Structural features and exploration targets of platform margins in Sinian Dengying Formation in Deyang-Anyue Rift, Sichuan Basin, SW China

Based on the seismic, logging, drilling and other data, the distribution, structural types and mound-shoal hydrocarbon accumulation characteristics of platform margins of the Sinian Dengying Formation in the Deyang-Anyue Rift and its periphery were analyzed. Four types of platform margins are developed in the Dengying Formation, i.e., single-stage fault-controlled platform margin, multi-stage fault-controlled platform margin, gentle slope platform margin, and overlapping platform margin. In the Gaoshiti West-Weiyuan East area, single-stage fault controlled platform margins are developed in the Deng 2 Member, which trend in nearly NEE direction and are shielded by faults and mudstones, forming fault-controlled–lithologic traps. In the Lezhi-Penglai area, independent and multi-stage fault controlled platform margins are developed in the Deng 2 Member, which trend in NE direction and are controlled by synsedimentary faults;the mound-shoal complexes are aggraded and built on the hanging walls of the faults, and they are shielded by tight intertidal belts and the Lower Cambrian source rocks in multiple directions, forming fault-controlled–lithologic and other composite traps. In the Weiyuan-Ziyang area, gentle slope platform margins are developed in the Deng 2 Member, which trend in NW direction;the mound-shoal complexes are mostly thin interbeds as continuous bands and shielded by tight intertidal belts in the updip direction, forming lithologic traps. In the Gaoshiti-Moxi-Yanting area, overlapping platform margins are developed in the Deng 2 and Deng 4 members;the mound-shoal complexes are aggraded and overlaid to create platform margin buildup with a huge thickness and sealed by tight intertidal belts and the Lower Cambrian mudstones in the updip direction, forming large-scale lithologic traps on the north slope of the Central Sichuan Paleouplift. To summarize, the mound-shoal complexes on the platform margins in the Dengying Formation in the Penglai-Zhongjiang area, Moxi North-Yanting area and Weiyuan-Ziyang area are large in scale, with estimated resources of 1.58×1012 m3, and they will be the key targets for the future exploration of the Dengying Formation in the Sichuan Basin.

Petrophysical properties and their influencing factors of carbonates in the fourth member of Sinian Dengying Formation, Sichuan Basin, SW China

Based on the measurements of petrological, petrophysical and elastic properties of the samples of different sedimentary facies in the fourth member of Sinian Dengying Formation (Deng 4 Member) in the Sichuan Basin, the diagenetic processes of reservoirs of different sedimentary facies and their controls on the petrophysical properties were discussed. The results show that cracks and mineral composition jointly control the petrophysical properties, and both are significantly influenced by sedimentary environment and diagenesis. The microbial dolomite of mound-shoal facies mainly experienced multi-stage dolomitization, penecontemporaneous dissolution, tectonic rupture and hydrothermal/organic acid dissolution processes, giving rise to cracks and dissolved pores. The grannular dolomite of inter-mound-shoal bottomland or dolomitic lagoon facies mainly underwent mechanical compaction, burial dolomitization and tectonic-hydrothermal action, creating cracks and intercrystalline pores. The diagenesis related to crack development increases the pressure- and saturation-dependent effects of samples, leading to significant decrease in the compressional wave impedance and Poisson's ratio. Dolomitization changes the properties of mineral particles, resulting in a Poisson's ratio close to dolomite. The muddy, siliceous and calcareous sediments in the low-energy environment lead to the decrease of impedance and the differential change of Poisson's ratio (significantly increased or decreased). The samples with both cracks and dissolved pores show high P-wave velocity dispersion characteristics, and the P-wave velocity dispersion of samples with only fractures or pores is the lowest.

