Kuqa Basin Research Articles

Deep-learning-based natural fracture identification method through seismic multi-attribute data: a case study from the Bozi-Dabei area of the Kuqa Basin, China

Fractures play a crucial role in tight sandstone gas reservoirs with low permeability and low effective porosity. If open, they not only significantly increase the permeability of the reservoir but also serve as channels connecting the storage space. Among numerous fracture identification methods, seismic data provide unique advantages for fracture identification owing to the provision of three-dimensional information between wells. How to accurately identify the development of fractures in geological bodies between wells using seismic data is a major challenge. In this study, a tight sandstone gas reservoir in the Kuqa Basin (China) was used as an example for identifying reservoir fractures using deep-learning-based method. First, a feasibility analysis is necessary. Intersection analysis between the fracture density and seismic attributes (the characteristics of frequency, amplitude, phase, and other aspects of seismic signals) indicates that there is a correlation between the two when the fracture density exceeds a certain degree. The development of fractures is closely related to the lithology and structure, indirectly affecting differences in seismic attributes. This indicates that the use of seismic attributes for fracture identification is feasible and reasonable. Subsequently, the effective fracture density data obtained from imaging logging were used as label data, and the optimized seismic attribute near the well data were used as feature data to construct a fracture identification sample dataset. Based on a feed-forward neural network algorithm combined with natural fracture density and effectiveness control factor constraints, a trained identification model was obtained. The identification model was applied to seismic multi-attribute data for the entire work area. Finally, the accuracy of the results from the training, testing, and validation datasets were used to determine the effectiveness of the method. The relationship between the fracture identification results and the location of the fractures in the target reservoir was used to determine the reasonableness of the results. The results indicate that there is a certain relationship between multiple seismic attributes and fracture development, which can be established using deep learning models. Furthermore, the deep-learning-based seismic data fracture identification method can effectively identify fractures in the three-dimensional space of reservoirs.

Prospects for Eocene potash deposition in the East Tethyan domain: New evidence from fluid inclusions and chlorine isotope of halite crystals in the Kuqa Basin, NW China

The level of the Tethys Sea increased several times during the deposition of Mesozoic to Cenozoic sediments in the Kuqa Basin in the Tarim Block. The basin contains Paleogene to Neogene units containing evaporite-rich layers, which are favourable sources for economic concentration of potash (K-rich salt). However, the basin’s geology is poorly understood, lacking knowledge of the source of the brine, paleo-water temperatures, and the degree of evaporation. As a result, the exploration for potash mineralisation has been limited. This study documents the geochemistry, fluid inclusions, and Cl-isotope of Early Eocene halite crystals from the Kuqa Basin. The results show that the homogenisation temperatures of primary fluid inclusions in halite range between 20° and 57 °C, which is close to the temperature range of the seawater in the Tethyan sea during the same period. The average value of ∼ 32 °C is taken as the average temperature of the salt-forming brine in the Early Eocene. The homogenisation temperatures of the inclusions are generally high, with a maximum temperature of ∼ 57 °C, which is consistent with the global Paleocene–Eocene thermal maximum (PETM) climatic event, indicating that the salt-forming brine has experienced long-term and sustained high temperatures and short-term extremely high temperatures. This is consistent with the high temperatures recorded during the deposition of potash in other parts of the earth, such as ∼ 59 °C for the Silurian Michigan Basin in the USA and ∼ 58 °C for the Quaternary Lop Nur region in Mongolia. In addition, the Br content of the halite samples is between 58 and 125 ppm, and the Br × 103/Cl value is between 0.11 and 0.21, which is consistent with the evaporation curve of seawater, indicating that there is a marine environment favourable for potash deposition. The value of δ37Cl is between −0.7 and −0.14 ‰ showing that the salinity of the brine evolved to a higher concentration, especially during the later stage of potash deposition. This indicates that the potential for the presence of potash deposits in the Early Eocene Kuqa Basin is high.

