The Altyn Tagh Fault is a large strike-slip fault that forms the northern boundary of the Tibet Plateau. Many researches have been done on the Altyn Tagh Fault is abundant and come to the consensus that the Cenozoic sinistral strike-slip movements of the Altyn Tagh Fault began around 50 Ma. The sinistral strike-slip displacement is 200 to 400 km^[1,2,3,4,5]. The Qaidam Basin, situated on the southern side of the Altyn Tagh is a large petroliferous basin. Oil and gas exploration in the basin began in 1954. By 2016, 29 oil and gas fields had been discovered, with geological oil and gas reserves equivalent of 10.2×10⁸ t^[6]. The Cenozoic tectonics and hydrocarbon accumulation in the Qaidam Basin have been extensively studied^{[6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]}. The discovery of bedrock reservoirs in the Dongping and Niuzhong areas of the basin opened up a new field of oil and gas exploration in the basin. As of 2015, the proven natural gas reserves in the basin amounted to 519×10⁸ m³. The Dongping and Niuzhong areas in the south slope of Altyn Tagh Mountain are distinct from the southwestern part of the basin in terms of Cenozoic tectonic evolution and hydrocarbon accumulation process. Moreover, these areas are complex in tectonic deformation and dominated by fault-related structural traps. Therefore, in-depth research on the Cenozoic tectonic evolution and it hydrocarbon significance of this area is needed to further understand the Cenozoic activities of the Altyn Tagh fault and the hydrocarbon accumulation process and to guide oil and gas exploration in the basin. In this work, we used high-resolution seismic data to study the Cenozoic structures that restrict the accumulation of Jurassic source rocks in the Niuzhong and Dongping areas. Moreover, we draw on drilling, logging, and other data to explore the hydrocarbon significance of the Cenozoic tectonic evolution in this region.

The Qaidam Basin is located at the northeastern margin of the Tibet Plateau. The basin in triangular shape is bounded by the eastern Kunlun fault in the south, the Altyn Tagh Fault in the northwest, and the Qilian fault zone in the north. As the Qaidam Basin is a unique in-plateau basin bordered by two large strike-slip fault zones (Altyn strike-slip fault zone and East Kunlun strike-slip fault zone), its Cenozoic tectonic evolution is shaped both temporally and spatially by these two large strike-slip boundary faults. In Paleogene, the basin was mainly affected by the sinistral strike-slip movement of the Altyn Tagh Fault; the southwestern part of the basin was in an extensional environment, while the northern margin in front of the Qilian Mountains was in a compressional-torsional environment. Since the Neogene, due to the sinistral strike-slip and the northward migration of the East Kunlun fault, a series of transpression structures came up in the southwestern part of the basin^[16,22,24]. The Qaidam Basin is controlled by the Altyn Tagh Fault and East Kunlun fault. These two large sinistral strike-slip faults and the northern margin of the Qilian fold-and-thrust belt acted jointly in time and space, giving rise to the unique strike-slip superimposed structure of the Qaidam Basin since the Cenozoic^{[16, 22, 24]}. In the southwestern part of the Qaidam Basin, a group of NW-SE trending structures developed extensively, forming a series of linearly distributed folds on the surface, such as Yingxiongling and Nanyishan. Some researchers believe that the formation of this kind of structure is related to the thrust structure^{[10-12, 25-27]}. In recent years, new research has shown that the NW-SE trending structures in the southwestern part of the basin are a series of structures related to strike-slip fault. These folds are controlled by the East Kunlun fault and the Altyn Tagh Fault jointly, are characterized by flower structure on profile, and mostly began to form after the Miocene^{[24, 27-31]}. The Niuzhong and Dongping areas of the basin along the Altyn slope also have a series of E-W trending faults, such as the Niubei fault. Wu et al.^[14] and Zhao et al.^[19] considered this group of faults is closely related to the Neogene sinistral strike-slip movement of the Altyn Tagh Fault. Three sets of oil and gas systems have formed within the Qaidam Basin. They are the Jurassic freshwater lake marsh facies in the North Qaidam margin and in the middle part of the Altyn slope, the Paleogene-Neogene salinized lake source rocks in the West Qaidam area, and the Quaternary biogas source in the East Qaidam area in the basin^[6], giving rise to three sets of oil and gas systems with different features. So far, wells making breakthroughs in oil and gas exploration in the middle section of the front part of Altyn mountain, such as Dongping 1 and Niuxin 1, are all located near the junctions of E-W and NW-SE trending structures. The Cenozoic evolution of these two groups of structures constrained the reservoir formation of Jurassic source rocks in the Dongping and Niuzhong areas.