Hydrocarbon accumulation conditions and exploration targets of the Sinian Dengying Formation in the Southeastern Sichuan Basin

No breakthrough has been made in petroleum exploration of the Sinian Dengying Formation in the southeastern Sichuan Basin as the major factors controlling the hydrocarbon accumulation of the Dengying Formation have not been well established,including the development period and distribution of the platform margin,the effectiveness of the source-reservoir combination and the reliability of the Cambrian pre-salt structural trap.Based on the interpretation of seismic data and hydrocarbon-source correlation,the distribution of the platform margin of the Sinian Dengying Formation in the southeastern Sichuan Basin was mapped.Moreover,the gas source rocks of the Dengying Formation were clarified according to geochemical characteristics of fluid inclusions,and the pre-salt structures were further confirmed by seismic forward modeling and experiments of prestack depth migration.Results showed that the sedimentary facies of the 4th member of Dengying Formation in the southeastern Sichuan Basin was platform margin of gentle slope type.The platform margin reservoir of the Dengying Formation and the Lower Cambrian source rocks constitute a favorable source-reservoir combination.The carbon isotope characteristics of gas in fluid inclusions indicate that some of the oil/gas in the Dengying Formation were contributed by the Doushantuo Formation source rocks.As the difference in seismic velocities between carbonate rocks and gypsum/salt beds may cause a reduction in the amplitude of pre-salt structures,the low-amplitude structures identified on current seismic profiles are underestimated instead of overestimated.Accordingly,the Sinian structural traps are confirmed to develop in the deep pre-salt sequences in the low-steep anticline belt,southeastern Sichuan Basin.On the basis of the above new understanding,we propose two types of hydrocarbon migration modes for the Sinian Dengying Formation in southeastern Sichuan Basin,including lateral migration of hydrocarbons generated from the Cambrian Qiongzusi Formation and vertical migration of hydrocarbons derived from the Sinian Doushantuo Formation.Lithologic traps at the platform margin and pre-salt structural traps within the platform are favorable targets for natural gas exploration.

Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation, East Sichuan Basin, China

Hydrothermal Dolomite （HTD） is present in the Upper Sinian （Upper Proterozoic） Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The δ13C and δ18O isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while STSr/86Sr is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in S7Sr/S6Sr isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320℃. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are： （1） dolomite host rock →sphalerite-galena-barite-fluorite; （2） dolomite host rock →saddle dolomite →quartz; （3） dolomite host rock →saddle dolomite→bitumen; （4） dolomite host rock →saddle dolomite →barite. The mean chemical composition of the host dolomite matrix and HTD didn＇t change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation ＆ expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement. The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%-5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis （Z2-C1） and the later one to the Emei taphrogenesis （D2-T2）. But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.

Occurrences and Formation Mechanisms of Botryoidai Structures from the Sinian Dengying Formation,Sichuan Basin,China

ObjectiveThis study aims to characterize the occurrences and interior structural features of botryoidal structures from the Sinian Dengying Formation in the Sichuan Basin of Southwestern China, and to shed light on their formation mechanism.

Hydrocarbon accumulation and exploration prospect of mound-shoal complexes on the platform margin of the fourth member of Sinian Dengying Formation in the east of Mianzhu-Changning intracratonic rift, Sichuan Basin, SW China

Drilling,seismic and logging data were used to evaluate the hydrocarbon accumulation conditions of the mound-shoal complexes in the platform margin of the fourth member of Sinian Dengying Formation in the east side of the Mianzhu-Changning intracratonic rift in the Sichuan Basin.The four understandings are:(1)The platform margin belt of the Deng 4 Member can be divided into three sections,northern,middle and southern;the middle section is at the core of the Gaoshiti-Moxi paleouplift and the structural high now,while the southern and northern sections are at the slope of the paleouplift and the structural lows now;the three sections have similar development characteristics and reservoir features of platform margin mound-shoal complex.(2)In the margin of the east side of the rift,there are several faults nearly perpendicular to the platform margin belt,the faults divide the platform margin belt into rugged paleo-landform,and the high part developed platform margin mound-shoal complexes and the reservoirs are good in physical properties,while the low part developed inter-beach depression and no mound-shoal complexes,where the reservoirs are poor in physical properties.(3)The six groups of faults nearly perpendicular to the platform margin belt divide the platform margin belt into seven large mound-shoal complexes which have similar hydrocarbon accumulation conditions and accumulation evolution process and are rich in petroleum.(4)The inter shoal depressions between the mound-shoal complexes are characterized by tighter lithology,which can block the updip direction of the mounds and shoals at the lower part of the slope of the paleouplift and are favorable for the later preservation of mound-shoal gas reservoirs.This has been proved by Well Jiaotan 1 and Heshen 2 drilled successfully.The mound-shoal complexes on the platform margin of the structural slope area have a good exploration prospect.

