Northern Tarim Research Articles

Long-lived Cenozoic positive relief of the south-Eastern Tian Shan: Insights from provenance analyses of the northwestern Kuqa Depression sediments

The Eastern Tian Shan, one of the most active intracontinental orogenic belts, should bear important information on the mechanisms of intracontinental mountain building. In this study, detrital zircon U-Pb geochronological analysis of the Cenozoic Tiereke section succession in the northern Tarim Basin-Eastern Tian Shan convergence zone, combined with stratigraphic results, reveals the provenance histories of the Cenozoic sediments in the northwestern Kuqa Depression of northern Tarim Basin. Nine Cenozoic samples obtained from the strata of the Kumugeliemu Group to the lowest Kangcun Formation of the Tiereke section exhibit high resemblance to provenances of the south-Eastern Tian Shan, indicating existence of a prolonged eroding relief within this mountain since the early Cenozoic (∼54 Ma). This prolonged state of eroding relief requires continuous intracontinental deformation of the Eastern Tian Shan to sustain a provenance region, implying immediate deformation responding to the initial Indian-Eurasian collision. Furthermore, the south-Eastern Tian Shan served as the only source region for the northwestern Kuqa Depression of northern Tarim Basin, demonstrating that the upper-crustal boundary between the south-Eastern Tian Shan and the northern Tarim Basin was possibly stationary during most of the Cenozoic Era (∼54 Ma to ∼9.7 Ma). Such a stationary basin-orogenic boundary would favor the predictions of the pure-shear-like thickening model for the intracontinental mountain building during this period. We suggest that the long-lived mountain building of the Eastern Tian Shan may be due to its small convergence rate.

Basin filling and magmatic response to the migration of two-stage rifts: New insights into the late Neoproterozoic tectonics of the northern Tarim Craton

The spatiotemporal evolution of late Neoproterozoic sedimentary basins and their tectono-magmatic context provide essential information regarding the continental rift process of the Tarim Craton during the fragmentation of the Rodinia supercontinent. However, the incomplete exposure of strata and magmatism in the distal continental margin hinders the understanding of different rift phases that trigger continental breakup. The Sawapuqi region is currently located at the northernmost margin of the Tarim Craton and presents an uninterrupted sedimentary sequence and volcanic rocks from the latest Precambrian to the Cambrian period. Herein, we perform an integrated analysis of the Sawapuqi sub-basin and provide critical insights into the Ediacaran stratigraphic and tectono-magmatic evolution that shed light on the final rifting phase. The Sawapuqi sub-basin preserves regressive and transgressive successions, ranging from thick-bedded siltstones and siliceous rocks at the lower unit of the Sugetbrak Formation, through shore gravel-bearing laminated sandstone accumulation (upper unit), and restricted platform stromatolitic carbonates (Qigebrak Formation), to Cambrian mature passive margin sedimentation. An alkaline basaltic layer is interbedded within the coarsening-upward sedimentary packages of the early Ediacaran, recording the final rift-related magmatic pulse in the northwestern Tarim margin. The geochemical signatures of these basalts suggest a relatively shallow-depth derivation from the enriched mantle reservoir in an intraplate setting. The detrital zircon provenance of the coarse-grained siliciclastic to pebbly feldspathic debris sandstone sequence of the upper Sugetbrak Formation indicates the hybrid sediment infills from local metamorphic basements and Neoproterozoic igneous rocks, with progressively increased contribution from the coeval granites in the northern periphery of the Tarim Craton. The stratigraphic evolution from syn-rift clastic rocks to post-rift carbonates and the transition of detrital provenance observed in the Sawapuqi sub-basin suggest the development of a breakup succession at the end of the early Ediacaran period. The margin-localized continental extension and mantle-derived volcanism during the early Ediacaran differed from the distributed extension and bimodal magmatism during the Cryogenian, indicating two-stage extensional modes and subsidences prior to the lithospheric breakup. The rift system of the latter stage, which migrated oceanwards, eventually led to the continental breakup in the northwestern margin of the Tarim Craton. Comparable late Neoproterozoic stratigraphy and magmatic pulses between the Tarim Craton and Yili-Central Tianshan Block (YCTB) support their reconstructions as two conjugate rifted margins. This study indicates that the rift-drift tectonics triggered by this prolonged episodic rifting between the Tarim Craton and the YCTB was closely associated with the final fragmentation of Rodinia.

Seismic analysis of fault damage zones in the northern Tarim Basin (NW China): Implications for growth of ultra-deep fractured reservoirs

Seismic analysis of fault damage zones in the northern Tarim Basin (NW China): Implications for growth of ultra-deep fractured reservoirs

Does the fertile island effect in tamarisk nebkhas enhance in the northern Tarim Basin in the context of global warming?

