四川盆地震旦系陡山沱组烃源岩地球化学特征与有机质富集机制

Geochemical Characteristics of the Sinian Doushantuo Formation Source Rocks of the Sichuan Basin: Implications for the Organic Matter Accumulation

深层古老层系在油气勘探中的重要性正越来越受到重视,四川盆地震旦系陡山沱组是中国南方这一领域的潜在烃源岩层系,但过去因关注少而研究程度低。为加深对其理解,文章以样品条件较好的四川盆地东北缘城口明月乡剖面为例,采用岩石学、主、微量及稀土元素等综合分析的方法,系统探讨了陡山沱组泥页岩的地球化学特征和有机质富集机制。结果表明,陡山沱组发育了一套富有机质的泥页岩,主体沉积于次氧化—还原水体环境,显著的Eu正异常表明沉积时受热液活动影响明显。烃源岩有机质富集显著受控于沉积环境,同时由于沉积时整体处于较低的沉积速率,使得自下而上随沉积速率增大,有机质逐渐富集。此外,热液活动促进了有机质富集。陡山沱组具有资源潜力,但非均质性发育,可能与冰期—间冰期的海平面升降有关。

Deep ancient strata system is getting more and more attention in the field of hydrocarbon exploration,among which the Sinian Doushantuo Formation of the Sichuan Basin is considered as one of the most potential source rock.During the past decades,few work aimed at this formation had been taken.To deepen understanding of the Doushantuo Formation as a set of potential source rock,taking well-sampled Minyuexiang Section of Chengkou County of northeastern of Sichuan Basin as a case,the geochemical characteristics and organic matter accumulation mechanism of Doushantuo shale are systematically discussed by means of comprehensive analysis of petrology,major,trace and rare earth elements.The results show that the organic-rich black shale developed in the Doushantuo Formation was mainly deposited in suboxic-anoxic environment,and significant positive Eu anomalies indicate that the Doushantuo Formation wasinfluenced by hydrothermal activity during the sedimentation process.The organic matter accumulation is strongly controlled by the anoxic sedimentary environment.Due to the low sedimentation rate,the organic matter gradually enriched with the increase of deposition rate from bottom to top.In addition,hydrothermal activity led to the elevated accumulation and burial of organic matter.Further,the Doushantuo Formation has certain oil and gas potential,but its heterogeneity is developed,which may be related to the eustatic change during the glacial-interglacial period.