the Upper Permian reefs of Changxing stage in Eastern Sichuan,rocks from Tieshan 5 Well and Tiandong 10 well represent typical sponge reefs;rocks from Bandong 4 Well and Shuanglongchang 15 Well have not only the deposits of reef facies,but also crinoidal or echinoderm banks;rocks from Wolonghe 117 Well,Huanglongchang 1 Well and Yun'anchang 14 Well are mainly composed of carbonate platform deposits;rocks from Dachigan 24 Well consist of two types of reef facies:sponge framework reefs and sponge baffling reefs.Dolomites in the Permian reefs were formed of saline water mixed with fresh water during subaerial exposure of reefs above the sea level,then suffered late dolomitization during deeper burial stage,so they have evident geochemical characteristics of late dolomitization.The original deposits of these dolomites are of framework;not only they could be transformed by reef rocks,but also by those deposits of echinoderm banks,even common carbonate platform depostis.So in finding dolomite reservoirs of reef types,one should pay attention to find dolomite reservoirs formed of bank deposits as well as carbonate platform deposits.
In view of status of petroleum exploration in marine carbonates in southern China,a dynamic and practical classification scheme is presented for source-reservoir-cap rock combinations based on sequence stratigraphy study. It divides, for the first time, the source-reservoir-cap rock combinations in this region into 5 types,including boundary type, low water type, transgression type, high water type and composite type.Various types of combinations have different sedimentary and geological features and different petroleum geological significances. The boundary type is characterized by poor effectiveness in timing; the low water type is characterized by its wide distribution,small reservoir bodies and optimum migration conditions; the transgression type has strong sealing capability with small reservoir bodies; the high water type is rich in petroleum and is characterized by its largest reservoir bodies,widest distribution and regional and even globel comparability; while the composite type is characterized by its large, wide and stable distribution and hydrocarbon accumulations of multiple sources,multiple stages and multiple geneses.
The Northeast China is not a Hercynian folded metamorphic belt(or orogenic belt),but a compound terrain pieced together after the Early Carboniferous.We name it "Jiamusi-Xingmeng Terrain".The Upper Paleozoic is not the crystalline basement for Meso-Cenozoic basins in Northeast China,but marine and continental sedimentary cover of para-caprock nature.The Late-Paleozoic marine sequences have well-developed dark mudstones and carbonates,which is the evidence for hydrocarbon potential.These sequences are important new targets for exploring oil and gas in Northeast China.We propose that the Meso-Cenozoic basins in Northeast China could be divided into three basin groups-the west,the middle and the east separated by the Jiayin-Mudanjiang fault and the Nenjiang-Kailu fault.The genetic type,characteristics of petroleum geology and the temporal and spatial evolution pattern of each basin are also discussed in this paper.
That the grain sizes are between 0.2～50 μm have fractal property.Acoording to the principle of the fractal geometry,the expressions of pore size distribution curve,capillary pressure curve,J function curve,relative permeability curve and equilibrium contact angle of the reservoir rocks could be derived.Leverett.et.al show that the reservoir rocks with the similar properties have unique J fanction curve.The expression derived for J function curve indicates that the reservoir rocks of this kind have the similar fractal dimension and pore tortuosity.The expression of the capillary pressure and J function curve could be used to calculate the fractal dimension of the reservoir rock pore structure and its change after the water sensitivity or acidizing.This method is simpler than scanning electron microscopy.
