Genesis of authigenic kaolinite and its indicative significance to secondary pores in petroliferous basins: A case study in the Dongying Sag, Bohai Bay Basin, Eastern China

Abstract

Porosity of sandstone generally declines as the formation burial depth rises, during which transformation of unstable minerals (e.g. feldspar) in sandstone significantly contributes to abnormally high porosity zones. It is still controversial that whether transformation of feldspar being massive in sandstone can lead to formation of considerable secondary pores. As the main diagenetic product of feldspar transformation, namely authigenic kaolinite, in terms of the physical and chemical features, acts as indicators of the transformation process of feldspar and the consequent formation of secondary pores. In this study, a series of analyses are implemented on sandstone samples collected from the Dongying Sag, Bohai Bay Basin, Eastern China, including microscopic observation, SEM, EDX, XRD, isotope, and fluid inclusion analysis, with specific attention to genesis and formation process of authigenic kaolinite as well as their indicative significance to secondary pores. It is demonstrated that there are mainly three types of authigenic kaolinite (denoted as K1, K2, and K3), the occurrence and geochemical characteristics of which are greatly different. Based on contact relationship between authigenic kaolinite and other diagenetic products, the three authigenic kaolinite are distinguished into two stages, namely the first-stage K1and the second-stage K2 and K3, which are of obviously different genesis. Specifically, K1 is formed under the effect of acidic fluids that are rich in carboxylic acids. In contrast, K2 and K3 are formed under the effect of acidic fluids that are rich in CO2. During formation of K1, plagioclase dissolution pores are formed and well preserved, and carbonate cements get well preserved, showing no sign of dissolution. During formation of K2 and K3, K-feldspar is kaolinitized as K2, and a small amount of Si and Al elements migrates with short-distance and precipitates as K3 in the vicinity of K2. During this process, K-feldspar is transformed into kaolinite, accompanied by dissolution of carbonate cements and massive formation of secondary pores.