Characteristics and control factors of feldspar dissolution in gravity flow sandstone of Chang 7 Member,Triassic Yanchang Formation,Ordos Basin,NW China

To clarify the formation and distribution of feldspar dissolution pores and predict the distribution of high-quality reservoir in gravity flow sandstone of the 7^(th) member of Triassic Yanchang Formation(Chang 7 Member)in the Ordos Basin,thin sections,scanning electron microscopy,energy spectrum analysis,X-ray diffraction whole rock analysis,and dissolution experiments are employed in this study to investigate the characteristics and control factors of feldspar dissolution pores.The results show that:(1)Three types of diagenetic processes are observed in the feldspar of Chang 7 sandstone in the study area:secondary overgrowth of feldspar,replacement by clay and calcite,and dissolution of detrital feldspar.(2)The feldspar dissolution of Chang 7 tight sandstone is caused by organic acid,and is further affected by the type of feldspar,the degree of early feldspar alteration,and the buffering effect of mica debris on organic acid.(3)Feldspars have varying degrees of dissolution.Potassium feldspar is more susceptible to dissolution than plagioclase.Among potassium feldspar,orthoclase is more soluble than microcline,and unaltered feldspar is more soluble than early kaolinized or sericitized feldspar.(4)The dissolution experiment demonstrated that the presence of mica can hinder the dissolution of feldspar.Mica of the same mass has a significantly stronger capacity to consume organic acids than feldspar.(5)Dissolution pores in feldspar of Chang 7 Member are more abundant in areas with low mica content,and they improve the reservoir physical properties,while in areas with high mica content,the number of feldspar dissolution pores decreases significantly.

Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China

High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation,the Binhai area of Qikou Sag,providing an example of medium-and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin.By using data of cores,cast thin sections,scanning electron microscope and physical property tests,the sedimentary facies,physical properties and main control factors of the high-quality reservoirs were analyzed.The reservoirs are identified as deposits of slump-type sub-lacustrine fans,which are marked by muddy fragments,slump deformation structure and Bouma sequences in sandstones.They present mostly medium porosity and low permeability,and slightly medium porosity and high permeability.They have primary intergranular pores,intergranular and intragranular dissolution pores in feldspar and detritus grains,and structural microcracks as storage space.The main factors controlling the high quality reservoirs are as follows:(1)Favorable sedimentary microfacies of main and proximal distributary gravity flow channels.The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow,and the effect of deposition on reservoir properties decreases with the increase of depth.(2)Medium texture maturity.It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing,and was good for the formation of the primary intergranular pores.(3)High content of intermediate-acid volcanic rock detritus.The reservoir sandstone has high content of detritus of various components,especially intermediate-acid volcanic rock detritus,which is good for the formation of dissolution pores.(4)Organic acid corrosion.It was attributed to hydrocarbon maturity during mesodiagenetic A substage.(5)Early-forming and long lasting overpressure.A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone,and is responsible for the preservation of abundant primary pores.(6)Regional transtensional tectonic action.It resulted in the structural microcracks.

Structural characteristics of continental carbonate-rich shale and shale oil movability: A case study of the Paleogene Shahejie Formation shale in Jiyang Depression, Bohai Bay Basin, China

Based on rock mineral and geochemical analysis, microscopic observation, physical property measurement, and thin laminae separation test, etc., the characteristics of typical laminae of the Paleogene Shahejie Formation carbonate-rich shale in the Jiyang Depression were analyzed, and the organic matter abundance, reservoir properties, and oil-bearing properties of different laminae were compared. Typical shale storage-seepage structures were classified, and the mobility of oil in different types of shale storage-seepage structure was compared. The results show that the repeated superposition of mud laminae and calcite laminae are the main layer structure of carbonate-rich shales. The calcite laminae are divided into micritic calcite laminae, sparry calcite laminae and fibrous calcite vein. The mud-rich laminae are the main contributor to the organic matter abundance and porosity of shale, with the best hydrocarbon generation potential, reservoir capacity, and oil-bearing property. The micritic calcite laminae also have relatively good hydrocarbon generation potential, reservoir capacity and oil-bearing property. The sparry calcite laminae and fibrous calcite vein have good permeability and conductivity. Four types of shale storage-seepage structure are developed in the carbonate-rich shale, and the mobility of oil in each type of storage-seepage structure is in descending order: sparry calcite laminae enriched shale storage-seepage structure, mixed calcite laminae enriched shale storage-seepage structure, fibrous calcite vein enriched shale storage-seepage structure, and micritic calcite laminae enriched shale storage-seepage structure. The exploration targets of carbonate-rich shale in the Jiyang Depression Shahejie Formation are different in terms of storage-seepage structure at different thermal evolution stages.

