Geochemical characteristics and exploration significance of ultra-deep Sinian oil and gas from Well Tashen 5,Tarim Basin,NW China

The Well Tashen 5(TS5),drilled and completed at a vertical depth of 9017 m in the Tabei Uplift of the Tarim Basin,NW China,is the deepest well in Asia.It has been producing both oil and gas from the Sinian at a depth of 8780e8840 m,also the deepest in Asia in terms of oil discovery.In this paper,the geochemical characteristics of Sinian oil and gas from the well were investigated and compared with those of Cambrian oil and gas discovered in the same basin.The oil samples,with Pr/Ph ratio of 0.78 and a whole oil carbon isotopic value of31.6‰,have geochemical characteristics similar to those of Ordovician oils from the No.1 fault in the North Shuntuoguole area(also named Shunbei area)and the Middle Cambrian oil from wells Zhongshen 1(ZS1)and Zhongshen 5(ZS5)of Tazhong Uplift.The maturity of light hydrocarbons,diamondoids and aromatic fractions all suggest an approximate maturity of 1.5%e1.7%Ro for the samples.The(4-+3-)methyldiamantane concentration of the samples is 113.5 mg/g,indicating intense cracking with a cracking degree of about 80%,which is consistent with the high bottom hole temperature(179℃).The Sinian gas samples are dry with a dryness coefficient of 0.97.The gas is a mixture of kerogen-cracking gas and oil-cracking gas and has Ro values ranging between 1.5%and 1.7%,and methane carbon isotopic values of41.6‰.Based on the equivalent vitrinite reflectance(R_(eqv)=1.51%e1.61%)and the thermal evolution of source rocks from the Cambrian Yu'ertusi Formation of the same well,it is proposed that the Sinian oil and gas be mainly sourced from the Cambrian Yu'ertusi Formation during the Himalayan period but probably also be joined by hydrocarbon of higher maturity that migrated from other source rocks in deeper formations.The discovery of Sinian oil and gas from Well TS5 suggests that the ancient ultra-deep strata in the northern Tarim Basin have the potential for finding volatile oil or condensate reservoirs.

The pre-Sinian rift in central Sichuan Basin and its control on hydrocarbon accumulation in the overlying strata

Based on the latest seismic data, resistivity profile, outcrop evidence and logging data, the structural features of basement in Sichuan Basin and its control on the hydrocarbon accumulation in the Sinian-Cambrian strata was discussed. It was found that a NE striking pre-Sinian rift was developed across the whole basin. Controlled by a series of rift-parallel normal faults, horst-graben structures were developed inside the rift, large horst-graben structures and later activity of their boundary faults controlled the distribution of beach facies of the overlying strata. The horst-graben structures induced the formation of local highs of ancient landform and controlled the successive development of overlapped bioherm beach facies in long-term marine setting from the Sinian period to the Permian period, and as a result a widely distributed favorable sedimentary facies belt was developed. The pre-Sinian rift and later activities of related normal faults controlled the development of the grain beach and karst reservoirs and the deposition of high quality source rock, which form structural-lithologic traps. Through comprehensive evaluation, two large structural-lithologic composite trap favorable exploration areas in the south and north of the Gaoshiti-Moxi area, were selected.

Formation and destruction processes of upper Sinian oil-gas pools in the Dingshan-Lintanchang structural belt, southeast Sichuan Basin, China