AbstractWith the intensification of global warming, uncertainty regarding ecosystem evolution trends in drylands has increased. The objective of this article was to test whether the desertification process of tamarisk shrubland was intensified by comparing fertile island effects and soil ecological stoichiometry characteristics at different time periods. The tamarisk shrubland in the “shrubland” and “shrub duneland” stages in the northern Tarim Basin was selected, and soil samples were collected from the tamarisk nabkhas and from the interspaces between tamarisk nebkhas at the 0‐ to 30‐cm soil layers in 2009 and 2018. Soil carbon (C), nitrogen (N), and phosphorous (P) were analyzed, and the C/N, C/P, and N/P ratios were calculated. In comparison with 2009, fertile island effects were completely collapsed in the shrub duneland stage, with no obvious changes in the shrubland stage in 2018. The average soil C/N ratio decreased from a value greater than 25 in both stages. The average value of soil C/P ratio was below 200 in the shrubland and shrub duneland stages, with almost no marked change after nine years. The soil N/P ratio in our study significantly increased in 2018 than in 2009, but the soil N/P ratio was still much less than 11.9. Significantly positive correlations were observed between the relative interaction index (RII) of soil organic carbon (SOC) and soil total nitrogen (TN) and the soil C/P and N/P ratios in the tamarisk nebkhas. These results suggested that (1) fertile island effects were positive in relation to soil stoichiometry in tamarisk nebkhas in most cases; (2) the desertification process was accelerated in the shrub duneland stage, with no obvious progress during the shrubland stage; and (3) the loss of soil C from the collapsed fertile islands could therefore accelerate global warming.

Segmentation and lateral growth of intracratonic strike-slip faults in the northern Tarim Basin, NW China: influences on Ordovician fault-controlled carbonate reservoirs

Intracratonic strike-slip faults have been recognized as a major factor controlling the formation of fracture-cave carbonate reservoirs in deep buried basins, yet which properties and how the strike-slip faults influence reservoir distribution and their connectivity are still ambiguous. This uncertainty significantly restricts hydrocarbon exploration and development, such as in the Fuman oilfield, northern Tarim Basin, NW China. Using a high-resolution 3D seismic reflection survey and borehole data, we investigated the geometry and kinematic evolution of the FI17 fault zone in the Fuman oilfield. This fault zone is characterized by a single fault zone, pop-up or pull-apart structures, right-stepping en echelon normal faults, and much smaller displacement (<30 m) normal fault arrays from bottom to top. The FI17 fault zone consists of four genetic segments, including the extensional strike-slip duplex, Riedel left-lateral shear, right-stepping horsetail splay, and horizontal slip segments in map view. In particular, the formation of the ∼18 km Riedel shear zone is characterized by the growth and linkage of segmented shear faults (synthetic and secondary synthetic shears). We observed that the large-scale fault-controlled fracture-cave reservoirs are distributed in positions with wider fault zones, which are characterized by overlapping of neighboring secondary shear faults. Furthermore, the reservoir width examined in this study is natural logarithmic correlated (positively) to the fault zone width. The reservoirs linked by the same shear faults show better internal connectivity. The spatial coherence between fault geometry and reservoir features indicates that segmentation and lateral growth of intracratonic strike-slip faults controls the occurrence of fracture-cave reservoirs, which may provide support for reservoir prediction in the Fuman oilfield and other deeply buried fault-controlled carbonate reservoirs in general.

Mesozoic-Cenozoic Topographic Evolution of the South Tianshan (NW China): Insights from Detrital Apatite Geo-Thermochronological and Geochemical Analyses