The Dongping-Niuzhong area is located in the middle part of the Altyn slope and in the northwest of the Qaidam Basin, about 45 km away from the main Altyn Tagh Fault (Fig. 1). The remote sensing image shows (Fig. 1b) that a group of E-W trending faults, represented by the Niubei fault, have developed in the northern part of the Dongping and Niuzhong areas. This group of faults passes through the bedrock uplift between Niuzhong and the main Altyn Tagh Fault. The current regional topography of this area is less undulating, which is in stark contrast to the widely developed northwest anticline in the southwestern part of the basin.

In this study, we intend to investigate the Cenozoic tectonic activities in the Niuzhong area, and understand its evolutionary process. We selected three seismic sections in the study area (the position of each section is shown in Fig. 1c) to explain the results. Using a set of 3D data, we performed similarity seismic slice (Fig. 2) and produced a top surface distribution map. The 3D seismic data used in this work were processed and interpreted with KINGDOM software at the geophysical exploration center, geophysical exploration research institute, PetroChina Qinghai Oilfield Company. In this work, the stratigraphic identification of seismic sections was conducted based on drilling and logging data from the geophysical exploration center of the Qinghai oilfield. Stratigraphically, the sections can be divided into seven formations. These are, in chronological order: the Paleogene Lulehe Formation (E₁₊₂), the Paleogene Lower Ganchaigou Formation (E₃; within this formation, T₅ is the seismic interface showing the bottom boundary of the heading side and T₄ is the seismic interface showing the bottom boundary of the hanging side), the Neogene Upper Ganchaigou Formation (N₁; the seismic interface of the bottom boundary is T₃), the Neogene Lower Youshashan Formation (N₂¹; the bottom boundary is T₂), the Neogene Upper Youshashan Formation (N₂²; the bottom boundary is T₂), the Neogene Shizigou Formation (N₂³), and the Quaternary Qigequan Formation (Q₁) ^{[16, 32]}.

In Fig. 2, the coherence attribute slices of the 3D seismic data clearly show the Cenozoic tectonic geometry of the Niuzhong area. Two groups of faults occur in this area: the Niubei fault, and the Niudong I and Niudong II faults. Of them, the N-E trending Niubei fault is shown in both the deep and the shallow coherence attribute seismic slices of the Niuzhong area, while the NW-SE Niudong I and Niudong II faults are only displayed in the deep coherence attribute seismic slice in the Niuzhong area (Fig. 2b, 2c).

This fault is the most representative near E-W trending fault in Dongping-Niuzhong area. The coherence attribute seismic slice (Fig. 2b) shows that in its deeper section, the Niubei fault appears as a simple E-W trending fault (Fig. 2c); in the shallow middle section, it is divided into one fault in the north and another in the south, but the two faults merge again in the eastern section (Fig. 2b). According to the remote sensing image (Fig. 2a), the Niubei fault cuts through the current water system and shows features of sinistral strike-slip.

In section A-A° (Fig. 3), the Cenozoic in the Niuzhong area is generally high in the north and low in the south. The shal-low part of the Niubei fault is nearly vertical, and the deep part dips northward, cutting through the Cenozoic section. As a result of the faulting thrust component, the Youshashan Formation and above formations in the hanging side of the fault uplift toward the surface, while the Cenozoic below the Youshashan Formation is missing. The seismic event of the Youshashan Formation in the footwall of the fault has a significant decrease to the high point, which is in sharp contrast with the relatively stable layer thickness distribution of the underlying strata.

In section B-B° (Fig. 4), the Cenozoic in the Niuzhong area is also high in the north and low in the south, and from the middle section, the Niubei fault bifurcates into two branching faults leaning slightly northward, cutting the whole Cenozoic. The Cenozoic below the Youshashan Formation is missing on the hanging wall of the fault. The seismic event of the lower Youshashan Formation significantly decreases towards the high point, whereas the underlying strata change little in thickness.

The Niuzhong area has small surface height difference. In the remote sensing image, no surface trace of the NW-SE trending fault could be seen. Through seismic data, however, it can be seen that a group of faults represented by the Niudong I and Niudong II occurs in the Niuzhong area (Fig. 2c). The top structural map of the Lulehe Formation (Fig. 5a)and the Jurassic residual thickness map (Fig. 5b) both show that the Niudong I and Niudong II faults have sinistral strike-slip features.

In profile C-C° (Fig. 6), the Niudong I and Niudong II faults are nearly vertical and dip southwest in the deep part, cutting the Ganchaigou Formation and formation below in Cenozoic. Affected by the fault, the Youshashan Formation and other Cenozoic formations below it have noticeable bending and deformation, forming folds. The effects of the Niudong I and Niudong II faults on the formations above the Xiayoushan Formation gradually weaken from deep to shallow depth, and the near-surface strata are hardly affected. The Lower Ganchaigou Formation differs widely in thickness on the two sides of the Niudong I fault, and thins in the hanging wall of the fault noticeably.

