Pore Classification and Structural Characteristics of Oligocene-Pliocene Shale Reservoirs in the Western Qaidam Depression.

Oligocene-Pliocene shale reservoirs in the Western Qaidam Depression represent typical mixed shale deposits characterized by moderate organic matter (OM) abundance and sufficient OM maturity, indicating substantial shale-oil resource potential. Here, a comprehensive study was conducted to analyze the reservoir characteristics of different shale types, including the Upper Xiaganchaigou (late Oligocene), Shangganchaigou (Miocene), and Xiayoushashan (early Pliocene) Formations in the Western Qaidam Depression. Our analysis focused on the pore structural characteristics of shale reservoirs, employing X-ray diffraction, casting thin sections, scanning electron microscopy, low-temperature nitrogen adsorption, and nuclear magnetic resonance (NMR) as investigative techniques. Our results show that (1) the study area comprises five typical shale types: lime shale, argillaceous shale, limestone, argillaceous limestone, and mudstone. The best hydrocarbon source rock conditions are found in the lime shale and argillaceous shale. (2) Inorganic pores, including dissolution pores, intergranular pores, bedding fractures, structural fractures, and intraparticle pores in clay minerals, are the main pore types found in the studied samples and constitute the primary reservoir space for shale oil. On the basis of fractal dimensions obtained through NMR, the pores can be classified as micropores (<100 nm), mesopores (100-1000 nm), or macropores (>1000 nm). Mesopores are the main contributors to porosity. (3) The development of micropores is positively correlated with clay mineral content. The development of mesopores and macropores is influenced by the quartz, feldspar, dolomite, and calcite contents. Calcite content exhibits a negative correlation with porosity, suggesting that later-stage pore cementation hinders shale reservoir development. (4) The five typical shale reservoirs in the study area can be categorized into three types. Type I reservoir lithologies include lime shale and argillaceous shale; type II reservoir lithologies include limestone and argillaceous limestone; and type III reservoirs comprise mudstone. Type I and II reservoirs are of better quality than type III.