Abstract The present-day topography of Tianshan is the product of repeated phases of Meso-Cenozoic intracontinental deformation and reactivation, whereas the long-term Mesozoic topographic evolution and the timing of the onset of Cenozoic deformation remain debated. New insights into the Meso-Cenozoic geodynamic evolution and related basin-range interactions in the Tianshan were obtained based on new detrital single-grain apatite U-Pb, fission-track, and trace-element provenance data from Mesozoic sedimentary sequences on the northern margin of the Tarim Basin. Detrital apatite U-Pb age data from Early-Middle Triassic clastic rocks show two prominent age populations at 500–390 Ma and 330–260 Ma, with a paucity of ages between 390 and 330 Ma, suggesting that sediment source is predominantly from the northern Tarim and South Tianshan. From the Late Triassic to Early Jurassic, the first appearance of populations in the 390–330 Ma and 260–220 age ranges indicates that the Central Tianshan-Yili Block and Western Kunlun Orogen were source regions for the northern margin of Tarim Basin. In the Cretaceous strata, south-directed paleocurrents combined with the decrease in the 390–330 Ma age population from the Central Tianshan-Yili Block imply that South Tianshan was uplifted and again became the main source region to the Baicheng-Kuqa depression during the Cretaceous. Our new apatite fission-track data from the southern Chinese Tianshan suggest that rapid cooling commenced at c. 30 Ma along the southern margin, and the Early Mesozoic strata exposed on the southern flank of the Tianshan underwent c. 4–5 km of late Cenozoic exhumation during this period. This age is approximately synchronous with the onset of exhumation/deformation not only in the whole Tianshan but also in the interior of the Tibetan Plateau and its margins. It suggests that far-field, N-directed shortening resulting from the India-Asia collision was transmitted to the Tianshan at that time.

Silurian‐Devonian Lithospheric Thinning and Thermally Softening Along the Northern Margin of the Tarim Craton: Geological Mapping, Petro‐Structural Analysis and Geochronological Constraints

AbstractWhile the western part of northern Tarim Craton has long been considered as a Paleozoic passive margin, a pronounced Silurian‐Devonian magmatism developed on eastern part of this margin may indicate different but active margin setting. In this contribution, detailed structural mapping, petro‐structural analysis, and geochronological investigations were conducted in the Korla area, eastern part of northern Tarim Craton. Three main generations of fabrics were recognized. The earliest pervasive fabric is an originally sub‐horizontal metamorphic S1 foliation that is in part associated with migmatization characterized by high temperature/low pressure metamorphic mineral assemblages, interpreted as reflecting crustal extension. S1 foliation was affected by D2 contraction forming regional‐scale F2 upright folds associated with sub‐vertical axial planar foliation S2. D3 is marked by development of NW‐SE oriented dextral fault, asymmetric mega‐folding of S2 and spaced NW‐SE‐striking S3 foliation, likely in response to dextral transpression. Geochronological data indicate that D1 extension occurred from ca. 420 to 410 Ma, D2 contraction started around 410 Ma and lasted till 400 Ma or later, and D3 transpression was ongoing around ∼370 Ma. Integrated with regional data, an updated geodynamic model is proposed by interpreting the Central Tianshan, South Tianshan and NE Tarim Craton as an early Paleozoic supra‐subduction system. We suggest that the Silurian‐Devonian event reflects thermal softening and horizontal stretching of the supra‐subduction crust, resulting in drifting of the Central Tianshan continental arc from the proto Tarim Craton in association with opening of the South Tianshan back‐arc basin in‐between.

Geochronology, geochemistry, and Sr–Nd–C–O isotopic composition of the Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, northwest China

Carbonatites are the predominant source of rare earth element (REE) resources globally, but their origins and petrogenesis remain unclear. Currently, >600 carbonatites have been found worldwide, but only a few (<14%) of these carbonatites contain REE deposits. Some carbonatites are rich in REEs but do not host high-grade REE deposits. The Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, China, are ideal for investigating why some carbonatites do not host REE mineralization. We undertook a detailed petrographic, geochronological, and isotopic study of the Qieganbulake carbonatites. In previous studies, a carbonatite at Qieganbulake was dated to ca. 811 Ma. However, in this study, ages of ca. 926 Ma were obtained for phlogopite and baddeleyite in carbonatite dykes rather than pyroxenites or phlogopitelites-bearing carbonatite complex. Such ages of ca. 926 Ma are consistent with a previously published apatite U–Pb age. 40Ar/39Ar dating of phlogopite yielded a plateau age of 928 ± 17 Ma and secondary ion mass spectrometry U–Pb dating of baddeleyite yielded a concordia age of 926 ± 8 Ma, implying the carbonatite was emplaced at ca. 926 Ma. Laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of monazite yielded a 206Pb/238U intercept age of 811 ± 27 Ma, indicating that the main REE mineralization stage was coeval with later carbonatite emplacement at ca. 811 Ma. The studied Qieganbulake calcio-carbonatite samples are REE-rich (864–1495 ppm) and consist of calcite (62.18–93.92 vol%), apatite (1.10–24.82 vol%), dolomite (0.37–10.48 vol%), and lesser amounts of phlogopite, olivine, monazite, magnetite, and vermiculite. The calcite, apatite, and monazite (<1 vol%) have REE contents of 757–1111 ppm, 5127–11,556 ppm, and 45.35–59.88 wt%, respectively, and account for the high REE contents of the carbonatites (883–1539 ppm). The small amount of monazite in this carbonatite suggests it is a prospective rather than a confirmed ore deposit. Petrographic studies, oxygen values of 5.74 and 7.51 ‰ and characteristics of trace element for apatites with less monazite around suggest they have magmatic origin with less alteration. The 926 Ma Qieganbulake carbonatite has high Sr isotope ratios (0.70580–0.70639) and negative εNd(t) values (−7.6 to −11.8), indicative of an enriched mantle source. δ13CPDB and δ18OVSMOW values of the carbonatites vary from –3.3‰ to –4.1‰ and 9.7‰ to 12.0‰, respectively, indicative of a contribution from recycled crustal C that was probably derived from a subducted oceanic slab. Above characteristics of carbonatite with ∼926 Ma are different with those of carbonatite occurred at ∼811 Ma, suggesting two carbonatite emplacements in Qieganbulake. Based on our results and the regional geology, we infer that the 926 Ma Qieganbulake carbonatite was derived by partial melting of a subcontinental lithospheric mantle source that had been metasomatized by early Neoproterozoic subduction during circum-Rodinia subduction in the northern Tarim Craton. Early crystallization of apatite sequestered the REEs, resulting in limited REE mineralization. In addition, the limited hydrothermal alteration and ligands available, such as F− and SO42−, hindered REE transportation and precipitation.

