玛湖凹陷风城组碱湖烃源岩生烃机理及资源量新认识

Fengcheng Alkaline Lacustrine Source Rocks of Lower Permian in Mahu Sag in Junggar Basin:Hydrocarbon Generation Mechanism and Petroleum Resources Reestimation

碱湖烃源岩的生烃特征和机理是学科研究的前缘与难点。以准噶尔盆地玛湖凹陷下二叠统风城组为例,采用烃源岩人工剖面、自然剖面、油气特征标定相结合的方法,对这套碱湖烃源岩的生烃特征和机理进行了研究,并据此重新计算了资源量。结果表明,风城组碱湖烃源岩的生烃特征突出表现为转化率高、连续生烃、多期高峰、生油窗长、油质轻、油多气少,不同于传统的湖相优质烃源岩。这种独特的生烃机理在于烃源岩的有机和无机组成2方面。风城组碱湖烃源岩的生烃母质以菌、藻类为主,特别是菌类发育,因此生烃具有油多气少、转化率高、连续生烃、油质轻的特色;风城组的矿物组成独特性在于兼具发育碱类矿物和火山矿物,二者对生烃分别起延滞和催化作用,使得烃源岩可以早期生烃、持续生烃、多期高峰、生油窗长。基于这种特殊的生烃特征和机理,重新评估了研究区的资源量,生油量较前期认识提高了25%,生气量减少了13%,这更贴近于目前的勘探实际,对确定下步勘探开发方向具有重要意义。

Hydrocarbon generation characteristics and mechanism of alkaline-lacustrine source rocks are one of the research leading edges and difficulties. Based on a case study in the Lower Permian Fengcheng formation in the Mahu sag of Junggar basin, by using comprehansive methods of artificial section and natural outcrop of the source rocks as well as oil and gas characteristic calibration, the paper studies hydrocarbon generation characteristics and mechanism of the alkaline-lacustrine source rocks, based on which petroleum resources are reestimated. Results show that the Fengcheng alkaline-lacustrine source rocks exhibit some unique characteristics in hydrocarbon generation such as high conversion efficiency, continued hydrocarbon generation, multi-phase generation peaks, long oil-generation window, light oil and high oil to gas ratio, all of which are different from conventional lacustrine high quality source rocks. The unique hydrocarbon generation mechanism should be attributed to organic and inorganic compositions of the source rocks. The bioprecursors in the source rocks are dominated by bacteria and algae, with bacteria much more developed. The unique mineral composition of the Fengcheng formation is characterized by the existence of both alkaline and volcanic minerals that play retarding and catalytic roles on hydrocarbon generation, respectively, which allows early and continued hydrocarbon generation, multi-phase generation peaks and long oil-generation window of source rocks. According to these, the reestimation of petroleum resources in the study area is as follows： the oil generating volume is increased by 25% and the gas generating volume is decreased by 13%, which are more accordant with the present-day exploration results and have significance for making future exploration strategies .