The Silurian in the Tarim Basin covers nearly 25×10⁴ km², and is distributed in the north depression, southwest depression, and Central Tarim uplift, Northern Tarim uplift and Bachu uplift and other major tectonic units in the basin ^[1,2,3]. According to the third resource evaluation results in China, the geological reserves of the Silurian in the platform basin of the Tarim Basin are 5.98×10⁸ t. However, by 2018, the Silurian in the whole basin had only submitted 3P oil reserves of 8011.97×10⁴ t, which is extremely low in exploration. Based on various drilling data, previous researchers conducted studies on the characteristics, geneses, formation periods and evolution history of Silurian bituminous sandstones in Central Tarim and Northern Tarim areas ^[4,5,6,7], and clarified the genetic types of Silurian bituminous sandstones ^[4] and three primary reservoir-forming periods including Late Caledonian, Late Hercynian and Yanshan-Himalayan ^[8,9]. The amount of oil and gas loss in the Late Caledonian ^{[1, 6]} and the role of residual bitumen to oil and gas preservation and its potential for secondary hydrocarbon generation were discussed ^[10]. The sedimentary characteristics and reservoir types of the Silurian in key areas were combed ^{[11,12,13,14,15,16]} to point out that the structural background of the paleo-uplift has a certain controlling effect on the large-scale hydrocarbon accumulation in Silurian ^[8], the studies of the Silurian hydrocarbon accumulation patterns and distribution laws of inherited paleo-uplifts and their surrounding areas such as Central Tarim Basin and Northern Tarim Basin were intensively conducted ^{[2-3, 5, 8]}. Although the Silurian outcrops are widely exposed in the Shajingzi structural belt of the Keping faulted uplift in the northwest of the Tarim Basin, which provides direct data for stratigraphic division, lithology identification, and sedimentary formation analysis ^[17,18], the research on the potential of hydrocarbon accumulation is still extremely weak. Especially, the complex tectonic movements caused the Lower Paleozoic to directly expose in high-steep occurrence, and it is generally believed that oil and gas can’t be effectively preserved in the Silurian.

In 1990 to 2003, oil companies deployed several NW-trending 2D seismic lines along the Shajingzi fault but the seismic imaging quality was poor. The seismic sections showed that the strata were under monocline setting, which is difficult for hydrocarbon enrichment. Oil and gas companies deployed no exploration wells and the exploration rights were withdrawn in 2012. The authors believe that there are two major geological problems that have restricted the exploration breakthrough of Silurian oil and gas in the Shajingzi structural belt for a long time: (1) The unclear understanding of the distribution and evolution of the Shajingzi fault and its associated faults limits the study of formation and distribution of potential traps under the background of monoclines in the Shajingzi structural belt. (2) Bituminous sandstone formed after the destruction of initial oil reservoir is generally seen on Silurian outcrops in the Shajingzi structural belt, and there is a lack of systematic analysis and practical exploration of later hydrocarbon accumulation conditions. The stratigraphic characteristics and hydrocarbon potential research in the Silurian Shajingzi structural belt is crucial for clarifying Silurian lithofacies paleogeography environment, exploring hydrocarbon accumulation in the complex piedmont zone and evaluating Silurian exploration scheme in the northwest Tarim Basin.

Based on the continuous research and tackling on key problems on the stratigraphic distribution and hydrocarbon potential in the Shajingzi tectonic belt in 2013- 2018, Well Xinsudi 1 (XSD1 for short) drilled in the Shajingzi structural belt obtained industrial gas flow for the first time in the Silurian of northwestern Tarim in 2018. In 2020, the neighboring Well Xinsucan 1 (XSC1 for short) obtained better oil and gas show in the same layer of the Silurian. Based on data analysis of drilling, oil testing, geochemical and geological characteristics, and field profile measurements in the Shajingzi area, combined with target processing and fine interpretation of 2D seismic data, comprehensive researches on the structural evolution, Silurian deposition and hydrocarbon accumulation in the Shajingzi structural belt were conducted to determine the factors and primary controlling factors to hydrocarbon accumulation, thereby to establish a hydrocarbon accumulation model for Silurian, to evaluate the resources potential of the northwestern Tarim Basin, and to point out favorable exploration areas in the future.