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

The Well Tashen 5 (TS5), drilled and completed at a vertical depth of 9017 m in the Tabei Uplift of the Tarim Basin, NW China, is the deepest well in Asia. It has been producing both oil and gas from the Sinian at a depth of 8780–8840 m, also the deepest in Asia in terms of oil discovery. In this paper, the geochemical characteristics of Sinian oil and gas from the well were investigated and compared with those of Cambrian oil and gas discovered in the same basin. The oil samples, with Pr/Ph ratio of 0.78 and a whole oil carbon isotopic value of −31.6‰, have geochemical characteristics similar to those of Ordovician oils from the No. 1 fault in the North Shuntuoguole area (also named Shunbei area) and the Middle Cambrian oil from wells Zhongshen 1 (ZS1) and Zhongshen 5 (ZS5) of Tazhong Uplift. The maturity of light hydrocarbons, diamondoids and aromatic fractions all suggest an approximate maturity of 1.5%–1.7% Ro for the samples. The (4-+3-) methyldiamantane concentration of the samples is 113.5 μg/g, indicating intense cracking with a cracking degree of about 80%, which is consistent with the high bottom hole temperature (179 °C). The Sinian gas samples are dry with a dryness coefficient of 0.97. The gas is a mixture of kerogen-cracking gas and oil-cracking gas and has Ro values ranging between 1.5% and 1.7%, and methane carbon isotopic values of −41.6‰. Based on the equivalent vitrinite reflectance (Reqv = 1.51%–1.61%) and the thermal evolution of source rocks from the Cambrian Yu'ertusi Formation of the same well, it is proposed that the Sinian oil and gas be mainly sourced from the Cambrian Yu'ertusi Formation during the Himalayan period but probably also be joined by hydrocarbon of higher maturity that migrated from other source rocks in deeper formations. The discovery of Sinian oil and gas from Well TS5 suggests that the ancient ultra-deep strata in the northern Tarim Basin have the potential for finding volatile oil or condensate reservoirs.

Characteristics and Geological Significance of High-Frequency Cycles in Salinized Lake Basins: The Paleogene Kumugeliemu Group in the Xinhe Area, Northern Tarim Basin

