unconventional shale systems a comparative study of the in source sweet spot developed in the lacustrine chang 7 shale and the marine barnett shale abstract the upper triassic chang 7 shale and the mississippian barnett shale two remarkable shale systems that are considered promising unconventional shale oil gas targets in china and the usa respectively have different kerogen types that formed under lacustrine and marine environments in this study seventy six and ninety seven drilled core samples were collected separately from the zhang 22 well ordos basin and the mesquite 1 well fort worth basin respectively for determining the hydrocarbon generation retention and migration behaviors as well as the intrasource fractionation in the two shale successions geochemical indications of potentially producible oil as indicated by the oil crossover effect and the tmax shift phenomenon as well as the geochemical and molecular compositions of retained hydrocarbon suggest two categories of intervals in the chang 7 and barnett shale systems namely the in source sweet spot and the generative unit oils retained in the in source sweet spot have migrated from the adjacent generative unit and have been subjected to stratigraphic fractionation over geological time these oils are proportionally rich in aliphatic fraction and moderate chain length moieties which significantly improve the fluid mobility and producibility the oil quality improved by fractionation the weak adsorptive affinity of oil to organic matter and the good shale frackability all provide promise for the exploration and production potential of shale oil from the chang 7 and the barnett shale systems

a unique lacustrine mixed dolomitic clastic sequence for tight oil reservoir within the middle permian lucaogou formation of the junggar basin nw china reservoir characteristics and origin abstract the middle permian lucaogou formation in the jimusaer sag of the southeastern junggar basin nw china was the site of a recent discovery of a giant tight oil reservoir this reservoir is unusual as it is hosted by lacustrine mixed dolomitic clastic rocks significantly differing from other tight reservoirs that are generally hosted by marine lacustrine siliciclastic calcitic sequences here we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field petrological geophysical including seismic and logging and geochemical data field and well core observations indicate that the lucaogou formation is a sequence of mixed carbonate mainly dolomites and terrigenous clastic mainly feldspars sediments that were deposited in a highly saline environment the formation is divided into upper and lower cycles based on lithological variations between coarse and fine grained rocks in particular dolomites and siltstones are interbedded with organic rich mudstones in the lower part of each cycle whereas the upper part of each cycle contains few dolomites and siltstones tight oil accumulations are generally present in the lower part of each cycle and dolomites and dolomite bearing rocks are the main reservoir rocks in these cycles including sandy dolomite dolarenite dolomicrite and a few dolomitic siltstones optical microscope back scattered electron and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon related fluids

permian sedimentary tuff tight reservoirs in the santanghu basin nw china abstract sedimentary tuffs comprise important tight oil reservoirs in the middle permian tiaohu formation in the santanghu basin this paper describes three research advances made on these reservoirs 1 there are many sets of sedimentary tuff in the second member of the tiaohu formation and they are the sweet section for tight oil exploration vitric crystal vitric and argillaceous tuffs are the main reservoir types of which the first two are more favorable 2 a model was established to show how the volcanic edifice controls the type and distribution of the volcanic lacustrine tuff deposition the crystal vitric tuff is distributed in the lacustrine deposits nearest to the volcanic crater and the vitric tuff is distributed on both sides of the active volcanic belt a certain distance from the crater and this is the most favorable distribution area for sedimentary tuff later devitrification resulted in formation of a large number of micro pores and this is the main reason for the presence of favorable reservoirs 3 the sedimentary tuff reservoir has undergone successive geological processes from formation of secondary dissolution pores then hydrocarbon charge and accumulation to compaction the tight oil accumulation is the result of the coupling of the diagenetic evolution of the tuff in the tiaohu formation and the hydrocarbon generation and charging from the shale in the lucaogou formation

oil and gas developments in western canada in 1989 there were 5513 wells drilled in western canada during 1989 30 fewer than in 1988 total meterage drilled was 6 459 797 m for an average well depth of 1172 m exploratory drilling in 1989 dropped by 13 and development drilling by 40 from 1988 levels british columbia was the only area in western canada with increased activity the industry continued to acquire more acreage for less total bonus at provincial crown sales throughout western canada during 1989 3 564 827 ha were acquired for a total bonus of 484 872 000 footnote 5 although the total disposition of crown lands decreased only 4 the average bonus dropped to 136 ha down 26 from 1988 in alberta the most active exploration areas were the southern and eastern mesozoic gas play the western deep devonian gas play the oil plays on and adjacent to the peace river arch and the middle devonian carbonate oil plays in the northwest in british columbia the most active exploration areas were the mesozoic and paleozoic gas plays in the northeastern portion of the province for the first time in 5 years crown drilling rights were issued within the fraser valley in the southwest there was a focus on gas exploration in the province as pipeline reversal was implemented permitting access to canadian and california markets through the alberta transportation network in manitoba exploration and development activity continued to decline in northern canada exploratory drilling success offering of crown lands for exploration approval of gas exports and indications that many of the native lands claims may be approaching settlement all provided exploration encouragement for the next decade exploration activities occurred throughout the northwest territories from the offshore beaufort to the southern border in saskatchewan active exploration areas were shallow mesozoic gas plays in the western portions of the province and deep paleozoic oil plays in the south

factors controlling prolific gas production from low permeability sandstone reservoirs implications for resource assessment prospect development and risk analysis low permeability reservoirs from the greater green river basin of southwest wyoming are not part of a continuous type gas accumulation or a basin center gas system in which productivity is dependent on the development of enigmatic sweet spots instead gas fields in this basin occur in low permeability poor quality reservoir rocks in conventional traps we examined all significant gas fields in the greater green river basin and conclude that they all occur in conventional structural stratigraphic or combination traps we illustrate this by examining several large gas fields in the greater green river basin and suggest that observations derived from the greater green river basin provide insight to low permeability gas charged sandstones in other basins we present evidence that the basin is neither regionally gas saturated nor is it near irreducible water saturation water production is both common and widespread low permeability reservoirs have unique petrophysical properties and failure to fully understand these attributes has led to a misunderstanding of fluid distributions in the subsurface an understanding of multiphase effective permeability to gas as a function of both varying water saturation and overburden stress is required to fully appreciate the controls on gas field distribution as well as the controls on individual well and reservoir performance low permeability gas systems such as those found in the greater green river basin do not require a paradigm shift in terms of hydrocarbon systems as some have advocated we conclude that low permeability gas systems similar to those found in the greater green river basin should be evaluated in a manner similar to and consistent with conventional hydrocarbon systems to date resource assessments in the greater green river basin have assumed a widespread continuous type resource distribution failure to recognize some of the fundamental elements of low permeability reservoirs has led to an underappreciation of the risks associated with exploration and development investment decisions in these settings and likely a significant overestimation of available resource levels

organic matter rich shales of china abstract organic matter rich shales are the main target rocks for unconventional oil and gas exploration and development across the world in china shale gas geological resources are estimated as approximately 110 1012 m3 with recoverable gas reserves of ca 20 1012 m3 recoverable shale oil reserves are estimated as ca 5 109 t a total 35 important organic matter rich shale units have been recognized from mesoproterozoic to cenozoic strata across the entire china these shales are categorized according to their origin under marine marine nonmarine transitional and lacustrine conditions shales of marine origin with ca 9 1012 m3 recoverable resources dominate china s potential in terms of total volume of organic carbon currently the most favorable marine shales for oil and gas exploration are found in the sichuan basin within the lower cambrian qiongzhusi formation and in the wufeng longmaxi formations of uppermost ordovician through lower silurian a fortuitous combination of of sea level variations of paleo productivity of tectonic activity causing development and migration of partially closed deep basin depocenters and of sediment accumulation rates controlled the extensive deposition and distribution of organic matter rich shales in these wufeng and longmaxi formations organic matter rich shales in marine nonmarine transitional facies associated with coal measures occur in north china within the carboniferous and permian and in south china within the permian these carboniferous permian organic matter rich shales are important source rocks for the gas fields in the ordos and sichuan basins abundant organic rich shales are also widely distributed within coal bearing clastics and coal measure shales of fluvial lacustrine and swamp facies in upper triassic to middle jurassic successions of many basins lacustrine organic rich shales were deposited during the permian through neogene in various freshwater to saline lake settings lacustrine organic matter rich shales are the main oil source rocks in the songliao bohai bay ordos and junggar basins lacustrine algae contributed to the rain of organic matter and the preservation of organic matter and distribution of organic rich shale was controlled by lake currents water depth and oxygen poor conditions with enhanced preservation when buried by turbidity currents algal blooms were partly induced by trace nutrients from volcanic ash falls in all of these lacustrine basins seawater intrusion into the freshwater lake of the songliao basin promoted some episodes of black shales saline lacustrine basins such as middle permian junggar basin contain organic rich dolomite mudstone that mainly formed during hot climate conditions when the lakes had high salinity and stratified water columns that deprived the bottom waters of oxygen thereby preserving massive amounts of organic matter laminated calcite rich mudstone in the saline lacustrine settings formed in more brackish waters under stable warm conditions and weak biological activity the modeling of the factors controlling the distribution of organic matter rich shales within china s basins is important for the exploration and development of unconventional oil and gas resources

