Abstract:

Abstract:The western part of Kashi Depression lies in the conjunction area of the Tarim Basin, the Pamir Syntax, and the south West Tian Shan. The thick sediments accumulated in this depression were derived from and preserve a record of late Cenozoic tectonic uplift of the south West Tian Shan. This paper presents results of anisotropy of magnetic susceptibility (AMS) analysis of the late Cenozoic Tierekesazi section in the foreland of the south West Tian Shan. The late Cenozoic successions of the Tierekesazi section are continental sediments with a general upward increase in particle size and show a progressive change from fluviallacustrine facies to fan delta facies, alluvial fan facies and diluvial fan facies. Rock magnetic results of the late Cenozoic successions indicate that the magnetic minerals are mainly hematite, except for the lower Pakabulake Formation, which is dominated by magnetite. Results of the late Cenozoic AMS suggest a synsedimentary fabric with weakdeformation. Since 22.1 Ma directions of tectonic strain in the foreland of the south West Tian Shan are mainly N-S directed compressional stress, which is consistent with the continuous convergence of the Pamir and the south West Tian Shan in the late Cenozoic. At the time of deposition of the Anjuan Formation and the Xiyu Formation, directions of compressional strain changed from N-S directed to NNE-SSW directed, likely reflecting the activities of the Talas-Fergana fault. Right slip of the Talas-Fergana fault (TFF) might absorb part of the deformation amount of the south West Tian Shan in the late Cenozoic, leading to smaller deformation in the foreland of the southern West Tian Shan than in the foreland area of the South Tian Shan in the east of the TFF. As a result, late Cenozoic foreland thrustings in the area east of the TFF were more active and better developed than in the foreland area west of the TFF.

Abstract:The continuous collision and convergence of the Indian and the Eurasian plates initiated during the early Cenozoic resulted in large-scale intra-continental deformation within the Eurasian plate. This event reactivated the Tian Shan orogenic belts, which propagated the thrust toward the Tarim basin, forming the present southern Tian Shan fold-thrust belts with significant spatial variation in deformation along the strike of the belts. In this paper, we analyze the spatial variation of deformation and its possible controlling factors based on the seismic profile interpretation and balanced cross-sections across the structural belts. This study suggests that the western segment of the southern Tian Shan fold-thrust belts can be subdivided into the Bashibu lake segment, the Wuqia-Atushi-Kashi segment, and the west Keping segment. Among them, a series of double-thrust structures and back-load basins were mainly developed in the Bashibu lake tectonic segment. The deformation characteristics of the Wuqia-Atushi-Kashi tectonic section were manifested in two levels: stacking structure and tectonic wedge developed in the deep part, and fault-propagating folds and fold belts reconstructed by southward thrust faults developed in the shallow part. The deformation of west Keping was more intense and also manifested in two levels: stacking structure developed in the deep part, fault-propagating folds, fold belts reconstructed by southward thrust faults and recoiling structural system developed in the shallow part. Based on the geology of the study area, it is concluded that the strata deformation variation in the western section of the southern Tian Shan fold-thrust belts may be related to the northward advance of the Pamir block since the Cenozoic era, the activity of the Taras-Fergana right strike-slip fault, the influence of the activation of preexisting faults and ductile slippage.

Abstract:The “stacking” structure and basement involved thrust structure developed in the foldthrust belts of the southwestern Tian Shan are obviously different from the thin skin thrust structure of the Kalpin foldthrust belts on the eastern side. In this paper, based on structural profile interpretation, and using discrete element numerical simulation research and single factor variable control method, we have designed five sets of numerical simulation experiments to analyze the effect of controlling factors of structural evolution in the research area. We include the model of preexisting fracture, the model of décollement and the comprehensive model of pre-existing fracture, and décollement with different cohesion. The experimental results show that the cohesion of the décollement controls the difficulty in the upward breakthrough of preexisting faults. When the cohesion of the décollement is high, it is easy to form basement involved thrust structure along the preexisting faults. When the cohesion of the décollement is low, the roof faults of thrust system develop in the décollement and the fault below the décollement is prevented from spreading upward. With the active pre-existing faults is easy to form thin skin imbricate structures. In the area where the low cohesion décollement is developed, the deformation front propagates faster and farther forward. The deformation primarily occurs in the upper unit and mainly contains the thrust structure and triangular belt, while the structural deformation of the lower unit is relatively weak. The “stacking” structure and basement involved thrust structure that are developed in the foldthrust belts of the southwestern Tianshan were mainly controlled by the gypsum-salt layer of the Paleogene and the pre-existing faults of the pre-Cenozoic.

