Structural characteristics and implications on oil/gas accumulation in north segment of the Longmenshan piedmont, northwestern Sichuan Basin, SW China

Abstract

By integrating surface geology, seismic data, resistivity sections, and drilling data, the structural deformation characteristics of the frontier fault of thrust nappes were delineated in detail. The frontier fault of thrust nappes in northwest Scihuan Basin is a buried thrust fault with partial exposure in the Xiangshuichang—Jiangyou area, forming fault propagation folds in the hanging-wall and without presenting large-scale basin-ward displacement along the gypsum-salt layer of the Triassic Jialingjiang Formation to the Triassic Leikoupo Formation. The southwestern portion of the frontier fault of thrust nappes (southwest of Houba) forms fault bend folds with multiple ramps and flats, giving rise to the Zhongba anticline due to hanging-wall slip along the upper flat of the Jialingjiang Formation. In contrast, the northeastern portion of the frontier fault of thrust nappes (northeast of Houba) presents upward steepening geometry, leading to surface exposure of Cambrian in its hanging-wall. With the frontier fault of thrust nappes as the boundary between the Longmenshan Mountain and the Sichuan Basin, the imbricated structural belt in the hanging-wall thrusted strongly in the Indosinian orogeny and was reactivated in the Himalayan orogeny, while the piedmont buried structural belt in the footwall was formed in the Himalayan orogeny. In the footwall of the frontier fault of thrust nappes, the piedmont buried structural belt has good configuration of source rocks, reservoir rocks and cap rocks, presenting good potential to form large gas reservoirs. In comparison, the hanging-wall of the frontier fault of thrust nappes north of Chonghua has poor condition of oil/gas preservation due to the surface exposure of Triassic and deeper strata, while the fault blocks in the hanging-wall from Chonghua to Wudu, with Jurassic cover and thicker gypsum-salt layer of the Jialingjiang formation, has relative better oil/gas preservation conditions and thus potential of oil/gas accumulation. The frontier fault of thrust nappes is not only the boundary between the Longmenshan Mountain and the Sichuan Basin, but also the boundary of the oil/gas accumulation system in northwestern Sichuan Basin.