introduction to unconventional petroleum systems ben law is a consultant and sole proprietor of pangea hydrocarbon exploration llc his research interests include basin centered gas and coalbed methane systems prior to his consulting position he was a member and chief of the u s geological survey western tight gas sand project and regional coordinator of south asia for the u s geological survey world energy project he received b s and m s degrees from san diego state university california john b curtis is associate professor and director petroleum exploration and production center potential gas agency at the colorado school of mines he is an associate editor for the aapg bulletin and the mountain geologist as director of the potential gas agency he works with a team of 145 geologists geophysicists and petroleum engineers in their biennial assessment of remaining united states natural gas resources the collection of articles included in this theme issue of the aapg bulletin originated in the aapg

a new possible giant hydrocarbon generated formation the upper triassic source rock in southwestern junggar basin nw china abstract the triassic formation is a possible new giant hydrocarbon generated formation in northwest china and mid asia taking the upper triassic formation in the sikeshu sag in junggar basin as an example based on the comprehensive analysis on the geochemical characteristics of the cores and the dark mudstone of the outcrops and reservoir formation conditions we have evaluated the upper triassic source rocks by comparing with those in the ulungu depression and reached the following findings firstly the upper triassic formation is mainly composed of dark mudstone and sandy mudstone deposits and the hydrocarbon source rock is mainly distributed in the middle and upper parts with a thickness range of 100 150 m and area of 3500 km 2 secondly the source rock moderate in organic matter abundance with toc range of 1 3 has the material basis for hydrocarbon generation thirdly the organic matter has high percentage of sapropelinite and is dominated by type ii 2 fourthly the degree of the thermal evolution is moderate and the source rock with r o higher than 0 7 has a distribution area of about 1800 km 2 providing the conditions of massive hydrocarbon generation fifthly the source rock has great burial depth and wide distribution the source rock with a r o of higher than 0 7 and thickness of more than 100 m has an area of around 1400 km 2 implying huge resource potential sixthly the next step exploration should focus on highly mature hydrocarbon generation central area in the upper triassic lower jurassic in the east of the sag to search for and confirm favorable traps the research findings have important reference value for promoting the resource status of deepening the understanding of reservoir formation and clarifying the exploration direction in the sikeshu sag and other periphery mid asia areas

formation distribution and resource potential of the sweet areas sections of continental shale oil in china abstract shale oil refers to oil stored in organic rich shale it is an important part of unconventional oil and provides a large amount of resources based on the systematic study of the geological characteristics of shale oil in china s major continental basins the geological connotation of the sweet area section in shale oil is proposed in this paper and it refers to areas sections that can be preferentially explored and developed in oil bearing shale formations under current economic and technical conditions the formation conditions and distribution characteristics of continental shale oil in china are analyzed with a focus on the geological properties of the triassic yanchang formation in the ordos basin the cretaceous qingshankou formation in the songliao basin and the permian lucaogou formation of the jimusar sag in the junggar basin it is found that the sweet areas of shale oil with medium high maturation are mainly located in thick shales with a ro greater than 0 9 and the sweet sections are mainly located in the middle and lower parts of the shale formation the geological resources of continental shale oil in the major continental basins in china are estimated to be approximately 3700 108 tons the in situ conversion process icp may be a key technology for the effective development of medium to low mature shale oil the scale maximum burial depth and present depth of the organic rich shale are proposed as the three key parameters for the optimization of pilot sites by taking the triassic yanchang formation in the ordos basin as an example using icp it is expected that the recoverable resources of china s continental shale oil will be approximately 400 500 108 tons the terminology of man made reservoir is proposed in this paper in an man made reservoir the overall industrial output of the sweet areas sections is formed by artificial intervention and the construction of an underground fracture network system with man made permeability continental shale oil may be another revolutionary source of unconventional oil and is expected to be first successfully produced in china

continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum the ordos basin north central china abstract global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps the characteristic features include the following a hydrocarbon source and reservoir coexist porosity and permeability are ultra low nano pore throats are widely distributed hydrocarbon bearing reservoir bodies are continuously distributed there is no obvious trap boundary buoyancy and hydrodynamics have only a minor effect and darcy s law does not apply phase separation is poor there is no uniform oil gas water interface or pressure system and oil or gas saturation varies examples of unconventional hydrocarbon accumulations are the mesozoic tight sandstone oil province and the upper paleozoic tight sandstone gas province in the ordos basin north central china generally continuous hydrocarbon accumulation over a large area is a distinguishing characteristic of unconventional hydrocarbon sources because of the great potential of unconventional petroleum resources it is believed that research on such resources will be at the forefront of the future development of petroleum geology

analysis of the charging process of the lacustrine tight oil reservoir in the triassic chang 6 member in the southwest ordos basin china with the success of bakken tight oil tight sandstone oil and shale oil and eagle ford tight oil in north america tight oil has become a research focus in petroleum geology in china tight oil reservoirs are predominantly distributed in lacustrine basins the triassic chang 6 member is the main production layer of tight oil in the ordos basin in which the episodes timing and drive of tight oil charging have been analyzed through the petrography fluorescence micro spectrometry micro thermometry and trapping pressure simulations of fluid inclusions in the reservoir beds several conclusions have been reached in this paper first aqueous inclusions with five peaks of homogenization temperatures and oil inclusions with three peaks of homogenization temperatures occurred in the chang 6 reservoir beds the oil inclusions are mostly distributed in fractures that cut across and occur within the quartz grains in the quartz overgrowth and calcite cements and the fractures that occur within the feldspar

origin of calcite cements and their impact on reservoir heterogeneity in the triassic yanchang formation ordos basin china a combined petrological and geochemical study abstract calcite cement is a typical product of fluid rock interaction in petroleum reservoirs and its migration has a major impact on reservoir heterogeneity a case study of a closed diagenetic system was carried out on the upper triassic yanchang formation of ordos basin china the most successful exploration region for tight sandstone oil in china the investigated sandstones are mainly lithic arkoses q31f42l27 three occurrences of calcite cement are the most important diagenetic minerals affecting reservoir quality early calcite completely filling intergranular volumes late calcite filling pores after chlorite coating distributed in oil layers or locally replacing feldspar and crystal stock calcite typically distributed in dry layers 13c values of the sandstones are in the range 9 69 to 2 05 indicating that calcite is derived mainly from thermodynamic decarboxylation of organic matter dark mudstone interbedded with sandstone and hydrocarbon emplacement are the two crucial sources of organic fluid providing sufficient ca2 mn2 fe2 and mg2 for calcite precipitation through interaction with feldspar and other silicates such as dissolution and replacement together with transformation of smectite to illite manganese was continuously enriched in calcite during burial diagenesis driven by compaction of interbedded mudstone and fluid overpressure calcite migrated widely into calcite cemented zones cczs where fluid rock interaction was strong such as in the margins of sandstone bodies intrastratal sandstones with original high permeability zones and thin sandstones thereby forming an important organic co2 sink potential high quality reservoirs are distributed mainly in the central part of the closed system restricted by calcite cemented zones and are an exploration target our data suggest that frequently alternating sandstone and mudstone sequences comprise a complete organic co2 source sink system controlling the migration of calcite cement in a closed diagenetic system and reservoir heterogeneity by fluid rock interaction

geochemical comparison between gas in fluid inclusions and gas produced from the upper triassic xujiahe formation sichuan basin sw china abstract natural gas in the xujiahe formation of the sichuan basin is dominated by hydrocarbon hc gas with 78 79 methane and 2 19 c2 hc its dryness coefficient c1 c1 5 is mostly

environmental response to volcanic activity and its effect on organic matter enrichment in the permian lucaogou formation of the malang sag santanghu basin northwest china abstract lacustrine organic rich fine grained rocks are widespread in the second member of the permian lucaogou formation p2l2 in the malang sag santanghu basin with complex lithology and significant changes of lithofacies due to frequent volcanic activity in this study the mineralogical and petrological characteristics types of organic matter om and effect of volcanism on the sedimentary environment and om accumulation were analyzed using organic carbon and oxygen isotopes as well as major and trace element data the results revealed that the p2l2 was characterized by mixed deposition of volcanic ash and carbonate minerals with four main rock types tuff dolomitic tuff tuffaceous dolomite and dolomite the fine grained rocks dominantly contain type i and ii1 kerogens which were in a low mature stage and exhibited excellent original hydrocarbon generative potential when volcanic activity was strong large amounts of nutrients were released by the volcanic ash which then entered the lake in turn promoting the blooming of phytoplankton and causing the 13c values in the tuff to decrease in addition the paleotemperature decreased for a short time and the suboxic euxinic environment was conducive to the preservation of om resulting in high total organic carbon toc content in the tuff in contrast during intermittent or weak periods of volcanic activity the climate was hotter and drier and evaporation resulted in a sustained increase of salinity consequently the growth of algae was restricted which brought about an increase in the 13c values and relatively low toc content in the dolomite the relationships among primary productivity redox conditions clastic influx proxies and toc content illustrate that primary productivity was the most vital control on the accumulation of om high primary productivity was mainly driven by volcanic ash falls meanwhile high salinity and suboxic euxinic conditions were beneficial to the preservation of om giving rise to a high degree of om enrichment