Origin of paratacamite in the Kuqa Basin, Xinjiang, China: Defined by Cu isotope values

Isotopes of Cu in some copper minerals have been used to trace the origin of metallogenic materials. However, little is known about paratacamite, a sedimentary-type Cu mineral whose formation is closely related to brines and evaporites. The Kuqa Basin is filled with a huge thickness of evaporite deposits and is located in the northern margin of the Tarim Basin, Xinjiang, northwestern China. Here, we report a study of paratacamite from the Kuqa Basin and infer its genesis from the mineral assemblage and Cu isotopes of paratacamite. Paratacamite is widely distributed in basin deposits coexisting with saline minerals in fractures and joint planes in fault/fracture zones. Cu isotopic compositions (δ65Cu) of paratacamite range from −0.49‰ to 1.02‰, with a mean of 0.41‰. These values suggest that Cu in the paratacamite originated from Cu-bearing brine that was derived from the leaching of metallic ions from surrounding rocks and migrated along and upward through faults, fractures, and joint planes and ascended to the surface while cooling. On the basis of the paragenetic association of paratacamite with saline minerals in the studied samples, the occurrence of paratacamite is inferred to have been controlled by a crystallisation sequence of saline minerals with accompanying evaporation and concentration of brines from which gypsum, secondary glauberite, paratacamite, and halite were crystallised in turn. Differences in Cu isotopic compositions among the samples from the four studied mineralisation occurrences of Kezier, Yanshuigou, Daxiagu, and Dishui may be due to variation in the processes of Cu enrichment involving brines, such as Cu extraction from different rock layers, the migration and evaporation of Cu-bearing brine, and mineral deposition.

A Rapid Cu Enrichment Mechanism from Cu-Bearing Brine in Kuqa Basin, Xinjiang, China: Controlled by Crystallized Sequence of Saline Minerals

Sediment-hosted copper deposit is usually related to arid climate, ancient saline lake basin, and brine. The Kuqa Basin filled with giant-thickness evaporite units is located in the northern Tarim Basin, Xinjiang, China. It is famous for sandstone-hosted Cu deposits formed by synsedimentary processes. However, our recent studies reveal that Cu enrichment is closely related to brine on the surface of clastic rocks in the basin. It is shown that green Cu mineral coexisting with halite and gypsum occurred in the fractured fault belts of sandstones or was precipitated with halite on the surface of maroon clay in the scallops of sandstone. By SEM, EDS, and geochemical analysis methods on Cu-mineralized solid samples and brines, respectively, combined with previous geological evidence, our studies demonstrate that green Cu mineral is paratacamite, and it occurred with gypsum, halite, secondary glauberite, natural copper, and probably lead chloride on the fractured fault belts of sandstones or surface of clay. Meanwhile, the precipitation of paratacamite is controlled by a crystallized sequence of saline minerals accompanying with evaporated-concentrated course of brine in which gypsum, secondary glauberite, paratacamite, and halite are crystallized in turn. The Cu-bearing brine derived from meteoric waters and ancient seawater has a powerful capacity to leach metallic ions from its surrounding rocks and can be formed in a very short time (10 days is OK) in normal pressure and temperature conditions; also, the cycle of surface-Cu enrichment (mineralization) is only a few months (no more than 5). These indicate that a rapid Cu enrichment mechanism from Cu-bearing brine occurs on the earth’s surface of the evaporite basin. The mechanism might be supposed to make an enormous amount of metal mineralization in a short time if considered from a large-scale spatial viewpoint. By contrast with the lengthy geological period, the short evolutionary cycle of Cu enrichment (mineralization) is obviously different from the previous cognition.

Whether the Middle Eocene Salt-Forming Brine in the Kuqa Basin Reached the Potash-Forming Stage: Quantitative Evidence from Halite Fluid Inclusions

The Kuqa Basin is an important potentially potash-bearing basin in China, and thick salt-bearing strata were deposited under the influence of multistage Tethyan transgression-regression cycles during the Eocene. At present, research on the process of potash formation in the Kuqa Basin has mostly focused on traditional salt mineralogy, whole-rock geochemistry, and evaporite sedimentary evolution characteristics. However, research on the original ore-forming parent fluid directly related to potash formation has not yet been carried out, directly hindering further evaluation of potash mineralization. Therefore, this paper takes the internal factors controlling potash formation as the starting point and analyzes the physical and chemical properties, such as the homogenization temperatures ( T h ) and chemical compositions, of primary halite fluid inclusions. A total of 220 T h data from fluid inclusions were obtained, and the temperatures ranged from 9.4 to 54.1°C, indicating a high-temperature brine environment conducive to the rapid deposition of the potash deposit. In total, 22 halite fluid inclusions were analyzed for chemical components. The highest KCl content reached 0.59%, which was higher than the lowest industrial grade of potassium-rich brine (0.5%), indicating that the brine experienced a high degree of evaporation and concentration during the salt-forming period and reached the potash precipitation stage. This paper provides quantitative data on the evolution of the sedimentary environment in the Kuqa Basin and supports future potash exploration.