Abstract:The eastern Kuqa depression is rich in oil and gas resources, and its structural deformation mechanism is closely related to the Southern Tianshan orogenic belt. Based on conjugate joints in outcrops and distribution of residual Cretaceous, the characteristics of the palaeo uplift and palaeo stress field property in the eastern Kuqa depression at late Cretaceous were analyzed. Conjugate joints analysis in the Tugerming, Kezilenuer, Kuqa river, Kapushaliang river, Jidike and western Kuqa river outcrops indicates that the compressive stress direction of eastern Kuqa depression was NNW-SSE at Cenozoic while NE-SW at Mesozoic. The recovery of prototype basin demonstrates that the sedimentary center of Kuqa depression was roughly along well Dabei 1-Tubei 2-Kela 2, and developed Qiulitage-Xinhe-Yaha-Tiergen palaeo uplift belt at Cretaceous, which leads to thick residual Cretaceous in the north and thin residual Cretaceous in the south. Under the background of regional uplift and denudation at late Cretaceous, the east of Kuqa depression lift and been denudated due to tectonic compression, which results in the thinning of residual Cretaceous, and therefore the residual Cretaceous is thick east and thin west. The Tugerming anticline in the eastern Kuqa depression was a longstanding inherited sedimentary palaeo uplift at Carboniferous-Triassic, and became a tectonic palaeo uplift by further uplift at late Cretaceous, which leads to further thinning of residual Cretaceous around the anticline. The long axis strikes of the Tugerming anticline and residual Cretaceous isolines around the anticline and Well Kubei 1 and Kuqa river outcrop are all NW-SE, which further confirms that the palaeo tectonic stress direction was NE-SW at late Cretaceous in the eastern Kuqa depression. The late Cretaceous uplift and tectonic stress field in the eastern Kuqa depression is primarily related to the regional uplift in the north margin of Tethys basin group caused by the hyperplasia and collision of geological terrain in the Tethys orogenic belt.

Abstract:The Paleogene gypsum- salt rocks were deposited in the western Kuqa depression at the southern piedmont of Tian Shan. Trillions of cubic meters of nature gas fields were discovered within the Mesozoic strata below the gypsum- salt layer, an important gas source for the westeast gas transmission in China. The sub- salt in Kuqa depression is a target for oil and gas exploration. At present insufficient understanding of the formation mechanism of the salt structure, which requires restoration of salt structure balance, hinders gas exploration. We have, in this paper, by integrating seismic, drilling and surface geological data from four selected sections of the western Kuqa depression, undertaken tectonic balance restoration using the StructureSolverTM (SS) restoration software developed by StructureSolver Company. We have restored the seismic image before compression deformation of the supra- salt layer, salt layer, and sub- salt layer to rebuild the pattern of the seismic profiles before compression deformation. The results reveal that two stages of salt structures were developed in the western Kuqa depression: piercing salt dome developed in the Oligocene to Miocene, and extruded salt structures developed in the Pliocene to Quaternary. Under compaction, the salt dome developed in the early stage evolved into salt sheets and salt walls, thrust faults and salt anticlines developed in the supra- salt layer, and imbricated thrust faults and critical- taper wedge developed in the sub- salt layer. The differential loading of the overlying salt layer (gravity) was the reason for the development of the salt dome in the western Kuqa depression. The salt domes in the western Kuqa depression are distributed in front of the piedmont alluvial fan and on both sides of Quele alluvial fan. Critical taper wedge developed in the sub- salt layer in western Kuqa depression. The critical taper wedge theory is used to study the relationship between the slope of the top surface of the wedge and the angle of the bottom detachment layer. The slope of the top surface changes abruptly signifying variation in the angle of the detachment layer, thus indicating that the Paleozoic basement normal fault was involved in the wedge.