The Keping faulted uplift, which is located in the northwest of the Tarim Basin (Fig. 1), is a part of the Cenozoic South Tianshan fold-thrust system ^[19] and defined as the first-order tectonic unit of basin ^[17]. The Wensu uplift located in the upper section of the Keping faulted uplift and is the secondary tectonic unit. The Shajingzi fault is the boundary between the Keping faulted uplift and the northern depression of the first-order tectonic units of the basin, and it is also the boundary between the Wensu uplift and the Awati sag of the secondary tectonic units ^[17] (Fig. 1b). The Shajingzi fault belt strikes in the northeast-southwest direction. It is connected to the Kepingtage fault in the southwest and intersects with the Acha fault. It reaches the Karayuergun fault in the northeast with deep wedge-shaped thrusts, Shajingzi fault in narrow sense, and shallow extensional faults, total three sets of fracture systems ^[19,20,21]. According to the field outcrops and actual drilling data, the strata in the middle and eastern sections of the Keping fault uplift are better developed (Fig. 2). Downwardly, they are the Quaternary (Q), Neogene (N), Paleogene (E), Permian (P), Carboniferous (C), Devonian (D), Silurian (S), Ordovician (O), Cambrian (—C), Sinian (Z) and Mesoproterozoic Aksu Group (Pt₂ak), with hiatus of the Cretaceous (K), Jurassic (J) and Triassic (T) ^[17]. The Silurian is extensively exposed in the middle and eastern sections of the Keping faulted uplift (Fig. 1b), including the Upper Keziertag Formation ((S₃-D)k), the Middle Yimugantawu Formation (S₂y), the Lower Tata Ertag Formation (S₁t), and the Kepingtage Formation (S₁k). Among them, the Keziertag Formation is mainly composed of mauve and brownish-red thick and massive silty-fine sandstones, partially intercalated with gravel-bearing sandstone, conglomerate and mudstone. The Yimugantawu Formation is composed of mauve mudstone. The Tata Ertag Formation is mainly composed of dark mauve and mauve thin-medium fine sandstone and siltstone. The Kepingtage Formation is mainly composed of gray-green and dark-gray fine sandstone and argillaceous siltstone.

There are many Lower Paleozoic outcrops in the west part of the Shajingzi structural belt, with generally larger dip angles (30°-75°). Bituminous sandstone can be observed in the lower member of the Kepingtage Formation in outcrop profiles such as Sishichang and Dawangou. In the profiles such as Qigebulake, Xiaoerbulak, Sishichang and Dawangou, there are also outcrops of Cambrian- Ordovician source rocks (Fig. 1b).

In 2018, Oil and Gas Resources Survey Center of China Geological Survey deployed and implemented Well XSD1 in Shajingzi structural belt, with a completion depth of 2882 m in the Ordovician Dawangou Formation. The drilled strata and the thicknesses are shown in Fig. 2. Among them, the Silurian Kepingtage Formation can be divided into three sections including interbeds of sandstone and mudstone in the upper member (thickness of 126 m), mudstone in the middle member (thickness of 53 m), and interbeds of sandstone and mudstone in the lower member (thickness of 182 m). The coring of the Silurian Kepingtage Formation in Well XSD1 shows 1.45 m and one layer of oil-soaked sandstone and 6.49 m total 6 layers of oil-traced sandstone. The water dripped on the core appears as pearl shape, and the fluorescent dripping displays bright yellow radial pattern. There was gas bubble and oil spills during the immersion of sandstone samples in water. The core samples have porosity of 5%- 8% and permeability of (0.5-1.5)×10^-3 μm² from the core test, which refers to a low-porosity and low-permeability reservoir. From the comprehensive interpretation of the Kepingtage Formation in Well XSD1, there are 6.9 m of 4 gas layers and 11.8 m of gas-water layer (Fig. 3).