Sequence sedimentary evolution and reservoir distribution in the third and fourth members of Sinian Dengying Formation,Gaomo area,Sichuan Basin,SW China

Based on comprehensive analysis of cores,thin sections,logging and seismic data,the sequence stratigraphy and sedimentary evolution of the third and fourth members of Sinian Dengying Formation(Deng 3 and Deng 4 members for short)in the Gaomo area of Sichuan Basin were investigated,and the favorable zones for reservoir development in the Deng 3 Member and Deng 4 Member were predicted.(1)Two Type I and one Type II sequence boundaries are identified in the Deng 3 and Deng 4 members.Based on the identified sequence boundaries,the Deng 3 and Deng 4 members can be divided into two third order sequences SQ3 and SQ4,which are well-developed,isochronal and traceable in this area;the SQ3 thins from west to the east,and the SQ4 thins from northwest to southeast.(2)The sedimentary environment from the depositional period of SQ3 to SQ4 has experienced the evolution from mixed platform to rimmed platform,and the platform rimmed system on the west side is characterized by the development of platform margin microbial mound and grain shoal assemblages.The intraplatform area is a restricted platform facies composed of a variety of dolomites,and there are local micro-geomorphic highlands of different scales and scattered intraplatform mounds and shoals.(3)The Deng 4 Member reservoirs,with obvious facies-controlled characteristic,are mainly distributed in the upper part of high-frequency upward shallow cycle and the high-stand systems tract of the third-order sequence vertically,and are more developed in the platform margin belt than in the intraplatform belt,and more developed in the Gaoshiti platform margin belt than in the west Suining platform margin belt on the plane.(4)Three types of favorable reservoir zones of Deng 4 Member have been finely delineated with 3D seismic data;among them,the mound and shoal facies zones developed in the ancient highlands of the intraplatform are the first choice for the next exploration and development of the Deng 4 Member.

Reservoir characteristics and genetic differences between the second and fourth members of Sinian Dengying Formation in northern Sichuan Basin and its surrounding areas

This study investigated the characteristics and genesis of reservoirs in the 2^(nd) and 4^(th) members of Sinian Dengying Formation in northern Sichuan and its surrounding areas, on the basis of outcrop, drilling cores and thin section observation and geochemical analysis. The reservoirs of 2^(nd) member are distributed in the middle part of the stratum. The reservoir quality is controlled by supergene karst and the distribution of mound-shoal complex. The bedded elongated isolated algal framework solution-cave and the residual "grape-lace" cave, which are partially filled with multi-stage dolosparite, constituted the main reservoir space of the 2^(nd) member. There is no asphalt distribution in the pores. The pore connectivity is poor, and the porosity and permeability of the reservoir is relatively low. The reservoirs of 4^(th) member were distributed in the upper and top part of the stratum. The reservoir quality is controlled by burial dissolution and the distribution of mound-shoal complex. The bedded algal framework solution-pores or caves, intercrystalline pores and intercrystalline dissolved pores constituted the main reservoir space of the 4^(th) member. It's partially filled with asphalt and quartz, without any dolosparite fillings in the pores and caves. The pore connectivity is good. Most of the 4^(th) member reservoirs had medium-low porosity and low permeability, and, locally, medium-high porosity and medium permeability. Affected by the development of mound-shoal complex and heterogeneous dissolution, the platform margin along Ningqiang, Guangyuan, Jiange and Langzhong is the most favorable region for reservoir development. Deep buried Dengying Formation in the guangyuan and langzhong areas should be the most important hydrocarbon target for the future exploration.

