Abstract:

Lithofacies characterization is fundamental to the geologic evaluation and favorable layers and plays selection in shale gas exploration. Based on the outcrops and drilling data, a lithofacies classification method for marine shale was developed to finely characterize the lithofacies of the Upper Ordovician WufengLower Silurian Longmaxi shale through analysis of mineral composition, thin section, geochemical and logging data. Four preliminary conclusions were obtained: (1) Ternary diagram combined with sedimentary microfacies is an effective method in the classification of lithofacies types and their association for marine shale. (2) Six types of lithofacies were developed in the WufengLongmaxi shale in the Southern Sichuan Basin and they can be further classified into three types of lithofacies association, among which siliceous and calcareoussiliceous mixed shales are typical in (semi)deep water environment, argillaceous and argillaceouscalcareous mixed shales are dominant in semideep to shallow water environment, and argillaceoussiliceous mixed shale is developed in both shallow and deep water environment. (3) The basic features of each lithofacies association with different genesis, including mineral composition, matrix fabric, palaeontology, organic matter richness and log response, vary greatly with the decrease of water depth, stronger hydrodynamic force, and the increase of terrestrial input. (4) With the control of sea level change, the lithofacies associations deposited in deep water shelf, including siliceous, calcareoussiliceous shale and argillaceoussiliceous mixed shale are principal producing pay, while the lithofacies associations deposited in semideep to shallow water shelf, including argillaceous, argillaceoussiliceous shale and argillaceouscalcareous mixed shale, are the caprock of the producing pay. They form a selfsealed and selfstored reservoircap association vertically, and the former, which is rich in siliceous and organic matter, is favorable lithofacies to serve as producing pay, and the latter, which is rich in clay and deficient in organic matter, is favorable lithofacies to serve as capping bed.

Key words: Sichuan Basin, Lower Silurian, Longmaxi Formation, organicrich shale, ternary diagram, lithofacies types, lithofacies association, favorable lithofacies