The lower Cambrian Niutitang Formation hydrocarbon source rocks at the Dingshan- Lintanchang structure in the southeast Sichuan Basin were of medium-good quality with two excellent hydrocarbon-generating centers developed in the periphery areas, with a possibility of forming a medium to large-sized oil-gas field. Good reservoir rocks were the upper Sinian （Dengying Formation） dolomites. The mudstone in the lower Cambrian Niutitang Formation with a good sealing capacity was the cap rock. The widely occurring bitumen in the Dengying Formation indicates that a paleo oil pool was once formed in the study area. The first stage of paleo oil pool formation was maturation of the lower Cambrian source rocks during the late Ordovician. Hydrocarbon generation from the lower Cambrian source rocks stopped due to the Devonian-Carboniferous uplifting. The lower Cambrian source rocks then restarted generation of large quantities of hydrocarbons after deposition of the middle Permian sediments. This was the second stage of the paleo oil pool formation. The oil in the paleo oil pool began to crack during the late Triassic and a paleo gas pool was formed. This paleo gas pool was destroyed during the Yanshan-Himalayan folding, uplifting and denudation. Bitumen can be widely seen in the Dengying Formation in wells and outcrops in the Sichuan Basin and its periphery areas. This provides strong evidence that the Dengying Formation in the Sichuan Basin and its periphery areas was once an ultra-large structural-lithologic oil-gas field, which was damaged during the Yanshan-Himalayan period.

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.

Formation and evolution of Sinian oil and gas pools in typical structures, Sichuan Basin, China

The only major breakthrough in the exploration of the Sinian of the Sichuan Basin has been the Weiyuan gas field. Taking the typical structures in the Sichuan Basin as examples, an apatite fission track simulation was applied to constrain the thermal evolutionary history of the source rocks in this study. Combined with trap formation and evolution, the formation, destruction, and accumulation history of the Sinian reservoirs were analyzed from a dynamic perspective. The Sinian reservoirs underwent several primary stages： the paleo-reservoir formation and destruction in the late Silurian, hydrocarbon recharge in the Permian-Triassic, the cracking of oil to gas and dissolved-gas in the late Triassic-Mid-Late Jurassic, and the exsolution and accumulation of dissolved-gas as a result of episodic uplift since the Late Cretaceous. The exsolution process of dissolved-gas is of great significance to the accumulation of natural gas. The formation of the Weiyuan gas field is also related to this process. The Sinian in the Sichuan Basin has a broad exploration prospect, and exploration targets focused on the Leshan-Longnüsi Paleo-uplift tectonic zone with weak influences on potential hydrocarbon reservoirs from the late tectonism and transformation.

Distribution Features of the Nanhua-Sinian Rifts and their Significance to Hydrocarbon Accumulation in the Tarim Basin

On the basis of reprocessing 34 new two-dimensional spliced long sections(20,191 km) in the Tarim Basin, the deep structure features of the Tarim Basin were analyzed through interpreting 30,451 km of two-dimensional seismic data and compiling basic maps. Seismic interpretation and geological analysis conclude that the Nanhua-Sinian strata are a set of rift-depression depositional systems according to their tectonic and depositional features. The rift valley formed in the Nanhua Period, and the transformation became weaker during the late Sinian Period, which eventually turned into depression. From bottom to top, the deposited strata include mafic igneous, tillite, mudstone, and dolomite. Three major depocenters developed inside this basin during the rift stage and are distributed in the eastern Tarim Basin, the Awati area, and the southwestern Tarim Basin. Among them, the rift in the eastern Tarim Basin strikes in the near east-west direction on the plane and coincides with the aeromagnetic anomaly belt. This represents a strong magnetic zone formed by upwelling basic volcanic rock along high, steep normal faults of the Nanhua Period. Controlled by the tectonic background, two types of sedimentary systems were developed in the rift stage and depression stage, showing two types of sequence features in the Sinian depositional stage. The Nanhua System appears as a wedge-shaped formation, with its bottom in unconformable contact with the base. The rifting event has a strong influence on the current tectonic units in the Tarim Basin, and affects the distribution of source rock in the Yuertus Formation and reservoir beds in the Xiaoerbulake Formation in Lower Cambrian, as well as the gypseous cap rock in Middle Cambrian. The distribution features of the rifts have important and realistic significance for determining the direction of oil and gas exploration in the deep strata of the Tarim Basin. Comprehensive analysis suggests that the Tazhong region is the most favorable zone, and the Kalpin-Bachu region is the optimal potential zone for exploring sub-salt oil and gas in deep Cambrian strata.