Characterization of the Microbial Dolomite of the Upper Sinian Dengying Formation in the Hanyuan Area of Sichuan Province,China

The algal dolostone of the Upper Sinian Dengying Formation （corresponding to the Ediacaran system） in the Upper Yangtze Platform of China possesses a rich diversity of microorganisms and is an ideal site for the study of ancient microbial dolomite. We focused on algal dolostone and its microbial dolomite in the Hanyuan area of Sichuan Province, China. The macroscopic petrological features, microscopic morphology, texture characteristics of the fossil microorganisms and microbial dolomite, and geochemical characteristics were investigated. We found rich fossil microorganisms and microbial dolomites in the laminated, stromatolithic, uniform and clotted （algal） dolostones. The microorganisms present were mainly body fossils of cyanobacteria （including Renalcis, Girvanella, Nanococcus, and Epiphyton） and their trace fossils （including microbial mats （biofilms）, algal traces, and spots）. In addition, there was evidence of sulfate-reducing bacteria （SRB）, moderately halophilic aerobic bacteria, and red algae. The microbial dolomites presented cryptocrystalline textures under polarizing microscope and nanometer-sized granular （including spherulitic and pene-cubical granular） and （sub） micron-sized sheet-like, irregular, spherical and ovoidal morphologies under scanning electron microscope （SEM）. The microbial dolomites were formed by microbialiy induced mineralization in the intertidal zone and lagoon environments during the depositional and syngenetic stages and microbially influenced mineralization in the supratidal zone environment during the penecontemporaneous stage. The microbial metabolic activities and extracellular polymeric substances （EPS） determined the morphology and element composition of microbial dolomite. During the depositional and syngenetic stages, the metabolic activities of cyanobacteria and SRB were active and EPS, biofilms and microbial mats were well-developed. EPS provided a large number of nucleation sites. Accordingly, many nanometer-sized pene-cubical granular and （sub） micron-sized sheet-like microbial dolomites were formed. During the penecontemporaneous stage, SBR, cyanobacteria, and moderately halophilic aerobic bacteria were inactive. Furthermore, nucleation sites reduced significantly and were derived from both the EPS of surviving microorganisms and un-hydrolyzed EPS from dead microorganisms. Consequently the microbial dolomites present nanometer-sized spherulitic and micron-sized irregular, spherical, and ovoidal morphologies. Overall, the microbial dolomites evolved from nanometer-sized granular （including spherulitic and pene-cubical granular） dolomites to （sub） micron-sized sheet-like, irregular, spherical and ovoidal dolomites, and then to macroscopic laminated, stromatolithic, uniform, and clotted dolostones. These findings reveal the correlation between morphological evolution of microbial dolomite and microbial activities showing the complexity and diversity of mineral （dolomite）-microbe interactions, and providing new insight into microbial biomineralization and microbial dolomite in the Precambrian era.

Characteristics and development model of karst reservoirs in the fourth member of Sinian Dengying Formation in central Sichuan Basin,SW China

The reservoir space,types and distribution characteristics of karst carbonate gas reservoirs in the fourth member of Sinian Dengying Formation(Deng 4 Member)in central Sichuan Basin are analyzed based on the drilling,logging and seismic data.A development model of karst reservoirs is constructed to support the high-efficiency development of gas pools.The research shows that the reservoirs in Deng 4 Member have mainly small-scale karst vugs and fractures as storage space,and can be divided into three types,fracture-vug,pore-vug and pore types.The development patterns of the karst reservoirs are determined.On the plane,the karst layers increase from 65 m to 170 m in thickness from the karst platform to the karst slope,and the high-quality reservoirs increase from 25.0 m to 42.2 m in thickness;vertically,the reservoirs at the top of Deng 4 Member appear in multiple layers,and show along-bedding and along fracture dissolution characteristics.The reservoirs at the bottom are characterized by the dissolution parallel to the water level during the karstification period,and have 3-5 large-scale fracture-cave systems.Based on the reservoir development characteristics and the genetic mechanism,three types of reservoir development models of karst reservoir are established,i.e.,bed-dissolved body,fracture-dissolved body and paleohorizon-dissolved body.The construction of karst reservoir development models and seismic response characteristics of the three types of reservoirs can provide parameter for well placement and trajectory design,and substantially improve productivity and development indices of individual wells and gas reservoirs.The designed production capacity of the gas reservoir has enhanced from the initial 3.6 billion to 6 billion cubic meters,making the profit of the reservoir development increase noticeably.

