Advances in well log-based assessments of fine-grained sedimentary rocks

Fine-grained sedimentary rocks, which serve as the source rocks and reservoirs of tight/shale oil and gas, are the focus and frontiers of petroleum geology. The log-based assessments of these rocks hold great significance for the exploration and exploitation of unconventional hydrocarbons. In this study, the advances in both domestic and international log-based assessment techniques are systematically analyzed for fine-grained sedimentary rocks. The analytical results indicate that the combination of conventional and emerging logging techniques allows for the assessment of more than seven properties of fine-grained sedimentary rocks, including lithology, physical and electrical properties, oil-bearing capacity, oil mobility, wettability, brittleness, and source rock property. The log-based assessment of fine-grained sedimentary rocks has, therefore, further evolved into the assessment of their reservoir property, oil-bearing capacity, oil mobility, and fracability, collectively known as the "new four properties". Specifically, the reservoir property of these rocks is assessed based on parameters such as lithology, lithofacies, pore type, microscopic pore structure, lamellation, total porosity, and effective porosity. Their oil-bearing capacity is assessed using parameters like clay mineral content, TOC content, free hydrocarbon content, oil saturation, oil occurrence, and movable oil content. The oil mobility in fine-grained sedimentary rocks is assessed according to parameters such as maturity, formation pressure, crude oil density and viscosity, and gas/oil ratio. The fracability of these rocks is assessed using parameters like the respective compositions and contents of clay and brittle minerals, Young's modulus, Poisson's ratio, and the maximum and minimum principal stresses. The formation micro-imaging (FMI) logging slicing technology enables the manual identification and assessment of meter-, millimeter-, and even micron-scale laminae in fine-grained sedimentary rocks. Additionally, log data facilitate the identification and assessment of the lithofacies of fine-grained sedimentary rocks. The techniques and methods for log-based assessments of fine-grained sedimentary rocks are evolving from conventional and emerging logging techniques toward artificial intelligence approaches based on mathematical statistics.