Organic Matter Preservation and Hydrocarbon Potential of Microbialites: Insights From TOC Reconstruction in the Leikoupo and Dengying Formations, Sichuan Basin

Abstract

ABSTRACT Microbialites vary widely in organic matter abundance and preservation, making their hydrocarbon‐generating potential uncertain. Here, we compare microbialites from two contrasting depositional settings in the northwestern Sichuan Basin: the Middle Triassic Leikoupo Formation, deposited on a restricted evaporitic carbonate platform, and the late Ediacaran Dengying Formation, deposited on a relatively open shallow‐water carbonate platform. We integrate petrographic observations, inorganic and organic geochemical data, microbial cultivation experiments and thermal simulations to evaluate organic matter preservation and semi‐quantitatively reconstruct primary total organic carbon (TOC). The Leikoupo microbialites formed under more saline, anoxic, lower‐energy and more productive conditions, whereas the Dengying microbialites formed under less saline, more oxic, higher‐energy and less productive conditions. These contrasting environments favoured better organic matter preservation in the Leikoupo Formation. Residual TOC in the Leikoupo microbialites ranges from 0.04% to 1.20% (mean 0.25%), whereas reconstructed primary TOC averages 0.43%. This difference suggests that thermal evolution depleted the original organic carbon inventory and that present‐day TOC underestimates primary organic matter abundance. The Dengying microbialites contain less organic matter and record poorer preservation. Organic matter in the Leikoupo microbialites is predominantly microbial in origin and has a relatively high hydrocarbon conversion efficiency. Overall, salinity, redox conditions, hydrodynamic energy and paleoproductivity jointly controlled organic matter enrichment and preservation. Microbialites deposited in restricted evaporitic settings may have acted as localized hydrocarbon‐generating intervals, although their regional contribution was probably limited.