Late Hercynian Research Articles

Genetic mechanism of multiphase states of Ordovician oil and gas reservoirs in Fuman oilfield, Tarim Basin, China

Abundant oil and gas are yielded from Ordovician reservoirs in the Fuman oilfield of Tarim basin and behave in multi-phase states at comparable burial depths, including light oil, volatile oil, and condensate gas. Oil and gas phase are distributed in the west and east, respectively, with heavier oil property in the north than that in the south. However the genetic mechanism of oil and gas phase states in the study area remains obscure. This has been examined based on fluid inclusions and geochemical parameters of light hydrocarbons and diamondoids in this paper. The results show that most Ordovician reservoirs were charged by oils in the late Caledonian and the late Hercynian, and the oil maturity charged in the same period decreased from south to north. In the Himalayan, dry gas and condensate oil-wet gas were respectively filled into the Yuke area in the east and local areas in the west. The multi-period differential charging of oil and gas with different properties is the main cause of the current oil and gas phase states. Two and three periods of oil charging are responsible for the formation of light and volatile oil, respectively, in western regions. Condensate gas reservoirs in Yuke area were formed by three periods of oil and gas charging, with the dominance of dry gas in the Himalayan. The phase states were transformed into volatile oils with the decrease in the charging intensity of dry gas to the west. The oil property becomes lighter from north to south is mainly related to the oil maturity charged during the same period, but also affected by the gradually decreasing cracking degree of oils from south to north. The current oil and gas phase states were ultimately produced by the genetic model of “multi-period differential charging and spatial differential cracking”.

Petrogenesis and tectonic setting of Early Permian–Late Triassic granitoids in the Qinling Orogenic Belt: Constraints from petrology, geochemistry and zircon U–Pb–Hf isotopes

The widely distributed Late Hercynian–Indosinian granites in the West Qinling Orogenic Belt are keys to better understand the tectonic–magmatic evolution of the West Qinling Orogenic Belt. In this paper, granitoids from the Zhacanggou area of Guide Basin in the western section of the West Qinling Orogenic Belt were studied on petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopes. Zircon U–Pb dating yield granitoids in the Zhacanggou area were emplaced at 228.3 ± 4.4 Ma, 265.2 ± 2.2 Ma and 277.8 ± 2.5 Ma, respectively. The whole-rock geochemical compositions of these granitoids belong to the weakly peraluminous calc-alkaline to metaluminous high-K calc-alkaline series, and exhibit different degrees of negative Eu anomalies, loss of high-field strength elements (Nb, P and Ti), and enrichment of large ion lithophile elements (Rb, Th, U and Sr). The εHf (t) values of Early–Middle Permian and Late Triassic granitoids range from −12.0 to 5.3 and –10.6 to −5.0, and corresponding two-stage model ages (TDM2) of 956 to 2051 Ma and 1581 to 1936 Ma, respectively. A summary of geochronology for granitoids formed in the West Qinling Orogenic Belt during the Late Hercynian–Indosinian indicating magmatic activities concentrated in the Late Triassic (210–230 Ma) and Permian–Middle Triassic (235–277 Ma). These granitoids were both formed by partial melting of ancient crust, which then mixed with enriched lithospheric mantle, and the former has a higher mantle contribution than the latter. The early granitoids were associated to northward subduction of the Mianlue oceanic slab, while the late granitoids were formed in the transition from collision to extension.

Differential deformation of a strike-slip fault in the Paleozoic carbonate reservoirs of the Tarim Basin, China

Ultra-deep carbonate reservoirs buried over 7000 m are distributed around strike-slip faults, which control hydrocarbon accumulation and enrichment in the Tarim Basin, China. This study proposes a fracture development index (FDI) method for analyzing the width of the strike-slip fault zone based on 3D seismic data. Based on the FDI method and geological and geophysical data, we analyze the structural characteristics of the SB5 strike-slip fault and the scaling of fault zone width and throw.In map view, at depth the strike-slip fault is characterized by single fault segments, extensional overlaps, and contractional overlaps, at intermediate depths by left-stepping en echelon normal faults, and in shallow layers by right-stepping en echelon normal faults. In cross section, we find composite flower structures and positive flower structures and medium-shallow layer negative flower structures or deep layer negative flower structures and medium-shallow layer negative flower structures. The formation time of deep, middle, and shallow layer faults is the middle Caledonian, Late Caledonian, and Late Hercynian. The heterogeneity of the strike-slip fault zone varies from strong to weak from contractional overlaps, to extensional overlaps, and single fault segments. Strike-slip fault-related fractures are dominated by three sets intersecting with strike-slip faults at small angles. Where throw is less than 100 m, the fault zone width and throw have a good power law relationship. Where the throw is over 100 m, the width tends to diminish with throw.