Evaluation of Hydrocarbon Generation Using Structural and Thermal Modeling in the Thrust Belt of Kuqa Foreland Basin, NW China

The Kuqa Basin is a typical foreland basin in northwest China, characterized by compressive foreland fold-and-thrust belts and a regionally distributed huge salt layer. A large number of overthrust faults, fault-related folds, and salt-related structures are formed on the thrust belt due to strong compression and structural deformation, causing difficulty in simulation of the basin. In this study, modeling of the thermal history of the complicated compressional structural profiles in the Kuqa foreland basin was successfully conducted based on the advanced “Block” function introduced by the IES PetroMod software and the latest geological interpretation results. In contrast to methods used in previous studies, our method comprehensively evaluates the influence of overthrusting, a large thick salt layer with low thermal conductivity, fast deposition, or denudation on the thermal evolution history. The results demonstrate that the hydrocarbon generation center of the Kuqa foreland basin is in the deep layers of the Kelasu thrust belt and not in the Baicheng Sag center, which is buried the deepest. A surprising result was drawn about the center of hydrocarbon generation in the Kuqa foreland basin, which, although not the deepest in Baicheng Sag, is the deepest part of the Kelasu thrust Belt. In terms of the maturity of the source rock, there are obvious temporal and spatial differences between the different structural belts in the Kuqa foreland basin, such as the early maturation of source rocks and the curbing of uplift and hydrocarbon generation in the piedmont zone. In the Kelasu thrust belt, the source rock made an early development into the low mature-mature stage and subsequently rapidly grew into a high-over mature stage. In contrast, the source rock was immature at an early stage and subsequently grew into a low mature-mature stage in the Baicheng Sag–South slope belt. The time sequence of the thermal evolution of source rocks and structural trap formation and their matching determines the different accumulation processes and oil and gas compositions in the different structural belts of the Kuqa foreland basin. The matching of the multistage tectonic activity and hydrocarbon generation determines the characteristics of the multistage oil and gas accumulation, with the late accumulation being dominant. The effective stacking of the gas generation center, subsalt structural traps, reservoir facies of fine quality, and huge, thick salt caprocks creates uniquely favorable geological conditions for gas enrichment in the Kelasu foreland thrust belt.

Origin and circulation of saline springs in the Kuqa Basin of the Tarim Basin, Northwest China

It is widely accepted that hydrogeochemistry of saline springs is extremely important to understand the water circulation and evolution of saline basins and to evaluate the potential of potassiumrich evaporites. The Kuqa Basin, located in the northern part of the Tarim Basin in Northwest China, is a saline basin regarded as the most potential potash-seeking area. However, the origin and water circulation processes of saline springs have yet to be fully characterized in this saline basin. In this study, a total of 30 saline spring samples and 11 river water samples were collected from the Qiulitage Structural Belt (QSB) of the Kuqa Basin. They were analyzed for major (K+, Ca2+, Na+, Mg2+, SO42−, Cl− and HCO3−) and trace (Sr2+ and Br−) ion concentrations, stable H-O-Sr isotopes and tritium concentrations in combination with previously published hydrogeochemical and isotopic (H-O) data in the same area. It is found that the water chemical type of saline springs in the study area belonged to the Na-Cl type, and that of river water belonged to the Ca-Mg-HCO3-SO4 type. The total dissolved solid (TDS) of saline springs in the QSB ranged from 117.77 to 314.92 g/L, reaching the brine level. On the basis of the general chemical compositions and the characteristics of the stable H-O-Sr isotopes of saline springs, we infer that those saline springs mainly originated from precipitation following river water recharging. In addition, we found that saline springs were not formed by evapo-concentration because it is unlikely that the high chloride concentration of saline springs resulted in evapo-concentration and high salinity. Therefore, we conclude that saline spring water may have experienced intense evapo-concentration before dissolving the salty minerals or after returning to the surface. The results show that the origin of salinity was mainly dominated by dissolving salty minerals due to the river water and/or precipitation that passed through the halite-rich stratum. Moreover, there are two possible origins of saline springs in the QSB: one is the infiltration of the meteoric water (river water), which then circulates deep into the earth, wherein it dissolves salty minerals, travels along the fault and returns to the surface; another is the mixture of formation water, or the mixture of seawater or marine evaporate sources and its subsequent discharge to the surface under fault conditions. Our findings provide new insight into the possible saltwater circulation and evolution of saline basins in the Tarim Basin.