Multi-Stage Fluid Charging and Critical Period of Hydrocarbon Accumulation of the Sinian Dengying Formation in Central Sichuan Basin

Objective The natural gas exploration of the Sinian reservoirs in the central Sichuan Basin has made a significant breakthrough in recent years, and has thus attracted much attention among geologists. The Sichuan Basin is known to have complicated geological settings, which has experienced multiple stages of tectonic evolution, fluid charging and hydrocarbon accumulation. This research aims to determine the geochemical characteristics of each stage of fluids, the features and time interval of fluid activity in different geologic periods, and further to restore the critical period and geological age of the hydrocarbon accumulation.

Geochemical characteristics of upper Sinian Dengying dolomite at northern Yangtze platform: implication for Pb-Zn metallogenesis

1 Introduction and geological setting Several Pb-Zn ore deposits(i.e.,Mayuan Pb-Zn deposit)occurring inthe upper Sinian Dengying dolomitesin northern Yangtze platform have been discovered in recent years,interesting research of

Characteristics and main controlling factors of reservoirs in the fourth member of Sinian Dengying Formation in Yuanba and its peripheral area, northeastern Sichuan Basin, SW China

Based on the abundant outcrops and 3 D seismic data in Yuanba and its surrounding areas in northeastern Sichuan Basin,the characteristics,main controlling factors and development regularities of reservoirs in the fourth member of Sinian Dengying Formation(shortened as Deng 4 Member)were analyzed comprehensively.The results show that the reservoirs were mainly developed in microbial dolomite associated with the microbial mound and dolarenite associated with grain beach,and had intergranular dissolution pores,inter-frame dissolution pores and karst caves as main storage space.The reservoirs were large in scale and strong in heterogeneity.The analysis of the reservoir genetic mechanisms shows that the reservoirs in the fourth member of Dengying Formation were the products of karstification on mound-beach facies deposited in the second episode of Tongwan movement.The karstification had the marks of eogenetic karstification such as granophyric karst system and half-dissociation zone.The reservoirs were controlled by the sedimentary facies belt,the mound beach facies was the decisive factor in reservoir development.Further analysis indicates that the Yuanba area had a geological background of well-developed platform margin mound beach facies,so mound beach bodies can be taken as a favorable exploration target.

Breakthrough and significance of technology on internal multiple recognition and suppression: A case study of Ordovician Dengying Formation in Central Sichuan Basin, SW China

Aimed at the serious mismatch between the synthetic seismogram and the real data of the Sinian Dengying Formation and the Lower Cambrian Qiongzhusi Formation in the GS1 well area, Sichuan Basin, four aspects of internal multiples identification and suppression were studied. Firstly, a forward modeling method of internal multiple based on reflectivity method was developed. Through eight means such as post-stack and pre-stack forward modeling of internal multiple, and combined with VSP data, it was demonstrated that well-seismic mismatching in this area is mainly caused by the internal multiples. Secondly, the simulation results of internal multiple forward modeling using the stripping method combined with the internal multiple periodicity analysis showed that four groups of overburden layers with velocity inversion were the main sources of the internal multiples. Thirdly, by identifying internal multiples accurately and using suppression technology based on pattern recognition, an effective and replicable suppression scheme suitable for these formations was established, overcoming the difficulty of the small speed difference between internal multiple and primary reflection wave which makes the current methods ineffective. Fourthly, an evaluation index of internal multiple intensity was proposed, and the internal multiple intensity distribution diagram of the fourth member of Dengying Formation(Deng-4 Member) in Gaoshiti-Moxi area was compiled. This scheme greatly improved the well-seismic match, and the strata sedimentary features are clearer on the new seismic profiles with higher lateral resolution, with which smaller faults and geological anomalies can be identified and a series of the bead reflections in the Dengying Formation are first discovered. The coincidence rate of reservoir prediction based on seismic waveform classification was increased from 60% to 90%, and that of hydrocarbon detection based on dual phase medium theory was increased from 70% to 100%.