Dynamic background on formation, evolution, and later reformation of the Late Paleozoic–Early Mesozoic Basin in Songliao and its significance for hydrocarbon exploration

Located on the Songnen block of the Northeast Asia block group, the Songliao Basin is sandwiched by three major plates in Siberia, North China, and the Pacific Ocean. The recognition of the basin's properties and the dynamic processes of formation, evolution, and late reformation of the Late Paleozoic–Early Mesozoic Basin (LP-EMB), which has suffered from intense multi-stage, and various types of late reformation, are always controversial. On the basis of previous research on the regional tectonic geological background, combined with petrochemical and chronological data from core samples and seismic data, the attributes, evolution process, and late reformation of the LP-EMB and its dynamic environments have been deeply analyzed. Since the Late Paleozoic, it has successively experienced the development of Late Hercynian to Early–Middle Indosinian rifts and subsequent multi-stage superimposed transformation stages, which can be divided into the near-north-trending thrust reformation in the Late Indosinian, the near-west-trending thrust reformation in the Early Yanshanian, the differential extensional reformation in the early Late Yanshanian, the strike-slip shearing and deep burial reformation in the middle Late Yanshanian, and the strike-slip compression fold transformation in the late Late Yanshanian to the early Himalayan. The formation of the LP-EMB was mainly controlled by back-arc extension caused by the subduction and retreat of the paleo-Pacific Plate and partly by the closure of the Paleo-Asian Ocean between the North China Plate and the Northeast Asia micro-block group. The later reformation stages were closely related to the collision extrusion, strike-slip activity, and deep mantle activity caused by either the relative convergence movement between Songnen and other micro-blocks in the Northeast China micro-block group or by the remote collision effect of the Northeast China micro-block group and the surrounding plates. The inner fault zone (body), which was formed by multi-phase fault cutting, and the top weathering and denudation unconformity surface, are potential sites of hydrocarbon accumulation, from which natural gas has been transported along the fault and fracture belt into the weathering crust and the inner fracture zone in the Upper Paleozoic–Lower Mesozoic (UP-LM). This study significantly expands our knowledge of the tectonic evolution and gas exploration of the Songliao Basin.

The Hercynian tectonics in the Tassili-n-Ajjers area, Algeria: A possible continuous stress-strain regime?

This work shed some light on the multiple tectonic phases occurred during the late Hercynian Orogeny that is related to the global plate tectonic geodynamics and their effects in the Tassili-n-Ajjers plateau area, which is located in the south-eastern edge of Saharan platform in Algeria, and known to be the southern margin of the Paleozoic Illizi Basin, wherewell preserved Paleozoic sedimentary rocks outcrop. Three Hercynian tectonic phases have been evidenced in the area are: Early Carboniferous (E1), Late Carboniferous-Permian (E2), and a Late Permian-Early Triassic (E3). The Hercynian tectonic regime in the Tassili-n-Ajjers area that was dominated by strike-slip movements during E1 and E2 tectonic phases changed into an extensional regime during E3 tectonic phase. Paleo-strain determination, folds axis distribution and the kinematic analyses of the strain field of the Hercynian active tectonic show a continuous deformation regime rather than discrete tectonic phases. Moreover, the progressive deformation regime is strongly supported by the absence of tectonic unconformities between Late Devonian/Early Carboniferous - Early Permian. At a large-scale, this continuous deformation appears as a result of a progressive closure of Rheic and western Paleotethys Oceans, and the clockwise rotation of African plate which occurred during the Hercynian/Variscan orogeny. Therefore, we could consider that the most part of Algerian Saharan platform has registered the far-field effects of this orogeny and provides a suitable illustration of an intracontinental deformation.

The Contribution of Petroleum Charging Episodes to Different Strike-Slip Fault Zones in the Shunbei Area, the Tarim Basin, NW China

In recent years, newly discovered ultra-deep fault-karst reservoirs in the Shunbei area of the Tarim Basin have greatly increased the prospectivity of the basin and become a hotspot for further hydrocarbon exploration. In this study, the diagenetic sequences of the Lower-Middle Ordovician reservoir were established through observation of thin sections under transmitted light, reflected light, and cathode luminescence. The hydrocarbon charge history in the Shunbei area was reconstructed and associated with a series of diagenetic events using fluid inclusion microthermometric measurements combined with one-dimensional basin modeling. The results show that the Shunbei area has experienced three hydrocarbon charging episodes in total, in the late Caledonian, late Hercynian, and late Himalayan. Finally, the relationship between measured QF-535 factors and the API gravity of crude oil was used to quantify the level of contribution of each of the petroleum charging episodes to the No. 1, No. 5, and No. 7 fault zone reservoirs. The contribution of early hydrocarbon charging episodes gradually increases from west to east in the Shunbei area. This increase was driven by different tectonic events throughout geological history and the subsequent evolution of paleo-structural patterns. This study has a significant impact on marine carbonate reservoir evaluation in strike-slip fault zones in China.