Salinized lake basins have distinctive sedimentary response characteristics, similar to marine shallow-water carbonate platforms. High-frequency cycles can also be used to reveal more sedimentological information, such as relative lake-level fluctuations, lithofacies sequence combinations, and paleogeographic evolution. In this article, a comprehensive study on the stratigraphic shelf delineation and high-frequency cycles of the Paleozoic Kumugeliemu Group in Xinhe area, northern Tarim Basin, was performed using drilling cores, logging curves, and seismic analyses. As a result of the study, the following data were obtained: the three sets of marker beds in the Kumugeliemu Group in the study area could be divided into a bottom sandstone component (E1-2 km1), a lower gypsum mudstone component (E1-2 km2), a salt rock component (E1-2 km3), and an upper gypsum mudstone component (E1-2 km4) by petrology vertical overlay combination and isochronous tracking correlation, which constituted two third-order cycles (ESQ1, ESQ2). They were further divided into seven fourth-order cycles (Esq1–Esq7). Due to the droughty and saline lacustrine depositional system background, the internal rock fabric changed frequently and showed a periodic vertical overlay pattern. Stratified gypsum salt, gypsum mud (sand) rock, and gypsum rock were used as the cycle interface. A single cycle was mainly characterized by an upward shallower depositional sequence of rapid lake transgression followed by a slow lake regression, composed of massive sandstone–lamellar mudstone–lime dolomite–gypsum rock, massive sandstone–lamellar mudstone–gypsum rock (gypsum salt), massive sandstone–massive gypsum mud (sand) rock–gypsum rock, and other cycle structure types. The complete sedimentary cycle was superposed by a single cycle and compared by the inter-well thickness difference, indicating that the study area had a paleogeomorphology pattern of “West-Low–East-High”. The thickness of the cycles decreased gradually from bottom to top vertically, and five sedimentary stages were determined, i.e., freshwater, brackish, brackish water, salt lake, and semi-saltwater, reflecting the evolutionary process of increasing salinity, lake basin filling, and gradual salinization and shrinkage.

Seismic velocity structure of the crust and uppermost mantle beneath the Tien Shan and its adjacent areas

The Tien Shan and its adjacent areas have been a prime place to understand the process of continental collision, the mechanism of mountain building and the interaction of tectonic blocks. In this study, we collect seismic data recorded by 74 broad-band stations from the China Provincial Digital Seismic Networks and the Regional Kyrgyzstan and Kazakhstan Networks between January, 2007 and September, 2009. A joint inversion technique that combines three types of datasets (receiver functions, phase velocities of Rayleigh wave measured from both ambient noise and teleseismic earthquake data) is applied to image the crustal and upper mantle structure beneath the Tien Shan and its adjacent areas. The average crustal thickness in the study area is about 50 km, however, the Moho depth extends to ∼70 km beneath the Kyrgyz Platform near the southwestern Tien Shan. Our velocity models show a good correlation with subsurface geological features at shallow depths: low velocities are predominantly observed beneath the basins due to thick sedimentary layer, whereas high velocities are mainly distributed beneath the mountain ranges due to crystalline basement rocks. In the upper mantle a low velocity zone is obviously observed beneath the western Tien Shan. Both the crust thickness and S wave velocity structure of the Tien Shan and its adjacent regions display obvious horizontal and vertical heterogeneities from west to east, which suggests that the far-field effects of the collision between Eurasian plate and Indian plate plays an important role in the tectonic activity of the Tien Shan. The apparent velocity heterogeneities beneath the northern Tarim Basin may indicate that the Tarim Basin may have been eroded and damaged by upwelling hot materials from the upper mantle.

Metamorphism and chronology of Paleoproterozoic mafic granulite from the Kuluketage area, northern Tarim and its tectonic applications

Mafic granulites provide key insights for understanding the composition and evolution of the lower crust but are rarely reported in northern Tarim. In the present study, a systematic petrographic, phase equilibrium modeling and zircon U-Pb geochronological study of a Paleoproterozoic mafic granulite (garnet two-pyroxene granulite) from the Kuluketage area, northern Tarim were conducted. Phase equilibrium modeling demonstrates a decompression–heating clockwise P-T path involving the prograde metamorphic stage (M1) at ∼ 950 °C / 12 kbar, the peak metamorphic stage (M2) at ∼ 1020 °C / 10 kbar, and the retrograde stage (M3) at ∼ 880 °C / 7 kbar. The decompression heating P-T path generally occurs in the post-collisional lithospheric extension setting which may contribute the UHT metamorphism of the M2 stage. Zircon U-Pb geochronological study reveals four age peaks at ∼ 2.45, ∼ 2.3, ∼ 1.91, and ∼ 1.85–1.78 Ga. We interpret the ∼ 2.45 Ga magmatic event as an older captured zircon age corresponding to a widely reported magmatic event of ∼ 2.4–2.5 Ga in Kuluketage. The slightly younger ∼ 2.3 Ga magmatic event may correspond to the crystallization age of the mafic granulite’s protolith, however, rarely reported in northern Tarim. The metamorphic age of ∼ 1.91 Ga represents a collisional-related high-pressure metamorphic age corresponding to the M1 stage and the age peaks of ∼ 1.85–1.78 Ga may represent post-peak cooling events corresponding to the M2-M3 stage.