Genesis of ultra-deep dolostone and controlling factors of largescale reservoir:A case study of the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin

This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirs,focusing on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin.The study involves petrology,microscale X-ray diffraction,trace element analysis,and C-O-Sr-Mg isotope experiments to provide a detailed analysis.The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrix dolostone and four types of cement.The Dengying Formation,which developed under a sedimentary background of a restricted platform,contains special microbial and microcrystalline dolostones.The dolomite grains are small(<30μm)and have a low order degree(Min=0.55),with large unit cell parameters and an extremely high Na content(Max=788 ppm).The^(87)Sr/^(86)Sr value of the dolostone is consistent with contemporaneous seawater,while the δ^(13)C andδ^(18)O values are lower than those of the contemporaneous seawater.The δ^(26)Mg value is small(Min=-2.31‰).Powder crystal,fine-crystalline,and calcite dolostones with coarser and more ordered crystals exhibit similar δ^(13)C and^(87)Sr/^(86)Sr values to microbial and microcrystalline dolostone.During the sedimentary period of the Dengying Formation,ancient marine conditions were favorable for microbial survival.Microorganisms induced the direct precipitation of primary dolomite in seawater,forming microbial and microcrystalline dolostones during the seawater diagenesis period.During the subsequent diagenesis period,dolostones underwent the effects of dissolution-recrystallization,structures,and hydrothermal fluids.This resulted in the formation of dolostone with coarser crystals,a higher degree of order,and various types of cement.The Longwangmiao Formation was developed in an interplatform beach characterized by special particle dolostone.The particle dolostone has a large grain size(>30μm),high order degree(Min=0.7),small unit cell parameters,high Na content(Max=432 ppm),and low Fe and Mn content.The δ^(26)Mg and δ^(13)C values are consistent with the contemporaneous seawater,while the δ^(18)O and^(87)Sr/^(86)Sr values are higher than those of the contemporaneous seawater.There is mutual coupling between multiple-period varying δ^(26)Mg values and sedimentary cycles.The dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater.The thickness and scale of the dolostone in the Longwangmiao Formation are controlled by the periodic changes in sea level.The period of dolostone development from the Sinian to the Cambrian coincides with the transition from Rodinia’s breakup to Gondwana’s convergence.These events have resulted in vastly different marine properties,microbial activities,and sedimentary climate backgrounds between the Sinian and the Cambrian.These differences may be the fundamental factors leading to the distinct origins of dolostone formed in the two periods.The distribution of sedimentary facies and deep tectonic activities in the Sichuan Basin from the Sinian to the Cambrian is influenced by the breakup and convergence of the supercontinent.This process plays a key role in determining the distribution,pore formation,preservation,and adjustment mechanisms of ultra-deep dolostone reservoirs.To effectively analyze the genesis and reservoir mechanisms of ultra-deep dolostone in other regions or layers,especially during the specific period of supercontinent breakup and convergence,it is crucial to consider the comprehensive characteristics of seawater properties,microbial activities,sedimentary environment,and fault systems driven by tectonic activities.This can help predict the distribution of high-quality and large-scale ultra-deep dolostone reservoirs.