Major period and process of hydrocarbon accumulation of deep reservoirs — A case study of Tuoputai area in Tarim Basin, China

Deep hydrocarbon reservoirs in China's Tarim Basin are characterized by multistage accumulation, but the major period and the process of hydrocarbon accumulation are still ambiguous. By integrating analyses of fluid inclusions, light hydrocarbon and diamondoid, and reservoir bitumen, we determined the contribution of oils charged in different periods and reconstructed the hydrocarbon accumulation process of Ordovician reservoirs in the Tuoputai area, which is significant to expand deep petroleum exploration in Tarim Basin. The results show that the abundance of yellow to yellowish-green (34.6%–42.1%) and bluish-green oil inclusions (42.1%–46.2%) is relatively high, followed by the blue oil inclusions (15.4%–23.1%). The Th values of the corresponding coexisting aqueous inclusions are mainly in 70–80 °C, 90–100 °C, and 120–130 °C, respectively. The hydrocarbon charging is mainly in the late Caledonian and the late Hercynian, followed by the Himalayan. However, oils charged in the late Caledonian experienced strong biodegradation with the tectonic uplift in the early Hercynian and were transformed into heavy oils and carbonaceous bitumen. The recharged oils have not been altered by biodegradation, thermal cracking, gas washing, and thermochemical sulfate reduction, the amount of oils charged in the Himalayan was relatively small. And the calculated maturity of oils, determined by vitrinite reflectance (Rc = 1.3%–1.5%), is similar to the thermal evolution degree of the Є1y source rock in late Hercynian (1.3%–1.6%) and far less than that in Himalayan (1.8%–2.0%). Large amounts of oils accumulated in the late Hercynian made a major contribution to the current reservoirs. Deep hydrocarbon reservoirs in Tuoputai area were mainly got accumulated when the formation was shallowly buried and then preserved with the deep burial.

Characteristics, formation mechanism and evolution model of Ordovician carbonate fault-controlled reservoirs in the Shunnan area of the Shuntuogole lower uplift, Tarim Basin, China

The Ordovician carbonate fault-controlled reservoir in the Shunnan area is substantially different from the typical karst reservoir closely related to an unconformity. The development of fault-controlled reservoirs is controlled by the deep and large strike-slip fault in the Middle Caledonian period. The characteristics, diagenesis types, diagenetic fluid evolution characteristics, fluid-rock interactions, and reservoir response of fault-controlled reservoirs were investigated systematically by analyzing 3D seismic, logging, and drilling core data. Production data, reservoir petrology, and chemical filling geochemistry were also used. The formation and evolution of fault-controlled reservoirs are affected by the coupling of the structure, lithofacies, and fluid. The dolomitization in the quasi-syngenetic period in the early stage of Episode I in the Middle Caledonian provided the material basis for the development of fault-controlled reservoirs. The Mg-rich high-salinity thermal fluid in the late Caledonian and the Early Hercynian provided a constructive transformation for the development of fault-controlled reservoirs. In contrast, the transformation of reservoirs by the Si-rich magmatic thermal fluid in the late Hercynian was mainly destructive, and the action range of the deep thermal fluid was strongly controlled by strike-slip fault zones. The releasing bend and superimposed parts of different structural styles in the fault zone have high activity intensity. These are areas where dissolution and transformation of the fluid occur preferentially. Dolomite is often more conducive to the development of fault fracture zones and promotes the dissolution and transformation of the deep thermal fluid. Therefore, the strike-slip fault cutting through the basement causes the hydrothermal fluid to enter the fault, dissolving carbonate minerals and forming a hydrothermal-modified fault-controlled reservoir composed of fractures, dissolution pores, and dolomite intercrystalline pores. The results provide a new approach and direction for the study of different types of deep and ultra-deep carbonate reservoirs.

Characteristics of late reformation and petroleum prospect of Carboniferous strata in the Bayanhot Basin, N. China

The Bayanhot Basin is located at the intersection of the Alxa Block, Ordos Basin, and Corridor Transitional Zone. Although the Carboniferous palaeo‐sedimentary and tectonic features are of great significance in discovering the scientific issues of the influential range of the Qinlin‐Qilian Ocean and palaeo‐Asian Ocean and the relationship of the North China Block and Alxa Block, as well as in evaluating hydrocarbon prospects, it is difficult to restore the Carboniferous features because of intense late reformation. It is necessary to systematically discuss the late reformation characteristics. It is shown in this paper that Carboniferous strata have mainly experienced two extensive tectonic movements during the Late Hercynian–Indosinian and Yanshanian periods and had four reformation types: structural compression, uplift to erosion, superimposed and deep burial, and thermodynamic reformation, based on comprehensive analysis of seismic profiles, wells, aeromagnetic data and tectonic characteristics of the surrounding regions. Carboniferous strata were partly eroded by the compressional folding during Late Hercynian–Indosinian movement, and were forcefully eroded by thrusting and uplift during Yanshanian movement, especially movement in the Late Jurassic, which determined the present thickness of the Carboniferous strata. The deep overlap of the overlying Jurassic, Lower Cretaceous, Palaeogene, and Neogene rocks in different areas was beneficial to the maturation of Carboniferous source rocks. The magmatic rocks exposed by well drilling and shallow aeromagnetic anomaly belts show heating reformation, which accelerated the maturation of source rocks to some extent. The total strength of the Carboniferous late reformation was stronger in the western area than in the eastern area and greater in the northern area than in the southern area. The multiple periods and various types of reformation were closely related to the regional closure of the Palaeo‐Qinling‐Qilian Ocean in the south, Palaeo‐Asian Ocean, and Mongolia–Okhotsk Ocean in the north and the subduction of the Palaeo‐Pacific Plate in the east. The Carboniferous strata in eastern and southern basins will be the prospective favourable areas for the petroleum exploration.