The E-W faults, represented by the Niubei fault, and the NW-SE trending faults both show thrust characteristics on section (Figs. 3-6). They also have noticeable horizontal displacement on the plane (Figs. 2 and 5). Therefore, both groups of faults are identified as sinistral strike-slip transpression faults.

The Lower Ganchaigou Formation differ significantly in thickness in the hanging wall and foot wall of the Niudong I and Niudong II faults, and thins in the hanging wall, which means that the Niudong I and Niudong II faults became active when the Lower Ganchaigou Formation deposited during the Oligocene. The influence of the NW-SE trending faults on the Lower Youshashan Formation and formations above gradually weakened from bottom-up, and the near-surface strata are almost unaffected by the faults (Fig. 6). Considering together with features of the NW-SE trending faults in the Niuzhong area are shown only in deep coherence attribute map (Fig. 2c), it is concluded that the activity of the Niudong I and Niudong II faults weakened significantly after the deposition of the Youshashan Formation (after the Miocene).

The seismic event of the Youshashan Formation south of the Niubei fault significantly decreases towards the high point (Figs. 3 and 4). It can be concluded that the deposition of the Youshashan Formation happened in the same period as the activity of the Niubei fault. In the northern part of the Niubei fault, the strata below the Youshashan Formation are missing, and the Upper and Lower Youshashan Formations are in angular unconformable contact, some researchers considered that this meant the E-W faults, represented by the Niubei fault, became inactive before the deposition of the Upper Youshashan Formation^[10,11,12]. However, that the Niubei fault cuts through the current water system and cuts the Niudong I and Niudong II faults proves that the activity of the Niubei fault started later than the Niudong I and Niudong II faults, and continues to now. Combined with the evidence of growth strata south of the fault, we infer that the activity in the Niubei fault began during the Miocene. The strong vertical uplift at the beginning of the fault activity led to the uplift and erosion of the Cenozoic below the Youshashan Formation in the northern part of the fault, so the Cenozoic below the Lower Youshashan Formation in this part is missing. Later, the vertical uplift rate of the Niubei fault reduced, and the Youshashan Formation began to deposit in the Niuzhong area, resulting in the unconformity between the Upper and Lower Youshashan Formation.

Based on the evolutionary process of the basin during the Cenozoic, and evidence presented in the previous sections, we inferred the Cenozoic tectonic evolution process in the Dongping-Niuzhong area. By using oil and gas drilling data and consulting previous studies on the southwestern part of the Qaidam Basin, we also examined the hydrocarbon significance of the Cenozoic tectonic evolution in the Dongping and Niuzhong areas.

The gas reservoirs in Dongping and Niuzhong areas are Jurassic coal-type gas reservoirs. The reservoir layers include the bedrock weathering crust and conglomerate and sandstone in the Lulehe and Ganchaigou Formations. High-angle cracks controlled by shear stress in the reservoir layers are important for improving their storage performance. Hydrocarbon charging began in the middle and late stages of the Oligocene^{[33,34,35,36,37]}. The wells where breakthrough has been achieved in oil and gas exploration, such as Dongping 1 and Niuxin 1, are located in the junction between the NW-SE trending tectonic structure near the inherited Jurassic sag and the E-W trending structures near the Altyn Mountains (Fig. 7). The natural gas concentration is higher in the vicinity of the source rocks and faults.