Abstract:The subsalt structural geometry in front of the north structural belt is quite different in eastern and western segments of the Kuqa depression. The subsalt structure is composed of an imbricate thrusting belt in the Kelasu structure of the western segment, while it is a faulted anticline in the Dongqiu structure of the eastern segment. In this paper, based on seismic interpretation, five discrete element models are designed to explore the influence of pre- existing faults, syntectonic sedimentation and properties of salt layer on the structural geometry and evolution of fold- and- thrust belts, as well as the causes behind the different structural features of the eastern and western segments in the Kuqa depression. The simulation results show that the deformations of units above and below the salt layer are decoupled. A pre- existing fault can affect the deformation propagation, causing the differences in subsalt wedge shape, fault numbers and suprasalt structural geometry. Syntectonic sedimentation could enhance the deformation decorrelation above and below the salt layer, while a stronger salt property decreases it. Comparing the simulation results with the structural features of the eastern and western segments in the Kuqa depression, we suggest that a pre- existing fault could be the controlling factor of the structural variations between eastern and western segments. In the eastern Kuqa depression, the location of pre- existing fault first deforms, forming the Dongqiu fault and the Dongqiu anticline above it. Besides, the pre- existing fault absorbs most of the shortening, and prevents the deformation from propagating forward, thus causing an out of sequence deformation propagation. In the western Kuqa depression, with no interruption from pre- existing fault, the deformation of subsalt units sequentially propagates forward and forms an imbricate thrusting belt.

Abstract:The formation of the Pamir salient is related to pre-existing arcuate weak zones. However, the mechanism for the reactivation of the weak zones during the early stage of the Indo-Eurasian collision remains unknown. In this study, we apply analogue modeling to address this issue. Based on similarity principle of scales, we have set up two models with pre-existing weak zone in combination of either non-décollement or décollement. Our modeling yields the following outcomes. ① The modeling result validates the conception that a pre-existing arcuate weak zone can produce the Pamir salient. ② Under the condition of a pre-existing arcuate weak zone without décollement, deformation propagates northward, with the strain loaded in the pre-existing weak zone during late deformation stage. Southdipping thrust faults propagate forward, widening the fold-and-thrust belt. The topography exhibits a slope dipping to the north, showing a northward decreasing wedge. ③ Under the condition of a pre-existing arcuate weak zone and décollement, the former was reactivated at a very early deformation stage, with the strain immediately loaded in the weak zone. Subsequently, the deformation progressively propagates northward from the moveable wall. When the shortening amount is large enough, frontal thrust emerges at front of the weak zone. The deformation architecture consists of several popup structures, resulting in a topography characterized by alternatively arranged ranges and valleys. ④ A décollement is necessary for the strain be exerted immediately on the arcuate weak zone at the northern boundary of the Pamir in the initial stage of the Indo-Eurasian collision. The confirmation of the existence, depth and interface of the early Cenozoic décollement requires further studies.

