Shale Gas Reservoir Evaluation by Geophysical Measurements: A Case Study of the Upper Ordovician-Lower Silurian in the Fenggang Block, Northern Guizhou Province

With the aid of geophysical measurements, including seventeen two-dimensional (2D) seismic lines and the well logging curves of well FGY1, the structure and reservoir characteristics of the Upper Ordovician-Lower Silurian strata in the Fenggang block, northern Guizhou Province, were analyzed thoroughly to identify desert areas and favorable intervals. The results show that Longmaxi-Wufeng is the most prospect-rich formation, consisting of a thick succession of overmature black shale, this formation remaining partially in the Suiyang, Fenggang and Jianchaxi synclines. The Longmaxi-Wufeng shale, especially the lower member, was deposited in a reducing low-energy environment with relatively high U content and a low Th/U value. In this shale, the organic matter type (sapropelic and humic-sapropelic), total organic carbon (TOC) content, gas content, gas adsorption capacity, vitrinite reflectance and brittle mineral content are profitable for shale gas preservation and development. The fractures of this shale were closed because of its high overburden pressure. The gas adsorption capacity of this shale increases with increasing TOC content and R-o. In the Longmaxi-Wufeng Formation at well FGY1, the most favorable intervals are in the depth ranges of 2312.4-2325.1 m and 2325.8-2331.1 m.