geochemical characteristics of natural gases related to late paleozoic coal measures in china abstract by 2013 more than 20 gas fields related to late paleozoic coal measures have been discovered in china primarily distributed in the bohai bay ordos sichuan and junggar basins the total proven reserve in these gas fields is more than 3200 109 m3 accounting for 30 1 of the proven gas reserves in china after analyzing the molecular compositions stable carbon isotopes and helium isotopes of 375 gas samples genetic types and origins of the natural gas as well as the secondary alterations it experienced are discussed in this study most of the natural gas related to the late paleozoic coal measures are coal type gas with some of the gases from the sichuan basin and the jingbian gas field in the ordos basin being oil type gases generated from marine mudstones in the permian longtan formation and marine carbonates in the ordovician majiagou formation respectively the equivalent vitrinite reflectance values based on stable carbon isotopes of natural gases vary from 0 8 to 2 5 indicating the mature to over mature stage positive carbon isotope series 13c ch4 13c c2h6 and 13c ch4 13c c3h8 the form of 13c ch4 13c c2h6 is primarily resulted from the mixing of oil type gases of different maturities while the form of 13c ch4 13c c3h8 is primarily resulted from the mixing of coal type gases of different maturities high concentrations of hydrogen sulfide in natural gas from the sichuan basin and the chenghai gas field in the bohai bay basin are resulted from thermochemical sulfate reduction tsr which causes to some extent the fractionation of carbon isotopes by contrast the fractionation of methane carbon isotopes resulting from the tsr effect is not clear carbon dioxide from the bohai bay and junggar basins is primarily biogenic generated via the thermal decomposition of organic matter while that from the ordos basin sichuan basin and chenghai gas field is generated via the thermal decomposition of carbonates carbon dioxide related to the tsr effect has also contributed to the sichuan basin and chenghai gas field

geochemical characteristics of the triassic chang 7 lacustrine source rocks ordos basin china implications for paleoenvironment petroleum potential and tight oil occurrence abstract the fact that high quality lacustrine source rocks generally shale and mudstone control the formation and distribution of conventional and unconventional oil and gas reservoirs in lacustrine basins has been well accepted in petroleum exploration and exploitation taking the chang 7 lacustrine source rocks in the ordos basin as an example several points having been reached are as follows 1 the chang 7 source rocks possess an excellent source rock potential and the shales hold a better potential than the mudstones 2 the paleoenvironments of the chang 7 source rocks were sub reducing to sub oxidizing conditions and fresh and brackish water depositional environments with a maximum water depth of 150 m the organic matter origins of the source rocks are mainly plankton algae bacteria and other aquatic microorganisms 3 the beginning time of the oil generation of the chang 7 source rocks is at 165 ma and the peak oil generation occurred during 115 95 ma the cumulative amounts of oil generation are up to 4711 103 t km2 and the ratios of peak generation amounts to cumulative generation amounts are 50 the beginning timing and peak generation timing of the shales are earlier than those of the mudstones respectively and the cumulative oil generation amount of the shales is higher than that of the mudstones indicating a better oil generation potential of the shales 4 due to the more remained oil possessed in the shales the hydrocarbon expulsion threshold of the chang 7 shales 2560 m is deeper than that of the chang 7 mudstones 2080 m 5 the occurrences of the chang 8 6 tight oils are predominantly controlled by the outer boundary of the chang 7 source rocks distribution while the transition areas between thickness toc and ro high value centers are the accumulation and enrichment zones the chang 7 shales controlled the occurrence of the chang 8 tight oil reservoirs and the chang 7 mudstones controlled the occurrence of the chang 7 and chang 6 tight oil reservoirs

major and trace element geochemistry of the lacustrine organic rich shales from the upper triassic chang 7 member in the southwestern ordos basin china implications for paleoenvironment and organic matter accumulation abstract lacustrine organic rich shales are widespread in the upper triassic yanchang formation chang 7 member of the ordos basin however information on the short term climate evolution within the chang 7 member is still lacking and the mechanism of organic accumulation in the shales remains controversial to address these issues this study analyzes the palaeoenvironment and depositional setting of different intervals in the chang 7 member of the southwestern ordos basin and constrains the main control factors on accumulation and preservation of organic matter based on organic major and trace element data the lower chang 73 shale which contains a high abundance of thermally mature type i and ii kerogen has excellent original hydrocarbon generative potential the lower chang 73 interval has the most humid climatic condition the strongest weathering condition a euxinic bottom water condition a high paleo productivity and a low terrigenous input during the period of upper chang 73 and chang 72 the climate became somewhat drier and hotter which was marked by relatively weak weathering conditions suboxic to anoxic conditions low paleoproductivity and high terrigenous input the chang 71 was characterized by a moderately humid climate moderate weathering conditions paleoproductivity and terrigenous input the relationship among the primary productivity redox conditions clastic influx indicators and toc content suggests that primary productivity was the most critical factor controlling the enrichment of organic matter the high primary productivity during the early chang 73 interval was probably driven by the deposition of volcanic ash and was influenced by marine incursion events a warm and humid paleoclimate combined with input of volcanic ash and seawater resulted in eutrophic nutrient rich and stratified surface water during the early chang 73 period which led to algal blooms with high primary productivity a stratified water column with high primary productivity together with oxygen deficit bottom water conditions and low terrigenous input resulted in the greatest enrichment of organic matter in the lower part of chang 73 shale

geological control factors of micro oil distribution in tight reservoirs abstract understanding the oil distribution characteristics in unconventional tight reservoirs is crucial for hydrocarbon evaluation and oil gas extraction from such reservoirs previous studies on tight oil distribution characteristics are mostly concerned with the basin scale based on lucaogou core samples geochemical approaches including soxhlet extraction total organic carbon toc and rock eval are combined with reservoir physical approaches including mercury injection capillary pressure micp and porosity permeability analysis to quantitatively evaluate oil distribution of tight reservoirs on micro scale the emphasis is to identify the key geological control factors of micro oil distribution in such tight reservoirs dolomicrites and non detrital mudstones have excellent hydrocarbon generation capacity while detritus containing dolomites siltstones and silty mudstones have higher porosity and oil content and coarser pore throat radius oil content is mainly controlled by porosity pore throat radius and hydrocarbon generation capacity porosity is positively correlated with oil content in almost all samples including various lithologies indicating that it is a primary constraint for providing storage space pore throat radius is also an important factor as oil migration is inhibited by the capillary pressure which must be overcome if the reservoir rock with suitable porosity has no hydrocarbon generation capacity pore throat radius will be decisive as tight reservoirs are generally characterized by widely distributed nanoscale pore throats and high capillary pressure hydrocarbon generation capacity plays an important role in reservoir rocks with suitable porosity and fine pore throats because such reservoir rocks cannot be charged completely the positive correlation between hydrocarbon generation capacity and oil content in three types of high porosity lithologies detritus containing dolomites siltstones and silty mudstones supports this assertion

controls on hydrocarbon accumulation by facies and fluid potential in large scale lacustrine petroliferous basins in compressional settings a case study of the mesozoic ordos basin china abstract large scale lacustrine basins in compressional settings are an important type of petroliferous basin and the study of the characteristics of hydrocarbon accumulation in such basins is of great significance this synthesis paper takes the mesozoic ordos basin central china as a case study to establish lateral and vertical hydrocarbon accumulation patterns in this basin type and highlights how facies and fluid potential control hydrocarbon accumulation in such basins based on systematic analyses of the mesozoic source rocks sedimentary facies migration and accumulation patterns and characteristics of the oil reservoirs the hydrocarbon accumulation characteristics in the mesozoic ordos basin are discussed the analysis indicates that sedimentary facies controlled the characteristics and distribution of hydrocarbon source rocks and reservoirs while diagenetic facies controlled the quality and distribution of the reservoirs excess pressure and fluid potential controlled hydrocarbon migration directions and paths whereas facies and potential controlled the locations of oil accumulations and traps it is concluded that source rocks controlled oil bearing regions sedimentary facies controlled oil bearing plays and fluid potential controlled the locations of accumulations for shallow intervals because fluid migration is mainly controlled by buoyancy paleotopographic highs played an important role in controlling hydrocarbon accumulations for deep layers chang 7 to chang 10 members since buoyancy is less than the capillary pressure excess pressure caused by the undercompaction of mudstone and hydrocarbon generation at depth provides the driving force for lateral and vertical migration of oil and becomes the main migration force the reservoir accumulation characteristics differ laterally between regions of the basin at the periphery of the basin reservoir units are stacked in a step wise manner whereas at the center of the basin multiple reservoir intervals are vertically stacked in the form of multistorey accumulations vertically layers with different burial depths have different accumulation characteristics the paleotopography controlled hydrocarbon accumulation in shallow intervals instead excess pressure fluid potential and paleotopography controlled accumulation in deep intervals this understanding could be used to inform prospecting in other large scale lacustrine petroliferous basins in compressional settings such as the eocene green river basin in the u s a and the middle triassic gunnedah basin in australia