Formation of inter-salt overpressure fractures and their significances to shale oil and gas: A case study of the third member of Paleogene Shahejie Formation in Dongpu sag, Bohai Bay Basin

Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation(Es3)of Dongpu sag in Bohai Bay Basin as an example,the origin of overpressure,development characteristics,formation and evolution mechanism,formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification,scanning electron microscopy observation,analysis of fluid inclusions,logging data analysis,and formation pressure inversion.The results show that overpressure is universal in the salt-lake basin of Dongpu sag,and under-compaction caused by the sealing of salt-gypsum layer,pressurization due to hydrocarbon generation,transformation and dehydration of clay minerals,and fault sealing are the 4 main factors leading to the occurrence of overpressure.The overpressure fractures are small in scale,with an average length of 356.2μm and an average underground opening of 11.6μm.But they are densely developed,with an average surface density of 0.76 cm/cm2.Moreover,they are often accompanied by oil and gas charging,and thus high in effectiveness.Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging,approximately 25-30 Ma ago and 0-5 Ma ago.Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas.They contribute 22.3%to the porosity of shale reservoir and 51.4%to the permeability.They can connect fracture systems of multiple scales,greatly improving the quality of shale reservoir.During the development of shale oil and gas,inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures,giving rise to complex and highly permeable volumetric fracture networks,improving hydraulic fracturing effect and enhancing shale oil and gas productivity.

Geochemistry of source rocks in the Es_3 and Es_4 members in the Shahejie Formation of the Western Depression, Liaohe Oilfield, China

The Western Depression of the Liaohe Basin is the major exploration area of the Liaohe Oilfield, and its main source rocks consist of the third and fourth members of the Shahejie Formation (Es3 and Es4). These source rocks are widely distributed in the depression, with semi-deep lake and fan delta as the main sedimentary facies, brown oil shale and black gray-dark gray mudstone as the main rocks, and a total thickness of 270-1450 m. The kerogens are mainly of the types I and IIA, and partly of the type IIB and least of the type III. The Ro values range from 0.4%-0.8%, indicating an evolution stage from immature to mature. The maturity of Es4 source rocks is rela-tively high, reaching the early mature stage, but their distribution and thickness are lower than those of Es3. Besides, according to biomarker analysis, it is thought that the source rocks of Es3 and Es4 are characterized by mixed input, and most of the source rocks were formed in the brackish water-saline and strongly oxygen-free environment. Fur-thermore, the Qingshui, Niuxintuo and Chenjia sags are believed to possess greater potential for hydrocarbon gen-eration and expulsion, for they are source rocks with a larger thickness, have higher organic carbon contents, belong to better organic matter types and possess higher maturities.

Hydrocarbon Reservoir Model of Glutenite in the Fourth Member of Shahejie Formation in Northern Dongying Depression

The glutenite in the fourth member of Shahejie Formation(Es^4) in northern Dongying depression straightforwardly penetrated into the muddy bathyal -abyss facies.The conditions of reservoir is very superior:(1) the high quality thick bedded oil source rock is developed very well in the Lijin sag and Minfeng sag of the study area,and it has the higher capability of generating hydrocarbon;(2) the

Effect of Shale Reservoir Characteristics on Shale Oil Movability in the Lower Third Member of the Shahejie Formation, Zhanhua Sag

