A Comparison of Generation–Retention–Expulsion in Felsic and Carbonate Laminated Shale by Semi-Open Thermal Pyrolysis: Implications for Shale Oil Exploration

Paleogene lacustrine shale is a key source rock for large oil reserves in China and a major target for shale oil exploration. However, differences in the chemical characteristics of felsic and carbonate shales during burial and thermal evolution remain poorly understood. This study evaluates hydrocarbon generation and expulsion efficiency in these shale types using pyrolysis experiments on lower Paleocene Kongdian Formation samples (Type I) from the Eastern China Sedimentary Basin. Results show that felsic shale has higher hydrocarbon generation capacity than carbonate shale. During pyrolysis, carbonate shale retained ~119 mg/g more oil but expelled 184 mg/g less than felsic shale. Felsic shale reached peak oil generation and retention faster but with lower retention efficiency. The larger volume of residual hydrocarbons in felsic shale facilitated earlier expulsion onset, higher yields of gaseous hydrocarbons, and superior gas expulsion efficiency. While both shales exhibited similar thermal evolution trends for hydrocarbon gases, methane proportions and gas-oil ratios (GOR) differed significantly. Carbon loss was comparable during the oil window, but felsic shale lost more carbon overall. At higher temperatures, n-alkanes in residual oil decreased sharply, with lighter oil retained at advanced maturity, increasing GOR and reducing heavy hydrocarbons. These findings demonstrate the effective hydrocarbon potential of medium-high TOC felsic and carbonate shales.