Quantitative temperature recovery from middle Eocene halite fluid inclusions in the easternmost Tethys realm

Middle Eocene climatic reconstruction has long been the focus of geoscientists. However, quantitative temperature records for this interval remain lacking. The easternmost Tethys realm is especially problematic, because the traditional climatic proxies are limited by the presence of widespread and thick evaporite deposits. In this study, we present the first quantitative seawater temperatures records revealed by homogenization temperatures (Th) of primary halite fluid inclusions from the Middle Eocene Xiaokuzibai Formation of the Kuqa Basin in western China. A total of 231 Th data were obtained from two halite samples, with values ranging from 12.8 to 37.3 °C. Average Th values of 22.4 °C and 23.3 °C are in good agreement with the previous studies of climatic proxies. By combining our results with previously reported temperature records, we infer that the entire Tethys realm experienced similar climate conditions during the Middle Eocene, including a maximum seawater temperature above 34 °C, which may be related to the Middle Eocene Climatic Optimum.

Impact of an interbedded viscous décollement on the structural and kinematic coupling in fold-and-thrust belts: Insights from analogue modeling

Fold-and-thrust belts (FTBs) can be segmented both across and along strike because of various factors including tectonic and stratigraphic inheritance. In this study, we investigated along/across-strike structural interactions in a FTB propagating toward a foreland which displays contrasted lithological sequences. A set of analogue models was performed in a compressional box where a single viscous level of varying width was interbedded within a frictional series. The tectonic interaction between the viscous and the frictional provinces was tested both along and across strike. Results indicate that a frictional province influences the along-strike tectonic evolution of an adjacent viscous province. This influence decreases when the width of the viscous province increases. The frictional provinces control the taper, structural style, obliquity of the structures' trend and kinematics of the shallow deformation front of the viscous province. Results evidence how far a frictional province can impact the deformation of an adjacent viscous province. For frictional-viscous wedges, it appears that the critical taper theory, which is generally applied in 2-D, should be likely considered in terms of 3-D. Moreover, the kinematics of the deep deformation front shows mutual influences between the adjacent viscous and frictional provinces.Experimental results are compared to natural examples in the Kuqa Basin (Southern Tian Shan, China) and the Salt Range (Pakistan), and give an insight to a better understanding of the dynamics of fold-and-thrust belts bearing a viscous décollement, such as salt.

Paratacamite from Gypsum Veins in Sandstones of the Kuqa Basin, Xinjiang, China: Implications for a New Epigenetic Cu Enrichment Mechanism

AbstractThe Kuqa Basin filled with Paleogene evaporite series is located in the northeast of the Tarim Basin, Xinjiang, China. It is famous for sandstone‐hosted Cu deposits formed by synsedimentary processes. However, our recent studies reveal that there has been another Cu mineralization mechanism in this basin. Field investigations show that there is a close relationship among faults, salt domes, and brine. Cu deposits are mainly located in two east–west‐trending anticlinal belts in the basin, adjacent to salt domes in the belts. Cu minerals in gypsum veins of the Jidike and Kangcun formations have been investigated by SEM, EDS, and X‐ray diffraction methods. The occurrence of paratacamite in gypsum veins has been reported to coexist with glauberite and halite in the joint planes of sandstones. In addition, it occurs accompanying residual crystal salt encrustation in limestone fractures, or in sandstones in dendritic form. These features indicate that the surface‐Cu enrichment in the Kuqa Basin might have originated from Cu‐bearing brine in the underlying evaporite units, which migrated upward along fractures. In addition, the presence of H2S in the east–west fault belt in the Kuqa Basin, and the discovery of surface sulfur, calcium carbonate, and covellite, suggest thermochemical sulfate reduction near salt domes in the deeper parts of the rock units. This process resulted in the generation of reduced brine and provided a favorable environment for Cu enrichment. Therefore, the surface‐Cu mineralization near salt domes is interpreted to be the result of Cu‐bearing brine migrating upward to the surface along faults (or joints) following the intrusion of deep salt domes. The geological evidence indicates the presence of reducing brine and Cu‐bearing brine near the salt dome in the deeper rocks of the Kuqa Basin, thus making the intrusive contact zone of the salt dome a favorable site for the epigenetic enrichment of Cu. Our study demonstrates that Cu enrichment in the Kuqa Basin resulted not only from synsedimentary deposition but also through epigenetic enrichment associated with salt dome intrusion and brine‐rich fluids.