To reveal the effect of shale reservoir characteristics on the movability of shale oil and its action mechanism in the lower third member of the Shahejie Formation(Es3l), samples with different features were selected and analyzed using N2 adsorption, high-pressure mercury injection capillary pressure(MICP), nuclear magnetic resonance(NMR), high-speed centrifugation, and displacement image techniques. The results show that shale pore structure characteristics control shale oil movability directly. Movable oil saturation has a positive relationship with pore volume for radius > 2 μm, as larger pores often have higher movable oil saturation, indicating that movable oil is present in relatively larger pores. The main reasons for this are as follows. The relatively smaller pores often have oil-wetting properties because of organic matter, which has an unfavorable effect on the flow of oil, while the relatively larger pores are often wetted by water, which is helpful to shale oil movability. The rich surface provided by the relatively smaller pores is beneficial to the adsorption of immovable oil. Meanwhile, the relatively larger pores create significant pore volume for movable oil. Moreover, the larger pores often have good pore connectivity. Pores and fractures are interconnected to form a complex fracture network, which provides a good permeability channel for shale oil flow. The smaller pores are mostly distributed separately;thus, they are not conducive to the flow of shale oil. The mineral composition and fabric macroscopically affect the movability of shale oil. Calcite plays an active role in shale oil movability by increasing the brittleness of shale and is more likely to form micro-cracks under the same stress background. Clay does not utilize shale oil flow because of its large specific surface area and its block effect. The bedding structure increases the large-scale storage space and improves the connectivity of pores at different scales, which is conducive to the movability of shale oil.

Shale Lithofacies and Sedimentary Environment of the Third Member,Shahejie Formation,Zhanhua Sag,Eastern China

Researches into shale lithofacies,their sedimentary environments and relationship benefit understanding both of sedimentary cycle division and unconventional hydrocarbon exploration in lacustrine basins.Based on a 100~300-m-thick dark shale,mudstone and limestone encountered in the lower third member of the Eocene Shahejie Formation(Es3l member)in Zhanhua Sag,Bohai Bay Basin,eastern China,routine core analysis,thin sectioning,scanning electron microscopy(SEM),mineralogical and geochemical measurements were used to understand detailed facies characterization and paleoclimate in the member.This Es3l shale sediment includes three sedimentary cycles(C3,C2 and C1),from bottom to top,with complex sedimentary characters and spatial distribution.In terms of the composition,texture,bedding and thickness,six lithofacies are recognized in this succession.Some geochemical parameters,such as trace elements(Sr/Ba,Na/Al,V/Ni,V/(V+Ni),U/Th),carbon and oxygen isotopes(δ^(18)O,δ^(13)C),and total organic carbon content(TOC)indicate that the shales were deposited in a deep to semi-deep lake,with the water column being salty,stratified,enclosed and reductive.During cycles C3 and C2 of the middle-lower sections,the climate was arid,and the water was salty and stratified.Laminated and laminar mudstone-limestone was deposited with moderate organic matter(average TOC 1.8%)and good reservoir quality(average porosity 6.5%),which can be regarded as favorable reservoir.During the C1 cycle,a large amount of organic matter was input from outside the basin and this led to high productivity with a more humid climate.Massive calcareous mudstone was deposited,and this is characterized by high TOC(average 3.6%)and moderate porosity(average 4%),and provides favorable source rocks.

Enrichment and exploration of deep lacustrine shale oil in the first member of Cretaceous Qingshankou Formation, southern Songliao Basin, NE China