quantitative identification of diagenetic facies and controls on reservoir quality for tight sandstones a case study of the triassic chang 9 oil layer zhenjing area ordos basin abstract the identification of diagenetic facies is essential for prospecting hydrocarbon reserves and for hydrocarbon exploration in tight oil reservoirs a comprehensive method for determining quantitative index of reservoir diagenetic facies rdi using core analysis techniques hierarchical cluster analyses hca and grey correlation analyses gca was proposed the petrological characteristics physical properties and pore structures of the chang 9 tight oil sandstones are analysed based on thin section analysis x ray diffraction xrd scanning electron microscopy sem cathode luminescence cl and mercury injection capillary pressure micp tests the properties of the chang 9 sandstones were controlled by different forms of diagenesis compaction and cementation of calcite have reduced or occluded the pore volume in the sandstones but not all cementation is deleterious to reservoir quality chlorite cementation helped preserve relict primary porosity and the direct dissolution of feldspars and unstable lithic fragments has resulted in enhanced pore connectivity following petrographic observations and quantitative analysis the sandstones were divided into four diagenetic facies calcite cementation facies intense compaction facies porosity preserved by grain coating chlorites facies and dissolution facies the intense compaction facies and calcite cementation facies are responsible for the evolution of low quality reservoirs while the reservoirs with porosity preserved by grain coating chlorites facies and dissolution facies are of better porosity and permeability the distribution of fractures improves reservoir quality by enhancing the dissolution of unstable grains and providing convenient channels and storage spaces for oil and gas migration

geochemical characteristics of gases from the largest tight sand gas field sulige and shale gas field fuling in china abstract this study performed a detailed geochemical analyses of the components stable carbon isotopes of alkane gas and co2 stable hydrogen isotopes of alkane gas and helium isotopes of reproducing gas from the largest tight gas field sulige and shale gas fuling field in china the comparative study shows that tight gas from the sulige gas field in the ordos basin is of coal derived origin which is characterized by a positive carbon and hydrogen isotopic distribution pattern 13c1 13c2 13c3 13c4 2h1 2h2 2h3 i e the carbon and hydrogen isotopes increase with increasing carbon numbers carbon dioxide from this field are of biogenic origin and the helium is crust derived shale gas from the fuling shale gas field belongs to oil derived gas which has complete carbon and hydrogen isotopic reversal of secondary alteration origin 13c1

diagenetic alteration pore throat network and reservoir quality of tight gas sandstone reservoirs a case study of the upper paleozoic sequence in the northern tianhuan depression in the ordos basin china the sulige gas field in the ordos basin is the largest gas field in china the studied area the northern tianhuan depression is located west of the gas field and is considered as a potential gas producing area the complex diagenetic alteration history and its effects on the pore throat network and reservoir quality of the tight gas sandstones are explained the upper paleozoic first member of the shanxi formation and eighth member of the shihezi formation tight gas reservoir rocks are composed of coarse to medium grained sandstones and are dominated by sublitharenites the most important diagenetic factor for porosity reduction was compaction followed by cementation diagenetic mineral phases include quartz overgrowth calcite cement and replacement and kaolinite cement and replacement with minor heulandite and chlorite pressure dissolution of quartz grains and quartzite fragments is the main source of silica for the early and late quartz overgrowth cementation formed mainly from 120 c to 140 c the hydrolysis and alteration of volcanic material and mica flakes released some silicon calcium and aluminum ions calcium and carbon in calcite cements could be derived from surrounding mudstones calcite replaced the quartz overgrowths the formation of kaolinite as cement and replacements was controlled by the interactions of silica aluminum ph and temperature 140 c because of deformation of detrital components mica flakes and ductile rock fragments and diagenetic replacement clear negative correlations between reservoir quality and contents of the diagenetic minerals are lacking a complex and isolated pore throat network was created by strong diagenetic transformation based on analysis of thin sections three dimensional x ray micro computed tomography and constant rate mercury injection data the samples with relatively good reservoir quality porosity 10 and permeability 0 5 md are characterized by dissolution pores and primary intergranular pores and commonly contain more and larger pore throats both in volume counts 1800 and size 0 02 per unit volume and diameter 1 20 m than the samples with poor reservoir quality the intercrystalline pores of kaolinite are very widespread and considered favorable for oil and gas accumulation because some asphalt was found within these pores this study may be used by analogy to better understand diagenetic factors controlling the formation of tight gas sandstone reservoirs and predict the development of pore throat systems

temporal and spatial aspects of the cusp inferred from local and global ground and space based observations in a case study 1 global and local observations from both ground and space facilities are brought to bear to understand the origin of a stepped cusp ion dispersion observed 3 hours prenoon under an interplanetary magnetic field characterized by strongly positive by conditions under which the cusp is believed to be shifted postnoon our main aim is to distinguish the temporal from the spatial cusp features the local observations from svalbard are from approximately the 0900 1200 mlt sector while the in situ observations are from polar at 0900 mlt the global auroral observations were acquired by the earth camera on polar the aurora from the ground perspective appeared as a periodic sequence of brightening events located on either side of a midday gap aurora centered at about 1030 mlt at noon the aurora appeared as a semiperiodic sequence of brightening events these were concurrent with enhancements of the overhead ionospheric hall current polar observed two steps of a staircase ion energy latitude dispersion these two steps were in turn concurrent with two consecutive auroral brightening events we argue from this complement of observations that the stepped cusp was temporal in nature and resulted from a time varying reconnection process global uv imagery was employed to complement the ground observations for the purpose of separating the temporal from the spatial aspects the imager captured namely two bands of strong uv emission in the prenoon and postnoon sectors between which there appeared a persistent weak uv aurora we interpret this to be an underlying spatial substratum patchy and bursty uv enhancements coincided with the transients seen in the local ground and space measurements corroborating further our distinction between space and time in the cusp comparing these inferences with recent work on the response of the dayside aurora to reconnection at the magnetopause we find a close correspondence in addition we document the field aligned currents associated with the stepped cusp which in turn compare favorably with recent theoretical ideas this work is a contribution to the current debate on the nature of the stepped cusp and suggests that multiple data sets are essential to study this problem

geologic significance and optimization technique of sweet spots in unconventional shale systems abstract crude oil in unconventional shale systems present as tight oil and shale oil accumulates inside an oil kitchen in formations with coexistent sources and reservoirs organic matter present in oil shale is not yet mature and requires heating to convert it into crude oil oil exploration in shale systems involves the exploration of shale oil retained in source rocks and tight oil rich zones located near source rocks tight oil is a type of realistic unconventional oil resources in china the marked increase in potential shale oil reserves and exploration of these reserves will result in a shale oil revolution similar to that experienced for shale gas based on a systematic comparison of geologic features of shale systems in the us and china the geologic significance of the sweet spots in shale systems is proposed this zone contains an abundance of unconventional oil in shale systems that can be explored and developed under current economic and technical conditions the sweet spot zone refers to the zone in the tight oil rich zone which has industrial value within the scope of matured high quality source rocks on the plane the sweet spot interval refers to the high productivity interval of tight oil which has industrial value through artificial stimulation the main aim of oil exploration in shale formations is to identify the sweet spots the distribution of the economic sweet spots in shale systems is evaluated by overlapping the geologic engineering and economic sweet spots resource assessment techniques the identification of logging data properties high resolution 3d seismic surveys horizontal well production from well pads and artificial reservoir development of sweet spots in oil bearing shale formations can assist efficient development of oil globally shale formations contain a significant volume of oil reserves currently stimulated reservoir volume srv techniques in horizontal wells in marine shale gas formations in the united states have average peak productivity cycles of 10 15 years to achieve commecial oil production in lacustrine shale systems in china it is important to utilize large formation thicknesses and the high abundance of organic matter in addition the development of practical and economic techniques will result in an increase of productivity of tight oil and shale oil by 30 50 million tons as well as the economic development of oil in lacustrine shale systems in china

the characteristics and significance of conventional and unconventional sinian silurian gas systems in the sichuan basin central china abstract oil and gas discoveries within proterozoic cambrian strata and the majority of shale gas production occurs in relatively stable tectonic regions and characterised by moderate thermal maturity r o o 2 5 the system comprises two components namely a conventional sinian cambrian dolomite hosted gas resource and an unconventional cambrian silurian shale gas deposit both represent trillion cubic meter accumulations the migration accumulation type sinian cambrian gas system is a result of numerous processes including 1 paleo high trapping within a relatively stable tectonic setting 2 appropriate source materials located in a paleo rift trough 3 large scale dolomite grainstone reservoirs located near the paleo high 4 adequate gas produced from in situ crude oil cracking and 5 gypsum salt and shale cap rock preservation the self contained source reservoir cambrian silurian shale gas system is localized by deepwater organic rich shelf facies characterized by high silicon and calcium contents the recent discovery of these two massive gas fields in old and over matured sedimentary strata has effectively extended the upper maturity limit of r o over 3 0 which may indicate the efficacy of broadening current natural gas exploration areas thus potentially increasing global natural gas resources significantly furthermore the discovery may spark interest in natural gas exploration in widely distributed old strata thus encouraging co development of conventional and unconventional petroleum accumulations