Inclusion Characteristics of the Fracture-Cave Calcite of Ordovician Yingshan Formation and Its Indication to the Formation of Paleokarst Reservoir in the Northern Slope of Tarim Basin Center Area, China

A karst fracture-cavity was developed in the Ordovician Yingshan Formation in the northern slope of the Tarim Basin center area (Tazhong area); however, the area experienced multistage karst and diagenesis in the later period, and the reservoir was heavily reformed, characterized by complex distribution. Based on the testing of the inclusions in the karst fractures and fracture-cave calcite, this study summarizes the physical and chemical characteristics of the inclusions in the northern slope of Tazhong. The major inclusions were liquid and gas–liquid phases, while the minor ones were hydrocarbon phases. Based on the salinity–homogenization temperature diagram, the gas–liquid inclusions could be divided into seven types: low-temperature low-salinity inclusions (the lowest salinity was 0.88%, which is lower than that of present sea water; the lowest temperature of the inclusions was 49°C); low-temperature high-salinity inclusions; medium-temperature low-salinity inclusions; medium-temperature medium-salinity inclusions; medium-temperature high-salinity inclusions; high-temperature medium-salinity inclusions; and high-temperature high-salinity inclusions. The development of inclusions was characterized by high temperature and high salinity or low temperature and low salinity. In addition, the low-temperature and low-salinity inclusions near the unconformity surface were well-developed. The high-temperature and higher-salinity inclusions far from the unconformity surface were also well-developed. A burial history analysis showed three stages of karstification or filling in the area: exposed karstification in the Middle Caledonian, shallow burial filling in the Middle Hercynian, and burial filling in the Late Hercynian. By determining the inclusions in calcite associated with mud in the karst caves and combined with previous studies, the Middle Caledonian was found to be the period associated with the eogenetic karstification of meteoric water and seawater. This is the key factor in the formation of karst reservoirs in the Tazhong area. The large-scale calcite growth in the Middle Hercynian and Late Hercynian burial periods led to further filling of the early karst reservoir, reduced reservoir space, and enhanced reservoir heterogeneity.

Key oil accumulation periods of ultra-deep fault-controlled oil reservoir in northern Tarim Basin, NW China

A giant fault-controlled oilfield has been found in the ultra-deep (greater than 6000 m) Ordovician carbonate strata in the northern Tarim Basin. It is of great significance for hydrocarbon accumulation study and oil exploitation to determine the key oil accumulation periods. Based on detailed petrographic analysis, fluid inclusion association (FIA) in calcite samples filling in fractures from 12 wells were analyzed, and key accumulation periods of the strike-slip fault-controlled oilfield was studied by combining oil generation periods of the source rocks, formation periods of the fault and traps, and the fluid inclusion data. (1) There are multiple types of FIA, among them, two types of oil inclusions, the type with yellow fluorescence from the depression area and the type with yellow-green fluorescence from the uplift area with different maturities indicate two oil charging stages. (2) The homogenization temperature of the brine inclusions in FIA is mostly affected by temperature rises, and the minimum temperature of brine inclusions symbiotic with oil inclusions is closer to the reservoir temperature during its forming period. (3) FIA with yellow fluorescence all have homogenization temperatures below 50 °C, while the FIA with yellow-green fluorescence have homogenization temperatures of 70–90 °C tested, suggesting two oil accumulation stages in Middle–Late Caledonian and Late Hercynian. (4) The Middle–Late Ordovician is the key formation period of the strike-slip fault, fracture-cave reservoir and trap there. (5) The oil generation peak of the main source rock of the Lower Cambrian is in the Late Ordovician, and the oil accumulation stage is mainly the Late Ordovician in the depression area, but is mainly the Early Permian in the uplift area. The key oil accumulation period of the strike-slip fault-controlled reservoirs is the Late Caledonian, the depression area has preserved the primary oil reservoirs formed in the Caledonian, while the uplift area has secondary oil reservoirs adjusted from the depression area during the Late Hercynian. Oil reservoir preservation conditions are the key factor for oil enrichment in the strike-slip fault zone of northern Tarim, and the Aman transition zone in the depression is richer in oil and gas and has greater potential for exploration and development.