The lower member of the Kepingtage Formation in Well XSD1 at the depth interval of 2525.5-2528.5 m (3 m and one layer) has a maximum casing pressure of 4.8 MPa for small-scale sanding fracturing gas testing, a daily gas production of 1.260 5×10⁴ m³, a cumulative gas production of 3.781 5×10⁴ m³, a daily water production of 16.38 m³, and a cumulative water production of 55.57 m³. It is concluded from the test result that it is a “gas-water layer”. A the depth intervals of 2377-2386 m and 2409-2413 m (13 m total 2 layers) in the upper member of the Kepingtage Formation in Well XSD1, the maximum casing pressure of the middle-sized sanding fracturing gas test is 1.03 MPa, the average daily gas production is 1.681 7×10⁴ m³, the cumulative gas and oil productions of 29.109 5×10⁴ m³ and 2.16 m³ respectively. The test conclusion is “gas layer”. Well XSD1 obtained the Silurian industrial gas flow in the northwestern Tarim Basin for the first time, achieving a major breakthrough since the exploration of the Silurian in the northwestern Tarim Basin in the 1950s.

Well XSC1 is located in the updip direction of the Well XSD1 (the two wells are about 1150 m apart). It aims at the exploration of Cambrian subsalt and also reveals two sets of favorable sandstone sections in the upper and lower members of the Kepingtage Formation. The thicknesses of the two sets of sandstones in the Kepingtage Formation in the two wells are similar. However, the Silurian sandstone in Well XSC1 is about 78-110 m higher than that of the Well XSD1, representing better oil and gas show. The core of the Kepingtage Formation reveals 5.41 m total 8 layers of oil-soaked sandstone, 2.1 m total 2 layers of sandstone with oil patch, and 6.83 m total 3 layers of sandstone with oil trace. The core samples have porosity of 6%-8% and permeability of (0.5-1.5)×10^-3 μm² from the core test. It is concluded by comprehensive interpretation that there are 59 m total 12 gas layers, showing large thickness of gas layers and great exploration potential. Oil and gas testing of the Well XSC1 is now conducted upwardly.

Relevant parameters of natural gas in the upper and lower members of the Silurian Kepingtage Formation in Well XSD1 are listed in Table 1. The natural gas compositions of the two members are basically the same, with relatively high content of methane. The crude oil in the lower member of the Kepingtage Formation has a density of 0.883 3 g/cm³, a viscosity of 16.98 mPa•s at 50 °C, a wax content of 3.4%, and a freezing point of 4 °C. The crude oil belongs to conventional thin oil (medium oil) characterized by low viscosity and low wax content. In the lower member of the Kepingtage Formation, the pressure of 24.47 MPa in the middle of the test layer (2527 m) was obtained with formation pressure coefficient of 0.987 and temperature of 41.88 °C. Besides, in the upper member of the Kepingtage Formation, the pressure of 24.27 MPa in the middle of the test layer (2395 m) was obtained with formation pressure coefficient of 1.033 and temperature of 41.05 °C.

The crude oil from the upper member of the Silurian Kepingtage Formation in Well XSD1 is dominated by saturated hydrocarbons (52.87%-72.65%), followed by aromatic hydrocarbons (19.25%-29.67%) and nonhydrocarbons (5.80%-11.36%). Further, it is characterized by relatively high bitumen content (2.31%-16.35%). The saturated/aromatic hydrocarbon ratio of 1.78-3.77 (averaging 2.78) and low nonhydrocarbons/bitumen ratio of 0.44-2.51 (averaging 1.32) are similar to those of crude oil in the Silurian in Central Tarim (averaging 2.00 and 1.49, respectively) ^[22]. The crude oil from Well XSD1 has a certain preponderance of pristane with a Pr/Ph ratio of 0.78-0.85. The relationship between DBT/P (dibenzothiophene/phenanthrene) and Pr/Ph ratio indicates that the Silurian crude oil/bitumen (sand) is typically originated from marine facies (generally less than 1), showing parent source rocks developed in reducing original depositional environment. The regular sterane in crude oil from Well XSD1 is distributed in an asymmetric “V” shape (nearly inverted “L” shape) with low amplitudes in the left and high amplitudes in the right (Fig. 4), with lower rearranged sterane (rearranged sterane/regular sterane ratio of 0.21-0.27), lower C₃₀-rearranged hopane content (C₃₀-rearranged hopane/C₃₀ hopane ratio of 0.10-0.13), and lower gammacerane index (0.13-0.15). Oil-oil correlation shows that the geochemical characteristics and indicators of Silurian crude oil in the Shajingzi structural belt are mostly similar to that of crude oil charged in the Early Silurian in the Central Tarim area, indicating that their geneses are generally similar. The oil of the latter is sourced from the mixed Cambrian-Ordovician ^[22]. The oil-rock correlation shows that although the biomarker parameters of the Silurian crude oil from Well XSD1 are not highly correlated with the Cambrian- Ordovician source rocks obtained from shallow parts (outcrops and wells), the carbon isotope composition of hydrocarbon families and the sulfur isotope composition of individual hydrocarbon are relatively close to those of the Cambrian Yuertusi Formation shale, showing better affinity between the two. The result indicates that the Yuertusi shale has an important contribution to hydrocarbon generation. It is considered by comprehensive analysis that the Silurian crude oil from Well XSD1 comes mainly from Cambrian-Ordovician source rocks in the deep Awati sag, with the dominance of Cambrian source rock.