Evolution of the foreland basins on either side of the Tian Shan and implications for lithospheric architecture

The previous understanding of Tian Shan's lithospheric architecture can be summarized into three models: unidirectional underthrusting, bidirectional underthrusting, and mantle upwelling. These models make distinctive predictions regarding the migration and subsidence histories of the foreland basins on either side of the range. Seismic profiles crossing both foreland basins document their migration and subsidence history. We analyze a seismic profile across the Southern Junggar foreland basin (SJFB) to the north of the range, which demonstrates that the basin migrated northward at ∼3.1 mm/yr during 22–16 Ma. Combining this with previously known migration rates of the Northern Tarim foreland basin (NTFB) to the south of the range, we constrain the migration of these two foreland basins relative to their basements. We then employ an elastic plate model to simulate the subsidence profiles across these two basins at ∼20 Ma, 13–16 Ma, ∼5 Ma, and 0 Ma constrained by back stripping well log data and seismic records of the basin onlaps. Our modelling reveals the propagation rates of the southern and northern Tian Shan thrust wedges and the evolution of their topographic loads. Our analysis and modelling demonstrate that the NTFB has the characteristics of a retro-foreland basin, whereas the SJFB resembles a pro-foreland basin during early-middle Miocene times. This suggests that the Tian Shan's growth during this period was dominated by underthrusting of the Junggar lithosphere. Furthermore, our analysis and modelling reveal a convergent pattern between the Tarim lithosphere and the Tian Shan changed from lithospheric indentation to lithospheric subduction after the middle Miocene, as the plate setting of the NTFB and SJFB changed. The Tian Shan had begun to uplift before the middle Miocene, with the orogen's height reaching half of its present elevation before the late Miocene.

Long-lived paleo-uplift controls on Neoproterozoic–Cambrian black shales in the Tarim basin

Neoproterozoic–Cambrian black shales are one of the most important hydrocarbon source rocks worldwide, including in the Shunbei and Fuman regions in the Tarim Basin, China. However, the hydrocarbon generation from these shales is complex, with liquid petroleum found in reservoirs at depths of 6000–8000 m. Based on drilling, seismic, and well testing data, the ultra-deep tectonic evolution of the northern Tarim Basin, and its effects on hydrocarbon generation from black shales were investigated. During the Neoproterozoic, the northern Tarim Basin was filled with rift and passive continental margin sedimentary rocks between the north and central Tarim paleo-uplifts. A sequence of lower Cambrian black shales was developed on the slope around the paleo-uplifts over a wide area, which are thick and have high contents of total organic carbon. Even after multiple stages of tectonism, including the Caledonian, Hercynian, Indosinian, Yanshanian, and Himalayan orogenies, the north (Tabei) and central paleo-uplifts were preserved. The Caledonian, Hercynian, and Himalayan orogenies were the main periods of hydrocarbon generation. Four hydrocarbon-generation models were established, which are the Tabei-, Bachu-, Tazhong-, and Tadong-types. The current Ro values of the Tabei-type black shales are 1.0%–1.5%, whereas those of the Bachu-, Tazhong-, and Tadong-type black shales are >2.0%. Due to the long-lived paleo-uplifts and short-lived deep burial during the Himalayan Orogeny, the Tabei-type black shale is in the oil and early gas generation stage, which obviously differs from the over-mature Cambrian and Ordovician black shales in South China. Therefore, the long-lived paleo-uplifts not only affected the deposition and distribution of black shale, but also hydrocarbon generation processes and the depth limit of active source rock. The thermal evolution of black shale in the ultra-deep Tarim Basin requires further investigation.

Intracontinental convergence accommodated by underthrusting of Tarim Basin beneath central Tian Shan, Central Asia: Insights from Cenozoic upper crust deformation across the western Kepingtage fold-and-thrust belt in the northern Tarim Basin

The way in which intracontinental convergence caused by remote continent-continent collision has been accommodated remains debatable, but it is attributed to either pure shear thickening or continental subduction/underthrusting. Quantification of the deformation architecture of the upper crust and its shortening is vital for testing these competing models. For this purpose, we conducted quantitative analyses of four seismic reflection profiles along the western Kepingtage fold-and-thrust belt of the intracontinental convergence zone between the northern Tarim Basin and the central Tian Shan, a result of the remote India-Eurasia collision, which capture the structures of the upper crust and constrain the amount of shortening in the region. The structural analyses show a thin-skinned style of deformation of the western Kepingtage fold-and-thrust belt, with minimum north−south shortening estimates of ∼71.1 km (∼46.6% strain), ∼76.3 km (∼51.4% strain), ∼67.1 km (∼56.1% strain), and ∼53.0 km (∼49.9% strain), respectively, from east to west. Assuming that coupling shortening occurred across the entire crust and an initial crustal thickness of ∼41−46 km, these shortening ratios would produce a ∼83−95-km-thick crust along the northern Tarim Basin, which is significantly larger than the actual thickness previously reported. This inconsistency leads us to rule out the pure-shear thickening model, which predicts crustal-scale coupling deformation. Alternatively, our results imply that the crust of the northern Tarim Basin was deformed in a decoupling style. These results, combined with those of previous studies, lend support to the intracontinental underthrusting model. We suggest that both the upper crustal fold-and-thrust belts and the lower crustal underthrusting of the northern Tarim Basin accommodated intracontinental convergence.

