Formation of carbonate laminae in shale and their impact on organic matter in Dongying depression.

Carbonates, the main components of oil shale that influence oil and gas accumulation, are becoming increasingly significant in oil shale studies. This paper aims to examine the formation mechanisms of various carbonate minerals in shale and their impact on oil enrichment. In the Dongying Depression, two predominant types of carbonate minerals have been identified: micritic carbonate and grain carbonate. Micritic carbonate primarily forms through biogenic processes, where alternating carbonate and clay mineral laminae result from the periodic stratification of lacustrine water bodies. These layers are relatively thin. Micritic carbonate rocks contain low organic matter and predominantly feature narrow slit-like pores, leading to a tight pore structure that hinders oil shale accumulation. In contrast, grain carbonate formation is governed by diagenetic processes. Influenced by deep fluids, these carbonate laminae are mainly composed of lens-shaped, coarsely crystalline calcite and exhibit significant thickness. Grain carbonate rocks have a relatively high organic matter content, with pore spaces primarily consisting of bottleneck- and slit-shaped throats. This configuration enhances reservoir capacity and creates favorable conditions for oil shale accumulation.