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Evaluating the pre-Jurassic marine source rocks in China has been difficult because these rocks are generally too high- or over-maturated for most traditional methods to work. As to the remaining parameter TOC (%), its lower limit for recognizing the carbonate source rocks in China has been in dispute. Nineteen Phanerozoic sections in the Middle-Upper Yangtze Platform and the Guizhou-Hunan-Guangxi Basin have been studied in search for a different approach to complementing the traditional evaluation method for these source rocks. We have applied a geobiological approach to tracing the organic carbon (OC) output and accumulation from the living stage (primary productivity) to the post-mortem deposited remains, and finally to the preserved burial organics. Four biological and geological parameters are employed to represent the OC of the three stages. A series of proxies of these parameters are discussed and integrated to establish a geobiological evaluation system independent of TOC and other traditional methods. Here we use the Guangyuan section in Sichuan as an example for the geobiological evaluation. Our results indicate that in the argillaceous rocks, the geobiological parameters show the qualified source rocks in accordance with high TOC values; but in the carbonates, the good source rocks delineated by the geobiological parameters have a wide range of TOC, from 0.03% to 1.59%, mostly <0.3%. We suggest that it is still premature to set TOC=0.3% or 0.5% as the lower limit for the pre-Jurassic carbonate source rocks in South China. [ABSTRACT FROM AUTHOR]

The article explores the use of the geobiological approach in the assessment of Chinese carbonate source rocks. A series of paleoecological and biogeochemical parameters were employed to represented organic carbon (OC). It notes that clastic rocks based on geobiological parameters are qualified source rocks, while good source rocks from carbonates have low total organic carbon (TOC) values. It also notes the broad range of habitat types for carbonate source rocks in the Yangtze platform.