The salt chimney effect: delay of thermal evolution of deep hydrocarbon source rocks due to high thermal conductivity of evaporites

AbstractHalf of the topseals to the world's largest oilfields are evaporites. Rock salt has a thermal conductivity two to four times greater than that of other sedimentary rocks found in oil‐ and gas‐bearing basins. Strong heat conduction through evaporites can increase the geothermal gradient above evaporite deposits, resulting in a positive thermal anomaly and above‐average temperature while simultaneously decreasing the geothermal gradient below evaporites, resulting in a negative thermal anomaly. Most Triassic–Jurassic hydrocarbon source rocks in the Kuqa Basin, western China, are overlain by ~1500‐m‐thick Tertiary evaporites with underlying Cretaceous sandstones and mudstones. Directly measured strata temperatures indicate an obvious break in the steepness of the geothermal gradient above and below Paleogene evaporites, with a significantly steeper geothermal gradient above the evaporites. Simulations of the thermal evolution of source rocks based on data collected from well Kela‐2 indicate that if the thickness of evaporites (mainly rock salt and anhydrite rock) in overlying rocks above source rocks increases compared with the thickness of siliciclastic rocks in the overlying rocks, then strata temperatures and vitrinite reflectance in Jurassic source rocks will decrease accordingly. Our thermal simulations based on the thickness and thermal conductivity of evaporites accurately coincide with previous studies based on homogenization temperatures, hydrocarbon–water contact retrospection, and carbon isotope results from natural gases. The gas generation center located in the Kalasu Tectonic Belt today is also sealed in an evaporite‐related structural trap that formed at this time. Therefore, the speculated natural gas generation times not only correlate with the evaporite‐related structural trap formation, but the calculated maturity of deep source rocks below the evaporites also coincides with current gas reserves. And our studies can help to find the deep oils and gases under thick evaporites.

Spatial Distribution of Halite in Kuqa Basin from Paleogene to Neogene and Signification of Potash Survey

Evaporites with gigantic thickness had been developed in Kuqa Basin from Paleocene to early Miocene,and the sediment thickness changed from tens to thousands of meters.By 3D mine software,spatial distribution model of

(U-Th)/He thermochronometry constraints on the Mesozoic-Cenozoic tectono-thermal evolution of Kuqa basin: A case study of well TZ2

The oil and gas resources in Kuqa foreland basin are rich,however,there is a lack of effective researches of the tectonic thermal history in the Mesozoic and Cenozoic period,which is important for oil and gas exploration.Based on the tested apatite and zircon He ages data from drilling well samples,the He closure temperature of borehole apatites from Kuqa basin isdetermined(about 89℃)by analyzing the relationship between apatite ages of(U-Th)/He and depth/temperature in profile.Mesozoic-Cenozoic thermal history of the well TZ2will has been modeled by using the He ages and vitrinite reflectance(Ro)data.The modeling result shows that the forming time of Tuziluoke anticline started at 5 Ma,and it uplifted and eroded about 670 min Cenozoic period with the average erosion rate of 0.133mm/a.According to structural evolution analysis of Tuziluoke anticline,the time of gas-migrating fault activity and trap formation was determined,which demonstrated the time of gas accumulation in the Tuziluoke anticline was5Ma.The consistency of the times of the hydrocarbon generation and expulsion,fault activity and trap formation was key factor for gas accumulation in Tuziluoke anticline.Therefore,application of the(U-Th)/He technology to the tectono-thermal evolution of sedimentary basin has the important significance for oil and gas exploration in Kuqa basin.

