Tracing fluid sources and formation conditions of Miocene hydrocarbon-seep carbonates in the central Western Foothills, Central Taiwan.

Highlights • Seep carbonates are hotsted in Miocene marine mudstone of the Kuohsing area, Taiwan. • Carbon isotope signals were strongly altered, but REE compositions remain unaffected. • Negative δ13C of −47.6‰ was recognized as a near-primary isotopic signature. • Carbonates show MREE enrichment, no La and Ce anomalies, and supra-chondritic Y/Ho. • Carbonates were formed in reducing seep fluids with limited admixture of seawater. Abstract Hydrocarbon-seep carbonates were discovered to be hosted within Miocene marine mudstone in the central Western Foothills near the Kuohsing area, Central Taiwan. Here, we present field observations, petrography, mineralogy, stable carbon isotope data, as well as rare earth element compositions. We evaluate the role of diagenetic alteration on the Kuohsing seep carbonates and constrain their fluid sources and formation conditions. Four main morphologies in the field were observed, including cylindrical columns, massive forms, lenticular nodules, and stratiform bodies. Petrographically, the carbonates are predominantly composed of partially recrystallized calcite and dolomite, with abundant amounts of biogenic detritus, pyrite aggregations and unidentified filaments. Isotopic analyses show that carbon isotope values inversely vary with oxygen isotope values, indicating substantial modification of the carbon isotope compositions of the carbonates by diagenetic processes. Extremely negative δ13C values as low as −47.6‰ recognized as a near-primary isotopic signature recorded in the carbonates, an end member of the trend, reveals that methane was incorporated during the carbonate precipitation. On the other hand, the carbonates have well preserved primary REE compositions, exhibiting MREE enrichment, no La and Ce anomalies, and supra-chondritic Y/Ho ratios. These REE patterns suggest that the Kuohsing authigenic carbonates were precipitated from reducing seep fluids with limited admixture of seawater. [ABSTRACT FROM AUTHOR]