Pure shales in the first member of Qingshankou Formation(simplified as Qing 1 Member)in the southern Songliao Basin,i.e.,the semi-deep and deep lacustrine shales,are characterized by a high content of clay minerals and poor hydrocarbon mobility,making the development of shale oil difficult.According to the drilling and testing results,the shale of Qing 1 Member can be classified into 3 lithofacies,i.e.,bedded argillaceous shale,laminated diamictite shale,and interbedded felsic shale.The TOC and brittle minerals control the enrichment of shale oil,of them,TOC controls the total oil content,in other words,the total oil content increases with the increase of TOC;while the laminae made up of brittle minerals contain a large number of bigger intergranular pores which are favorable enrichment space for movable shale oil.In consideration of the origins of the 3 lithofacies,two shale oil enrichment models are classified,i.e.,the deep lacustrine high-TOC bedded argillaceous shale(Model-I)and the semi-deep lacustrine moderate-high-TOC laminated diamictite shale(Model-II).In the Model-I,the shale is characterized by high hydrocarbon generation ability,high total oil content,abundant horizontal bedding fractures,and vertical and high angle fractures locally;the complex fracture network formed by horizontal bedding fractures and vertical fractures improve the storage capacity and permeability of the shale reservoir,increase the enrichment space for movable oil.In the Model-II,the shale is characterized by good hydrocarbon generation ability and fairly high total oil content,and as the brittle laminae contain large intergranular pores,the shale has a higher movable oil content.Based on the two models,shale oil sweet-spot areas of 2880 km2 in the southern Songliao Basin are favorable for further exploration.Aimed at the difficulties in reservoir fracturing of the lacustrine shale with a high content of clay minerals,the composite fracturing technology with supercritical carbon dioxide was used in the shale oil reservoir for the first time,realizing large-scale volume fracturing in shale with a high content of clay minerals and strong heterogeneity,marking a breakthrough of oil exploration in continental shale with a high content of clay minerals in China.

The coupling of dynamics and permeability in the hydrocarbon accumulation period controls the oil-bearing potential of low permeability reservoirs:a case study of the low permeability turbidite reservoirs in the middle part of the third member of Shahejie

The relationships between permeability and dynamics in hydrocarbon accumulation determine oil- bearing potential （the potential oil charge） of low perme- ability reservoirs. The evolution of porosity and permeability of low permeability turbidite reservoirs of the middle part of the third member of the Shahejie Formation in the Dongying Sag has been investigated by detailed core descriptions, thin section analyses, fluid inclusion analyses, carbon and oxygen isotope analyses, mercury injection, porosity and permeability testing, and basin modeling. The cutoff values for the permeability of the reservoirs in the accumulation period were calculated after detailing the accumulation dynamics and reservoir pore structures, then the distribution pattern of the oil-bearing potential of reservoirs controlled by the matching relationship between dynamics and permeability during the accumulation period were summarized. On the basis of the observed diagenetic features and with regard to the paragenetic sequences, the reservoirs can be subdivided into four types of diagenetic facies. The reservoirs experienced two periods of hydro- carbon accumulation. In the early accumulation period, the reservoirs except for diagenetic facies A had middle to high permeability ranging from 10 × 10-3 gm2 to 4207 × 10-3 lain2. In the later accumulation period, the reservoirs except for diagenetic facies C had low permeability ranging from 0.015 × 10-3 gm2 to 62× 10-3 -3m2. In the early accumulation period, the fluid pressure increased by the hydrocarbon generation was 1.4-11.3 MPa with an average value of 5.1 MPa, and a surplus pressure of 1.8-12.6 MPa with an average value of 6.3 MPa. In the later accumulation period, the fluid pressure increased by the hydrocarbon generation process was 0.7-12.7 MPa with an average value of 5.36 MPa and a surplus pressure of 1.3-16.2 MPa with an average value of 6.5 MPa. Even though different types of reservoirs exist, all can form hydrocarbon accumulations in the early accumulation per- iod. Such types of reservoirs can form hydrocarbon accumulation with high accumulation dynamics; however, reservoirs with diagenetic facies A and diagenetic facies B do not develop accumulation conditions with low accumu- lation dynamics in the late accumulation period for very low permeability. At more than 3000 m burial depth, a larger proportion of turbidite reservoirs are oil charged due to the proximity to the source rock, Also at these depths, lenticular sand bodies can accumulate hydrocarbons. At shallower depths, only the reservoirs with oil-source fault development can accumulate hydrocarbons. For flat surfaces, hydrocarbons have always been accumulated in the reservoirs around the oil-source faults and areas near the center of subsags with high accumulation dynamics.

