Laminae characteristics of lacustrine shales from the Paleogene Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China: Why do laminated shales have better reservoir physical properties?

Abstract

Laminae are the most typical and representive sedimentary feature in organic-rich shales, and revealing their influences on the reservoir physical properties is vital to understand the oil enrichment mechanisms of lacustrine shale. This study investigated the laminae characteristics and their effects on the physical properties of the shales from the Second Member of the Kongdian Formation (Ek2). The research presented in this study is based on core and thin section observations, XRD analysis, low-pressure nitrogen physisorption (LNP), high-pressure mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM). The results show that the Ek2 shales developed massive and laminated sedimentary structures. The laminae can be categorized as siliceous laminae, dolomite laminae, clay-rich siliceous laminae, organic matter laminae, and calcite laminae. Analysis of SEM, LNP, MIP, and NMR show that macropores and micro-fractures contribute significantly to the total pore volume of laminated samples, while micropores and mesopores are the dominant pore types in massive samples. The pore structure and connectivity of the laminated samples are better than that of the massive samples. Comparison of laminar and massive mudstones in the Ek2 showed that the development of laminae physical properties that are more conducive for reservoir formation as a result of interlaminated fractures, optimized arrangement of minerals, and improved organic-inorganic interactions.