Analysis of the Oldest Carbonate Gas Reservoir in China——New Geological Significance of the Dengying Gas Reservoir in the Weiyuan Structure, Sichuan Basin

The Weiyuan Structure is the largest surface structure in the Sichuan Basin. However, the abundance of the Dengying Formation gas reservoir in the Weiyuan Structure is low. The height of the gas column is 244 m, but the integrated abundance is only 26.4%. After nearly 40 years of exploration, the Gaoshi1 Well and Moxi8 Well yielded gas flows that marked an important exploration success after the discovery of the Sinian Dengying Formation gas reservoir in the Weiyuan Structure, Sichuan Basin, Lower-Paleozoic in 1964. Combined with research examples of oil and gas migration and gas chimneys around the world, the authors used comprehensive geological-geophysical-geochemical research methods to provide a reasonable explanation of the low abundance of the gas reservoir in the Weiyuan Structure based on the surface and subsurface data. The latest research results show that(1) currently, the Weiyuan Structure is the apex of the Dengying Formation in the Mid-Sichuan Basin. The Guang'an, Longnüsi, Gaoshiti-Moxi, and Weiyuan structures are a series of traps in the Dengying Formation with gradual uplifting spill and closure points during the regional uplift of the Himalayan period. The natural gas of the Dengying Formation accumulated in different ways over a wide range and long distance in the Sichuan Basin.(2) At approximately 40 Ma, the Weiyuan area started to uplift and form the present structure, and it is the only outcropped area with the Triassic Jialingjiang Formation and Leikoupo Formation in the surface of the Sichuan Basin(except the steep structural belt in East Sichuan). Caused by the uplift and denudation, the core of the Weiyuan Structure has formed an escaping "skylight" for natural gas. The evidence of a gas chimney includes(1) the component percentage of non-hydrocarbon gas, which decreased from the bottom to the top,(2) the pressure coefficient is normal because the gas reservoir from the Upper Sinian to the Lower Permian commonly have a normal pressure coefficient(an average of 1.0), and(3) the isotope geochemistry of the argon mostly represents abiogenic characteristics of a deep source, and the 40 Ar/36 Ar ratio is as high as 2 855–5 222 in the Upper Permian. All of these characteristics provide sufficient evidence for a gas chimney effect. The characteristics of low abundance in the Weiyuan Structure can be a reference example for studying the late reconstruction of deep oil and gas reservoirs in the superimposed basins of western China.

Cementation and diagenetic fluid of algal dolomites in the Sinian Dengying Formation in southeastern Sichuan Basin

The Sinian Dengying Formation in China hosts one of the oldest gas-bearing reservoirs in the world.With a long geological history,it has developed into the reservoir rocks,whose quality depends on chemical dissolution and cementation.Despite presence of cavities and pores in the algal dolomites of the Dengying Formation,they were mostly filled with various forms of dolomite cements.Therefore,the investigation of these dolomites and their evolution is significant for analyzing the mechanism of pore space preservation.There are five types of cements in the Dengying Formation,which were well sampled for lab measurements such as thin section,cathodeluminescence,minor elements,isotope analysis of carbon,oxygen and strontium,in attempt to explore the fillers' geochemical characteristics and the diagenetic fluids.They are characterized as follows:firstly,fibrous rim dolomite cement is a typical product formed at sea bottom,and geochemically very similar to the matrix(micrite and microcrystalline dolomite),which could be a reflection of the geochemical characteristics of water at that time;secondly,the diagenetic fluids of the foliated dolomite cements tend to be inheriting,derived from the shallow burial "imprisoned" seawater;thirdly,fine-to medium-grained dolomite cements with fairly high strontium isotopes and low carbon and oxygen isotopes were precipitated chiefly in freshwater during the period of uplifting caused by Tongwan tectonic movement;fourthly,coarse-grained dolomite cements bear a relatively high percentage of Mn element,but low in Fe and Sr,with carbon and oxygen isotopes remarkably shifted to negative values,which all show that they are related to hydrocarbon maturation and migration,and corresponding Thermochemical Sulfate Reduction;lastly,saddle-shaped dolomite cements are hydrothermally associated,and they were formed in high-temperature brine,which had passed through the areas rich in radioactive strontium isotopes.