Structural characteristics, formation & evolution and genetic mechanisms of strizke–slip faults in the Tarim Basin

Exploration and development practices prove that intracratonic strike–slip faults control the development of carbonate reservoirs and the enrichment of oil and gas in the Tarim Basin. Intracratonic strike–slip faults are characterized by short displacement, small throw and great burial depth, so there are still many controversies on their structural evolution and formation mechanisms. In order to settle these controversies, this paper firstly analyzes the geometric and kinematic characteristics of the intracratonic strike–slip faults in the central Tarim Basin. Then, based on extensive high-accuracy 3D seismic data and 2D seismic data, the formation and evolution process of the intracratonic strike–slip faults in the Tarim Basin are discussed deeply, and their genetic mechanisms are explored. Moreover, the following research results are obtained. First, the strike–slip faults in the study area have the characteristics of vertical stratification, plane zonation and segmentation along the strike. Second, the movement directions of the deep transpressional strike–slip faults are simple. The NE and NNE strike–slip faults are sinistral, and the NNW strike–slip faults are dextral. However, the shallow transtensional faults are more different in the movement direction. The strike–slip faults have displacements of 400–1500 m, and they are classified as intracratonic strike–slip faults with small slip distance. Third, the strike–slip faults were formed in the middle Caledonian, and some faults were reactivated in the late Caledonian–early Hercynian, late Hercynian, and Yanshanian–early Himalayan. The distribution of faults is different in different periods. Fourth, the formation of the strike–slip faults in the middle Caledonian is controlled by the basement heterogeneity and the compression action from the closure and subduction of the Proto-Tethys Ocean. The transtensional faults are R-shear branch faults or tensile break faults generated by the later activation of middle Caledonian strike-slip faults, and their formation is controlled by the subduction and closure of oceanic basins around the basins in different periods. In conclusion, the structural characteristics, formation & evolution and genetic mechanisms of intracratonic strike–slip faults are in close relation to oil and gas, where great benefit of oil and gas exploration and development is achieved. Besides the Tarim Basin, the Sichuan Basin and the Ordos Basin have similar geological and structural settings, so the studies on typical basins will surely enrich and complete the theories on the intracratonic strike–slip faults in domestic micro land blocks and multi-cycle basins and point out the direction for the discovery of more oil and gas resource.

Genesis and preservation of the giant ultradeep Hadexun petroleum accumulation in the Tarim Basin, China

Reservoired liquid petroleum has limited potential to be preserved under high temperature conditions due to thermal cracking and potentially thermochemical sulfate reduction, and thus exploration targets in ultradeep strata are mainly gas and condensate gas. However, a giant ultradeep liquid petroleum accumulation, with an average depth of >6500 m, was discovered recently in the Hadexun area, Tarim Basin. In this study, integrated geochemical analyses were conducted on the black oil samples from the Hadexun area. Intact terpanes and steranes and low concentration of diamondoids were detected, indicating that these oils were generated at peak oil window and were barely altered by secondary geochemical process (thermal cracking or others) despite their depth. The associated wet gas with light isotopic profiles was classified as mature oil-derived gas co-generated with the oil. Petroleum accumulation analyses suggested that the favorable carbonate reservoir bodies in the Middle Ordovician Yijianfang Formation were attributed to karstification and weathering, and mainly distributed along the major strike-slip fault belts. Together with the thick mudstones in the overlying Upper Ordovician Sangtamu Formation, they formed a favorable reservoir-seal assemblage in the Hadexun area. Massive volumes of liquid petroleum were generated and expelled from the Lower Cambrian black shales in the Late Hercynian and migrated into the Ordovician carbonate reservoirs through the strike-slip fault system, and subsequently were exposed to stable tectonic conditions. Due to the rapid subsidence since the Neogene, insufficient temperature-time exposure eliminated the impact of thermal cracking; therefore, the petroleum accumulation was preserved in the liquid phase, and further indicates that huge petroleum resource potential remains in ultradeep strata in the Tarim Basin. • Oil/gas sourced from the Cambrian shales were barely altered by geochemical process. • Insufficient compensation of temperature and time led to the preservation of the ultradeep oil. • Oil/gas was preferentially enriched in favorable carbonate reservoir bodies along faults. • Liquid petroleum and condensate/gas are the main targets in Ordovician and Cambrian assemblages, respectively.

