Evaluation criteria for Paleozoic effective hydrocarbon source rocks

One of the focal points for geochemically assessing marine carbonate rocks for a long time in China is how to identify the threshold value of organic enrichment of efficient hydrocarbon source rocks. Some of the considerable views from Chinese authors spur people to believe that the carbonate rocks with TOC content low down to 0.1 % (wt. ) still could be as efficient source rocks. These views are as follows: (1) non-reservoir carbonate rocks typically contain less organic matter than shales but have higher hydrocarbon conversion, therefore, carbonate should have less critical source rock threshold value; (2) low organic enrichment in the present carbonate rocks in China is as a result of higher levels of thermal maturity, and so sample's initial characteristics should be recovered; and (3) the huge thickness of a sedimentary rock can compensate for low levels of organic enrichment. However, subsequent studies including many practical exploration investigations and laboratory simulation experiments do not completely support these views above. The carbonate rocks with TOC content low to 0. 1 % - 0.2%( wt. ) could not absolutely be as potential and/or efficient source rocks. Otherwise, hydrocarbon source rocks are largely ubiquitous and should never be a major limiting factor within exploratory programs. The organic-rich rocks within marine stratigraphic intervals or marine carbonates should mainly be shale, mudstone, marl and micrite limestone instead of pure carbonate. Geochemical studies have shown that (1) the same types of organic matter have similar hydrocarbon yield and no the case that carbonate source rocks have higher conversion than that of mudstone; (2) generation and expulsion process of hydrocarbon could generally not cause the organic enrichment down obviously for those rocks containing less 1. 0% (wt. ) of organic carbon content deposited in epicontinental sea environment; and (3) expulsion efficiency decreases with increasing gross source rock thickness. Therefore, it is concluded that threshold value of organic enrichment for a carbonate petroleum source rock could not below that for shale (0. 5 % (wt. TOC)), and threshold should be independent of source rock lithology. Efficient source rocks need not too large in thickness, but must contain higher organic enrichment ( 0. 5% (wt. TOC)) intervals and have a certain distribution scope.