Occurrence mechanism of lacustrine shale oil in the Paleogene Shahejie Formation of Jiyang Depression, Bohai Bay Basin, China

To determine the occurrence mechanism and mobility of shale oil in the Shahejie Formation in the Jiyang Depression, organic geochemistry analysis, thin-section petrological observation, low-temperature nitrogen adsorption, high-pressure mercury intrusion porosimetry, field emission scanning electron microscopy experiments were conducted on shale samples to reveal its storage mechanism, including pore size, ratio of adsorbed oil to free oil, mobility and its influencing factors, and mode of storage. Residual shale oil is mainly present in pores less than 100 nm in diameter under the atmospheric temperature and pressure. The lower limit of pore size for free oil is 5 nm, and the lower limit of pore size for movable oil occurrence is about 30 nm. The light components, low TOC and high porosity are the main factors contributing to the high proportion of movable oil. Each type of pore can contain residual shale oil, but not all pores have shale oil. Pore connectivity and surface wettability are the determinants of shale oil enrichment degree and enrichment state.

Origin of Quartz Cement in the Paleogene Sandstone Reservoir of Shahejie Formation,Bohai Bay Basin,China

The precipitation of authigenic quartz plays a significant role to reduce the reservoir characteristics and enhance the stiffness of the rock.The Es_(1) sandstone of Shahejie Formation is acting as a significant hydrocarbon producing rock in the Nanpu Sag.Various methods like thin section petrography,cathodoluminescence(CL),scanning electron microscope(SEM,with EDS),and electron microprobe analysis has been used to reveal the origin of quartz cement as well as to evaluate the effect of quartz cement on reservoir quality.The studied sandstone is classified as immature to mature feldspathic litharenite and lithic arkose and consists of quartz,feldspar,rock fragments and micas.Petrographic studies and SEM analysis shows that the authigenic quartz is acting a significant cement that reduces the reservoir quality.Whereas clay minerals(kaolinite and mixed layer illite to smectite)are dominant in the Es_(1) sandstone,that can reduce the reservoir quality.SEM,CL and thin section analysis reveal that there are two stages of quartz cement in the studied samples;that are pore filling authigenic cement and quartz overgrowth cement.Fluid inclusion homogenization temperatures shows that stages of quartz cement were developed with continuous process from 70℃ to 130℃.Quartz cements were generally originated from I/S reaction,feldspar dissolution,conversion of rock fragments and pressure solution.Feldspar dissolution(K-feldspar)and kaolinite to illite reaction is an insignificant silica source for the silica cement which is internally precipitated in a close system with diffusion transporting mechanism.Overall,quartz cement significantly enhance the rock strengthen and brittleness effectively as well as it reduce the overall reservoir quality.

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

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

Reconsideration of the sediment characteristics of the second member of the Triassic Xujiahe Formation in Sanhuiba,Huaying Mountain

The second member of the Triassic Xujiahe Formation in the Sanhuiba area in Huaying Mountain is an important section of sedimentary delta facies in the Chongqing Tianfu Geological Survey and Training Base. In spite of some initial efforts on its lithology and lithofacies, the existing research is yet to be completed and suffers from contradictory conclusions and inadequate evidence. Starting from the sedimentary microfacies types and cyclic features, we analyzed the vertical sequence of a sedimentary delta model and discovered that this section is a constructional delta deposit. Eight incomplete and asymmetric regressive cycles are marked off, given that frequent erosion surfaces appeared in the section. Based on the analysis of structural characteristics of every monocycle, we summarized the system of cycles, tracts and sequence characteristics of the overall section. In view of this systematic series of cycles, we discussed the features of the syndepositional tectonic movements in the area and conclude that, due to frequent erosion, migration and diversion of a sub-channel branch was brought about by oscillating movements of the crust. In a comparison with neighboring sections, we explored the cause of erosion and the relations between erosion and cycles. This study greatly strengthens existing research and theories, makes an important and comprehensive contribution to geological surveys and training in the area and provides essential suggestions for coal exploration, oil and gas evaluation and in general to exploration and development in the target stratum.

