Subsidence And Uplift Characteristics And Their Relationship With Fractures And Gas Reservoiring Characteristics,in The Upper Paleozoic Of Ordos Basin

The relationship between differences of subsidence&uplift and fracture in the upper Paleozoic of Erdos basin heavily effects oil and gas reservoir. Research level on it is low so far. In this article, the strata denudation at the end of late Cretaceous and deposition-subsidence features of Taiyuan formation, Shanxi formation, Shihezi formation, Shiqianfeng formation, early-middle Triassic, late Triassic, early Jurassic, middle Jurassic, early Cretaceous in Erdos basin was analyzed. Based on the analysis, sedimentation and denudation differences of main source rocks (Taiyuan formation) of upper Palaeozoic in Erdos Basin at the end of late Triassic, early Jurassic, Middle Jurassic, early Cretaceous and nowadays was studied. The relation between differences in subsidence&uplift and fracture’s formation and evolution was analyzed by observing fracture’s growth characteristics in the core and slice, and compression-elasticity-fracture experiments of rock. The difference of transport capability between the different mediums was compared on the permeability test. On the basis of the above research, concerning with the analysis of the driving force for hydrocarbon migration, and preliminary analysis of the formation characteristics of oil and gas reservoir related to fault and crack of the the upper Paleozoic of Erdos basin. Conclusion was as follows:1. Difference between sedimentary, subsidence, erosion and the main source rock’s settlement and uplift of the upper Paleozoic in Erdos Basin①. In Benxi formation, Yinchuan and Lingwu on the sides of the basin, are both the large subsidence area; In Taiyuan formation, large subsidence area are Ansai, Zizhou, Yanchang and Yichuan; In Shanxi formation, are Fuxian, Wuqi, Yichuan; In Xiashihezi formation, are Zhiluo, Fuxian, Yichuan; In Shangshihezi formation, are Shuanghe, Xiasiwan, Ganquan; In Shiqianfeng formation, are Zichang, Ansai, Ganquan; In early and middle Triassic, are Mizhi, Yulin; In late Triassic, are Huanxian, Huachi, Zhengning; In early Jurassic, are Ansai, Yanan, Ganquan, Zhiluo; In middle Jurassic, are Yuancheng, Huachi, Baibao; In early Cretaceous, are Jiyuan, Huanxian. On the whole, larger subsidence area are always located in the northwest and southwest of basin,especially in the southwest.Since the end of late Cretaceous, Erdos Basin has been entered main erosion period, and large amount in east, decreasing amount to center and west, which formatting the modern structure of the high-east low-west.②. Main source rocks of Taiyuan formation of upper Palaeozoic in Erdos basin is at end of late Triassic sedimentary period, and the largest subsidence area are mainly located in Huanglong-Tongchuan and Qingjian-Suide lines over3400m; At end of early Jurassic, subsidence area is located in laojundian-Tongchuan-Baishui ine over3600m; At end of middle Jurassic, subsidence area is located in Zichang-Laojundian line over3800m and western Maerzhuang over3800m; At end of early Cretaceous, subsidence area is located in Maerzhuang-Jiyuan-Fanxuel line over4400m. On the whole, the largest subsidence area at end of late Triassic to early Jurassic is relatively stable; then moving to southwest of the basin at end of middle Jurassic to early Cretaceous and subsidence range decreases gradually towards northeast of basin.2. The differences of subsidence&uplift and fracture in the upper Paleozoic of Erdos basin heavily effects oil and gas reservoir. Subsidence burial process can be divided into three stages:compaction, elastoplastic compression and fracture. There are parallel crack, low-angle inclined crack and vertical crack growing up in the upper Paleozoic in Erdos Basin. Among them, parallel crack having slippage, most are found in the mud rock. The parallel cracks always grow up in the rock level and different litho logy interface. The vertical cracks always grow up in the structural homogeneity of rock. Generally, the vertical crack cannot cut through the parallel crack and they compose a form like "工". When basin uplifted and eroded, not only crack grow, but also crack growing in subsidence period expanse.3. Porous sandstone, mudstone and cracks have different transport performance. Commonly, in Porous sandstone and mudstone, the parallel permeability is greater than the vertical (Parallel permeability is1.013-3.811times of vertical permeability, average2.113times in mudstone; parallel permeability of sandstone is1.07-29.6times of vertical permeability, average2.66times). The permeability of fractured rock is greater than the permeability of porous rock(Fractured mudstone permeability is2.55-253.7times of porous mudstone, average118.36times; fractured sandstone permeability is7718.1-752278.4times of porous sandstone, average259037.9times.).4. Area of maximum source area in three key geological period in upper Paleozoic of Erdos Basin are:Maerzhuang-Jiyuan-Fanxue line at end of Early Cretaceous (maximum settlement period), southwestern Dabaju-Shancheng-Xiaonangou and northwestern Tiesukemiao-Gongkahan line in late Cretaceous (erosion period), and Tiesukemiao-Bulagesumu line in nowadays structure(largest regional structure burial depth). The maximum source area is mainly controlled by the main source rocks’ subsidence range.5. Under the certain gas range and gas amount, gas concentrations change with the distance between the main source area and erosion boundary. The farer the main source area distance, the nearer the erosion boundary, the more amount of gas lateral migration, and the more gas loss; to the contrary, the nearer the main source area distance, the farer the erosion boundary, the less gas loss and the less amount of gas lateral migration.