成熟探区有效烃源岩的评价方法——以辽东湾地区为例

油气勘探成熟区预测资源量与勘探发现不符,问题可能在于有效烃源岩(特别是深层烃源岩)及其油气生成潜力没有得到正确地评价。在辽东湾地区,首先利用地球化学和地球物理手段对各凹陷中5套烃源岩进行了分层评价和油气生成模拟实验,计算得到油气生成强度;然后利用烃源岩埋藏过程中油气的生成模型、孔隙度变化模型及其油气饱和度,计算得到油气排出率,并根据烃源岩与砂岩的不同组合形式对油气排出效率进行校正,得到油气排出强度;最后将油气生成强度和排出强度与油气田储量规模进行了相关分析,根据相关性确定有效烃源岩的性质、分布以及对油气聚集的控制规律。结果表明,在辽东湾地区,生油强度大于25×10~4t/km^2(排油强度大于15×10~4t/km^2)的烃源岩可以形成储量规模大于500×10~4t的油田,为有效烃源岩。其中,生油强度大于200×10~4t/km^2(排油强度大于125×10~4t/km^2)的烃源岩可形成储量规模大于5 000×10~4t的油田,为优质烃源岩;而当生油强度低于25×10~4t/km^2(排油强度小于15×10~4t/km^2)时,没有规模储量的油气田,此类烃源岩为非有效烃源岩。与以往以有机碳含量的下限值分析为核心的有效烃源岩评价不同,这种利用油气生成强度和排出强度开展有效烃源岩评价的方法有利于推动中国东部成熟探区的油气精细勘探。

Evaluation method of effective source rock in mature exploration area: a case study of Liaodong Bay

There are contradictions between the discovered oil and gas reserves and the predicted resources in mature exploration area. It may be because the effective source rocks (including deeply buried source rocks)and their hydrocarbon generation potential are not properly evaluated. Different from the previous evaluations focusing on the minimum of TOC in effective source rocks, a new evaluation method of the resource rocks in Liaodong Bay is proposed in this study. In this area, using geochemical and geophysical methods, the stratified evaluation and hydrocarbon generation simulation experiments are conducted on five sets of source rocks in each depression, so as to calculate out the oil and gas generation intensity. Then, the hydrocarbon generation model, porosity variation model and oil saturation in the source rock burial process are applied to calculate out the oil and gas expulsion rate, and the oil and gas expulsion efficiency is corrected according to the different assemblages of source rock and sandstone, so as to obtain the oil and gas expulsion intensity. Finally, the correlation analysis is made between hydrocarbon generation intensity, expulsion intensity and oil and gas field reserves; on this basis, this study determines the nature and distribution of effective source rocks and the control laws of oil and gas accumulation. This study shows that in Liaodong Bay, the source rocks with the oil generation intensity greater than 25×10⁴t/km², i.e., the oil expulsion intensity greater than 15×10⁴t/km², can accumulate and be developed into the oil field with reserves greater than 500×10⁴t, which are deemed as effective source rocks. Among these, the source rocks with the oil generation intensity greater than 200×10⁴t/km², i.e., the oil expulsion intensity greater than 125×10⁴t/km², can accumulate to form the oil field with reserves greater than 5 000×10⁴t, which are considered as high-quality source rocks. If the oil generation intensity is less than 25×10⁴t/km² and the oil expulsion intensity is less than 15×10⁴t/km², the source rocks are defined as non-effective source rocks, where no oil fields have been discovered. The evaluation method using hydrocarbon generation intensity and expulsion intensity for effective source rocks is favorable for further fine oil and gas exploration in mature exploration areas of East China.