The carbon isotopic composition of Silurian gas from Well XSD1 is lighter, and it is generally attributed to Type I-II kerogen. The methylcyclohexane index value of Silurian light hydrocarbons is 33.33, indicating that it is originated from sapropelic parent material. It is concluded from the light hydrocarbon triangle diagram that it belongs to oil-prone gas. Further, the diagram of (δ¹³C₂-δ¹³C₁) versus δ¹³C₁ also indicates that natural gas belongs to the oil-prone type. The natural gas is characterized by C₁/C₂₊ value of 31.89-45.41, with the dominance of methane and rare heavy hydrocarbon gas, drying coefficient (C₁/C_1-5) of 0.97-0.99, belonging to dry gas. The light hydrocarbon heptane value of 30 and paraffin index of 2 indicate that the natural gas is in mature-high mature stage, reflecting higher maturity of natural gas. From the comparison of carbon isotopic composition of methane and ethane, there is no evident characteristics of thermal cracking at high temperature.

The Shajingzi structural belt is adjacent to the Awati hydrocarbon generation sag. It is currently known that the Lower Paleozoic mainly develops three sets of marine source rocks (the Cambrian Yuertusi Formation, Ordovician Sargan Formation and Yingan Formation) ^[23,24], which can directly observed on multiple outcrops. Well XSD1 reveals the source rocks in the Yingan and Saergan Formations. Well XSC1 reveals the source rocks in the Yingan, Saergan and Yuertusi Formations. The outcrops and wells reveal that the source rocks have the similar thickness and the same lithological combinations, but the measurement results of organic carbon and hydrogen index of the outcrop samples are lower due to weathering. The information on the properties of soluble organic matter is insufficient. According to laboratory measurements and well-seismic calibration analysis, the black mudstone of the Yuertusi Formation is characterized by TOC of 1.33%-16.79%, vitrinite reflectance R_o of 1.48%- 2.00%, dominated by types I-II₁ kerogen, with the thickness about 10-55 m. It is distributed from the east and west of the Keping faulted uplift to the northern depression. It is the best set of source rock found in the Tarim Basin. The black mudstone of the Saergan Formation is characterized by TOC of 1.15%-9.10%, vitrinite reflectance R_o of 0.75%-1.20%, dominated by types I-II₁ kerogen, about 4-50 m thick. It is developed in the central and western parts of Keping faulted uplift-Awati sag, and is an important set of source rock in the northwest. The test results of marl samples in the Yingan Formation show that it has poor hydrocarbon generation capacity, which is a set of ineffective source rock.

