Structural characteristics of continental carbonate-rich shale and shale oil movability: A case study of the Paleogene Shahejie Formation shale in Jiyang Depression, Bohai Bay Basin, China

Abstract

Based on rock mineral and geochemical analysis, microscopic observation, physical property measurement, and thin laminae separation test, etc., the characteristics of typical laminae of the Paleogene Shahejie Formation carbonate-rich shale in the Jiyang Depression were analyzed, and the organic matter abundance, reservoir properties, and oil-bearing properties of different laminae were compared. Typical shale reservoir-flow structures were classified, and the mobility of oil in different types of shale reservoir-flow structure was compared. The results show that the repeated superposition of mud laminae and calcite laminae are the main layer structure of carbonate-rich shales. The calcite laminae are divided into micritic calcite laminae, sparry calcite laminae and fibrous calcite vein. The mud-rich laminae are the main contributor to the organic matter abundance and porosity of shale, with the best hydrocarbon generation potential, reservoir capacity, and oil-bearing property. The micritic calcite laminae also have relatively good hydrocarbon generation potential, reservoir capacity and oil-bearing property. The sparry calcite laminae and fibrous calcite vein have good permeability and conductivity. Four types of shale reservoir-flow structure are developed in the carbonate-rich shale, and the mobility of oil in each type of reservoir-flow structure is in descending order: sparry calcite laminae enriched shale reservoir-flow structure, mixed calcite laminae enriched shale reservoir-flow structure, fibrous calcite vein enriched shale reservoir-flow structure, and micritic calcite laminae enriched shale reservoir-flow structure. The exploration targets of carbonate-rich shale in the Jiyang Depression Shahejie Formation are different in terms of reservoir-flow structure at different thermal evolution stages.