We present a detailed structural analysis of geometry and temporal development of the Cenozoic faults in the southwestern part of the Qaidam Basin—the largest petroliferous sedimentary basin and the only one producing oil and gas within the Tibetan Plateau, northwest China—based on three-dimensional (3D) seismic and well-log data. The Cenozoic faults are mostly steep (50–70°), basement-involved reverse faults trending primarily west-northwest and secondarily north–south. The two fault sets are commonly linked to each other by west-northwest-oriented faults bending either southward at their eastern tips or northward at the western tips, and originated possibly from inversion of pre-existing extensional faults in response to the far-field effect of the Cenozoic India–Eurasia collision. Faults or fault segments active during different time periods were identified from isopach maps and verified via seismic reflection profiles, showing that the faults became active as early as the Paleocene in the southern part of the southwest Qaidam Basin and propagated dominantly northward and subordinately eastward over time. Measurement of throws on major faults indicates that fault activity intensified over time and culminated since the mid-Miocene. These faults have been an important factor in forming the oil fields in the southwest Qaidam Basin by improving permeability, forming anticlinal traps, and acting as conduits for oil migrating from source rocks to the reservoirs.
The geological characteristics of bedrock gas reservoirs and the reason for the enrichment and high production of gas in the Dongping area of the Qaidam Basin are studied based on logging data, image log data, core observation, thin section analysis, reservoir microscopic study and cap rock condition evaluation. The main lithology of the bedrock reservoirs in the Dongping area is granite and granite gneiss. The reservoir space mainly consists of fractures, dissolution pores and micro-pores, among which massive matrix micro-pores and dissolution pores are the key factors for the high and stable gas production in the study area. Due to the Tertiary salty environment, the fractures and pores 0 to 18 meters from the bedrock top are filled with gypsum and calcite, forming good "top-sealing" cap rock, this special reservoir-cap rock combination in wide distribution results in the high production of these gas reservoirs. There are two types of gas reservoirs: one is fracture-pore reservoirs at the top of the bedrock, mainly distributed 20−50 m below the "top sealing" cap rock, strongly controlled by tectonic background, and high and stable in gas production; the other is fractured reservoirs inside the bedrock, large in gas-bearing depth, great gas-bearing differences, abrupt change in lateral direction, and high but not stable in production.
Gas Hure Oilfield is an area rich in oil and gas in the Qaidam Basin and contains E31 and N1-N21 oil reservoirs. Through conventional column chromatographic extraction of 48 oil samples, this paper investigates the oil migration and charge direction using nitrogen compounds, sandbody distribution in E31 and N1-N21, and the E31 reservoir dissection. Oil in the I+II and III+IV oil layers of the E31 reservoir migrates from the middle-north of the anticline to its south and north, and its oil source areas are the Mangya hollow and the Gas rift-subsidence. The injection spots of the III+IV oil layer are the two sides of well 9-7 in the middle of the anticline, and the injection spots of the ?+? oil layer are wells 8-34 and 9-38. The N1-N21 reservoir oil is mainly from the Mangya hollow and migrates upwards along faults to reservoirs. Faults and sandbodies are the main transport channels in the field and most reservoirs are anticlinal.
Based on detailed field survey in the western Qaidam Basin, combined with petroleum exploration practices in recent years, this study suggests that the Qaidam Basin is a strike-slip superimposed basin jointly controlled by the left-lateral strike-slip Altyn Tagh and East Kunlun faults. The Altyn Tagh fault acts as the major controlling boundary, while the East Kunlun fault only controls the local evolution of the southern edge of the basin. To the east, the northern Qaidam-Qilian Shan thrust-fold belt passively accommodates the northeastward displacement along the Altyn Tagh fault through NE-SW directed shortening. In the India-Asia collision background, the left-lateral strike-slip faulting along the Altyn Tagh fault initiated from the early Eocene, forcing the Qaidam Basin to move northeastward, causing the thrust and slip deformation of the NW-SE faults in northern Qaidam margin-Qilian Mountain area, and the deposition of the Cenozoic since the Paleogene Lulehe Formation. During the Paleogene, the northern Qaidam Basin developed coarse-grained sediments like that in a foreland basin, forming poor quality source rocks; while the southwestern Qaidam Basin was an extensional sag basin where good quality source rocks deposited. By the early Miocene, left-lateral faulting along the Kunlun fault became active, leading to the formation of a series of en-echelon faults (e.g. the Kunbei fault, the Arlar fault and the Hongliuquan fault). These faults gradually migrated northward, their kinematics changing from left-lateral strike-slip motion to NE-SW transpression. Strike-slip-related structures controlled by those faults (e.g. the Yingxiongling structure) are ideal places for oil and gas accumulation from Paleogene source rocks. To sum up, the Qaidam Basin is a strike-slip superimposed basin jointly controlled by the left-lateral strike-slip Altyn Tagh and East Kunlun faults. The temporal and spatial superimposition of these two strike-slip faults during the Cenozoic controlled the evolution of the basin as well as the oil and gas accumulation.
