The intermontane Mae Sot basin, Thailand, contains carbonate-rich oil shales in which oil yield, mineralogy, and major and trace element geochemistry are related to the rock type. Higher grade oil shale is significantly enriched in silicate minerals (quartz, feldspars, clays, and analcite) relative to carbonates, and in dolomite relative to calcite. Couplets of organic-rich and mineral-rich laminae increase in mean thickness from 0.080 mm to 0.670 mm from high to low-grade oil shale. The organic material consists mainly of lamalginite, and yields up to 341 L of oil/MT (81.8 gal/ton) on retorting. Fish and vascular-plant fragments are the main megafossils.

Mappable oil shale sequences 10 m (33 ft) thick are interstratified with marlstone-sandstone sequences 70 m (230 ft) thick. Markov chain analysis applied to stratigraphic sequences indicates that low-grade oil shale passes gradually into high-grade oil shale, and vice versa. The strata are interpreted as the deposits of extensive fresh to brackish lakes, with oil shale sequences formed in perennial, stratified lakes and marlstone-sandstone sequences formed in shallow perennial lakes and subaerially exposed flats. Dolomite in the oil shales was authigenic. The systematic changes in mineralogy and geochemistry between rock types probably reflect a gradient in redox potential from the center to the margins of the lake. Sandstone and siltstone beds and some carbonate laminae are interpret d as density flow deposits and form a significant proportion of the sequences. The episodic deposition of oil shales reflects changes in lake level within the enclosed basin, probably due to climatic fluctuations on a 24,000 to 46,000-year scale.