Compressional salt tectonics and synkinematic strata of the western Kuqa foreland basin, southern Tian Shan, China

AbstractThe synkinematic strata of the Kuqa foreland basin record a rich history of Cenozoic reactivation of the Palaeozoic Tian Shan mountain belt. Here, we present new constraints on the history of deformation in the southern Tian Shan, based on an analysis of interactions between tectonics and sedimentation in the western Kuqa basin. We constructed six balanced cross‐sections of the basin, integrating surface geology, well data and a grid of seismic reflection profiles. These profiles show that the Qiulitage fold belt on the southern edge of the Kuqa basin developed by thin‐skinned compression salt tectonics. The structural styles have been influenced by two major factors: the nature of early‐formed diapirs and the basinward depositional limit of the Kumugeliemu salt. Several early diapirs developed in the western Kuqa basin, soon after salt deposition, which acted to localize the subsequent shortening. Where diapirs had low relief and a thick overburden they tended to tighten into salt domes 3000–7000 m in height. Conversely, where the original diapirs had higher relief and a thinner overburden they tended to evolve into salt nappes, with the northern flanks of the diapirs thrusting over their southern flanks. Salt was expelled forward, up dip along the mother salt layer, tended to accumulate at the distal pinch‐out of Kumugeliemu salt located at the Qiulitage fold belt. Furthermore, the synkinematic strata (6–8 km thick) of the Kuqa basin indicate that during the Cenozoic reactivation of the Tian Shan, shortening of the western Kuqa basin was mainly in the hinterland until the early Miocene. Then, compression spread simultaneously southwards to the Dawanqi anticline, the Qiulitage fold belt and the southernmost blind detachment fold at the end of Miocene. The western Kuqa basin has a shortening of ca. 23 km. We consider that ca. 9 km was consumed from the end of the Miocene (5.2/5.8 Ma) to the early Pleistocene (2.58 Ma) and another ca. 14 km have been absorbed since then. Thus, we obtain a ca. 3.4/2.8 mm year−1 average shortening from 5.2/5.8 to 2.58 Ma, followed by a 60–90% increase in average shortening rate to ca. 5.4 mm year−1 since 2.58 Ma. This suggests that the reactivation of the modern Tian Shan has been accelerating up to the present day.

Discovery, Genesis and Significance of Metallic Minerals in Evaporate Series in the Kuqa Basin

Abstract:A large‐scale evaporate series is developed in Paleogene‐Neogene strata in the Kuqa basin. The series is composed mainly of evaporate with thin beds of clastic rock (mainly mudstone and siltstone). In grayish white medium‐ and coarse‐grained sandstone in Miocene strata, the formation of copper minerals is in close connection with brine. In joint planes, which are developed in vertical strata, are filled with gypsum. Gypsum and copper‐mineralized sandstone contains enormous copper minerals, mainly atacamite. According to the SEM analysis for salt rock, gypsum rock, limestone, grayish green siltstone, grayish white medium‐coarse‐grained sandstone, some minerals are composed of metallic elements including Au, Ag, Cu, Zn, Pb, Co, Ni and U etc., in which Au occurs in a native form, Cu occurs in a native form or as atacamite in salt rock, gypsum rock and limestone, Ag occurs as silver sulfide in gypsum, and Zn, Pb, Co, Ni, U occur as compounds along with the above metallic ions in evaporate or clastic rock From SEM images, we can see that metallic elements or their compounds (oxides or sulfides) “take root” as grains in salt or gypsum crystals, which belong to primary chemical sedimentation along with evaporate, while some grains “float” on surface of salt or gypsum. In the former case, mineral grains were formed together with salt (gypsum) crystals; while in the latter case, minerals were enriched from internal metallic ions (Paleogene evaporate samples) or external metallic ions (Neogene gypsum samples) in the late stage of evaporate formation. The metallic ions in Paleogene evaporate samples might originate from weathered or denudated materials in the south Tianshan Mountains. The metallic ions in the Neogene evaporate samples might be from metal‐bearing brine, which migrated upward to surface along fractures and leached into evaporate (gypsum). Occurrence of metallic minerals and their compounds (elementary substance) in Paleogene evaporate proves that diversified metallic minerals exist in evaporate. The source of metallic ions in the Neogene evaporate series shows that evaporate could provide materials for late‐stage metallic mineralization.