Geochemical Indication of Sinian Bedded Siliceous Rocks in the Hunan-Guizhou-Guangxi Area and Their Environmental Significance

: In the Hunan-Guizhou-Guangxi area there have developed very thick bedded siliceous rocks of the late Sinian. The rocks have a fairly pure composition, with an average content of siliceous minerals exceeding 95%. They are relatively rich in Fe and Mn, and poor in Al, Ti and Mg. The Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn) and U/Th ratios and the Al-Fe-Mn and Fe-Mn-(Ni+Co+Cu)×10 triangle diagrams all show that they are hydrothermal sedimentary siliceous rocks. In the rocks the total amount of REEs is low, the δCe shows an obvious negative anomaly and the δEu a weak anomaly, and LREE>HREE, all indicating that they are products of hydrothermal processes. The δ30Si and δ18O values, as well as the formation temperature of the rocks all clearly show that the silica forming the rocks comes from hot water. Besides, analyses of the depositional environment of the rocks using the MnO/TiO2 ratio and the δCe and δ30Si values yield the same conclusion that they are formed in environments from continental marginal slopes of a bathyal environment to oceanic basins of a deep-sea environment.

Sedimentary and Geochemical Characteristics of Sinian Cap Carbonates in the Upper Yangtze Region

A global\|scale glaciation occurred at about 600 Ma ago. As a result, the Earth became the "Snowball Earth". The glaciation came to the end abruptly when atmospheric carbon dioxide increased to such an extent as to be about 350 times the modern level because of subaerial volcanic degassing. The rapid termination of glaciation would have led to warming of the Snowball Earth and extreme greenhouse conditions would have been created. The transfer of atmospheric carbon dioxide to oceans would give rise to the rapid precipitation of calcium carbonate in warm surface seawaters, thus forming the cap carbonate rocks as observed worldwide today.\; Regionally persisting, thin layers of carbonate rocks directly and ubiquitously overlie Proterozoic glacial deposits almost on every continent, and are commonly referred to as cap carbonates. Their unusual litho\| fabrics, stratigraphically abrupt basal and upper\|level contacts and strongly negative carbonate isotopic signatures (\{δ\{\}\+\{13\}C\-\{carb.\}\} values range from -7.0‰-0‰) suggest a chemical oceanographic origin, the details of which remain unknown. It is proposed that these enigmatic deposits are related to the destabilization of gaseous hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and internal basins.\; The authors carried out studies on the geochemistry, sedimentology and palaeontology of the Sinian cap carbonates in Guizhou and Hunan provinces, including the occurrence of cap carbonates of unusual fabrics, strongly negative carbon isotopic signatures, and a lot of bitumen nodules. From the results it is suggested that the cap carbonates were formed from solid methane seepage, and it is in agreement with Kennedy’s viewpoint (2001) that the cap carbonates resulted from the rapid precipitation of calcium carbonate in response to solid methane seepage.

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.

Sequence Stratigraphy and Relative Sea Level Changes in Sinian System of Yichang, Hubei Province

The study on outcrop sequence stratigraphy of the Sinian section in the Yangtze Gorges an important step in the comprehensive stratigraphy for the Sinian stratotype. In this paper, the base of the Nantuo tillite is selected as the bottom boundary of the Sinian, and the top of the Baimatuo Member of the Dengying Formation is taken as the top boundary. The Sinian lasted from 740 the to 570 Ma. According to the features of sequence boundary facies and the parasequence sets, two supersequences are recognized in the Sinian with six sequences. In this paper, the authors discuss the correlation of those sequences. On the basis of the subsidence rate and the thickness of decompressed deposits, the authors discuss the mechanism of tectonic and sedimentation of the Dengying Formation and the features of high-frequency rhythms as well as the 3rd-order sequences in the Yangtze Gorges.