On plane, the Shajingzi fault strikes in northeast- southwest, with a length of 163 km (Fig. 1b). The fault continued to be active from the Ordovician to the Neogene and controlled the formation and evolution of the Awati sag-Keping faulted uplift structure framework ^[19,20,21]. The Lower Paleozoic and part of the Upper Paleozoic are exposed in the northwest of the Shajingzi structural belt. Well XSD1 reveals the Upper Ordovician-Quaternary strata (Fig. 2), and Well XSC1 reveals the Cambrian- Quaternary strata. The rock-electric characteristics of each layer are quite different. Well-seismic calibration is carried out based on the high-quality seismic data by target processing, which can identify the distribution characteristics of main horizons and faults of the Shajingzi structural belt (Fig. 5). Previous researchers have clarified that the deep basement-involved wedge-shaped thrust structure of the Shajingzi fault zone, the Shajingzi fault in a narrow sense and the shallow extension structure. Especially, the Shajingzi fault in a narrow sense developed in the Late Permian-Neogene is a high-angle basement-involved compressional strike-slip fault with large scale and long active duration ^[19,20,21], effectively communicating the Awati hydrocarbon-generating sag and the Shajingzi structural belt, thereby to provide a good channel for oil and gas transport (Fig. 5). The oil and gas generated from the deep Cambrian-Ordovician source rocks in the Awati sag mainly migrated vertically along the Shajingzi fault and its induced faults during hydrocarbon accumulation period, and adjusted horizontally along the unconformity and the sandstone framework.

The sandstone types in the coring section of the Kepingtage Formation in the wells XSD1 and XSC1 are mainly medium-fine-grained lithic quartz sandstone, with a quartz content of 77%-88%, averaging 78.3%, a feldspar content of 3%-8%, averaging 5.7%, and a lithic content of 9%-18%, with an average of 15.3%. The clastic particles of sandstone are mainly medium to fine-grained sandstone. The lithic is mainly quartzite and a small amount of igneous rock, with occasional occurrence of phyllite and argillaceous fragments. The intergranular interstitial materials are mainly argillaceous matrix, calcite and secondary quartz. Calcite is characterized by medium- crystalline structure and plaque-like cemented particles. The secondary overgrowth of quartz is strong. The tested porosity of core samples was 4.8%-8.4%, with an average of 6%, and the permeability was (0.5-1.5)×10³ μm², with an average of 0.71×10³ μm² (Fig. 6), belonging to ultra-low porosity and low-permeability reservoir. The pore type is mainly intergranular dissolved pores, followed by intergranular dissolved pores and a small amount of micro- cracks (Fig. 7). The pore diameter is generally 0.1-0.2 mm, showing uneven distribution. Most of them are distributed in isolation and the throat is not developed, generally showing poor connectivity as a whole.

At the end of the Ordovician, the central Kunlun Block collided with the Tarim Block to form the ancient southern Tarim uplift and the ancient northern Tarim uplift, which controlled the paleogeographic pattern of the gulf opening westward in the Late Silurian ^[25,26]. In the Keping-Awati area, the composite sedimentary system of tidal-dominated delta and tidal-flat facies were developed (Fig. 8). The cores of upper member and lower member of the Kepingtage Formation in Well XSD1, Well XSC1 and neighboring outcrops showed the development of sedimentary buildups of ripple silt-fine sandstone and horizontal bedding silt-fine sandstone and clay layers, which are typically tidal-dominated delta deposits at the distal end of delta front far from the provenance, with weak hydrodynamic effect. The porosity of sandstone reservoir in the Kepingtage Formation in the Well XSD1 was worse than that of the sandbodies near the provenance in Central Tarim Basin and North Tarim Basin (7%-15%) ^{[7, 12]}. In addition, the currently measured maximum palaeostress in core from the Shajingzi structural belt was 85.7 MPa, which was 35 MPa greater than that of the Yingmaili area. The results indicate that strong tectonic compression experienced after the deposition of Silurian may further make the sandstone reservoirs more compact, resulting in the large-scale development of fractures visible in the outcrops and cores. In contrast, the deposit at the distal end of delta front is the key factor resulting in worse physical properties of the sandstone reservoirs in the lower and upper members of the Silurian Kepingtage Formation in the Shajingzi structural belt. It is speculated that the physical properties of reservoir are better near the provenance in the southwest and subtidal sandbodies in the middle of the basin (Fig. 6).

