Laminar characteristics of lacustrine organic-rich shales and their significance for shale reservoir formation: A case study of the Paleogene shales in the Dongying Sag, Bohai Bay Basin, China

Abstract

• Six types of the organic-rich shale laminae are reported in the Dongying Sag. • Discontinuous laminae were formed under high-energy hydrodynamic conditions. • Shale laminae containing favorable reservoir storage space are in the order of wavy laminar > lenticular laminar > weak laminar. • Thick and wavy laminae are the most favorable types for shale oil exploration. Shale oil exploration in China has demonstrated that laminated shales are favorable lithofacies for producing shale oil effectively. It is of great significance to study the type and reservoir properties of shales with different laminar structures. In this study, the laminae of Paleogene organic-rich shales in the Dongying Sag, Bohai Bay Basin were divided into two categories and six types based on thin section petrography, scanning electron microscopy (SEM), and geochemical characterization. The laminar types include thin parallel laminae, thick parallel laminae, wavy laminae, lenticular laminae, weak laminae, and sandy laminae. The first three types of laminae have good continuity and were formed under low-energy hydrodynamic conditions. In comparison, the latter three types are discontinuous and were formed under relatively high-energy hydrodynamic conditions with a strong terrigenous input. Significant differences were observed in the reservoir space types, microcracks, and oil-bearing properties among the different laminae. Analyses of porosity and oil saturation further revealed that laminated shales with high laminar continuity exhibit higher porosity, permeability, and oil saturation values than shales with poor laminar continuity. Shales with thick parallel laminar shale and wavy laminar shale possess favorable petrophysical parameters. Thus, from the perspectives of reservoir and oil-bearing properties, laminated shales with high laminar continuity are better than shales with poor laminar continuity for shale oil exploration in the Dongying Sag. The new findings provide a reference for the optimization and evaluation of favorable continental shale oil reservoirs in eastern China.