The structures of the Qaidam Basin are mainly Cenozoic ones^{[6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]}. A lot of research on the initial time of the Altyn Tagh Fault activity has been carried out. Most researchers, such as Yin A et al.^[1] and Cheng F et al.^[3], believe that the Cenozoic sinistral strike-slip movement of the Altyn Tagh Fault started around 50 Ma. During the Paleogene, affected by the sinistral strike-slip movement of the Altyn Tagh Fault, the relatively rigid basement of the Qaidam Basin gradually migrated northeast along the Altyn Tagh Fault. As a result, the earth crust of the northern margin of the Qaidam-Qilian mountain area shrank in the northeast-southwest direction. The northern margin area appeared as a foreland basin. At the meantime, the southwestern part of the Qaidam Basin had a background of relative tensile stress, where a series of tensional structures developed, forming the Mangya depression with abundant hydrocarbon-source rock^{[16, 24]}. Different from the tensile environment in the southwest of the basin, the Dongping and Niuzhong areas were in a transpression background in Oligocene. From the beginning of the late Oligocene, Niudong I and Niudong II faults began active, and a series of NW-SE trending east transpression faults began to take shape in the Dongping and Niuzhong areas. From the Oligocene to the Miocene, the NW-SE trending faults in the Niuzhong and Dongping areas continued active, giving rise to a series of folds, which together bore the transpression component in the Dongping and Niuzhong areas from Oligocene to Miocene. Located in a transpression fold area, the Oligocene-Miocene strata in the Dongping and Niuzhong areas mainly consist of coarse clastic deposits, but lack high-quality Cenozoic source rock. Despite this, the nearby inherited Jurassic sag has high-quality source rocks preserved, which can guarantee the supply of oil and gas for reservoir accumulation in the middle part of Altyn slope. In addition, the anticlines related to the early NW-SE trending faults with formation time matching the hydrocarbon generation period are the pointing area of ​​hydrocarbon migration. The widely developed NW-SE trending faults such as Niudong I and Niudong II connect the lower Jurassic source rock with the upper reservoirs, providing ideal channels for the oil and gas charging in Oligocene-Miocene (Fig. 8). The fracture system developed along the fault is an important factor for improving the physical properties of the reservoir layers.

Since the Miocene, the southwestern part of the basin has changed from the relative transtension background of the Paleogene period to the northeast-southwest transpression extrusion background, and the NW-SE trending transpression structures in and around the Yingxiongling area began to take shape, which became important sites for oil and gas accumulation (mainly with oil and gas from Paleogene source rock) in the southwestern part of the basin in late Cenozoic^{[16, 25]}. After the Miocene, the sinistral strike-slip movement speed of the Altyn Tagh Fault increased significantly^[2,3]. The flower structure of Altyn expanded into the basin. A series of E-W trending faults with an approximately 20° angle from the main Altyn Tagh Fault, represented by the Niubei fault, began to form in the middle part of the Altyn slope, representing the P-shear of the Altyn fault zone in the Riddle model. After the Miocene, the activity of E-W trending Niubei fault caused a large-scale uplift of the area between it and the main fault of Altyn, forming a bedrock uplift area. The E-W trending faults, together with the large-scale uplift of the bedrocks in northern Niubei, bore a large proportion of compression component in the Dongping and Niuzhong areas, resulting in a decrease of the NW-SE trending transpression tectonic activity. Consequently, Dongping and Niuzhong became the areas least affected by the Cenozoic tectonic movement in the Altyn slope after Miocene, which is conducive to the preservation of gas reservoirs.

In summary, geological factors favorable for Jurassic coal- type gas accumulation in the Dongping-Niuzhong area of ​​the middle part of the Altyn slope include the inherited Jurassic hydrocarbon-bearing sag, the NW-SE trending tectonic structure matching the hydrocarbon generation and discharge period, and the relatively stable structural preservation conditions after Miocene. Many favorable exploration targets are located in and around the Jurassic source rock sag. Apart from the areas with breakthroughs, including the Niuzhong and Dongping areas, the NW-SE trending tectonic trap areas formed in the early stage such as the E-II structure area also favorable exploration targets.

The NW-SE trending transpression structure of the Dongping and Niuzhong areas began active in the Oligocene Era, but the activities weakened after the Miocene. The E-W trending faults in the middle part of the Altyn slope took shape in the Miocene and stayed active until today. The NW-SE trending fault and the E-W faults are the principal pressure surface and P-shear of the Altyn strike-slip fault zone, respectively. They are the tectonic response of the Qaidam Basin to the Cenozoic sinistral strike-slip movement of the Altyn Tagh Fault. The consecutive activities of the two groups of structures have jointly shaped the Cenozoic tectonic evolution of the Dongping and Niuzhong areas.

In the inherited Jurassic sag, good source rocks have developed. Previous studies revealed that hydrocarbon charging of the region began in the middle and late Oligocene, matching the formation time of the NW-SE trending faults. These faults became effective channels of oil and gas migration in the Dongping and Niuzhong areas. The fault-related anticlines are favorable targets for oil and gas accumulation. The Niuzhong area has been less affected by Cenozoic tectonic movement since the Miocene, thus has better conditions for gas reservoir preservation. In the slope region of the Altyn Mountains, in the Jurassic sag and its adjacent areas, the NW-SE trending anticline traps formed in the early Cenozoic, such as the E-II structure, are also favorable exploration targets, worth more attention.

We thank those who helped us and provided support to our research work for this paper. Among them are Professor Daowei Zhang and Professor Yongshuwei Zhang of Qinghai Oilfield, and engineers including Qiquan Zhang, Changhao Zhang, Peng Wang and Bo Wang.