Abstract:Décollement is one of the main controlling factors affecting structural deformation of fold and thrust belts. The structural deformation of the Wushi and Kuqa fold and thrust belts in the northern margin of the Tarim Basin was mainly controlled by two sets of basal and shallow décollement, between which are differences in thickness and property along the strike. In this paper, with the objective of studying the influence of these "along the strike differences" on the structural deformation of the Wushi and Kuqa fold and thrust belts, four sets of experimental models were designed based on the analogue modeling method and the geological background of these belts. In the sandbox models, glass microbeads are used to simulate brittle décollement such as in shales and mudstones, and silicone gel is used to simulate the ductile décollement such as in salt rocks and gypsum salt rocks. There are two sets of décollements in the experimental models, in which the basal one is a uniformly distributed brittle décollement, and the shallow one has differences in thickness and property along the strike. The experimental results show that in the sandbox model with double décollements, thickness and property differences of the shallow décollement along the strike play a very important role in controlling the structural deformation and wedge taper of the fold and thrust belts. The specific results are as follows: ① when the thickness of the shallow décollement is variable along the strike, deformation on the thinner side of the shallow décollement restricts the deformation transfer to the thicker side. This results in a relatively short transfer distance of the deformation front of the model and the extension range of the deformation front on both sides of the model is basically the same. The number of faults on the thinner side is obviously larger than that on the thicker side, and the interval between them is smaller. The wedge taper gradually decreases from the thinner to the thicker side. ② When there is a difference in the property of the shallow décollement along the strike, the brittle side shows up-and-down deformation, while ductile side shows up-and-down layered deformation. Under the condition that both sides of the shallow décollement have the same thickness, the propagation speed of the ductile side is generally faster than the brittle side, but the extension range of deformation front on both sides of the final model is basically the same, and the wedge taper decreases gradually from the side of the brittle décollement to the side of the ductile décollement. When the thickness of the shallow brittle décollement decreases, the transfer distance of the deformation front on both sides of the model still tends to be consistent with the continuous increase of the shortening distance, and the overall wedge taper is larger, but the wedge taper on the brittle side is always larger than the one on the ductile side. In addition, the analogue modelling results verify that in the Wushi and Kuqa fold and thrust belts, the difference in the property of the shallow décollement along the strike is the main controlling factor that causes the structural style differences, but it is not the primary reason for the large-scale deflection of the deformation front of the fold and thrust belts.

Abstract:The collision between Indian and Asian causes that blocks, which are separated by faults, laterally slide along largescale strike- slip faults and Tibet plateau rapidly uplift. These are also two endmember models of the evolutionary deformation of the Tibet plateau: continental extrusion model and crustal thickening model. The paper designs three non- rigid bookshelf fault models and use particle image velocimetry (PIV) technique to monitor surface deformation. Experimental results show that the displacement field, velocity, shear strain and area strain are obviously different within non- rigid blocks before and after boundaries between non- rigid blocks is stressed. Based on the experimental results, two conclusions are drawn: ① the deformation of the eastern margin of the Tibet plateau is continuous and the interior of blocks can form the thrust belt, rift system and dextral strike- slip fault system;②since the Miocene, the evolution of the Tibet plateau is mainly experienced two stages, from early Miocene to late Miocene (22~8 Ma), it not only have sinistral strike- slip motion, but also block clockwise rotation, giving rise to a nearly east- west trending Qilian Shan- Nan Shan thrust belt, Jishi Shan thrust belt, Qimen Tagh thrust belt, Weihe Graben and nearly north- south trending dextral strike- slip faults; Since the late Miocene (8Ma), there has been little rotation, mainly experiences left strike- slip motion, forming a nearly northsouth thrust zone and rift valley system.

Abstract:The Caledonian paleo- uplift in the Sichuan Basin is located in the mid- west of basin, adjacent to the east of the Longmen mountain tectonic belt. Analysis of the formation and evolution of the Caledonian paleo- uplift and the dynamics of its genetic mechanism is of great significance for studying intraplate tectonic deformation and guiding oil and gas exploration. To achieve this, guided by the results of previous studies, we have undertaken new research using a large number of seismic reflection profiles and drilling data. In this study, by comparing and analyzing the stratigraphic correlation in different regions, we have determined the absence of Pre- Permian strata in the paleo- uplift area, as well as the distribution range of pinching point, unconformity truncation point, and onlap point in different strata. Based on these we provide a detailed description of the distribution characteristic of Caledonian paleo- uplift by using a combination of seismic profiles and cross well lines, thus revealing the periodic evolutionary characteristic of the Caledonian paleo- uplift. By researching the paleo- structural evolution of the Dengying Formation and combining it with the balanced section technique, we have divided the formation and evolution stages of the paleo- uplifts into five stages, namely, the embryonic stage (Z), the developmental stage (∈~S), the denudation and flattening stage (D~C), the stable burial stage (P~T2l) and the finalization stage (T3x~Current). Based on these results, we have further investigated the genetic mechanism of the Caledonian paleo- uplift, and revealed its evolutionary characteristics by taking into account the tectonic background and characteristics of the basement faults in the Sichuan Basin. It is mainly believed that the formation of Caledonian paleo- uplift is controlled by the regional tectonic movement and the segmental activities of basement faults. The differtial uplift, similar to the “grabenhorst” in the basement, resulted in a “great uplift to big depression” tectonic pattern in the basin, which in turn influenced the formation and evolution of the inherited paleo- uplift.

