The characteristics and formation mechanism of a regional fault in shale strata: Insights from the Middle-Upper Yangtze, China

Shale is a type of fracture-prone rock, but few regional faults are observed in shale strata. Here, we investigate a regional detachment fault in the Middle-Upper Yangtze (MUY) to understand the formation mechanism of a special regional fault in shale strata. Based on a lithological, mineralogical, and mechanical study as well as semi-quantitative model of displacement, we found that the formation of this detachment fault is closely related to the lithological association and flexural slip processes. The lithological association of this detachment fault is characterized by two thick competent layers separated by a thin weak interlayer at both the macroscopic and microscopic scales. When these layers are folded, the detachment fault zone is commonly present in a thin weak interlayer, namely, the Wufeng Formation at the macroscopic scale, and primarily along interfaces in laminar siliceous shales (LSS) and secondarily in K-bentonite interlayers within the LSS at the microscopic scale. The regional fault zone is characterized by parallel to strata surfaces and abrupt contacts with the protolith. The fabric of the fault core is obviously controlled by the flexural slip intensity and the number of slip events, as well as their direction and structural position. The natural shale gas obtained from Well HD-1 has proven that this regional detachment fault in shale strata is not only a migration pathway for hydrocarbons but also acts as an adsorption state reservoir for free gas from other areas. This regional detachment fault in shale strata is likely to advance the shale gas industry and to expand the exploration of conventional gas.