gas geochemical evidences for biodegradation of shale gases in the upper triassic yanchang formation ordos basin china abstract fractionation of chemical components and hydrogen carbon isotopic compositions associated with biodegradation of shale gases were determined from typical shale profiles of the upper triassic yanchang formation ordos basin china the results show that the light hydrocarbons c1 c5 of the shale gases were partially degraded by microorganisms and the degradation degree decreased with depth in the profile of well yy22 however the degradation degree of wells fy2 is basically unchanging at different depths and there is no obvious evidence for biodegradation in well yy18 biodegradation of c1 and c2 can occur and the degradation of hydrocarbon gas does not occur in any obvious sequence but instead there are only small differences between the relative degradation rates the relative degradation rates in descending sequence were c3 n c4 n c5 i c5 i c4 c2 c1 with intensification of degradation 13c1 of shale gas showed no obvious change whereas a marked alteration took place in 2hc1 the 13c values of c3 and n c4 did not alter significantly during early biodegradation thus use of exceptional heavy isotopic compositions of those two components to recognize moderate levels of biodegradation is limited in contrast the 13c and 2h values of c2 became more positive even at a low degree of biodegradation hence these are effective parameters for identifying biodegraded gases with a low degree of degradation no obvious change of isotopic composition was observed for the n c5 and i c5 during degradation due to the low relative degradation rates but high content of c1 and c2 biodegradation of c1 and c2 will severely destroy and decrease the gas content of shale gas there is a positive relationship between gas content and total organic carbon toc of shale samples without biodegradation in contrast samples that have experienced biodegradation show an obvious decrease in gas content with toc

reservoir characterization and hydrocarbon accumulation in late cenozoic lacustrine mixed carbonate siliciclastic fine grained deposits of the northwestern qaidam basin nw china abstract late cenozoic lacustrine mixed carbonate siliciclastic sequences in the northwestern qaidam basin nw china are mainly composed of high porosity and low permeability fine grained rocks but have big potentials of oil gas commercial production however how these fine grained rocks generate high quality reservoirs and accumulate hydrocarbons remains poorly known this study focuses on shallow buried shang youshashan formation middle late miocene n22 and shizigou formation late miocene pliocene n23 reservoirs and hydrocarbons in xiaoliangshan area of the northwestern qaidam basin we conduct mineralogy petrography and porosity and permeability analysis for reservoir rocks and biomarker analysis for hydrocarbons and potential source rocks the results indicate that the n22 high quality reservoirs are dominated by fractured marlstones and lime mudstones and constitute structural traps whereas the n23 hydrocarbon reservoirs are mainly composed of siliciclastic dominantly rocks and form stratigraphic lithologic traps similar biomarker ratios between crude oil samples and paleogene organic matter rich rocks reveal that hydrocarbons in the shallow system were likely derived from deep buried source rocks rather than late cenozoic lacustrine fine grained rocks themselves in this case large scale vertical fractures created by kink band structures in the northwestern qaidam basin are supposed to facilitate hydrocarbon migration and accumulation preliminary production data and exploration well oil gas test results demonstrate highest yields and thus large hydrocarbon potentials in the n22 marlstone and lime mudstone reservoirs collectively we suggest that relatively high permeability fractured and carbonate rich reservoirs on structural highs are favorable targets for future oil gas exploration in the northwestern qaidam basin this petroleum system provides a typical and significant example for hydrocarbon exploration and development in mixed carbonate siliciclastic fine grained rocks of nonmarine petroliferous basins

a new method for assessing tight oil with application to the lucaogou formation in the jimusaer depression junggar basin china this study proposes a new method for estimating tight oil resources in a closed petroleum system using an alternative mass balance model the proposed approach primarily derives resource estimates based on a large volume of rock eval pyrolysis data in addition to leveraging the cost effective straightforward and rapid process the proposed method has the flexibility to assess the potential of different types of resources including organic rich tight shale oil and organic lean tight oil the method is validated by assessing the oil resource potential of the middle permian lucaogou formation p2l in the jimusaer depression junggar basin china results indicate that the p2l contains 1 62 109 t 11 91 109 bbl of organic rich and 2 88 109 t 21 17 109 bbl of organic lean tight oil of the organic rich tight oil 21 comprises free oil resources amounting to 0 343 109 t 2 521 109 bbl the proposed approach deals sufficiently with the different resources in a petroleum system well and provides a useful tool for estimating the resource potential of an unconventional play

a type of continuous petroleum accumulation system in the shulu sag bohai bay basin eastern china abstract this study analyzed crude oils from the lower part of the third member of the eocene shahejie formation es 3 l and three prospective source rocks from the shulu sag bohai bay basin eastern china using a variety of organic geochemical methods biomarker characteristics were used to interpret source rock organic matter input and depositional environment and oil source rock correlation the biomarker data indicate that the crude oils originated from the es 3 l source rock which contains a mixture of plankton and land plant organic matters deposited in brackish fresh water under reducing conditions the oil in the es 3 l is self sourced instead of migrated from the overlying source rocks the petroleum generation potential of the es 3 l source rock was evaluated using organic geochemistry total organic carbon toc values for approximately 100 samples are between 1 02 and 4 92 wt and hydrogen indices range from 285 to 810 mg hydrocarbons g toc the es 3 l source rock contains mainly type ii and iii kerogen and most of the samples are thermally mature the data show that the es 3 l source rock has good potential for liquid hydrocarbon generation the es 3 l rock also acts as the oil reservoir having very low bulk porosity and permeability various types of storage space in the marlstone and carbonate rudstone in the es 3 l of the shulu sag include 1 fractures 2 intergranular pores 3 dissolution pores 4 organic matter pores 5 intragranular pores and 6 seams around gravels pore size ranges from nanometers to millimeters because the oil was generated and stored in es 3 l strata which lack any obvious trap and seal and have low permeability the unit represents a continuous petroleum accumulation

oil retention and intrasource migration in the organic rich lacustrine chang 7 shale of the upper triassic yanchang formation ordos basin central china the zhang 22 well was drilled in the ordos basin penetrating the chang 7 member of the triassic yanchang formation which cumulatively has more than 80 m 260 ft of black organic rich shale of oil window maturity using 76 samples collected every 1 m 3 ft from the well the effects of stratigraphic fractionation and petroleum expulsion within five intervals of the chang 7 shale were qualitatively and quantitatively documented the organic rich intervals 1 2 and 5 having an average total organic carbon toc of 6 79 wt and pyrolyzable hydrocarbon potential of 9 40 mg g rock i e the amount of hydrocarbons generated by pyrolysis between 300 c and 650 c s2 are defined as generative units in the chang 7 shale system compared to the in source reservoirs or sweet spots i e the third and fourth intervals which contain a lower average toc of 4 19 wt and an average s2 value of 7 17 mg g rock but the highest amount of free oil average total oil of 7 35 mg g rock geochemical and molecular compositions display distinctive differences between samples from these source and reservoir groupings for example bitumens from the generative units proportionally possess lower saturated hydrocarbons 56 66 than those from the in source reservoirs up to 81 the proportions of aromatic and polar compounds in the generative units are accordingly higher than in their counterpart the individual molecular weight distribution of sample extracts displays more light end moieties being enriched in the generative units by applying the compositional mass balance calculation the overall and compound specific expulsion efficiencies in the in source reservoirs are abnormally negative compared to the positive values in the generative intervals this finding in conjunction with the effects of the preferential retention of aliphatic hydrocarbons and the differential expulsion of light molecular weight compounds in the in source reservoirs indicates a short distance intrasource migration of generated petroleum into the sweet spot intervals intervals 3 and 4 from the overlying units intervals 1 and 2 and the underlying interval 5 furthermore when quantifying the total amount of retained petroleum in the shale system an amended assessment has been introduced to overcome the systematic misestimations if only unextracted values for the amount of thermally extractable hydrocarbons volatilized at 300 c mg hc g rock were considered thus the oil crossover effect temperature at which the rate of s2 generation is at maximum shift phenomenon and the hydrogen index being shifted to higher values after extraction all account for identifying intervals 3 and 4 as the in source reservoirs in this study we have not only identified a set of promising in source targets for shale oil exploration and production but we also presented the chemical and molecular composition for these shale oils we have additionally speculated for the intrasource migration model and further discussed the different expulsion efficiencies in the shale system upon the compositional mass balance calculation as well as the stratigraphic fractionation on differentiating the chemical compositions during migration the improved oil quality by fractionation the extra storage potential derived from microfossil quartz the weak adsorptive affinity of oil to organic matter and the good shale susceptibility to hydraulic fracturing all give a promising prospective for exploring and producing shale oil from the chang 7 shale system in the ordos basin

