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.

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.

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.

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.

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.

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.

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.

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%.

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.

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.

Types of biogenic quartz and its coupling storage mechanism in organic-rich shales: A case study of the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation in the Sichuan Basin, SW China

Biogenic quartz in the Upper Ordovician Wufeng Formation to Lower Silurian Longmaxi Formation(WufengLongmaxi)shale layers in the Sichuan Basin and its periphery is qualitatively analyzed and quantitatively characterized by organic petrologic,mineralogic,and geochemical methods to find out the coupling effect between organic matter and quartz.(1)There are two types of biogenic quartz in the shale layers:Type I quartz is submicron quartz appearing in clusters around the organic matter.Type II quartz is in nano-scale grain size and floats in spherical shape on organic matter,with grains in point-to-point or surface-to-surface contact;this type of quartz is mainly biologic origin and slightly affected by hydrothermal activity in local parts.(2)The reservoirs in the Wufeng-Longmaxi formations is consistent in distribution with biogenic silica content in them,and mainly concentrated at the bottom of the Wufeng-Longmaxi formations,and is thinner in the Changning and Weiyuan regions,while thicker in the Fuling region.(3)The biogenic quartz in the Wufeng-Longmaxi shale worked through the entire evolution process of hydrocarbon generation.The presence of biogenic quartz can enhance the development of organic matter pores and microcracks,and can effectively preserve the organic matter pores and residual intergranular pores,forming"biological silicon intergranular pores,organic pores and micro-fractures".This would benefit later hydraulic fracturing and result in high production/stable production of well.The coupling effect between biogenic quartz development and organic matter evolution and hydrocarbon generation is a critical factor for high-quality shale reservoir development.

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.

Petrogenesis of the Dengying Formation Dolomite in Northeast Sichuan Basin,SW China:Constraints from Carbon-Oxygen Isotopic and Trace Elemental Data

The widespread dolomite of the Sinian Dengying Formation in the Sichuan Basin(China)serves as one of the most important oil and gas reservoir rocks of the basin.Well WT1,as an exploration well,is recently drilled in the Kaijiang County,northeastern Sichuan Basin(SW China),and it drills through the Dengying Formation dolomite at the depth interval of 7500–7580 m.In this study,samples are systematically collected from the cores of that interval,followed by new analyses of carbon-oxygen isotope,major elements,trace elements,rare earth elements(REEs)and EP-MA.The Dengying Formation dolomites of Well WT1 haveδ13C values of 0.37‰to 2.91‰andδ18O values of-5.72‰to-2.73‰,indicating that the dolomitization fluid is derived from contemporary seawater in the near-surface environment,rather than the burial environment.Based on the REE patterns of EPMA-based in-situ data,we recognized the seawater-sourced components,the mixedsourced components and the terrigenous-sourced components,indicating the marine origin of the dolomite with detrital contamination and diagenetic alteration.Moreover,high Al,Th,and Zr contents indicate significant detrital contamination derived from clay and quartz minerals,and high Sr/Ba and Sr/Cu ratios imply a relatively dry depositional environment with extremely high seawater salinity,intensive evaporation,and strong influences of terrigenous sediment.

川北震旦系灯影组软沉积物变形构造及成因分析

四川盆地北缘震旦系灯影组发育软沉积变形构造,其成因机制尚不明确。结合野外露头对川北广元地区灯影组软沉积变形构造发育特征、分布及成因进行系统分析。研究表明,广元地区震旦系灯影组三段软沉积变形构造主要分布于中下部灰白色薄层灰岩及灰绿色—紫红色薄层泥岩中,可见液化脉、液化卷曲变形、液化角砾岩及层内断层4种典型构造。灯三段早期浅水、较安静水体沉积物受同期循环剪切应力及地层应力复合作用,为持续性古地震诱发的液化变形所致。垂向上可识别出2期不完整振动液化序列,表明灯三段至少存在2次持续的古地震活动。综合分析指示上扬子克拉通北部灯三段沉积期构造活动强烈,地震活动应与桐湾运动Ⅰ幕晚期裂陷槽的拉张活动相关。

