Abstract: The isolated Carboniferous carbonate platform in the southern margin of Pre-Caspian Basin is an important exploration target in the Caspian Sea area, Kazakhstan. Taking the representative K oil reservoir as the study object, thin sections, geochemical, logging, and geological data have been comprehensively used to evaluate the characteristics and oil and gas geological significance of the Carboniferous Visean–Bashkirian bitumen bearing carbonate reservoirs. The study results show that the distribution of bitumen was controlled by the open fracture system in high-energy platform margin and tectonic–diagenetic processes, while it was basically absent in the isolated pores in the platform zone. In response to the problem of overestimation of logging interpreted porosity caused by bitumen enrichment, a multi mineral quantitative inversion model constrained by bitumen physical phase has innovatively been constructed, in which bitumen is quantitatively characterized as a solid organic mineral component, achieving accurate calculation of bitumen content and significantly improving the accuracy of porosity interpretation (reducing average error by 20%). The geochemical indicators (T_max of 452–461 ℃, I_H<130 mg/g (HC/TOC)) indicate that bitumen was a product of high-temperature thermal cracking and possible TSR alteration, and its genesis might be related to local thermal anomalies in the context of regional tectonic–thermal events in the Hercynian period. The Bashkirian high abundance bitumen zone is a sign of the residual oil reservoir after cracking, which is mainly distributed in the high porosity and high permeability zone in the platform margin. The bottom boundary of bitumen occurrence reveals that the ancient oil-water contact is deeper than the current value, and the storage space system below the Visean oil reservoir unit indicates the potential preservation zone of deep primary oil reservoirs. This study elucidates the indicative significance of bitumen on the location of paleo oil reservoirs and reservoir heterogeneity, providing new ideas for fine evaluation of bitumen bearing carbonate reservoirs and petroleum exploration in deep formations.

Key words: bitumen, logging evaluation, paleo oil reservoir, carbonate platform, Pre-Caspian Basin