Abstract:

 The study of shale lithofacies is the basic work of shale gas geological evaluation and layer selection. This paper takes the Wufeng Formation-Member 1 of Longmaxi Formation in Changning area of Sichuan basin as the research object. By using outcrop, drilling, and logging data, combined with rock and mineral, thin section, and geochemical data, the mineral composition, lithofacies characteristics, and their influence on gas content of shale are systematically studied, the favorable lithofacies are determined, and the spatial distribution characteristics of shale lithofacies in Changning area are characterized. The results show that there are five lithofacies in this area: Clay-siliceous mixed shale, mixed shale, siliceous shale, calcareous-siliceous mixed shale, and calcareous shale. Vertically, the volume fraction of carbonate minerals gradually decreases from bottom to top, and the integral number of clay minerals gradually increases. The lithofacies is a transition of calcareous siliceous shale-siliceous shale-clay-siliceous mixed shale. The subsidence center of the study area is located in the northeast of Changning area, and the lateral thickness of the siliceous shale lithofacies shows an increasing trend from southwest to northeast. The siliceous shale has the characteristics of high biogenic siliceous mass fraction, high w(TOC), high brittleness, and high pore structure parameters, so it has the best gas content. Although the clay minerals in the clay-siliceous mixed shale can generate secondary pores through diagenetic transformation, their plasticity is high, and the pores are difficult to preserve due to compaction. In addition, the higher plasticity leads to undeveloped shale fractures, which further reduces the physical properties of the reservoir, and the clay-siliceous mixed shale has poor gas-bearing properties.

Key words: Changning area')">

Changning area, Wufeng Formation-Longmaxi Formation, shale, lithofacies, gas-bearing, Sichuan basin