Neogene coupling between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China

Based on the sedimentary and subsiding features of Kuqa foreland basin, this paper presents the following characteristics of Neogene coupling relationship between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China:(l) The Southern Tien Shan Orogen underwent Neogene uplifting of 4 km in height and the Kuqa Basin underwent Neogene subsidence of 4-6 km in depth accordingly beginning in 25 Ma; (2) The Southern Tien Shan Orogen moved continuously toward the Kuqa Basin, with largest structural shortening rate of greater than 53.7%, and the north boundary of the Kuqa Basin retreated continuously southward accordingly since the Miocene; (3) There are two subsidence centers with high subsiding rates and large subsiding extent, located in the eastern and western Kuqa Basin respectively, with the subsiding maximizing in the deposition period of Kuqa Formation.

Neogene coupling between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China.

Based on the sedimentary and subsiding features of Kuqa foreland basin, this paper presents the following characteristics of Neogene coupling relationship between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China: (1) The Southern Tien Shan Orogen underwent Neogene uplifting of 4 km in height and the Kuqa Basin underwent Neogene subsidence of 4-6 km in depth accordingly beginning in 25 Ma; (2) The Southern Tien Shan Orogen moved continuously toward the Kuqa Basin, with largest structural shortening rate of greater than 53.7%, and the north boundary of the Kuqa Basin retreated continuously southward accordingly since the Miocene; (3) There are two subsidence centers with high subsiding rates and large subsiding extent, located in the eastern and western Kuqa Basin respectively, with the subsiding maximizing in the deposition period of Kuqa Formation.

Neogene coupling between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China

Neogene coupling between Kuqa Basin and Southern Tien Shan Orogen, Northwestern China

Geometric tests and their application to fault-related folds in Kuqa

For brittle rock deformation, there are two main types of fault-related folds found in foreland thrust belts, i.e. fault-bend folds and fault-propagation folds. The predicted angular function for the quantitative geometric relationships between fault shapes and fold shapes in fault-bend folds and fault-propagation folds have been tested in seismic profiles; i.e. Deep Bashijiqike, Tugeerming, Kashangtokai, East Qiulitage, Deep East Qiulitage and Yakelike fault bend anticlines and Bashijiqike, Yiqikelike and South Yiqikelike fault propagation anticlines in the Kuqa rejuvenated foreland thrust belt. Test results for the transformation from seismic-time section into seismic-depth section using the seismic velocity spectrum can be used to identify whether the interpreted structures are reasonable. With geometric analysis and testing of the subsurface structures, it is possible to predict the existence of the fault-bend folds and fault-propagation folds in the Kuqa area. This study provides convincing proof that the Kuqa basin is a rejuvenated foreland basin.

Sequence Stratigraphy of the Desert System: A Case Study of the Lower Cretaceous in the Kuqa Basin in Xinjiang, Northwestern China

Abstract The Lower Cretaceous strata in the Kuqa Basin in Xinjiang are marked by a set of arid red beds. Several types of sedimentary facies can be identified in this set of arid red beds: mudstones of the plaza and intracontinental sebkha, aeolian sandstones, sandy conglomerates of the intermittent river, conglomerates of the pluvial fan, etc. These types of sedimentary facies constitute a typical desert system. Therefore, the Cretaceous strata in the Kuqa Basin provide a favorable condition for studies of sequence stratigraphic divisions of the desert system. With the rise and fall of the base level of the sedimentary basin, cyclicity is clearly revealed in stratigraphic records, which helps the identification of the third‐order sequences. Based on the cyclicity in stratigraphic records, 5 third‐order sequences can be found in the strata of the Early Cretaceous in the Kuqa Basin. These sequences comprise a second‐order tectonic sequence. The primary feature of these third‐order sequences is of an upward‐fining sedimentary succession formed by a succession of “coarse sediments of the alluvial system‐fine sediments of the lake system”. The result of this study shows that aeolian sandstones are the best reservoirs of natural gas in the Cretaceous strata in the Kuqa Basin, and that the Kela‐2 gas field is the first large gas field dominated by aeolian sandstone reservoirs in China.