Metallogenic age and genesis of the Zhaishang gold deposit in Minxian, Gansu Province

寨上金矿位于西秦岭大规模卡林-类卡林型金矿床集中地之中，是近二十年来探获的又一超大型金矿。通过对寨上金矿的6件矿石样品中的载金矿物黄铁矿进行Rb、Sr、Re、Os同位素分析，获得Rb-Sr、Re-Os等时线年龄分别为273±10Ma、273Ma±22Ma。该数据大于前人所获的二期成矿阶段年龄，在西秦岭已探明的金矿床中尚无报道，反映了寨上金矿更早一期的成矿年龄为273Ma，这也为该区域寻找金矿床拓宽了时间轴。Sr、Os、S同位素及稀土和微量元素研究表明，寨上金矿成矿物质为深源与壳源物质的混合。二叠纪时位于岷礼前陆盆地的吴家山隆起经历了强烈的隆升延展作用，岩浆热液沿断裂通道上升到有较高Au背景值盆地中的褶皱带，使地层中的Au活化富集，在构造有利部位卸载成矿，形成早期寨上金矿床，后期钨矿化及金矿化叠加在早期金矿床之上。寨上金矿第一期金成矿的年龄在时空上与阿尼玛卿洋盆闭合、共和坳拉谷俯冲碰撞、江里沟及中川喜马拉雅型岩体的侵入时间基本吻合，符合“地壳加厚有利于形成金矿床及喜马拉雅型花岗岩与金矿关系密切”的结论，对在该区域寻找同时代金矿床具有一定的指示意义。;Zhaishang gold deposit is located in the concentrated area of large-scale carlin-like gold deposits in West Qinling Mountains, and it is the second super-large gold deposit discovered in the past 20 years. Rb, Sr, Re, Os isotopes of the pyrite, the gold-bearing mineral of Zhaishang gold deposit, in six ore samples were tested in this paper to determine their metallogenic ages and ore-forming materials. The measured Rb-Sr and Re-Os isochron ages of the pyrite are 273±10Ma and 273Ma±22Ma, which are older than the ages of two metallogenic stages of the Zhaishang deposit obtained by the predecessors, i.e., a earlier metallogenic stage (the first stage of 273Ma) might be exist in this area. This new chronological data may broaden the time axis for searching gold deposits in this area since no same metallogenic ages have ever been reported in the explored gold deposits in West Qinling. Furthermore, analyses results of Sr, Os, S isotopes, REE and trace elements compositions indicate that the ore-forming material of the Zhaishang gold deposit is the mixture of deep source and crust source materials. Tectonically, the Wujiashan Rise which is located in the Minli fore-arc basin experienced a strong geothermal uplift and extension in the Late Hercynian Period, which resulted in the hydrothermal fluids rising along the fault channels to the folded belts of the basin with a high Au background, and the activated Au enriched in the formation and unloaded in suitable structural spaces to form the first stage Zhaishang gold deposit. The later two stages of W and Au mineralization superimposed on the early gold deposit. Compared with regional geological evolutionary events, the age of gold mineralization in the first stage of Zhaishang gold deposit is consistent with the time of the closure of the A'nyemaqen ocean basin in the West Qinling, the subduction collision of Gonghe aulacogen, and the intrusion of Jiangligou and Zhongchuan Himalayan-type plutons, which obeys the rule crustal thickening is conducive to the formation of gold deposits and Himalayan granites are closely related to gold deposits. It is of indicative significance for searching contemporary gold deposits in this area.

Geochemical Characteristics and Migration Pathways of Ordovician Carbonate Oil Reservoirs in the Tuoputai Area, Tarim Basin, Northwestern China

AbstractExploration potential is huge and the oil resources are rich in the Ordovician reservoirs of the Tarim Basin. However, the mechanism of hydrocarbon accumulation is complex and not yet fully understood. In the Tuoputai area, the hydrocarbon migration pathways and characteristics of deep hydrocarbon accumulation are revealed through analyses of the physical data of rich oil and gas, the geochemical parameters of oil, and fluid inclusions. The results show that the Ordovician oils in the Tuoputai area have the same geochemical characteristics as the mixed oil from the Lower Cambrian source rock and the Middle–Upper Ordovician source rock. The Ordovician reservoirs have been charged three times: in the late Caledonian, late Hercynian, and Himalayan stages. Oil charging occurred in the Hercynian stage, in particular, as it is the main filling period of hydrocarbon. The north‐northeast (NNE)‐trending TP12CX major fault, active in in these times and is dominant migration channel of hydrocarbon, but there is segmentation affected by the difference of activities. Oil maturity is higher in the south than in the north and is abnormally high near the major fault. Parameters related to migration indicate that oil migrated northeastward along the NNE‐trending TP12CX major fault and adjusted laterally along the secondary faults and weathering crust, forming the present characteristics of oil and gas distribution.