Abstract:A comparsion of regional stratigraphy, typical measured sections, lithofacies and depositional environments suggests that the western and eastern part of the southern Junggar Basin (roughly along 88°E) have distinctively different rock assemblages, stratigraphic sequences, sedimentary characteristics, and depositional environments. The western part of the southern Junggar Basin is characterized by the clastic-volcanic-volcaniclastic rock assemblages, while the eastern part is composed mainly of carbonate-volcanic-volcaniclastic rock assemblages. In the western part of the southern Junggar Basin, the Cisuralian-Guadalupian strata are generally absent or deeply buried. The Lopingian-Early Triassic strata are dominated by conglomerates with some sandstones and minor mudstones, revealing features of an alluvial fan system. The Middle-Late Triassic strata are composed principally of fine-grained clastic rocks which are interpreted as belonging to a lacustrine system. The depositional environment in the eastern part of the southern Junggar Basin is interpreted as a deep-water turbidite in the Pennsylvanian, followed by a shallow marine environment in the Cisuralian-Guadalupian. In the Guadalupian Lucaogou Formation, the oil shale and carbonate rock are broadly exposed in the Bogda Mountains, suggesting occurrence of a rifting-drifting tectonic cycle in this period. Subsequently, the whole Junggar Basin is characterized by the fluvial and lacustrine systems from the Lopingian, which consist mainly of fine to corase-grained clastic rocks. In the Middle-Late Triassic, the drainage system had expanded with a levelled process, and the eastern part of the southern Junggar Basin is composed mainly of deltaic and shallow lacustrine clastic rocks. This study shows that the Permian-Triassic sediments in the southern Junggar Basin and the adjacent regions are dominated by rifted and depressional sedimentation, but there are differences between the western and eastern part of the southern Junggar Basin in their rift level.

Abstract:The alkaline lacustrine deposits are distributed all over the world and formed under special tectonic and climate conditions with volcanic and hydrothermal activities. The old, deeply buried alkaline lacustrine deposit in the Lower Permian Fengcheng Formation of the Mahu Sag provides a unique opportunity to study its sedimentary and diagenetic evolution. In addition, the distribution of alkaline lake deposits in the Fengcheng Formation is closely related to the distribution of shale reservoirs, thus a study of their formation and evolution provides important support towards the study of the shale reservoirs.A comprehensive study of sedimentary characteristics and genesis of worldwide alkaline lacustrine deposits was carried out to provide new insights into the genesis of the alkaline lacustrine deposits in the Lower Permian Fengcheng Formation of the Mahu Sag, northwestern Junggar Basin. An overview of the literature indicates that both contemporaneous volcanic activity and specific climatic could control the formation of the alkaline lacustrine deposits. The formation of alkaline lacustrine deposits required hydrologically closed basins, adequate solute supply of sodium and carbon, low concentration of sulfate, and a strong evaporative environment. The relationship between the mineral types and burial depth of the global alkaline lacustrine deposits indicated that the minerals were transformed by diagenesis. Diagenesis resulted in the conversion of gaylussite to pirssonite and finally into shortite. Meanwhile, wegscheiderite was formed diagenetically from nahcolite and trona. Comparative analysis showed that shortite and wegscheiderite in the alkaline lacustrine deposits from the Fengcheng Formation were formed by diagenetic mineral transformation. Searlesite was formed from pyroclastic deposits under burial alteration and diagenesis, and reedmergnerite was related to hydrothermal activity. The characteristic minerals were associated with the sediments formed by weathering of surrounding alkaline volcanic rocks. The depocenter of the alkaline lacustrine deposits in the Fengcheng Formation was covered by high alkaline brines and the depositional environment was anoxic as a result of perennial salinitybased lake water stratification. The alkaline lacustrine deposits in the study area were controlled by specific climate, tectonism, volcanism and diagenesis.