Three sets of reservoir-caprock assemblages were developed in the Silurian in the Shajingzi structural belt from top to bottom (Fig. 9): (1) the thick-bedded mudstone caprock of the Yimugantawu Formation (S₂y) and the tidal channel sandstone reservoir of the upper member of the Tata Ertag Formation (S₁t₂), with mudstone thickness of about 300-500 m and sandstone thickness of about 150-300 m. (2) The mudstone caprock of the lower member of the Tata Ertag Formation (S₁t₁) and tide-dominated delta sandstone reservoir in the upper member of the Kepingtage Formation (S₁k₃), with mudstone thickness of about 50-100 m and sandstone thickness of about 80-150 m. (3) The mudstone caprock in the middle member of the Kepingtage Formation (S₁k₂) and the tide-dominated delta sandstone reservoir in the lower member of Kepingtage Formation (S₁k₁), with mudstone thickness of about 60-150 m and sandstone thickness of about 80-150 m. The average thickness of a single layer of sandstone in the first set of reservoir-caprock assemblage was 7-18 m, and the mud-to-stratum ratio was low, which was not conducive to the preservation of oil and gas. The oil and gas displays are rarely found in drilling. The average single layer thickness of sandstone in the second and third sets of reservoir-caprock assemblages was 2-9 m, and the mud-to-stratum ratio was relatively high, and the oil and gas preservation conditions were better. Therefore it is the main exploration target. The Well XSD1 obtained industrial airflow in the second and third sets of reservoir-caprock assemblages.

The formation of the Silurian reservoir-caprock assemblage is mainly controlled by the influence of sedimentary evolution. During the deposition period of the lower member of the Kepingtage Formation, the central part of the Tarim Basin was in inner neritic facies deposit and a large tidal-dominated braided river delta developed from south to north in the west prograded. The progradation was progressed to the current Wensu bulge of the Keping faulted uplift (Fig. 8), resulting in the formation of reservoir interval. From joint well correlation (Fig. 9), the provenance progresses roughly from the southwest to the northeast, and the thickness of the sandbody gradually decreases and the grain size becomes finer. During the deposition of the middle member of the Kepingtage Formation, the basin was extensive inner neritic mudstone deposit, which was a caprock section. During the depositional period of the upper member of the Kepingtage Formation, the depositional pattern was roughly the same as that of the lower member, with tidal-dominated deltas and subtidal sandbodies developed, showing the development of important reservoir intervals. During the deposition of the lower member of the Tata Ertag Formation, the basin was subsided as a whole and it was dominated by the deposit of shallow bay mudstone, which was a caprock interval. During the deposition period of the upper member of the Tata Ertag Formation, the topography of the basin was gentle, and tidal sandbodies such as tidal sand mats and sand ridges were developed. The deposits can be used as reservoir intervals. During the deposition of the Yimugantawu Formation, the basin was mainly shallow bay mudstone, which was a caprock interval.

As shown in Fig. 10, the Shajingzi fault began to be active from the end of the Ordovician and formed a thrust wedge during the Late Caledonian-Early Hercynian. The prototype of the fault belt and early structural traps were formed from the Late Hercynian to Early Indosinian. In the later period, the fault continued to be active and the traps continued to be adjusted. In the Himalayan period, the fault activation was terminated and finalized, which was a period of trap finalization ^{[17,18,19,20,21]}. The Shajingzi fault and its associated faults not only control the formation of traps, but also provide good migration channels for hydrocarbons generated by deep source rocks in the Awati sag. On the whole, the Silurian traps in the Shajingzi structural belt are dominated by structural traps such as fault blocks, fault noses, and anticlines. There are structural-lithological traps with updip pinching-out sandstone, which are characterized by the control of faults, sequential distribution along structural belt, and contiguous superimposition (Fig. 5).

The source rock of the Cambrian Yuertusi Formation in the Northwest Tarim Basin began to generate hydrocarbons in the Ordovician, reached the peak of oil generation during the Late Caledonian-Early Hercynian and the peak of gas generation during the Himalayas, currently in high mature-over mature stage ^[23]. The source rock of the Ordovician Saergan Formation began to generate hydrocarbons from the Early Hercynian Period, and entered the initial oil generation peak from the Late Hercynian-Early Indosinian period, currently in mature-over mature stage ^[24]. The analysis results of fluid inclusions in Well XSD1, Well Shun9, and Central Tarim and North Tarim indicate that there are multiple periods of hydrocarbon accumulation in the Silurian, mainly including the Late Caledonian-Early Hercynian, Late Hercynian, and Himalayan ^[8,9], which respectively correspond to Silurian bitumen, crude oil and natural gas. They match better with the hydrocarbon generation evolution history of Cambrian-Ordovician source rock. According to the comprehensive analysis of forward modeling and inversion of hydrocarbon accumulation, the formation of the traps in the Shajingzi structural belt was coupled greatly with the hydrocarbon expulsion period of the Cambrian-Ordovician source rock. After the traps were finalized during the Himalayan period, the oil and gas generated by deep source rocks can migrate to favorable Silurian traps along the channels of faults, sand bodies, and unconformities to accumulate, thereby to form the present reservoirs.

