Groundwater Characteristics And Hydrocarbon Preservation In Micang-Daba Piedmont Zone

Before experienced several tectonic movements, the original hydrocarbon preservation conditions in Micang-Daba piedmont zone and its margin were good. Looking for the left area where didn’t be destroyed become the first thing in the oil and gas exploration. Based on the analysis of cap rock features in the study area, the influences of tectonism and hydrological-geochemical characteristics, hydrocarbon preservation conditions in different tectonic units were studied; so did the spatial and temporal relationship between these factors and the preservation condition.Results as following were drawn during this research:(1) There were serveral kinds of cap rocks in the study area, such as: gypsum-salt rock (Middle-Lower Triassic Leikoupo and jialinjiang Formation), shale (Middle Jurassic Shangshaximiao, Xiashaximiao, Qianfoya Formation, Lower Jurassic Ziliujing Formation, Upper Triassic Xujiahe Formation, Lower Triassic Feixianguan Formation, Permian Formation, Middle-Lower Siluric and Cambrian Formation) and some dense argillaceous limestone.(2) Since the beginning of the Yanshan movement, Xuefeng, Qinling and Longmen mountain gradually moves close to the Sichuan basin, A series of overthrust nappes come into being(most happened in the period of late Yanshan and Himalayan movement since the Late Cretaceous). Tectonics seriously impact on hydrocarbon preservation conditions, especially the intense fracturing and uplifting and erosion. While, structural deformation intensity gradually weakened and the preservation conditions become better as long as closing to the center of basin. These come to a clear zonation and segmentation with hydrocarbon preservation conditions of the area.(3) The present meteoric water infiltrate through the fractures in the Micang and Daba piedmont zone, the infiltration gradually weaken from the piedmont to the basin. Duing to the lack of good regional cap, Paleozoic exposed area has strong atmospheric water infiltrations, hydrocarbon preservation conditions are poor. In the area with distribution of Lower Triassic gypsum rock and Jurassic mudstone caprock, the meteoric water infiltration is blocked, the overall preservation conditions are good.(4) The paleofluid activities in the strata of the Micang-Daba piedmont zone take place frequently. Early tectonic movements are conducive to the formation of hydrocarbon reservoirs in the Piedmont. While, the late Yanshan-Himalayan movements affect in the bad ways.(5) There are significant differences of preservation conditions in the study area.From the flauts zone to the basin, the study area can be divided to four parts: A、B、C、D. Part A:the hydrocarbon preservation conditions were completely destroyed; Part B:the hydrocarbon preservation conditions were seriously destroyed; Part C:because of the distribution of the gypsum-salt rocks, preservation conditions were significantly improved. Part D:with the stable stuctures, the preservations are good. The area from the northwest of Nanjiang block to the southeast of Zhenba block where the gypsum-salt rocks distributed and the piedmont where located the pressure seal boxes, are the most favorable hydrocarbon accumulation preservation units.