Lake Sequence Stratigraphy of the Shahejie Formation in the Zhanhua Hollow, Shandong Province

Two sequences can be identified in the sedimentary strata of the lower Tertiary ShahejieFormation in the Zhanhua hollow. The lowstand, lake-transgressive and highstand systemstracts were formed under the control of ancient structure, palaeotopography, palaeoclimate, sed-iment supply, marine-transgression, and so on. In the paper the authors present a composite se-quence stratigraphical section of the Shahejie Formation in this area, and expound the bounda-ries of parasequences, parasequences set and systems tracts and evolutions of lake sequencestratigraphy and sedimentology and discuss various factors controlling the lake level fluctuation.The differences between lake and marine sequence stratigraphy are also indicated in the paper.The lake sequence stratigraphical study should be based on a synthetic analysis of structural evo-lution, palaeomagnetism, palaeoclimatic changes, geological Well logs, seismic and logging data,palaeontological data, global sea level changes, and so on. The sequence stratigraphical evolutionis closely related to the formation, development and elimination of the whole basin and thesedimentary process.

Characteristics and evolution of lacustrine turbidite of Chang-7 Member of Yanchang Formation in northern Shaanxi

Lacustrine turbidite of Chang-7 Member in the studied area consists of siltstone and fine sandstone with respect to grain size,which is feldspathic lithic sandstone,syrosem arkose and arkose with respect to mineral constitution affected by provenance. There are such apparent signatures as lithology,sedimentary structure,sedimentary sequence and well logs,to recognize turbidite. During the paleogeographic evolution of Chang-7 Member,lake basin and deep lake are both at their maximum extent during Chang-73 stage,resulting in the deposition of Zhangjiatan shale with widespread extent and of turbidite with fragmental-like. Deep lake line is gradually moving toward lake center and turbidite sand bodies are gradually turning better with better lateral continuity,connectivity and more thickness,from stages of Chang-73,Chang-72 and Chang-71 ,which can be favorable reservoir in deep-water.

Genetic Mechanism of the Dolomite in Dolomitic Glutenite of the Shahejie Formation—A Case Study of QHD 29-2 Oilfield in Bohai Bay Basin, China

The dolomite in dolomitic glutenite of the shahejie formation in the eastern steep slope of the Shijiutuo bulge in Bohai is a high-quality reservoir, and the content of dolomite is positively correlated with reservoir physical properties. In this paper, by using thin section, core, wall core, geochemical data and analyzing petrology and mineralogy characteristic, we systematically analyzed the paleogeographic environment and genetic mechanism of this kind of dolomite and established the genetic models. The dolomite in the glutenite body has many characteristics of development, which is formed by three kinds of genesis: quasi-synergy dolomitization, buried dolomitization and hydrothermal dolomitization. The dolomite in glutenite is produced in the form of matrix, grain (sandstone, oolith), biological skeleton (conch, ostracod), clastic shell and dolomite cement. The minor elements, carbon and oxygen isotopes, trace minerals and paleontological combinations reveal that the paleogeographic environment was closed continental salt-brackish water bay, the climate was arid and hot, and the evaporation was strong. It provides favorable conditions for the production of the dolomite in dolomitic glutenite. There are three genetic models of dolomite. The first model is penecontemporaneous dolomitization. The climate was arid and hot, the aragonite and high-magnesium calcite deposited with sand and gravel. Due to the effect of evaporation, dolomitization occurred. The second model is buried dolomitization. The water from dehydration of clay minerals causes the Mg2+ in the high-magnesium formation migrating into the rock, leading to the occurrence of dolomitization. The third model is hydrothermal dolomitization. Deep faults can bring geothermal fluids into the overlying reservoir and form the hydrothermal dolomite.

Identification and geochemical significance of unusual C24 tetracyclic terpanes in Shahejie Formation source rocks in the Bozhong subbasin,Bohai Bay Basin

C_(24)tetracyclic terpanes are common compounds in source rocks and crude oils,and C_(24)17,21-secohopane is the most common and widely used source-related indicator.In this study,three unusual C_(24)tetracyclic terpanes were detected on the m/z 191 chromatogram of saturated hydrocarbons in the Shahejie Formation source rocks in the Bozhong subbasin.Based on the mass spectra characteristics,diagnostic ion fragments,retention time and comparisons with published literature,three unusual C_(24)tetracyclic terpanes were identified as 10β(H)-des-A-oleanane,10β(H)-des-A-lupane and C_(24)des-Ahopane.To the best of our knowledge,this is the first study to detect and publicly report these three compounds in source rock samples from the Shahejie Formation of the Bozhong subbasin,Bohai Bay Basin.The results indicated that 10β(H)-des-A-oleanane and 10β(H)-des-A-lupane likely originated from terre strial angiosperms,while C_(24)des-A-hopane likely originated fro m prokaryotic o rganisms.Te rrestrial angiosperms provide the material basis for the generation of compounds A and B,and the distribution and concentration of these two compounds are affected by thermal maturity.In the low maturity stage(0.5%<R_(0)<0.7%),compounds A and B are relatively enriched in the source rocks.

