Geologic analysis of Middle-Lower Jurassic shale reservoirs in Yuanba area, northeastern Sichuan Basin

Integrating with drillings, cores, outcrops and logging data, we made a comprehensive geologic analysis of the Middle-Lower Jurassic shale reservoirs in the Yuanba area of the northeastern Sichuan Basin, from the aspects such as organic-rich shale distribution, geochemical characterization, mineral composition, hydrocarbon store space property, shale porosity and permeability condition, and geomechanical nature. The organic-rich shales mainly generated from the Daanzhai member of the Ziliujing Formation to the 1st and 2nd members of the Qianfoya Formation. They are shallow to semi-deep lacustrine facies, and mainly consist of dark shales with limes and sand stones. Organic matters are rich, and of type Ⅱ₂. The average TOC content is 1.14%. The vitrinite reflectance (R_o) ranges from 1.4% to 1.6%, indicating for the mature and high-mature stages of condensate oil and wet gas. Generally, the Middle-Lower Jurassic shales have simple and stable structure with few fractures developed, hence are favorable for hydrocarbon generation and preservation. The lacustrine facies shales in the Daanzhai member are similar to the Eagle Ford shales in the North America in general. The dark shales in the Daanzhai member are 40-115 m thick with various pore types. Horizontal and micro fractures are well-developed. The average brittle mineral content, Young’s modulus and Poisson ratio are 52.7%, 37 GPa and 0.30, respectively, all contributing to the formation of fracture network.