Classification of lacustrine tight limestone considering matrix pores or fractures: A case study of Da'anzhai Member of Jurassic Ziliujing Formation in central Sichuan Basin, SW China

Abstract

To improve the prediction accuracy of sweet spots in tight reservoirs, the Da'anzhai Member limestone in Jurassic Ziliujing Formation, central Sichuan Basin was subdivided based on the relationship between characteristics of matrix pores and fractures and rock fabric, and the physical properties and oiliness of every type and the effect of different rock types on the natural productivity were discussed. The limestone reservoir has plenty, multi-type nano- to micro-meter micropores or microfractures. Bioclastics which mainly are bivalve shells, calcite or dolomite crystalline grains and silicate minerals are the three endmembers affecting the development of micropores or microfractures in the limestones. According to this, the limestone in the Da'anzhai Member is subdivided into 10 different types. Each type has the unique sedimentary and diagenetic history, and pore and fracture features. The results show that siliceous bivalve packstone and clay bivalve packstone have better storage property; bivalve-clastic grainstone and bivalve mudstone have higher permeability; clay bivalve packstone has higher oil content; and siliceous shell packstone, dolomitic shell packstone and argillaceous shell packstone can increase the supply ability of reservoirs. Lithologic differences lead to different pore-fracture and physical properties, which are the main reason of the different single well productivity in the Da'anzhai Member. Key words: tight limestone, matrix pores or fractures, rock types, physical property, oilliness, productivity features, Da'anzhai Member, Jurassic