Fracture characteristics and logging identification of lacustrine shale in the Jiyang Depression, Bohai Bay Basin, Eastern China

Abstract

Fracture has a vital effect on the storage and seepage ability of oil and gas in shale reservoirs, and its effective identification and evaluation is of great significance for shale oil and gas development. To analyze and evaluate fracture development characteristics in the lower sub-member of the third member of the Eocene Shahejie Formation (Es3L) and the upper sub-member of the fourth member of the Eocene Shahejie Formation (Es4U) in the Jiyang Depression, Bohai Bay Basin, core observations, thin sections, field emission scanning electron microscopy (FE-SEM), and well logging were performed. After data analysis, fracture types in the study area were characterized, relationships between shale oil types and shale oil production were discussed, and a new method was proposed to identify fractures using only conventional logging curves by combining discrete wavelet transform (DWT) and box fractal dimension with sliding window approach (SW box fractal dimension). Our results indicated that the fracture types according to geologic genesis are structural fractures (structural tension fractures, structural shear fractures, and structural tension-shear fractures), non-structural fractures (abnormal pressure fractures, diagenetic contraction fractures, bedding fractures, and dissolution fractures), and organic matter-related fractures. A close relationship occurs involving the types of shale oil and shale oil production, and fractured shale oil usually has the highest initial production. The fracture identification method using the DWT and SW box fractal dimension has good effects, and its accuracy is verified by Fullbore Formation MicroImager (FMI) logging in L69 well. Two high-production wells of shale oil in the study area (wells FY1 and Y187) were used to identify fracture development sections using the DWT and SW box fractal dimension, and excellent application results were obtained. In general, it is an efficient way to fully evaluate fracture development sections in shale reservoirs in logging profiles, and also provides new ideas for the fracture identification method using conventional logging curves in other shale oil regions.