Medusoid Fossils from the Sinian Xingmincun Formation of Southern Liaoning Province

Abundant medusoid soft-bodied metazoan fossils have been found in the Sinian Xingmincun Formation(with an isotopic age of 650 Ma) at Qipanmo, Qidingshan and Yangtun of the Dalian-Jinzhou area, southern Liaoning Province; in addition, a number of macroscopic algal fossils have been discovered for the first time in the Getun Formation overlying the Xingmincun Formation. Medusoid fossils include 3 genera and 6 species(all of which are new). Judging from their biological characters, these medusoid fossils are considered to have an intimate affinity with the Ediacara fauna in the terminal Precambrian of Australia, as their characters are close to each other. The authors suggest that these medusoids and the Edicara fauna might be the products of the same period. The discovery of the medusoid fauna in the Xingmincun Formation marks an important event in the evolution of the terminal Precambrian organisms in China, and it is of major practical significance in the study of the Sinian System of China and the correlation of the Sinian strata of northern and southern China.

Lithofacies paleogeography and exploration significance of Sinian Doushantuo depositional stage in the middle-upper Yangtze region, Sichuan Basin, SW China

In recent years, natural gas exploration in the Sinian Dengying Formation and shale gas exploration in Doushantuo Formation have made major breakthroughs in the Sichuan Basin and its adjacent areas. However, the sedimentary background of the Doushantuo Formation hasn't been studied systematically. The lithofacies paleogeographic pattern, sedimentary environment, sedimentary evolution and distribution of source rocks during the depositional stage of Doushantuo Formation were systematically analyzed by using a large amount of outcrop data, and a small amount of drilling and seismic data.(1) The sedimentary sequence and stratigraphic distribution of the Sinian Doushantuo Formation in the middle-upper Yangtze region were controlled by paleouplifts and marginal sags. The Doushantuo Formation in the paleouplift region was overlayed with thin thickness, including shore facies, mixed continental shelf facies and atypical carbonate platform facies. The marginal sag had complete strata and large thickness, and developed deep water shelf facies and restricted basin facies.(2) The Doushantuo Formation is divided into four members from bottom to top, and the sedimentary sequence is a complete sedimentary cycle of transgression–high position–regression. The first member is atypical carbonate gentle slope deposit in the early stage of the transgression, the second member is shore-mixed shelf deposit in the extensive transgression period, and the third member is atypical restricted–open sea platform deposit of the high position of the transgression.(3) The second member has organic-rich black shale developed with stable distribution and large thickness, which is an important source rock interval and major shale gas interval. The third member is characterized by microbial carbonate rock and has good storage conditions which is conducive to the accumulation of natural gas, phosphate and other mineral resources, so it is a new area worthy of attention. The Qinling trough and western Hubei trough are favorable areas for exploration of natural gas(including shale gas) and mineral resources such as phosphate and manganese ore.

Geological characteristics of the southern segment of the Late Sinian–Early Cambrian Deyang-Anyue rift trough in Sichuan Basin, SW China

Based on the latest drilling, seismic and field outcrop data, the geological characteristics(e.g. strata, development and sedimentary evolution) of the southern segment of the Late Sinian–Early Cambrian Deyang–Anyue rift trough in the Sichuan Basin are analyzed. First, the strata in the southern segment are complete. The first to second members of Dengying Formation(Deng 1 + Deng 2) are found with relatively stable thickness(400–550 m), and the third to fourth members(Deng 3+ Deng 4) show great thickness difference between the marginal trough and the inner trough, which is up to 250 m. The Cambrian Maidiping Formation and Qiongzhusi Formation in southern Sichuan Basin are relatively thin, with the thickness changing greatly and frequently. Second, the Deyang–Anyue rift trough extended southward during the Deng 4 period, affecting southern Sichuan Basin. Compared to the middle and northern segments of the rift trough, the southern segment is generally wide, gentle and shallow, with multiple steps, and alternating uplifts and sags, which are distributed in finger shape. Third, the Deng 1 + Deng 2 in southern Sichuan Basin records the dominance of carbonate platform and unobvious sedimentary differentiation, and the Deng 4 exhibits obvious sedimentary differentiation, namely, basin–slope–secondary slope–slope–secondary slope–platform margin–restricted platform, from the inner trough to the marginal trough. Fourth, the rift trough in southern Sichuan Basin has evolved in four stages: stabilization of Deng 1–Deng 2, initialization of Deng 3–Deng 4, filling of Maidiping–Qiongzhusi, and extinction of Canglangpu Formation.