Distribution characteristics of delta reservoirs reshaped by bottom currents:A case study from the second member of the Yinggehai Formation in the DF1-1 gas field,Yinggehai Basin,South China Sea

The Dongfang1-1 gas field(DF1-1)in the Yinggehai Basin is currently the largest offshore self-developed gas field in China and is rich in oil and gas resources.The second member of the Pliocene Yinggehai Formation(YGHF)is the main gas-producing formation and is composed of various sedimentary types;however,a clear understanding of the sedimentary types and development patterns is lacking.Here,typical lithofacies,logging facies and seismic facies types and characteristics of the YGHF are identified based on high-precision 3D seismic data combined with drilling,logging,analysis and testing data.Based on 3D seismic interpretation and attribute analysis,the origin of high-amplitude reflections is clarified,and the main types and evolution characteristics of sedimentary facies are identified.Taking gas formation upper II(IIU)as an example,the plane distribution of the delta front and bottom current channel is determined;finally,a comprehensive sedimentary model of the YGHF second member is established.This second member is a shallowly buried“bright spot”gas reservoir with weak compaction.The velocity of sandstone is slightly lower than that of mudstone,and the reflection has medium amplitude when there is no gas.The velocity of sandstone decreases considerably after gas accumulation,resulting in an increase in the wave impedance difference and high-amplitude(bright spot)reflection between sandstone and mudstone;the range of high amplitudes is consistent with that of gas-bearing traps.The distribution of gas reservoirs is obviously controlled by dome-shaped diapir structural traps,and diapir faults are channels through which natural gas from underlying Miocene source rocks can enter traps.The study area is a delta front deposit developed on a shallow sea shelf.The lithologies of the reservoir are mainly composed of very fine sand and coarse silt,and a variety of sedimentary structural types reflect a shallow sea delta environment;upward thickening funnel type,strong toothed bell type and toothed funnel type logging facies are developed.In total,4 stages of delta front sand bodies(corresponding to progradational reflection seismic facies)derived from the Red River and Blue River in Vietnam have developed in the second member of the YGHF;these sand bodies are dated to 1.5 Ma and correspond to four gas formations.During sedimentation,many bottom current channels(corresponding to channel fill seismic facies)formed,which interacted with the superposed progradational reflections.When the provenance supply was strong in the northwest,the area was dominated by a large set of delta front deposits.In the period of relative sea level rise,surface bottom currents parallel to the coastline were dominant,and undercutting erosion was obvious,forming multistage superimposed erosion troughs.Three large bottom current channels that developed in the late sedimentary period of gas formation IIU are the most typical.

Energy Dispersive X-Ray Spectroscopy of the Sohnari Member of Laki Formation from Southern Indus Basin of Pakistan

This study determines the geochemical and depositional environment analysis of the sediments of the Sohnari Member of the Laki Formation, Northern Kirthar Fold Belt of Pakistan. The Energy-Dispersive-X-Ray Spectroscopy (EDS) technique is used for the detection of major elements and the effects of shifting depositional climatic conditions of six representative samples which were acquired from the Sohnari Member of the Laki Formation at Lakhra area, Sindh, Pakistan. The sedimentological studies clarify that the sediments the Sonahri Member are relatively immature and most migrated in clastic mode. The availability of Silica shows that the Member was formed due to biochemical precipitation and detrital mode and was deposited at a fast rate of sediment deposition under the fluvio-deltaic depositional system. This is also deduced that the rapid rate of sediment deposition might be created a reducing atmosphere and allowing for the mineralization of sulphur.