ГЕОДИНАМІКА

The second part of the article studies the tectonic conditions and natural mechanisms of tectonic inversion of the Dnieper-Donets Basin and the Western Donets Graben. Method. The research uses the original method of reconstruction of fields of tectonic stresses and deformations. It also makes tectonophysical analysis of geostructures was used. The analytical base of the research consisted of the latest materials of geo-mapping, numerical modeling of deformations of the southern edge of the Eastern European platform and comparison of model and reconstructed stress fields. Results. In the geodynamic environment of the interference of the intraplate submeridional collision compression with the regional strike-slip stress field, the inversion deformations of the rift-like geostructure took place in the uplift-thrust and strike-slip modes. This led to significant horizontal movements of geomass of sedimentary rocks, deformation folding with the formation of three inversion floors - Late Hercynian (Saal-Pfalz), Early Alpine (Laramian) and Late Alpine (Attic). They formed structural ensembles of scaly tectonic covers of transverse displacement of geomass a from axial to onboard zones, folded covers of longitudinal approach from the Donbas Foldbelt and long linear anti- and synforms, the axes of which are oriented orthogonally to the direction of geomass advancement. Together they form the body of the Segment of Tectonic Wedging of geomass, which is distinguished as part of the Cover-Folded System of Tectonic Thrusting of regional scale. A feature of the tectonic framework of the Segment is the curvature of the planes of the main thrusts, which limit it, and smaller plumage thrusts, which control the folded covers of the thrust. It is associated with a change in the extension of the thrusts from the north-west in the territory of the Western Donets Graben to the western direction in the extreme south-east of the Basin. This causes the corresponding bending of the axes of the fracture anti- and synforms. Structural patterns of folding with a tendency to adapt the axes of folds to the extension of thrusts indicate significant horizontal displacements of geomas of the sedimentary stratum, which in conditions of limited geological space cause secondary deformations of linear folded forms. Due to the displacement of geomas from the zones of maximum compression in the axial part of the Graben to the zones of geodynamic shadow - in the direction of the Oryl depression and Graben boards, the West Donets Cover-Folded Tectonic Region was formed within the transition zone. Scientific novelty. The study completed an original kinematic model of tectonic inversion of the Western Donets Graben was completed. The mechanism of inversion, due to which the riftogenic structure is completely destroyed by folded deformations of platform orogeneses, is caused by the pressure of the "tectonic stamp" of the Donbas Foldbelt. Under its influence, a segment of tectonic wedge was formed in the Graben, which was diagnosed with oroclin of transverse extension of the sliding type. The body of the Oroclin is formed by echeloned, rock-articulated ensembles of anticlinal uplift -folds, synclines and scaly plates-covers of pushing. A geodynamic injection band was formed in the foreland of the Tectonic Orocline extension, where folded zones of geomas displacement were formed, which consist of coulisse articulated uplift-anticlines. At the top of the Orocline, at the ends of dynamically conjugate main thrusts, an advanced tectonic compression fan is formed. In the rear of the Oroclin – hinterland are tectonic sutures – the roots of the folding covers of the approach. Practical significance. Development of a structural-kinematic model of tectonic inversion of the Western Donets Graben will allow to improve the geodynamic model of tectonic inversion of the Dnieper-Donets paleorift, on the basis of which regional schemes of tectonic and oil-gas-geological zoning will be adjusted.

Silicon isotopic constraints on the genesis of cherts in the Ordovician sedimentary succession in Tarim Basin, Western China

• Hydrothermal fluids play an important role in the formation of cherts . • Si-O-Sr isotopes of the cherts to constrain the source and migration of the fluids. • Fault activity links the channels of hydrothermal migration during the late Hercynian. The Tarim Craton is the largest petroliferous basin in China, and thick chert have been found in the Ordovician carbonate reservoir during drilling. Owing to the tight chert, the storage performance may decrease and lead to drilling failure. In this study, silicon, oxygen, and strontium isotope compositions of chert were tested to constrain the source and migration of Si-rich fluids, which is helpful for understanding the reservoir and avoiding drilling failure. The Ordovician silicon dioxide in cherts have δ 30 Si values from +1.29‰ to +3.50‰, δ 18 O values from 20.21‰ to 27.50‰, and 87 Sr/ 86 Sr values from 0.709 to 0.714. The correlation between δ 30 Si- and δ 18 O reveals that the formation of cherts is related to the activity of low-temperature hydrothermal fluids, and the value of 87 Sr/ 86 Sr indicates the contribution of the deep crust source to the silicon-rich fluid. Comprehensive analyses showed that the deep thermal fluid was active during the formation of cherts. The late Hercynian tectonic movement led to the development and activity of deep and large faults, and the deep-source silica hydrothermal fluid entered the Ordovician reservoir along the fault from bottom to top, filling the pores, densifying the rocks, and causing deterioration of the storage performance. Therefore, through multi-geochemistry (Si-O-Sr isotopes), the spatial distribution characteristics of silica can be determined to avoid the distribution interval and member of silica during drilling. Moreover, the identification and distribution prediction of the genesis of cherts is of great significance for oil and gas exploration.