Based on the abovementioned analysis, the oil and gas accumulation model of “being controlled mainly by structure, and reservoir pattern in late period” for the Silurian reservoir in the Shajingzi structural belt is established (Fig. 11). It is clarified that the hydrocarbon source is mainly the mudstones in the deep Cambrian Yuertusi Formation and the Ordovician Sargan Formation in the Awati sag; the reservoirs are mainly sandstones of the Kepingtage Formation and the Tata Ertag Formation, and the caprocks are the mudstones of the Yimugantau Formation, the lower member of the Tata Ertag Formation, and the middle member of the Kepingtage Formation. The Shajingzi fault is large in scale and it has been active since the Caledonian, forming a series of derived faults. It not only controls the formation and distribution of Silurian structures and structural-lithological sandstone traps in the Shajingzi structural belt, but also provides a good channel for communicating deep source rocks and transporting oil and gas. There are multiple stages of oil and gas charging, primarily including Late Caledonian, Late Hercynian-Early Indosinian and Himalayan. Silurian traps were not finalized until the Himalayas ^[20], dominated by the charge of gas from the source rock in the Cambrian Yuertusi Formation. Therefore, the current oil and gas reservoirs, dominated by gas, are characterized by sequential distribution and contiguous superposition along the structural belt. Controlled by the Shajingzi fault and its associated faults, the Silurian oil and gas reservoirs in the Shajingzi structural belt are dominated by fault blocks (revealed by Well XSD1 and Well XSC1) and fault nose types. Oil and gas were mainly transported along the fault system and sealed by the overlying mudstone caprock and lateral faults, ultimately enrich in the structural highs. Anticline-type oil and gas reservoirs may be developed in the Silurian strata of the footwall of the Shajingzi fault in the Awati sag (the Triassic crude oil was encountered by Well SN2), and updip sandstone pinching-out structural-lithologic reservoirs are developed on the slope background of the Shajingzi structural belt. Adjacent to the Silurian unconformity at the distal end of the Shajingzi fault hanging wall, stratigraphic oil and gas reservoirs may be developed, but the oil reservoirs are more susceptible to degradation and thicken.

According to fine structural interpretation, 42 structural traps have been discovered in the Kepingtage Formation in the Shajingzi structural belt of the Tarim Basin, totally covering 668.9 km², and the traps are divided into 3 types based on identification, oil and gas shows and buried depth. Among which, there are Type I traps of 70.2 km², Type II traps of 516.2 km² and Type III traps of 82.5 km². There are 74 structural traps in the Silurian Kepingtage Formation in northwestern Tarim, with a total area of 8249.2 km², which are mainly distributed in the Keping faulted uplift, the Bachu uplift and the northeast of the Awati sag. Among them, a total area of the Type I traps, Type II traps, and Type III traps are of 2261.7 km², 1322.4 km², and 4665.1 km², respectively. The calculation formula of volumetric method for trap resources is:

According to the actual drilling and testing information of the wells XSD1 and XSC1, the gas-bearing column height of the Silurian Kepingtage Formation sandstone in the Shajingzi structural belt is about 300 m, the average thickness of the gas-bearing sand bodies in the upper member of the Kepingtage Formation is about 16.5 m, with an average effective porosity of 6%, an average water saturation of 40%, an average crude oil density of 0.88 t/m³, and an average crude oil volume coefficient of 1. According to Eq. (1), the resource abundance of Type I traps is estimated to be about 50×10⁴ t/km² (oil equivalent) and resource abundances of Type II and Type III traps are converted at 60% and 20% according to Type I traps respectively. Based on the ratio of 1018 m³ of natural gas per ton of crude oil, the natural gas trap resource of the Silurian Kepingtage Formation in the Shajingzi structural belt is up to 2.018×10¹¹ m³, and that of the Silurian Kepingtage Formation amounts to 2.03×10¹² m³ in the northwest of the Tarim Basin (approximately 15×10⁴ km²), showing huge exploration potential. It is expected to become an important target formation for increasing oil and gas reserves and production in the Tarim Basin.