Abstract:Carotenoids are a major group of pigments widely distributed in nature and are commonly preserved as derivatives in sedimentary records after diagenesis and catagenesis. Among them, the aromatic compounds are mostly associated with photosynthetic sulfur bacteria and have been used in paleoenvironment reconstruction. For example, the detection of aromatic carotenoids in sedimentary rocks has been interpreted as photic zone euxinia (PZE) during deposition, and further inferences can be drawn regarding paleowater depth, microbial communities, nutrient availability, etc. Aromatic carotenoidbased PZE have been widely used in oilsource rock correlation, exploring life evolution, understanding drivers of mass extinctions and as triggers of oceanic anoxic events. In this study, we assess the pattern of carotenoids in the Oligocene Anjihaihe Formation of the southern Junggar Basin to provide paleoenvironmental and paleoecological interpretations. Our results show predominance of chlorobactene and isorenieratane from green sulfur bacteria indicative of PZE during deposition. The dominance of green sulfur bacteria over purple sulfur bacteria in carotenoid input suggests a deep and large paleolake during deposition, in which the occurrence of PZE was likely related to the evaporative water conditions during the paleoclimatic shift from wet to dry conditions during late Oligocene. Additionally, we report a new diaromatic carotenoid with 38 carbon atoms that was potentially synthesized by cyanobacteria. Consistent with tectonic viewpoints, the postOligocene uplift of the north Tian Shan suggests a possibly larger paleolake during the Oligocene deposition than the current range of Anjihaihe Formation in the south of Junggar Basin.

Abstract:Correct identification and chronology of intrusive and volcanic rocks is a prerequisite for understanding the succession of regional magmatic events and establishing chronostratigraphic framework. Aksu area, located in the Northwest Tarim block, is known for the successively exposed Neoproterozoic stratum and various kinds of igneous rocks and thus attracts many Precambrian researchers. This work reports typical intrusive characteristics among the basic rocks in the Sinian beds based on remote sensing image recognition, field profile geological survey and petrological study in the southwest Aksu city. These intrusive characteristics include truncating Sinian sandstone layers, wrapping wall rocks and fine- grained tendency of crystal size from the basic rock bed interior to margins. These typical intrusive features, integrated with our U- Pb chronological studies on zircons, indicate that the basic rock in the Sinian beds in the southwest Aksu area are not entirely Sinian erupted basalts- a part of them are actually Early Permian (ca.290 Ma) intrusive dolerite sills which was contemporaneous with the main eruption episode of massive flood basalt during the Permian Tarim Large Igneous Province. We suggest that they are an important component of the Tarim Large Igneous Province. This remarkable magmatic thermal event probably had a great impact on remagnetization of Precambrian rocks in this district. Meanwhile, it also highlights the importance of carefully recognizing and distinguishing the igneous rock types when conducting research on Precambrian magmatism in the Northwest Tarim block.

Abstract:Despite its high potential, the degree of prospecting is low in the lower reservoir accumulated under the seal of the Lower Cretaceous Tugulu Group mudstones, in the southern Junggar Basin. The Dushanzi anticline is situated to the east of Sikeshu Sag. The first oil field in the southern Junggar Basin was found in the shallow Neocene sandstone reservoirs in the Dushanzi anticline. The hydrocarbon accumulation potential in the deeply buried reservoirs remains to be investigated. The DS1 well was drilled down to the Upper Jurassic Qigu Formation (J3q) with the aim of reaching a huge oil pool. A lot of bitumen directed along the fractures was found in J3q cores at a depth of 6410~6418 m. The J3qbitumen had Pr/Ph values of 0.12~3.48, the δ13Cvalues of -29.95‰ ~-25.3‰, high peaks of the gammacerane with isomorides, C29 steranes much more than C28 and C27 steranes, and plenty of diasteranes. According to the organic geochemical features, the bitumen should be a mixture of the Jurassic and the Cretaceous source rock generated hydrocarbons. Two stages of mature hydrocarbon migration were recognized by comprehensive experimental analysis of bitumen occurrence, Rb, Raman, GOI of hydrocarbon inclusions, and reservoir quantitative grain fluorescence. The Jurassic and Lower Cretaceous source rocks began to generate much hydrocarbon and tectonic traps developed in the Neogene in the Sikeshu Sag. Conditions for hydrocarbon accumulation were favourable in the late Cenozoic. However, the DS1 well was in the southern slope of the Dushanzi anticline, which should be the hydrocarbon pathway instead of a charging trap. Therefore, it will be important to accurately identify the structural traps for the deep-strata oil and gas exploration in the southern Junggar Basin.

