优质烃源岩评价标准及其应用: 以海拉尔盆地乌尔逊凹陷为例

“优质烃源岩控藏”的概念自提出后，正在逐步为勘探家所接受.但什么是优质烃源岩以及如何界定，则还缺乏共识和统一标准.针对这一难题，以海拉尔盆地乌尔逊凹陷为例，利用物质平衡原理计算的源岩排烃量与有机质丰度(TOC)关系曲线的拐点来确定优质烃源岩的有无及其下限标准.结果表明，在TOC较低时，所有源岩的排烃量有限，难以成为优质烃源岩；当TOC升高到某一阀值时，排烃量随着TOC的升高出现明显的增大.这一明显增大的拐点应该为优质烃源岩的下限，TOC高于该值的源岩即为优质烃源岩.虽然优质烃源岩的下限还与有机质的类型、成熟度等因素有关，但为简明实用，综合定量评价认识，推荐TOC＝2.0%作为确定优质油源岩的下限.该标准在研究区的应用表明，区内南二段、南一段优质烃源岩的生油贡献比例达到71%和87%，排油贡献比例更是高达85%和94%，优质源岩的分布与区内油藏的分布也有非常好的对应关系，证明了优质源岩的控藏作用. 

Evaluation Criteria of High-Quality Source Rocks and Its Applications: Taking the Wuerxun Sag in Hailaer Basin as an Example

The concept of "high-quality source-rock-control hydrocarbon accumulation" is being accepted gradually by explorers since being put forward. No evaluation criteria based on solid evidences such as the definition of high-quality source rocks and its identification method have been brought forward so far. To tackle this issue, we have identified in this paper, by taking the Wuerxun sag in Hailaer basin as an example, the existence of high-quality source rocks and the lower limit for evaluation of high-quality source rocks, based on the inflection point on the relationship curve of hydrocarbon expulsion (which is calculated by the material balance principle) versus total organic carbon (TOC). The results show that when TOC is low, all source rocks have limited hydrocarbon expulsion, thus they cannot be high-quality source rocks. Nonetheless, when TOC rises to some threshold, hydrocarbon expulsion increases significantly with TOC. This inflection point should be the lower limit of high-quality source rocks: those with TOC greater than the inflection point value are high-quality source rocks. For the sake of simplicity and practicability, it is recommended based on the recognition of quantitative evaluation that TOC=2.0% is regarded as the lower limit of high-quality source rocks, although the lower limit of high-quality source rocks is also related to the type and maturity of the organic matters in the source rocks and so on. The application of such criteria in the research area indicates that the high-quality source rocks in K₁n2 and K₁n1 contribute 71% and 87% to the oil generated respectively, and 85% and 94% to the oil expelled respectively. The distribution of high-quality source rocks is also closely related to the distribution of hydrocarbon reservoirs in the region, demonstrating that high-quality source rocks control hydrocarbon accumulation.