Although the Shajingzi structural belt is located on a monoclinic background, the Shajingzi fault is large and has long active period, which can control the formation of a series of traps and effectively communicate with the deep source rocks in the Awati sag. Both the upper and lower members of the Silurian Kepingtage Formation can form industrial gas flows, and it is possible to form large-scale hydrocarbon accumulation in the upper member of the Tata Ertag Formation locally. In addition, as long as effective traps are developed in the Cambrian, Ordovician, Triassic, and Carboniferous in the Shajingzi structural belt, large-scale oil and gas reservoirs are expected to be formed (Fig. 11), showing the huge exploration potential. From the Well XSD1, the lower porosity limit of the Silurian gas-bearing sandstone is only 5%, and the gas logging show is not evident during the drilling process. Based on this, the reexamination of several old wells in the northwestern Tarim has found new oil-gas-bearing intervals. The exploration results of the Shajingzi structural belt can provide reference for oil and gas exploration in other similar complex piedmont areas such as the middle and western sections of the Keping faulted uplift and the Bachu uplift.

The analysis of drilling samples from the Shajingzi structural belt in the Tarim Basin confirms that the Cambrian Yuertusi Formation and Ordovician Saergan Formation develop high-quality source rocks, while the Ordovician Yingan Formation doesn’t develop hydrocarbon source rock. The Silurian reservoirs of Well XSD1 mainly produce gas. The oil and gas are mainly derived from the deep Cambrian-Ordovician source rock in the Awati sag, dominated by the source rock in the Cambrian Yuertusi Formation.

The sandstone reservoirs of the upper and lower members of the Silurian Kepingtage Formation in Well XSD1 have porosity of 5%-8% and permeability of (0.5-1.5)×10^-3 μm², with well-developed fractures, belonging to fractured reservoir with low porosity and low permeability. The lower porosity limit of gas-bearing sandstone is currently known to be 5%.

There are three sets of reservoir-caprock assemblages in the Shajingzi structural belt, including: (1) mudstone caprock in the Yimugantawu Formation and tidal channel sandstone reservoir of the upper member of the Tata Ertag Formation, (2) mudstone caprock of the lower member of the Tata Ertag Formation and the tide-dominated delta sandstone reservoir of the upper member of the Kepingtage Formation, (3) mudstone caprock of the middle member of the Kepingtage Formation and the tide-dominated delta sandstone reservoir of the lower member of the Kepingtage Formation. Well XSD1 obtains industrial gas flow from the second and third reservoir- caprock assemblages.

The Shajingzi fault system has large scale and long activation duration. It communicates with deep high-quality source rocks in the Awati sag and controls the formation of a series of structural traps in the Shajingzi structural belt. Silurian traps have experienced multiple times of hydrocarbon accumulation and suffered destruction during the evolution process. After the finalization in the Himalayan period, they mainly received natural gas generated from the deep Yuertusi Formation source rock, and the natural gas occurs in the reservoirs so far, characterized by “being controlled mainly by structure, and hydrocarbon accumulation in the late period”.

The natural gas resource in Silurian traps in the Shajingzi structural belt amounts to 2.018×10¹¹ m³, and the Cambrian, Ordovician, Triassic, Carboniferous and other formations are expected to form large-scale oil and gas reservoirs with huge oil and gas exploration potential.

A—trap area, km²;

B_oi—average original crude oil volume coefficient;

GR—gamma ray, API;

H—average effective thickness, m;

N—trap resources or predicted geological reserves, 10⁴ t;

S_wi—average original water saturation of oil layers, %;

ρ_o—average crude oil density, t/m³;

ϕ—average effective porosity, %.