Timing the termination of the Doushantuo negative carbon isotope excursion: evidence from U-Pb ages from the Dengying and Liuchapo formations,South China

The Doushantuo negative carbon isotope excursion(DOUNCE) is the largest known marine inorganic carbon isotope anomaly. The origin of this pronounced negative excursion is still an enigmatic issue that attracts geologists. Time constraints on the excursion are the critical information that would provide insight into its genesis. In previous decades, the timing of its termination has been constrained by the widely cited zircon U-Pb age of 550.5 ± 0.8 Ma for the tuff at the top of the Miaohe Member at the Jiuqunao section in the Yangtze Gorges area, South China. However, results of recent studies indicate that the reliability of this time constraint needs to be re-evaluated. Here, a geochronological study was carried out using two K-bentonites from Fanglong in South China. A K-bentonite in the lower Dengying Formation yielded a U-Pb age of 557 ± 3 Ma, while a K-bentonite in the basal Liuchapo Formation yielded an age of 550 ± 3 Ma. Based on regional correlations between the Ediacaran successions in South China,the age(557 ± 3 Ma) for the K-bentonite in the lower Dengying Formation may serve as a second critical timing constraint for the ending of the DOUNCE. Combined with available estimates of the DOUNCE duration, our new data indicate that the DOUNCE has a maximum onset age ～570 Ma.

Paired δ^13Ccarb-δ^13Corg Evolution of the Dengying Formation from Northeastern Guizhou and Implications for Stratigraphic Correlation and the Late Ediacaran Carbon Cycle

This study provides δ^13C profiles from a lower-slope(Well ZK102)to basin(Bahuang Section)environment to better understand the temporal and spatial variability in δ^13Ccarb-δ^13Corg of the Yangtze Block during the Late Ediacaran.Our new δ^13C profiles together with the reported data suggest that the Upper Ediacaran successions from different depositional environments are generally bounded by negative δ^13Ccarb and/or δ^13Corg excursions in the underlying and overlying strata.Moreover,the Upper Ediacaran δ^13Ccarb profiles generally can be subdivided into two positive excursions and an interjacent negative excursion,whereas the paired δ^13Corg profiles from different depositional environments have individual variation trends.On the other hand,these data show a large surface-to-deep waterδ^13C gradient(~5‰variation in δ^13Ccarb,>10‰variation in δ^13Corg)which can be reasonably explained by the heterogeneity of the biological activities in the redox-stratified water column.Furthermore,the decoupled δ^13Ccarb-δ^13Corg pattern with large δ^13Corg perturbations at the lower slope precluded the existence of a large dissolved organic carbon reservoir at the Yangtze Block during the Late Ediacaran.Thus,the high δ^13Ccarb values in the Upper Ediacaran succession could be balanced by large amounts of buried organic carbon likely associated with high productivity.

Characteristics of dolomite karstic reservoir in the Sinian Dengying Formation,Sichuan Basin

The Sinian dolomite reservoir in Dengying Formation was developed extensively in the Sichuan Basin,and typically was the facies-controlled dolomite karstic reservoir.The development of this kind of the reservoir was related with the special deposits,diagenetic environment as well as the multi-phase and multi-type dissolution and fracturing actions.The arid-hot/dry paleo-climate was favourable to develop the large-scale microbial reef-shoal bodies and moundeshoal bodies,which was characterized by the organic framework and the sea floor hardening,thus forming primary matrix pore-type reservoir.In the background of the arid-hot/dry paleo-climate,the water-rock interaction was weak,the mineral stabilization process was slowly,and the vadose pisoliths,aragonite and high-magnesium calcite were well preserved,therefore,this diagenetic environment especially was beneficial for pore preservation.Moreover,the syngenetic and penecontemporaneous dissolution,especially three phases of weathered crust karstification due to three episodes of the Tongwan movement,led to further dissolution of spaces and form numerous non-fabric selective dissolution fractures and dissolution caves along faults and fractures.Multi-phase fractures,especially the late Yanshanian-Himalayan tectonic fractures,resulted in connection among pores,vugs and dissolution caves,and improved quality of the reservoir tremendously.