Located in east part of Yingxiongling structural belt in the Qaidam Basin, the Yingdong oilfield has a extremely complicated ground condition. Due to no significant discovery, this oilfield was considered to have no favorable geologic conditions for formation of oil or gas reservoir. In the past few years, with continuous improvement in the mountain 3D seismic surveys and logging data interpretation, some breakthroughs were obtained in 2010, and the Yingdong oilfield, the largest-scale reserves of a single reservoir with highest organic matter abundance, most favorable physical property and optimal development efficiencies in the Qaidam Basin, had been discovered, the production capacity was up to 0.55 × 106 t. Through detailed analyses of the Yingdong oilfield, some studies, such as hydrocarbon accumulation conditions and technical challenges, are carried out, and following conclusions can be achieved. The Yingxiongling area is located in Mangya hydrocarbon-generation sag in the west part of the Qaidam Basin, its oil sources are rich; the Neogene Xiayoushashan Formation and Shangyoushashan Formation are dominated by wide and gentle delta front—shore-shallow lacustrine sediments with interbeds of sandstone and mudstone, the sandbodies are widely distributed with favorable physical condition, and the mudstone is the key caprock, combined with high-quality Paleogene hydrocarbon source rocks, a complete source-reservoir-cap assemblage can be formed. Large-scale detachment faults of the Yingdong area connect high-quality Paleogene hydrocarbon source rocks with middle-shallow buried structural traps, thus, reservoirs formed in the early stage are modified, and at the same time, hydrocarbons formed in the later stage continue to migrate and accumulate; in this way, the deep and shallow faults form a relay-style hydrocarbon transport system, and hydrocarbons are accumulated in the shallow structural traps in the later stage; in this area, the middle-shallow faults have good lateral plugging performance which is favorable for preservation of oil and gas. For complex landforms and reservoir features in the Yingdong area, the integral 3D seismic acquisition, processing and interpretation technology is developed for complex mountain areas to provide a reliable foundation for hydrocarbon exploration. For some problems in the Yingdong oilfield like long oil/gas-bearing intervals, great difficulty in identification of fluids, the development mode of multiple oil/gas/water systems in the long intervals is established, and the geologic modeling technology with constraint of multiple conditions on complex fault blocks is also developed. Thus, hydrocarbon accumulation mechanism in the Yingdong oilfield is clear, and some complex key technology of engineering are well solved, providing necessary geologic theories and technical supports for high-efficiency development and rapid production construction in the Yingdong oilfield.
Seismic profile crossing southern boundary of Saishiteng Mountain shows the shear tectonic characteristic of fault zone at southern boundary of Saishiteng Mountain. In the west of Saishiteng, the fault zone thrusts towards top and north; in the center, the fault zone shows high angle, in the east (line 1001 profile), it represents as low-angle and basin-toward thrusted fault, which may be converted structure at the end of strike-slip faulted structure, in pattern of associated structure. The north boundary did not overthrust to basin; Wells Dongtai 1 and Gaqiu 1 at foothill of Saishiteng did not penetrate into new formations overlapped by the mountains but into associated products of high-angle faulted structure, which indicates that the mountain was less thrusted to the basin. So the structure deformation of Northern Qaidam Basin had been mainly strike slip since Cenozoic and the thrust nappe is associated with the strike-slip structure. The strike-slip faulted zone in north boundary showed reversal deformation, so it is not a favorable play to form original reservoirs; the masses between strike-slip trips that may be stable relatively, are favorable for forming reservoirs.
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