Abstract:A characterization of the geothermal regime of sedimentary basins and associated subsurface formation temperature is crucial for the assessment of hydrocarbon generation, preservation, and estimation of regional geothermal energy potential. The Upper Yangtze area of the South China is one of the longlasting regions for marine oil and gas exploration. Recently, it is the emerging area for shale gas exploration in China. The accurate characterization of the thermal regime in this area could constrain potential assessment of shale gas. However, further investigation is needed given the poor data coverage and associated inconsistency of previous studies. In this study we have combined the measured rock thermal properties and newly acquired steadystate temperature logging data with drill stem test temperature data to determine the presentday geothermal gradient and heat flow pattern, and estimate the subsurface formation temperaturesatdepth in the range of 1000~6000 m in the upper Yangtze area, along with temperatures at the bottom surfaces of two major Paleozoic marine source rocks in the Sichuan Basin. Our results demonstrate that this area has a moderatelow geothermal regime, and its presentday geothermal gradient and heat flow range (mean value) are 10~74℃/km (24℃/km) and 27~118mW/m2 (64mW/m2) respectively. Regional heat flow and temperature increase from the northeast to the southwest, showing a trend of "low in the northeast and high in the southwest". In the northeast, the west of Hubei and the north of Hunan province are characterized by low temperature areas; in the middle, the majority of the Sichuan Basin shows a moderate temperature; in the southwest, the Yunnan Province shows high temperature. This spatial variability of geothermal pattern in this area is mainly controlled by differential tectonics and magma activities. On the basis of predicted reservoir temperature and other geological and geochemical conditions, we propose that the shale system of the Lower Silurian Longmaxi Formation in ShizhuFuling, Weiyuan-Zigong-Luzhou and Yibin- Changning are likely to be the most favourable areas for shale gas exploration in this region.

Abstract:The borehole damage induced by earthquakes during hydrocarbon exploration and production is attracting growing attention. For safe production it is important to study the relationship between seismotectonics and borehole damage. A series of earthquakes, with maximum magnitude reaching M5.0, occurred on March 28th, 2019 in Mangya, Qinghai province. Their epicenters were located in the Shizigou structure and several wells in the Qaidam Basin were damaged during these earthquakes. Based on highresolution seismic and drilling data, this paper depicts the seismotectonics and the distribution of borehole damage for further investigation into the causes of the borehole damage. Results show that a suprasalt detachment structure and a subsalt positive flower structure comprise the Shizigou structure. The seismotectonics of these earthquakes involve a salt body with nonuniform thickness. The damaged wells during the earthquakes are mainly located in the NE limb of the Shizigou anticline and are close to the thickest part of the salt body. A plane which roughly consists of the borehole damage points is situated on the top surface of the salt body and is parallel to the layer. The result of the focal mechanism solution shows that one plane which dips steeply to the SSE mainly thrusts with a small left-lateral strike-slip component, whereas another plane which dips gently to the NE has both thrust and left-lateral strike-slip components. The two planes represent a branch of the subsalt basementengaged fault and the suprasalt detachment thrust fault respectively. Considering the uncertainty in the hypocenter location, either the subsalt branch fault or the suprasalt thrust detachment fault could be the causative fault, though the former is more likely. The reason for the borehole damage can be attributed to detachment along the top surface of the unstable salt during coseismic faulting. Thus, this paper reports an instance of borehole damage induced by earthquakes in salt tectonics.