Structural characteristics and storage control function of the Shun I fault zone in the Shunbei region, Tarim Basin

The Shunbei region, situated in the Tarim Basin, exhibits exploration potential. Previous exploration experience suggests that the Shun I strike-slip fault zone plays an important role in reservoir formation and hydrocarbon accumulation. In this study, based on the interpretation of three-dimensional seismic data, the structural characteristics of the Shun I strike-slip fault zone were identified. This study revealed that the Shun I strike-slip fault zone is characterized by vertical stratification, plane segmentation, multiple evolution staging, and heterogeneity of storage control. Influenced by tectonic activities during five periods, namely the early Caledonian, middle Caledonian, late Caledonian-early Hercynian, late Hercynian, and Indosinian-Himalayan periods, the Shun I strike-slip fault zone mainly developed high-angle strike-slip faults and en echelon normal faults during the Paleozoic. The Mesozoic–Cenozoic developed primarily small-scale extensional faults without spatial coupling with underlying strike-slip faults. The Shun I strike-slip fault zone consists of both hydrocarbon migration pathways and hydrocarbon traps. High-quality reservoirs generally develop along faults. Fractures in the damage zone of the Shun I fault control the formation of reservoir space. The Shun I strike-slip fault zone exhibits distinct segmentation characteristics. The Shun I strike-slip fault zone can be divided into nine segments of three types: vertical, pull-apart, and push-up. The discontinuity along the strike of the Shun I strike-slip fault zone partially controls the heterogeneity of the reservoir distribution, thus affecting reservoir connectivity. The nature of different segments of the strike-slip fault zone controls the scale and quality of reservoir development and restricts the formation of oil and gas reservoirs. Three types of oil control models were established for the Shun I strike-slip fault zone. Our findings indicate that the central main fault plane of the vertical segment pattern is the richest in hydrocarbons. Oil and gas accumulation along the fault plane of the flower structure in the pull-apart or push-up segments.

Evolution and Hydrocarbon Accumulation of the Cambrian-Ordovician Paleo-uplifts in the Southwest Tarim Depression, China

There is a close relationship between the evolution of the Cambrian–Ordovician paleo-uplifts and their hydrocarbon accumulation in the Southwest Tarim Depression. Using regional 2D seismic data interpretation, the structural morphology of the top of the Ordovician carbonate rocks in the Southwest Tarim Depression at different historical geologic stages is mapped. Combined with analysis of the discovered oil and gas reservoirs, the regularity of the hydrocarbon distribution in the Southwest Tarim Depression is investigated. Research shows that the evolution of the Cambrian-Ordovician paleo-uplifts in the Southwest Tarim Depression can be divided into four stages: (1) formation of the paleo-uplifts, (2) steady subsidence of the eastern paleo-uplift, (3) migration and adjustment of the western paleo-uplift and (4) strong subsidence and extinction of the paleo-uplifts. The Cambrian-Ordovician strata in the Southwest Tarim Depression experienced a seesaw-like structural reconstruction from an early north-dipping slope to a late south-dipping slope. During the structural reconstruction process, a pivot zone was located at the position of the current Maigaiti Slope, proving favorable for hydrocarbon accumulation and preservation. The fault zones in the eastern and western paleo-uplifts of the Maigaiti slope finalized during the Late Hercynian Period were favorable to preserving paleo-oil reservoirs. In contrast, the fault zones in the northern margin of the Maigaiti Slope, where the current tectonic uplift is located, were favorable for the accumulation of gas reservoirs.

The origin and accumulation of ultra-deep oil in Halahatang area, northern Tarim Basin

The giant Ordovician petroleum accumulations in the Halahatang area, northern Tarim Basin, China with burial depths of 7000–8000m are the deepest paleo marine carbonate oilfield ever discovered so far. By conducting comprehensive molecular geochemical analyses on 12 Halahatang oil samples, we suggested these oils and their associated gases are likely from the Cambrian source rock at mature stage; however, the oil maturity shows slight variation, it increases with the burial depth. Reservoir studies reveal that lateral fluid migration was hindered as a result of strong lateral reservoir heterogeneities, contrarily, vertical fluid migration was favorable due to developed fault system. Fluid inclusion and basin modeling analyses indicate a main oil and gas charge event during the late Hercynian period. Therefore, the formation of the giant Halahatang oilfields is attributed to abundant hydrocarbon generated from deep-buried source rocks, favorable vertical migration conduits related to fault system, and high-quality fracture-vuggy reservoirs in Ordovician strata. It is further proposed that Halahatang oilfields were formed in three evolution stages, including the reservoir formation during the Caledonian (after Silurian), the hydrocarbon accumulation during the late Hercynian (Permian), and continuous deep buried after the late Hercynian (since Triassic). Due to the low geothermal gradient and rapid burial in the late stage, oils in the Halahatang area did not reach the cracking threshold, resulting in abundant liquid oil accumulations preserved in the deep strata near the fault zone. The great exploration potential in the deep buried zones can be speculated based on this study.