Abstract: The origin of dolomite is a long-term argument in geology. In recent years, protodolomite has been discovered in hydrocarbon seeps, which provides critical evidence for solving the problem. Therefore, the authigenic carbonate found in hydrocarbon seeps is regarded as a kind of unique geochemical archives of fluid seeping in history. The carbonates studied in this paper occur as chimneys or lenses within the shale of Early Pliocene, the Yanshuikeng Formation, exposed at Chiahsien, southwestern Taiwan. Petrological and geochemical tests, such as petrography, mineralogy, carbon and oxygen stable isotopes, and rare earth element (REE) compositions, are made to trace the fluid sources and sedimentary environment. X-ray analyses suggest that the carbonate is mainly composed of dolomite (61.4%～88.0%). The δ¹³C values of dolomites range from −27.08‰ V-PDB to −10.58‰ V-PDB, suggesting a thermogenic methane and seawater associated carbon sources. The value of Ni/Co is around 12.54, and the original REE distribution shows a pattern of enrichment in the middle without Ce anomalies. All these results indicate that the Chiahsien dolomite was precipitated from reducing seep fluids. Under the action of microorganisms in the sub-reducing environment, HCO₃⁻ is increased and the SO₄²⁻ more consumed, which is in favor of the formation of seep dolomites.