Subsurface fracture characterization in a folded ultra-deep tight-gas sandstone reservoir: A case study from the Keshen gas field, Tarim Basin, China

In the Keshen gas field in the northern Tarim Basin, the main production interval is the ultra-deep (>6000 m), ultra-high pressure Lower Cretaceous Bashijiqike Formation. Understanding the characteristics and role of natural fractures in the reservoir is essential for efficient exploration and development. Data from borehole images (42 vertical wells) and cores (12 vertical wells) enabled a comprehensive characterization of fracture attributes, including mode, orientation, fill, density, and clustering as well as variations with respect to depth and structural position. Using image logs from 20 vertical wells and cores from 12 wells, we find that steeply dipping (>60°), E-W and N–S-striking fractures are the most common across the studied anticline. Fracture density is highest in the forelimb and near faults (2.85 fractures/m), compatible with fracture formation during folding and associated faulting. Fracture clusters 10+ m wide are also preferentially located within these structural zones and are spaced around 5–15 m apart. Fracture orientations, fill, and density varies with depth. We identify a depth transition horizon above and below which fracture characteristics, specially strike, dip, and resistivity change dramatically. We infer those changes are due to fold geometry, rheological differences associated with increasing pressure and temperature at extreme depths. We observe anhydrite-filled fractures located near the top of the formation, whose presence is associated with overlying evaporites. Vertical and lateral variations of fracture characteristics and distribution discovered in this reservoir may aid the interpretation and prediction of the fracture network in other ultra-deep sandstone reservoirs with a comparable setting or well performance.

Petrogenesis of the alkali basalt and trachy-andesite suite in the northern Tarim Basin, NW China: Implications for crust-mantle interactions controlled by the Permian mantle plume

The Permian Tarim large igneous province (LIP), which contains both large volumes of basalt and silicic volcanic sequences, has been intensively studied in the past decades, but some issues still remain controversial, including the petrogenesis of each component in the LIP, their interrelationships, and the geodynamic processes that produced them. In the Halahatang area of the northern Tarim Basin, a well-developed eruptive suite composed of basalt and trachy-andesite has been penetrated by numerous boreholes, which provide us an opportunity to better understand these issues. In this paper, we carried out an integrated study on the eruptive suite, including petrography, geochronology and geochemistry. The results indicate that the eruptive suite formed at around ∼290 Ma, generally synchronous with the main episode of magmatism within the Permian Tarim LIP. The basalts have similar compositions to typical oceanic island basalts (OIBs), which have alkaline, high-Ti and sodic compositions and show enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs). Whole-rock Sr–Nd isotopes are slightly enriched, with εNd(t) and (87Sr/86Sr)i values from −4.5 to −3.4 and 0.7065 to 0.7086, respectively. The trachy-andesites are alkaline, metaluminous and ferroan, and display highly enriched LREEs and LILEs. High Ga/Al ratios and Zr + Nb + Ce + Y values constrain their A-type affinity and A1-type nature. They have εNd(t) values from −8.2 to −4.5 and (87Sr/86Sr)i values from 0.7083 to 0.7189, more enriched than those of the basalts. Based on these results, we proposed that the different components in the igneous suite have close petrogenetic relationships. The alkali basalts may have been formed by mantle plume-derived melts coupled with somewhat assimilation of continental crust, while the trachy-andesites were products of variable fractionation of the basaltic magmas involving more continental crustal components in the magma evolution. In combination with the existing results in Tarim, we further suggested that the alkali basalt and trachy-andesite suite may have been generated via a crust–mantle interaction process related to the Permian mantle plume activity.

