Carbonate source rock with low total organic carbon content and high maturity as effective source rock in China: A review.

Decrease in TOC of the carbonate source rock with low total organic carbon (CSRLTOC) contents with hydrocarbon generation and expulsion (a, b, c) and the lower limit of TOC (LLTOC) decreases (d) with the increasing T max or VR. • The CSRLTOC (carbonate source rocks with low total organic carbon) can be effective to oil/gas accumulation in China. • The LLTOC (lower limit of total organic carbon) for effective carbonate source rocks at various mature stages is determined. • Some oil/gas fields have derived partly or mostly from the CSRLTOC. • The CSRLTOC should be conducted for more in-depth studies in the future. Carbonate rocks with high maturity and low total organic carbon content are widely distributed in China. Whether they can be effective source rocks remains controversial, but they have great significance for the evaluation of hydrocarbon generation potential and exploration prospects of carbonate rocks. Dataset reveals that as the depth or maturity increases, total organic carbon (TOC) content, atomic H/C ratio, and hydrocarbon generation potential of carbonate source rocks with low total organic carbon (CSRLTOC) all decrease because of hydrocarbon expulsion. Moreover, some pyrolysis experiments also indicate that the CSRLTOC are able to generate and expel a quantity of hydrocarbons. The above studies indicate that CSRLTOC can be effective source rocks for oil/gas pools. The unique features of carbonate rocks and CSRLTOC demonstrate that they are easier to expel hydrocarbons and contribute to oil/gas reservoirs than shale, and thus, the lower limit of TOC (LLTOC) of effective carbonate source rocks could be smaller than shale (0.5%). The LLTOCs of effective carbonate source rocks with type I-II 1 kerogen (mainly sapropelic group) at the levels of immaturity and low maturity, maturity, high maturity and over maturity are TOC = 0.5%, 0.5–0.3%, 0.3–0.2%, and 0.2–0.1%, respectively. If the carbonate source rocks are at the high-over mature stage, we could take 0.2% as the LLTOC. The present organic-poor source rocks with high maturity are possibly moderate or good source rocks at the low mature level in the geological periods, especially carbonate source rocks if type I-II 1 kerogen. Some typical oil and gas fields in China derive partly or mostly from the CSRLTOC, proving that CSRLTOC could be effective source rock in China. [ABSTRACT FROM AUTHOR]