The Geological Conditions And Exploration Prospect Of Paleozoic Shale In Lower Yangtze Area, Anhui

The evaluation parameters of gas shales used in North America and upper Yangtze region, China are utilized to evaluate the lower Yangtze area in Anhui Province. Sections measuring and sampling are conducted in fieldwork and geological data related to potential gas shales in the study area are collected. Core samples in three wells are collected and used for many experiments that associated with gas shales evaluation. Systematic research about the evaluation parameters, sedimentary environment and distribution features of target formations and tectonic evolution in the research area have been conducted. By comparing the shale gas geological and mining conditions with North America and upper Yangtze area, we proposed the area with highest shale gas potential. Our main conclusions are as follows:1. Lower Yangtze region has experienced three stages of tectonic evolution in Paleozoic, developing three types of basins. They are passive margin of Late Qrdovician-Sinian, foreland basin of the Late Ordovician-Silurian and Late Paleozoic craton basins, mainly developing the Lower Cambrian strata, the Upper Ordovician-the Lower Sinian strata and upper Permian strata respectively.2. Hetang formation, lower Cambrian mainly developed platform-clinothem facies. The strata thickness increases from the Northwest to the Southeast in the study area. The thickness of the black shales in Huangshan and Shitai area are more than 200 m while the largest thickness is more than 600 m. The Early Silurian developed a foreland basin. Overall, the thickness of Gaojiabian formation balck shale increases from the Northwest to the Southeast in the study area. The shale thickness in Huangshan area can be up to 254 m which is thicker than Chuzhou-Chaohu area. The thickness of Longtan formation decreases from the North east to the Southwest in the study area. It is about 200 m in Xuancheng-Jingxian area.3. The mean value of TOC is larger than 2.00% of the black shale in the study area. The organic matter of lower Cambrian and lower Silurian is mainly type I, and Gufeng and Dalong formation is mainly type I-II and Longtan formation is mainly type III.The Lower Paleozoic shales are generally over-matured and the Ro value of upper Permian is mostly from 1.3-2.0% which suggested mature to high-mature stage.4. X-ray diffraction results show that the average content of quartz, feldspar and pyrite of study strata is 77%-94%; the average content of carbonate minerals is below 20%; and clay mineral content is 5%-23% in average. Shale pore can be divided into organic pore, mineral pore and secondary pore/seam. The adsorption isothermal experiments show that the VL of Longtan, Wufeng and Hetang formations are 3.64 m3/t,1.42m3/t and 2.33 m3/t respectively.5. By comparing shale gas geological parameters with North American shale and Longmaxi shale in upper Yangtze region, many factors are taken into consideration for evaluating the study area. The study area is divided into 9 basins (syncline),21 units in total, and Gufeng-Longtan-Dalong Formations in Xuanguang Basin are proposed to be the most favorable exploration area.