Characteristics and Formation Mechanism of Multiscale Storage Spaces in Ancient Deep Tight Reservoirs: Examples from the Cambrian Yurtus Formation in the Northern Tarim Basin, China

AbstractUnder the background of complicated diagenetic fluids, high-temperature pressure in superimposed basin, the pore-forming and pore-preserving effects of deep tight reservoirs are complex, and the formation mechanism of high-quality reservoirs has always been the core issue. With the discovery of oil and gas in ultradeep drilling in Tarim Basin, the most unconventional tight reservoir of Lower Cambrian Yuertus Formation in Tabei area began to receive attention. Based on the research of typical field outcrops in Tabei area, the lithofacies, reservoir space differences, and reservoir formation mechanism are systematically analyzed. (1) The stratum has undergone complex multistage diagenetic fluid transformation, and the rock types are diverse with great differences; siliceous rock and dolomite are the main rock types, often in thin-medium layered distribution. (2) After long-term deep burial transformation, siliceous rock and dolomite can still retain a large number of effective storage spaces; hydrothermal dissolution pores, organic acid dissolution pores, atmospheric freshwater dissolution pores, and intercrystalline pores are the main pore types, which provide main storage spaces. Multiscale pores are developed in siliceous rock and dolomite, with good connectivity and good pore structure. (3) The large-scale storage spaces mainly come from the effective maintenance of primary pores, organic matter, and hydrothermal dissolution. The siliceous minerals and dolomite have stable properties and strong resistance to compaction and can effectively maintain the early pores. (4) The large-scale reservoir space was formed in the early diagenetic stage; the pressure-solution and cementation are the two most important diagenetic processes for reducing storage spaces; however, under the pore-forming and pore-preserving effects of early silicification and dolomitization, various types of dissolution, and oil charging, the tight lithology can still maintain effective storage spaces. The related research has important theoretical and practical significance for studying the formation mechanism of tight reservoirs in deep ancient strata and predicting high-quality reservoirs.

An aeromagnetic study of fault structures underneath the region across the Chinese Altai orogen, Junggar Basin, Tianshan orogen, and Tarim Basin

• We identified faults and volcanic suites in the Chinese Altai orogen, Junggar Basin, Tianshan orogen, and Tarim Basin regions. • The Junggar Basin has a basement composed of oceanic crustal material or continental-like material. • Mantle upwelling occurred during the Late Carboniferous–Cretaceous in the West Junggar orogenic belt . In this study, an aeromagnetic dataset is used to investigate the crustal structure in the region across the Chinese Altai orogen, Junggar Basin, Tianshan orogen, and Northern Tarim Basin, providing insights into the unclear nature of the basement of the Junggar Basin. The aeromagnetic data are for a region containing the Chinese Altai, Junggar Basin, Tianshan Mountains, and Tarim Basin. On the basis of magnetic anomalies, we identify several faults and volcanic suites in this region. According to the magnetic characteristics, gravity anomalies, and crustal thickness, we suggest that the Junggar Basin has a basement composed of oceanic crustal material or continental-like material. Mantle upwelling occurred in the West Junggar orogenic belt during the Late Carboniferous–Cretaceous. The faults we have identified provide useful insights into the tectonic development in the northwestern Central Asian Orogenic Belt.

The genesis of gas condensates and light oilsin the lower paleozoic of Tarim Basin, NW China: The exploration implications for ultra-deep petroleum

Almost 8 × 108 tons marine condensate reserveshave been estimated to be present in the Paleozoic to Cenozoic reservoirs of the Tarim Basin. Thethermal maturity of condensate gases are estimated to be greater than 2.2%REo (vitrinite reflectance equivalent),which are higher than the calculated maturity of thecondensates <1.9%REo. Therefore, the formation of gas-condensates might include a mixture of later cracked gases with earlieroil accumulations. The mixture model indicates that the relative content of oil-cracking gases varies from 42% to 98% in condensate gases, and the mixing extent of kerogen and oil cracking gas with various maturities could also lead to the carbon isotope reversal of methane and ethane. The positive correlation between the gas-oil ratio (GOR) of gas condensates and the maturity of cracking gas shows that the cracking gas is much more enriched in the eastern Tabei uplift of the northern Tarim Basin thanthe Tazhong uplift ofcentral Tarim Basin. The relative abundance of the triaromatic dinosteranes in the condensates coincides with the feature of hydrocarbons derived from Cambrian source rocks Furthermore, based on the multi-stagethermal maturity of the Cambrian source rocks, the formation mechanism of secondary gas condensates has been demonstrated and reconstructed. Recent exploration discoveries in the ultra-deep Cambrian of the Tabei and Tazhong uplifts, involving ZS-1 and LT-1wells, confirmed the cracking gases were derived from the post-high mature Manjar Depression and that liquid hydrocarbons could be maintained in the ultra-deep of Tabei uplift. The findings are helpful toestimate the exploration potential of the deeper cracking gases in the high-temperature and high-pressurereservoirs of the Tarim Basin.