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.

Sinian gas sources and effectiveness of primary gas-bearing system in Sichuan Basin, SW China

Based on correlation between geochemical characteristics of Sinian and Cambrian source rocks and discovered gas reservoirs,paleoand the analysis on geological conditions of reservoir formation,the sources of natural gas in the Sinian of Sichuan Basin have been discussed to sort out the contribution of Sinian source rocks to the gas reservoirs and effectiveness of Sinian primary gas-bearing system.Through the analysis of natural gas composition,carbon and hydrogen isotopes and effectiveness of Sinian accumulation assemblages,it is concluded that:(1)The natural gas derived from the Sinian source rock is characterized by low ethane content,heavy ethane carbon isotope and light methane hydrogen isotope,and obviously different from the gas generated by the Cambrian source rock.(2)The gas reservoirs discovered in Sinian Dengying Formation are sourced by Sinian and Cambrian source rocks,and the Sinian source rock contributes different proportions to the gas in the 4th member and the 2nd member of the Dengying Formation,specifically,39%and 55%to the 4th member in marginal zone and intra-platform,54%and 68%to the 2th member in the marginal zone and intra-platform respectively.(3)The effectiveness of the Sinian primary gas-bearing system depends on the gas generating effectiveness of the source kitchen,reservoir and combination of gas accumulation elements.For high-over mature marine source rocks at the Ro of less than 3.5%,besides gas generated from the thermal cracking of liquid hydrocarbon,the kerogen still has some gas generation potential by thermal degradation.In addition,the Sinian microbial dolomite still preserves relatively good-quality reservoirs despite large burial depths,which match well with other basic conditions for gas accumulation in central Sichuan paleo-uplift,increasing the possibility of Sinian primary gas-bearing system.The research results confirm that the Sinian primary gas-bearing system is likely to form large-scale accumulation.

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.

Application of mapping and dating techniques in the study of ancient carbonate reservoirs: A case study of Sinian Qigebrak Formation in northwestern Tarim Basin, NW China

Ancient marine carbonates experienced complex modifications,making it difficult to identify reservoir genesis and effective porosity before hydrocarbon migration.To solve these issues,we used element mapping and carbonate mineral laser U-Pb radiometric dating techniques to study the diagenetic environments based on geochemistry and diagenesis-porosity evolution based on geochronology of the dolomite reservoir of the Sinian Qigebrak Formation,northwest Tarim Basin.Two major understandings were obtained as follows:(1)Supported by petrographic observations,the element mapping,stable isotopes,strontium isotope,and cathodoluminescence tests were performed on different phases of dolomite cements precipitated in vugs and dissolved fissures.The results show that the dolomite reservoirs of the Qigebrak Formation went through freshwater,marine,extremely shallow burial,burial and hydrothermal diagenetic environments after synsedimentary dolomitization;the reservoir spaces were mainly formed in the synsedimentary period(primary pores)and freshwater environment(supergene dissolution pores)before burial;whereas the marine,burial and hydrothermal environments caused the gradual filling of reservoir space by dolomite cements.(2)Based on the above understandings,each phase of dolomite cement precipitated in the reservoir space was dated by the U-Pb radiometric dating technique,and the diagenesis-porosity evolution curves constrained by geochronology were established.The loss of reservoir porosity mainly occurred in the early Caledonian,and during the peak period of hydrocarbon generation of Yuertusi Formation source rock,the reservoirs still maintained at a porosity of 6%?10%.The above understandings provide a certain basis for the evaluation of accumulation effectiveness of the Sinian Qigebrak Formation,northwestern Tarim Basin,and provide a case for the application of mapping and dating techniques in the study of ancient carbonate reservoirs.

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.