Fractures in cores from the Lower Paleozoic Wufeng-Longmaxi shale in southern Sichuan Basin and their implications for shale gas exploration

Fractures are closely linked to the reservoir capacity of shale intervals, therefore also have a direct control over the test results of wells drilled into the intervals. A macro delineation based on cores and outcrops observation are combined with polarized microscopic and scanning electron microscopic images of samples to reveal the characteristics of fractures in the gas-bearing shale intervals of the Wufeng-Longmaxi Formations, southern Sichuan Basin. It shows that the shale intervals have highly-developed bedding-parallel and non-bedding-parallel macro and micro fractures, of which the bedding-parallel fractures take dominance and account for 75 % of the total macro fractures and 87 % of the total micro fractures. For macro fractures, the bedding-parallel fractures are generally foliation fractures and inter-layer sliding fractures, and the non-bedding-parallel fractures are oblique and vertical. For micro fractures, the bedding-parallel fractures are dominated by foliation fractures, while the non-bedding-parallel fractures are mostly fractures caused by abnormal pressure from hydrocarbon generation, diagenetic contraction fractures, and dissolution fractures. The density and distribution of the fractures are largely controlled by burial depth with the 3 500 m serving as a dividing line. For intervals below the line, fractures are mostly developed in the L1(1-3)layer; for intervals above the line, fractures are concentrated in the L1(1) layer. Density of both macro and micro fractures increases with increasing burial depth. There are wells showing that density of macro fractures in shale intervals below the line is ten times higher than those above the line. The fracture density is also controlled by TOC content and bedding types, which is well illustrated by shale intervals with high TOC content and bedding of striped siltstone type and graded (siltstone to claystone)type also having the highest fracture density. As fractures are essential to the reservoir capacity of shale intervals, the L1(1-3)and L1(1)layers contain the best quality shale intervals. However, with diagenetic contraction micro-fractures well developed, the bottom of the L1(2)also hosts some quality shale intervals. © 2022 Editorial Department of Oil and Gas Geology. All rights reserved.