Shale lithofacies types and reservoir characteristics from Ordovician Wufeng Formation to the first sub-member of the first member of Silurian Longmaxi Formation, northeast Zhaotong area

Abstract

Shale lithofacies has an important influence on shale reservoirs. It controls the hydrocarbon generation capacity, storage performance and fracturing performance of shale to a certain extent. In order to study the shale lithofacies characteristics, reservoir characteristics and evaluation of Wufeng-Longmaxi formations in the northeast Zhaotong area, a detailed research was carried out based on core, thin section, X-ray diffraction and various geochemical test data. Calcareous/siliceous mixed shale lithofacies, mixed shale lithofacies, clayey/siliceous mixed shale lithofacies, mixed siliceous shale lithofacies, clay-rich siliceous shale lithofacies are developed in the study area. There is a difference in the vertical distribution of lithofacies between the south and the north of the study area. The lithofacies from Wufeng Formation to the first sub-member of the first member of Longmaxi Formation (Long1-1 submember) presents the characteristics of transition from clayey shale lithofacies associa- tion to siliceous shale lithofacies association and then to mixed shale lithofacies association in the southern area, while the northern area shows a transition trend from mixed shale lithofacies association to siliceous shale lithofacies association and then to mixed shale lithofacies association. Horizontally, the lithofacies distribution is highly heterogeneous and has poor continuity. The reservoir characteristics of different lithofacies are different. Siliceous shale has good reservoir properties, with the characteristics of "high TOC content, high gas content and high brittle mineral content". The TOC content and gas content of clayey shale are high, but the content of brittle minerals is very low. The properties of mixed shale reservoirs are complex, and generally, the mixed shale reservoirs with high siliceous content have better properties. Based on TOC content, gas content and brittle mineral content, the shale lithofacies evaluation standard is established by combining analytic hierarchy process and entropy method, and the dominant lithofacies are determined. Mixed siliceous shale lithofacies, calcium-rich siliceous shale litho- facies, calcareous/siliceous mixed shale lithofaciesis are class Ⅰ dominant lithofacies; clay-rich siliceous shale lithofacies, mixed shale lithofacies and clayey/siliceous mixed shale lithofacies are class Ⅱ dominant lithofacies; mixed clayey shale lithofacies and silicon-rich clayey shale lithofacies are non-dominant lithofacies.

Keywords

• shale lithofacies
• reservoir characteristics
• reservoir evaluation
• wufeng formation
• ordovician
• longmaxi formation
• silurian
• zhaotong area