四川盆地蓬莱地区灯影组四段超深层白云岩储集层特征及主控因素

作为四川盆地超深层油气资源的主力产层之一,灯影组超深层碳酸盐岩储集层发育特征及其控制因素亟待进一步厘清。选取四川盆地内最新勘探突破的蓬莱地区灯影组灯四段超深层白云岩为主要研究对象,在野外露头、钻井岩心、显微薄片、阴极发光、扫描电镜及物性测试数据等资料的基础上,开展了储集层特征及主控因素等方面的研究。结果表明:(1)蓬莱地区灯四段储集岩主要包括凝块石白云岩、叠层石白云岩、泡沫绵层白云岩和砂屑白云岩;储集空间主要包括砂屑白云岩的晶间孔、晶间溶孔和溶洞,凝块石白云岩、叠层石白云岩和泡沫绵层白云岩的原生格架孔洞和后期溶蚀作用形成的溶蚀孔缝等。(2)凝块石白云岩和叠层石白云岩多发育孔洞缝型和孔洞型储集层,均具有较好的物性,孔隙度能达到4%以上,但前者平均渗透率可达2.43×10^(-3)μm^(2),后者仅为0.35×10^(-3)μm^(2)。(3)综合分析成岩演化史,认为灯四段储集层先后经历了沉积—准同生阶段、表生阶段、浅—中埋藏阶段和深层—超深层埋藏阶段的成岩改造。(4)蓬莱地区灯四段储集层发育具有自台地内部向台地边缘逐渐减弱的分布规律,总体上显示“相—断裂—溶蚀”三元综合成储模式。本研究明确了四川盆地蓬莱地区超深层白云岩储集层的特征及主控因素,可为后续超深层油气资源勘探开发提供理论基础。

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.

中上扬子地区震旦纪灯影组沉积期碳酸盐岩台地古地理格局及有利储集相带分布规律

【目的】中上扬子地区灯影组沉积期碳酸盐岩台地古地理格局及有利储集相带发育分布规律尚存争议,阻碍了灯影组油气勘探领域由绵阳—长宁拉张槽两侧向其他广大地区拓展。【方法】通过中上扬子地区灯影组30余条剖面(钻井)开展岩石类型划分及沉积相分析,【结果】进一步明确丘滩相以块状砂屑/鲕粒白云岩及柱状、穹窿状叠层石白云岩为特征,呈环带状分布在中上扬子台地(或内缓坡)边缘,向外过渡为中—外缓坡风暴岩或斜坡—盆地相区的滑塌角砾白云岩、泥质白云岩、硅质岩等。由于丘滩的阻挡作用,其内侧以低能的潮坪及潟湖沉积环境为主:其中潮坪相主要发育微生物纹层白云岩、穹窿—缓波状叠层石白云岩、凝块石白云岩夹砂屑白云岩、泥晶白云岩,纵向上常常形成米级—厘米级旋回;潟湖相主要由泥晶白云岩、含球粒/砂屑泥晶白云岩组成。相带时空分布表明灯影组在中上扬子地区总体呈现出进积的沉积趋势,内部包含2.5个层序,即三次进积(变浅)和两次退积(变深)旋回。【结论】由于灯影组在台地边缘—斜坡区域普遍遭到剥蚀,以及灯影组沉积期上扬子台地范围远超目前的板块边界,灯影组上部灯四段很少发现高能丘滩相带。在此背景下,局部地区(如松林—岩孔地区、中扬子台地周缘)灯影组中下部灯一段—灯二段(或蛤蟆井段、石板滩段)发育的丘滩相,是重要的有利储集相带。此外,处于海退体系域的灯二段中下部和灯四段上部(或相当层位)在中上扬子台地(或内缓坡)内部广泛发育微生物白云岩,也为储层发育有利相带。在中上扬子台地微生物白云岩广泛分布的背景下,进一步分析优质储层发育机理与分布规律才是找到油气接替区的关键。

四川盆地德阳-安岳凹槽形成机理及灯影组沉积新格局

四川盆地德阳—安岳凹槽两侧陡坎带震旦系灯影组天然气勘探不断取得新突破,是盆地内天然气增储上产的重要领域。为了明确该凹槽的形成机理、形成期次及沉积新格局,进一步指导灯影组气藏的深化勘探,采用最新大连片三维地震成果、结合钻井等资料,分析了各期陡坎之间、陡坎与台缘带之间的空间展布关系,探索了陡坎的演化规律,进一步明确了天然气的勘探方向。研究结果表明:①德阳—安岳凹槽陡坎总体表现为侵蚀谷特征,同沉积小断层的作用不明显;②灯影组二段(以下简称灯二段)发育Ⅰ、Ⅱ两个陡坎,灯影组四段(以下简称灯四段)发育一个陡坎,分别在桐湾Ⅰ和Ⅱ、Ⅲ幕侵蚀作用形成,这些陡坎与灯二段、灯四段台缘带无关;③灯二段沉积时期,受控于基底垒堑格局,发育由南向北进积的4~5排台缘带,呈北东向展布,其中蓬莱地区发育两个大型台缘带,台缘带丘滩体被陡坎Ⅱ截断,叠加岩溶作用形成优质储层。结论认为:①高石梯—磨溪地区灯四段与灯二段的陡坎叠合区发育灯四段陡坎型台缘带,而在灯四段槽上带,丘滩体仍呈北东向发育,表明该地区受陡坎Ⅱ与北东向古地貌双重因素控制;②蓬莱地区陡坎Ⅱ坡度变小,对灯四段台缘带控制作用减弱直至消失,主要发育2排北东向展布的缓坡型台缘带,即蓬莱①、②台缘带,蓬莱地区可作为今后天然气勘探重点关注目标区。