hydrocarbon accumulation processes in the dabei tight gas reservoirs kuqa subbasin tarim basin northwest china the dabei gas field is a recently discovered giant tight gas field in the kuqa subbasin western china the reservoir porosity and permeability mainly range from 1 to 8 and from 0 01 to 1 md respectively the hydrocarbon both gas and light oil accumulation processes in the tight sandstone reservoirs were studied based on detailed reservoir characterization thermal maturity of both gas and light oil hydrocarbon charge history regional tectonic compression and thrusting two episodes of oil and one episode of natural gas charge were delineated in the tight sandstone reservoir as evidenced by 1 similar sources but different maturities for the gas and light oil 2 the presence of abundant bitumen in the tight sandstone reservoir 3 the presence of both hydrocarbon gas inclusions and oil inclusions with two distinct fluorescence colors and 4 the presence of two groups of aqueous inclusions coeval with the petroleum inclusions with contrasting homogenization temperatures and salinities the oil inclusions with the blue white fluorescence color were determined to have been trapped at 5 4 ma whereas the gas charge may have occurred at circa 3 2 ma corresponding to a salinity change recorded in the aqueous inclusions the hydrocarbon accumulation processes appeared to be controlled by the tectonic compression of the south tianshan mountains intense tectonic compression caused thrust fault reactivation which provided pathways for hydrocarbon migration overpressure evolution of the reservoir indicates that an intense tectonic compression began at circa 5 ma which caused thrust activation and concomitant oil charge into the relatively porous part of the reservoir subsequent tectonic compression caused uplift and erosion associated with thrusting at the end of the kuqa formation deposition ca 3 ma with thrust faults and fractures acting as major migration pathways for the gas accumulation in the already tight sandstone reservoir resulting from both compaction and tectonic compression

generation of porphyry copper deposits by gas brine reaction in volcanic arcs we acknowledge research funding from bhp billiton a benjamin meaker visiting professorship to j m and a royal society wolfson research merit award and erc advanced grant critmag to j b

statistical analysis as a tool for assisting geochemical interpretation of the upper triassic yanchang formation ordos basin central china abstract the yanchang formation in the ordos basin is the most important petroleum play not only for conventional oil and gas accumulations but also for newly emerging shale oil and tight gas resources the molecular characterization of the basinwide source rocks predicts three groups of generative petroleum types paraffinic high wax oil mixed base paraffinic naphthenic aromatic low wax oil and gas and condensate supplementary to previous work 68 samples including the crude oils source bitumens and reservoir extracts from the yanchang petroleum play are analyzed the distribution of two terpane classes eight tricyclic terpanes and eight pentacyclic terpanes are determined with subsequent simultaneous rq mode factor analysis for a composite data set of these samples alongside 216 published crude oils worldwide with known facies descriptions thermal maturity has been evaluated as a consistent distribution at first using a combined method of a maturity related biomarker ts ts tm and aromatic parameters methyldibenzothiophene ratios to alleviate the maturity differences effect when discussing geochemical characterization the r mode factor analysis consists of the first two factors that are describing 45 present of the cumulative total variance in the data set and presents a sample grouping pattern in q mode factor analysis which is determined by different contributions of terpane associations i e the tricyclic c21 coupled with pentacyclic c26 c27 c28 and c30 in the same factor space three terpane associations the c26 and c28 terpanes the c21 and c30 terpanes and the c27 pentacyclic terpenes are respectively responsible for discriminating crude oil reservoir extracts and source bitumens in rq mode factor analysis molecular compositions further address more detailed interrelationships among three sample groups that crude oils and reservoir extracts are sharing close genetic relationships both in depositional environment typing and c27 c28 c29 sterane distribution samples from source rocks vary much significantly a mixing process which occurs after oils has been expelled from host source rocks into carrier units during accumulation in addition the migration contamination of c29 sterols when oils are cross through the chang 7 2 unit along migration pathways might also explain this lack of correlation between source rocks and oil reservoir system

review of diagenetic facies in tight sandstones diagenesis diagenetic minerals and prediction via well logs abstract the tight sandstones are characterized by low porosity low permeability complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones diagenetic facies which is the comprehensive description of the diagenesis and diagenetic minerals determines the formation and distribution of sweet spot by correlating the diagenetic facies to well log responses the subsurface distribution of porosity and permeability can be predicted however the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task this paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones and establishes a model for prediction of diagenetic facies via well logs as assessed from peer reviewed papers in the literature as well as from the authors personal experiences this review begins with reviewing the impacts of compaction cementation dissolution and various types of diagenetic minerals on reservoir quality evolution the definition and classification schemes of diagenetic facies are then discussed and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized the same diagenetic facies commonly display similar compositional and textural attributes matrix and cement as well as porosity systems the well log responses gr ac den cnl and rt of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections by translating the diagenetic facies to conventional well logs a predictable model which can be used for subsurface reservoir quality prediction is established then the theory of ecs logs is reviewed and the application of ecs logs in diagenetic facies evaluation is discussed at last the quantitative characterization for various type and degree of diagenesis is reviewed and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss cementational porosity loss and dissolution porosity content via well logs correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones this review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones

insight into pore throat size distribution and the controls on oil saturation of tight sandstone reservoirs using nuclear magnetic resonance parameters a case study of the lower cretaceous quantou formation in the southern songliao basin china pore throat structure characterization is crucial in tight oil sandstone reservoir studies to better understand pore throat size distribution and the effects on reservoir oil saturation petrographic scanning electron microscopy quantitative grain fluorescence qgf nuclear magnetic resonance nmr and pressure controlled mercury injection investigations were performed on a suite of tight sandstones from the fourth member of the lower cretaceous quantou formation k1q4 in the southern songliao basin china the tight sandstone samples showed similar compositional and textural maturity but different reservoir qualities at different depth intervals two oily indicators qgf index and qgf on extract spectra intensity varied significantly with respect to burial depth suggesting the existence of strong oil heterogeneities in the k1q4 tight sandstones oil emplacement occurred mainly in the primary pores associated with the absence of pore filling authigenic minerals pore throat size distributions had wider ranges in the oil rich layers than in the samples lacking oil emplacement the nmr pore throat radii ranged from 0 01 to 10 m in oil lacking samples but ranged from 0 01 to 100 m in oil rich layers oil signals were mainly detected in the pore throats with radii ranging from approximately 0 1 to 1 0 m in oil rich layers the nmr porosity can be divided into three parts based on transverse relaxation time t2 spectrum 1 immovable fluid 2 porosity of effective movable fluid pemf and 3 porosity of bound fluid bivariate plots of the ratios of various porosity types have significant effects in deciphering the oil saturation of tight sandstone reservoirs the pemf reflected well the reservoir qgf index among others therefore it can be considered as an effective nmr parameter to characterize pore size distribution and helpful in oil reservoir prediction a good relationship exists between pemf and geometric mean of the t2 distribution t2gm indicating that t2gm could also reflect reservoir oil saturation the pemf is mainly contributed by large pore throats and the poorly sorted pore throat distribution is better for effective movable fluids occurrence in tight sandstones the approach employed in this study provides a more comprehensive and accurate pore throat structure characterization in tight sandstone oil reservoirs generating effective parameters and convenient ways to evaluate reservoir oil saturation importantly it could be applicable for improving tight sandstone oil exploration and development efficiencies

resource potential and core area prediction of lacustrine tight oil the triassic yanchang formation in ordos basin china unlike liquid rich hydrocarbons in marine shale systems in north america the triassic yanchang oil is preserved in tight sandstone reservoirs deposited in a lacustrine setting the tight reservoir covers an area of 30 000 km2 11 600 mi2 and is characterized by ultralow permeability 0 001 1 00 md 0 001 10 3 1 00 10 3 m2 a low subsurface pressure coefficient 0 6 0 8 and low single well yield several tons 10 40 bbl to 10 tons 70 bbl per day the reservoir also features a high content of quartz feldspar and carbonate averaging 70 80 and low crude oil density of 0 84 g cm3 36 5 api with an oil saturation of approximately 60 80 for the first time this study employs the assessment of petroleum in place resources based on the genesis and statistics predicting method gspm and estimated ultimate recovery method for the tight oil in the ordos basin the advantage of gspm over previously reported methodologies is the suitable combination of genetic and statistical methods to estimate proximally sourced tight oil the results show that the total in place resource potential of the investigated tight oil is approximately 42 26 108 t 295 82 108 bbl with an average abundance of 15 104 t km2 99 104 bbl mi2 the recoverable resources for the 50 probability of the chang 7 tight oil accumulation is 3 11 108 t 21 77 108 bbl in the ordos basin the top two grades of the chang 7 tight oil core area cover the total area of three layers comprising 12 745 km2 4921 mi2 with 22 51 108 t 157 57 108 bbl of in situ resources furthermore the four most promising zones have been identified

