关于古生界烃源岩有机质丰度的评价标准

我国海相碳酸盐岩地球化学评价的焦点问题之一，是如何确定有其有机质丰度下限和划分有效烃源岩与非烃源岩的标准。长期以来，国内一些颇具影响的论点使人们相信，有机碳含量低到0．1％的碳酸盐岩仍能作为有效烃源岩。然而，国内外大量的勘探实践和实验室研究表明，有机碳含量为0．1％－0．2％的纯碳酸盐岩不能作为有效烃源岩，否则，有机质丰度不会成为油气勘探评价中的一个限定因素。在海相地层或海相碳酸盐岩中，能有效生烃的烃源岩是其中有机质丰度高的泥岩、泥灰岩和泥晶灰岩。碳酸盐岩或泥质岩中相同类型干酪根的有机质转化率相似，不存在碳酸盐烃源岩烃转化率高于泥岩的现象；生排烃过程一般并不会有机碳含量显著降低。因此，评价海相地层或碳酸盐岩地层中的烃源岩，沿用泥岩的有机质丰度下限值（有机碳含量为0．5％）是适合的。有效烃源岩的厚度不必很大，但有机质丰度必须高（有机碳含量大于0．5％），且要有一定的分布范围。

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.