Sedimentary Evolution And Resource Potential Of Upper Paleozoic, Linfen-Yuncheng Basin

Linfen-Yuncheng faulted basin is located in the middle south of Shanxi province, which is part of the Cenozoic Fenwei graben system. Previous exploration shows that it is lack of effective source rocks and other advantaged oil conditions in the Cenozoic strata in this area, so hydrocarbon potential is not optimistic. But it's worth of attention and study if the Mesozoic, Paleozoic strata underlying of Linfen-Yuncheng Cenozoic basin can form an industrial value of oil or gas reservoirs, similar to the adjacent Ordos basin and Qinshui basin. Given this, this study focused on the underlying Upper Paleozoic of the basin that has proven natural gas generation and reservoir, from the distribution and sedimentary characteristics, late reformation and hydrocarbon-generating characteristics of Upper Paleozoic, to explore the potential of resource of Upper Paleozoic in this area.Based on Seismic, gravity and magnetic data, combined with geothermal and hydrologic data, the author drawned up the Linfen-Yuncheng basin Pre-Cenozoic geological map. It's indicated that the distribution of Upper Paleozoic strata in Hongtong, Linfen, Xiangfen and Yicheng shows an overall "L" type. The east of Houma, the southwest of Wanrong and northwest of Yuncheng has sporadic distribution. Linfen-Hongtong area thickness is 500-900m, Xiangfen area is 300-500m, Houma sag and Yuncheng-Linyi sag is about 150m, Wanrong is 20-150m.Because of shortage of core data penetrating the Upper Paleozoic of this area, typical outcrops around the basin are surveyed in this study. According to lithology and its association, grain size characteristics and sedimentary structures, fossils markers, the Late Paleozoic Benxi Formation-Taiyuan Formation were deposited in epicontinental sea barrier-lagoon environments in this area. Delta was dominated facies during deposition of Shanxi Formation. Fluvial and temporary lacustrine facies were dominated during deposition of Lower Shihezi-Shiqianfeng Formations. During the Late Paleozoic, from early to late through experienced the evolution of coastal environment to the terrestrial environment, of which Benxi Formation, Taiyuan Formation and Shanxi Formation developed coal seam, carbonaceous shale and they are the favorable hydrocarbon source rocks.Based on the survey structure deformation of neighboring areas and analysis of the fission-track thermochronology, tectonic evolution of the Upper Paleozoic in Linfen-Yuncheng basin is reconstructed. Our studies show that there is a continuous sedimentation in Triassic and Upper Palaeozoic of Linfen-Yuncheng basin, and showing synchronous lifting with the Ordos basin which its west, during the Early-Middle Jurassic Ordos basin and Linfen-Yuncheng areas accepting stable deposition. During Late Jurassic, it was dominated by compression and uplift in the researched area. During Early Cretaceous, it may incorporate with the Ordos basin of its west, deposited sediments, but during the early of Late Cretaceous-Paleogene, it suffered strong compression and uplift, Mesozoic and part of Paleozoic strata were subjected to erosion. Since the Eocene, the Weihe basin to the north, faulted in turn, receiving deposits. Linfen sag which is north of researched area started faulting until the Pliocene, receiving deposits. While the mountains of both sides of basin, the lvliang mountain of the west and the eastern Huo mountain, Zhongtiao mountain, uplifted continuously along with the subsidence of the basin, remnants of Upper Paleozoic experienced deep-burying again.Based on the analysis of pyrolysis of source rocks in neighbouring outcrops of the basin, and compared with the geochemical characteristics of source rocks in Ordos basin and Qinshui basin, it's considered that the hydrocarbon generating potential of Upper Palaeozoic is similar to the Ordos basin's. Combined tectonic evolution of the Meso-Cenozoic of the basin with the adjacent areas,9 wells which are geothermal wells or hydrogeology drilling in the basin were used for thermal evolution history modelling and compared with well of Shancan 1 and well of Qincan 1. The study shows that source rock maturity in Linfen sag may be higher than Qinshui basin, better saved than Qinshui basin. Combined geochemical characteristics of source rocks, thermal evolution history modeling and preservation conditions of source rocks in the basin today, considered that Linfen sag was a favorable forecast area of coal-type gas exploration.