grain assemblages and diagenesis in organic rich mudrocks upper pennsylvanian cline shale wolfcamp d midland basin texas grain assemblages in the organic rich cline shale in the midland basin are dominated by components of extrabasinal derivation 11 4 to 98 5 vol average volume 82 6 major extrabasinal components include k rich clay minerals detrital quartz albite k feldspar micas and lithic fragments intrabasinal components include mainly biosiliceous allochems sponge spicules and radiolarians agglutinated foraminifera ca phosphate peloids clay rich peloids organomineralic aggregates intraclasts and other biocalcareous allochems authigenic minerals are most evident as grain replacements euhedral ankerite ca phosphate cement and precipitates in large pores a strongly localized spatial distribution of diagenetic products at micron to centimeter scales is observed in most siliciclastic samples except in biosiliceous allochem rich ones in which abundant intergranular pore filling clay size microquartz cement is observed compaction is evident in the cline shale because of low porosity and generally low cement volumes neither textural variation nor bulk mineral composition alone is sufficient to confidently decipher the rock bulk property e g total organic carbon and reservoir quality variation e g porosity and permeability however a good negative relationship between the ratio of extrabasinal to intrabasinal grains and favorable reservoir properties is observed in the cline shale specifically higher porosity permeability and total organic carbon are observed in samples representing the extreme end members of intrabasinal derived biosilica rich layers nickel a recognized proxy for paleoproductivity exhibits a positive relationship with intrabasinal grain content and reservoir properties x ray fluorescence based analysis of nickel can be a rapid and cost effective way to delineate favorable unconventional reservoir quality in the cline shale

formation of low permeability reservoirs and gas accumulation process in the daniudi gas field northeast ordos basin china abstract the daniudi gas field is a typical large scale coal generated wet gas field located in the northeastern ordos basin that contains multiple upper paleozoic gas bearing layers and considerable reserves of gas based on integrated analysis of reservoir petrology carbonate cement c o isotope geochemistry of source rocks and hc gas and numerical basin modeling a comprehensive study focusing on the formation of low permeability reservoirs and gas generation process uncovers a different gas accumulation scene in daniudi gas field the gas accumulation discovered was controlled by the reservoir permeability reduction and gas generation process and can be divided into two distinct stages by the low permeability reservoir formation time before the low permeability reservoir formation the less matured gas was driven by buoyancy migrated laterally towards ne and then accumulated in ne favorable traps during late triassic to early early cretaceous after the low permeability reservoir formation highly matured gas was driven by excessive pressure migrated vertically and accumulated in situ or near the gas generating centers during early to late early cretaceous the coupling relationship between reservoir diagenetic evolution and gas generation process controlled on gas accumulation of the daniudi gas field this study will aid in understanding the gas accumulation process and planning further e d of the upper paleozoic super imposed gas layers in the whole ordos basin and other similar super imposed low permeability gas layer basins

resource assessment perspectives for unconventional gas systems concepts are described for assessing those unconventional gas sys tems that can also be defined as continuous accumulations contin uous gas accumulations exist more or less independently of the wa ter column and do not owe their existence directly to the buoyancy of gas in water they cannot be represented in terms of individual countable fields or pools delineated by downdip water contacts for these reasons traditional resource assessment methods based on es timating the sizes and numbers of undiscovered discrete fields can not be applied to continuous accumulations specialized assessment methods are required unconventional gas systems that are also continuous accumu lations include coalbed methane basin centered gas so called tight gas fractured shale and chalk gas and gas hydrates deep basin and bacterial gas systems may or may not be continuous accumu lations depending on their geologic setting two basic resource assessment approaches have been em ployed for continuous accumulations the first approach is based on estimates of gas in place a volumetric estimate of total gas in place is commonly coupled with an overall recovery factor to nar row the assessment scope from a treatment of gas volumes residing in sedimentary strata to a prediction of potential additions to re serves the second approach is based on the production perfor mance of continuous gas reservoirs as shown empirically by wells and reservoir simulation models in these methods production characteristics as opposed to gas in place are the foundation for forecasts of potential additions to reserves

diagenetic constraints on the heterogeneity of tight sandstone reservoirs a case study on the upper triassic xujiahe formation in the sichuan basin southwest china abstract heterogeneity is an essential feature of tight sandstones because it is directly related to reservoir quality the upper triassic xujiahe formation in the sichuan basin was used as a case study to analyse the diagenetic constraints on the heterogeneity of tight sandstone reservoirs the three main diagenetic processes of interest are mechanical compaction cementation and dissolution a dissolution facies quartz cement facies and carbonate cement facies present sequentially from the centre to the margin of sand bodies abundant feldspars and rock fragments were dissolved to form secondary porosity or filled by carbonate cement authigenic chlorite coatings generated from eogenesis to the deep burial stage are thickest in high porosity samples the 13 c of carbonate cement ranges from 10 56 to 1 15 pdb suggesting multiple sources of carbon including carbonate fragments early inorganic carbonate cement and organically derived co 2 based on 18 o carbonate cementation occurred from 33 c to 110 c microthermometric data of aqueous inclusions indicate that quartz cement precipitated at a temperature range of 120 c 140 c a summary model of the diagenetic constraints on the heterogeneity of sandstones was constructed the spatial distribution of digenetic facies is controlled by differences in fluid flow velocity and ph fluids especially hydrocarbons migrated preferentially within the centre of sand bodies and caused dissolution dissolution generated secondary porosity and provided materials for authigenic mineral precipitation carbonate fragments were better preserved and superimposed with later carbonate cement along the sandstone mudstone boundary quartz overgrowths formed after carbonate cementation and developed in the transition zone between the intervals of carbonate cementation and dissolution consequently the reservoir quality of the xujiahe sandstones decreases gradually moving from the centre to the margin of the sand bodies

an improved hydrocarbon generation potential method for quantifying hydrocarbon generation and expulsion characteristics with application example of paleogene shahejie formation nanpu sag bohai bay basin abstract to evaluate petroleum resources it is important to quantify the hydrocarbon generation and expulsion characteristics of source rock the hydrocarbon generation potential method is an efficient model for calculating the characteristics of source rocks and evaluating the resource potential the previous hydrocarbon generation potential model was mainly established by measured rock eval pyrolysis data and due to inappropriate preservation measures an important component volatile hydrocarbon content of source rock may be significantly reduced prior methods may therefore not objectively reflect the properties of a source rock and cause unacceptable errors in the resource assessment this study proposes an improved hydrocarbon generation potential method the new method considers the initial hydrogen index hi0 and transformation ratio tr during the evolution of source rocks which can more realistically reflect the hydrocarbon generation and expulsion history of source rocks therefore the hydrocarbon evaporative loss qloss is calibrated and the hydrocarbon generation and expulsion features of source rocks are more accurately evaluated the proposed method is used to examine the effectiveness of the third member of the upper paleogene shahejie formation es3 in nanpu sag by utilizing the improved hydrocarbon generation potential model the hydrocarbon expulsion threshold of the es3 source rock is positioned at ro 0 875 the maximum hydrocarbon generation and expulsion potential qg and qe are 593 78 and 323 78 mg hc g toc respectively and the maximum hydrocarbon expulsion efficiency is 54 the maximum hydrocarbon generation and expulsion intensities are 500 104 t km2 and 250 104 t km2 respectively which locate source rock at the laoyemiao and gaoshangpu structural belts the amounts of generated and expelled hydrocarbon are 13 7 108 t and 6 9 108 t respectively and the resource potential of the tight oil of the es3 in the nanpu sag is 0 69 104 t compared with the previous method the improved hydrocarbon generation potential method limits the maximum qg and calibrates the qloss therefore the new method more objectively reflects the hydrocarbon generation and expulsion characteristics and resource prospect of the source rock

division of fine grained rocks and selection of sweet sections in the oldest continental shale in china taking the coexisting combination of tight and shale oil in the permian junggar basin abstract the permian formation in the junggar basin contains the oldest continental shale in china and has abundant petroleum resources the lithofacies and lithology of the shale strata in the lucaogou formation which are characterized by two sets of coexisting combinations of tight oil and shale oil in the jimsar sag eastern junggar basin are studied via experiments to explore their petrophysics geochemistry reservoir pores and oil bearing properties the results show that 1 the lucaogou formation in the jimsar sag consists of shallow deep lacustrine sediments and can be divided into six stratigraphic units vertically 2 fine grained sediments in the sag composed of shale dolomite and siltstone can be specifically divided into carbonate mudstone cm siliceous mudstone sm carbonate rock cc carbonate sandstone cs and siliceous sandstone ss 3 cm and sm are good source rocks for shale oil and p 2 l 2 and p 2 l 5 are the sweet sections for shale oil cc cs and ss are reservoirs of tight oil and p 2 l 1 and p 2 l 4 are the sweet sections for tight oil finally definitions are discussed and proposed for tight oil and shale oil

characteristics of micropores pore throats and movable fluids in the tight sandstone oil reservoirs of the yanchang formation in the southwestern ordos basin china thin section analysis field emission scanning electron microscopy constant rate mercury injection crmi x ray computed tomography x ct and nuclear magnetic resonance nmr were used to investigate pore systems pore sizes and pore throat distributions and determine evaluation criteria for tight oil reservoirs this research shows that the average porosity of the reservoirs in the chang 63 to chang 72 members of the yanchang formation of the ordos basin ranges from 4 to 16 with a corresponding average permeability of 0 1 to 0 4 md the main pore types in these tight oil reservoirs are dissolution pores and intergranular pores and the main pore throat types are laminated throats and control shaped throats based on experimental results the pore throat distributions differ among different testing techniques the results of this study indicate that a combination of crmi derived and x ct detected pore throat distributions is most suitable for calibrating nmr derived pore sizes over the full range of pore sizes the lower pore throat radius limit of effective reservoirs is 0 24 m and the lower permeability limit for the flow of movable fluid is 0 008 md based on a cluster analysis the studied tight oil reservoirs can be divided into three types compared with type iii reservoirs porosity 11 and permeability 0 15 md and type ii 8 porosity 11 and 0 05 md permeability 0 15 md reservoirs are of relatively good quality

thresholds of petroleum content and pore diameter for petroleum mobility in shale petroleum mobility in shale is closely correlated with the attributes of shale petroleum and pores however the relationship between these attributes is poorly understood to characterize petroleum mobility in self sourcing reservoirs a suite of mature eocene shales was selected and subjected to organic solvent extraction and both the raw and solvent treated samples were analyzed using pyrolysis nitrogen adsorption and x ray diffraction the results show that the pore surface area and pore volume of these shales are mainly controlled by their clay and quartz content rather than their organic matter om content and are limited by the presence of carbonates correlations of soluble om with pore surface area and volume after solvent extraction indicate that petroleum mobility of studied shales is initiated when the petroleum content reaches 0 70 wt of the rock and the pore diameter is over 12 1 nm these thresholds are established in the studied area and should be similar for the self sourcing reservoirs from similar sedimentary environments this work proposes a method to reveal the thresholds of petroleum content and pore diameter for petroleum mobility in self sourcing reservoirs which is useful in the assessment of petroleum producibility and is of significance for unconventional petroleum exploration and exploitation

geochemical characteristics of tight gas and gas source correlation in the daniudi gas field the ordos basin china abstract the molecular composition stable carbon and hydrogen isotopes and light hydrocarbons of the upper paleozoic tight gas in the daniudi gas field in the ordos basin were investigated to study the geochemical characteristics tight gas in the daniudi gas field displays a dryness coefficient c1 c1 5 of 0 845 0 977 with generally positive carbon and hydrogen isotopic series and the c7 and c5 7 light hydrocarbons of tight gas are dominated by methylcyclohexane and iso alkanes respectively the identification of gas origin and gas source correlation indicate that tight gas is coal type gas and the gases reservoired in the lower permian shanxi fm p1s and lower shihezi fm p1x had a good affinity and were derived from the p1s coal measure source rocks whereas the gas reservoired in the upper carboniferous taiyuan fm c3t was derived from the c3t coal measure source rocks the molecular and methane carbon isotopic fractionations of natural gas support that the p1x gas was derived from the p1s source rocks the differences of geochemical characteristics of the c3t gas from different areas in the field suggest the effect of maturity difference of the source rocks rather than the diffusive migration and the large scale lateral migration of the c3t gas seems unlikely comparative study indicates that the differences of the geochemical characteristics of the p1s gases from the yulin and daniudi gas fields originated likely from the maturity difference of the in situ source rocks rather than the effect of large scale lateral migration of the p1s gases

analysis of hydrodynamic factors in petroleum migration and entrapment the basic equations developed by m king hubbert to describe the water and buoyancy forces driving hydrocarbons through an aquifer can be expanded and combined with j a f plateau s equations on capillary pressure phenomena to allow study of oil migration and entrapment patterns the new equations are used to evaluate the sensitivity of entrapment processes to variations in temperature fluid densities and geologic parameters water drive is not important except when the tops of the petroleum transporting strata have very low dips in all other cases oil movement is governed by buoyancy drive increases in capillary pressure sufficient to cause petroleum entrapment seem important only when porosities drop below 5 the equations are also used in a computer analysis to combine the effects of all the variables and examine two hypothetical field cases specifically the computer model examines the impact of changes in transport strata transmissivity upon oil entrapment the resulting changes in potentiometric surface gradients in a structure of low dip can cause significant changes in both entrapment potential and location of the trap on the structure

hydrocarbon generation from confined pyrolysis of lower permian shanxi formation coal and coal measure mudstone in the shenfu area northeastern ordos basin china abstract the ordos basin is a significant gas producing region in china previous studies have shown that a large amount of the gas discovered in the upper paleozoic strata originated from carboniferous lower permian coal and its interbedded coal measure mudstone to compare the hydrocarbon generation characteristics and process two coal and coal measure mudstone samples from the shanxi formation in the shenfu area were used in confined gold tube pyrolysis experiments the analytical results revealed that both samples had poor oil generation potential with maximum c6 yields of 171 65 175 67 mg g toc for the coal and 50 12 51 86 mg g toc for the coal measure mudstone at the heating rate of 2 c h and 20 c h however their gas generation potentials are good and the coal exhibited higher values than the mudstone with final laboratory c1 c5 yields of 154 71 173 32 mg g toc for the coal and 82 52 98 96 mg g toc for the coal measure mudstone based on a comprehensive analysis of the indicators lnc2 c3 lnc1 c2 and 13c1 13c2 the hydrocarbon generation process can be divided into three stages the results of extrapolation to geological conditions revealed that the main gas generation stages of both source rocks in the shanxi formation began in the early cretaceous because of the regional tectonic uplift and the decrease in the geothermal gradient in the late cretaceous the c1 c5 volume yields of the coal and the coal measure mudstone reached plateaus of 78 4 ml g toc and 32 7 ml g toc respectively in the late cretaceous and the current transformation ratios of c1 c5 for the coal and the coal measure mudstone are only 32 1 and 22 8 respectively

clay mineral content type and their effects on pore throat structure and reservoir properties insight from the permian tight sandstones in the hangjinqi area north ordos basin china abstract clay mineral content and pore throat structure including type morphology size distribution and porosity and permeability are very important in factors that control gas storage in tight sandstone reservoirs in this study integrated thin section and field emission scanning electron microscope fe sem observation x ray diffraction xrd nuclear magnetic resonance nmr and rate controlled mercury porosimetry rcp analysis were used to investigate pore throat structure and the effects of clay minerals on pore throat connectivity size distribution and reservoir properties of tight sandstones in the middle permian successions of the hangjinqi area north ordos basin china results show that the tight sandstones have very low porosity and permeability averaged 9 76 and 0 968 md with diverse clay mineral compositions and grain sizes the main pore types are intergranular and intragranular dissolution pores intercrystalline and interstitial micropores most pores and throats are infilled with authigenic clay minerals such as acicular chlorite booklet kaolin and scaly and filamentous illite with minor silica and calcareous cements the pore and throat sizes in tight sandstones exhibit large variation with 147 648 m and 1 051 m in sandy conglomerates 160 998 m and 1 285 m in pebbly coarse grained sandstones 155 016 m and 0 868 m in coarse grained sandstones 158 134 m and 1 178 m in medium grained sandstones and 154 m and 0 379 m in fine grained sandstones respectively the results indicate that clay minerals affected the pore throat connectivity size distribution and reservoir properties of different lithofacies the properties of tight sandstone reservoirs are heavily influenced by illite content and throat size played a more important role influencing porosity and permeability this study helps to clarify the relationship of clay minerals pore throat structure and reservoir properties of the permian tight sandstones in the hangjinqi area and can guide associated work on tight sandstone reservoirs in the ordos basin and other basins worldwide

upper permian junggar and upper triassic ordos lacustrine source rocks in northwest and central china organic geochemistry petroleum potential and predicted organofacies abstract two sets of lacustrine source rocks in the junggar basin and ordos basin have been analysed by organic geochemical approach and organic petrographic technique one of these source rocks the junggar shales lucaogou formation and hongyanchi formation was deposited in a hydrologically closed brackish lake under a sub humid climate in higher latitude 39 43 n and consisted of organic rich profundal lake shales lucaogou formation and lean fluvial lacustrine lake margin mudstones the other one the chang 7 shale was deposited in a hydrologically open freshwater lake under a humid climate in a 31 n palaeolatitude and was characterized by high concentration of alginite rich organic matter the profundal lucaogou shale and chang 7 3 shale possess abundant total organic carbon high generative potential s2 and high hydrogen indices which rank as the best prolific source rocks in both basins followed by chang 7 1 shale hongyanchi formation and chang 7 2 are much leaner than their counterparts and contain relatively small portion of alginite but more terrigenous macerals this nature of organic matter makes them less prolific and becoming gas prone potential these differences in petroleum potential mirror depositional environment changes in source rocks formation potential source rocks in both basins have the potential for generating high wax oil but the brackish system clearly contains more pna oil the kerogen structures revealed by pyrolysis gas chromatography allowexplaining this trend the lucaogou chang 7 3 and chang 7 1 shales enriching in long and moderate alkyl chains share a nature of high wax oil generation trend in contrast the hongyanchi and chang 7 2 shales appears to be dominated by short to moderated chain length distribution and higher overall aromaticity to generate more pna oil even gas and condensate thermal liability was shown to be linked to kerogen structure in particular the presence of alicyclic and oxygen containing moieties and these in turn are correlate with total organic carbon content