Permian Lucaogou Formation in Jinan Depression, Junggar Basin Geochemical characteristics and oil source analysis of source rocks
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摘要: 【目的】近年,准噶尔盆地吉南凹陷井井子沟组常规砂岩油藏勘探取得重要进展,三级储量过亿吨,勘探潜力巨大,中二叠统芦草沟组作为凹陷内主力源岩层,目前研究相对薄弱。【方法】利用多项地球化学测试手段,对芦草沟组不同层段烃源岩地球化学特征进行对比,并据此厘定了二叠系油气来源。【结果】结果表明:①依据测井曲线特征可将芦草沟组分为三段,其中芦一、芦二段发育“优质”质量烃源岩,母质类型为Ⅰ-Ⅱ1型,处于低熟~成熟阶段,且芦二段生烃能力更强;芦三段主体为“非~一般”烃源岩,母质类型亦为Ⅰ-Ⅱ1型,受上覆火成岩影响,局部成熟度较高。②基于分子地球化学特征,芦草沟组沉积期整体处于偏还原环境中,有机质保存条件较好,但不同层段水体咸化程度有异,芦一段>芦三段>芦二段。在生物组成方面,证明了芦草沟组存在绿藻和蓝细菌(蓝藻)两种特征生烃母质,前者证据是与年代相关的生物标志物(C28甾烷),后者为β-胡萝卜烷和中链单甲基烷烃的检出,其中芦一、芦三段成烃生物中显示出蓝藻的主要贡献,芦二段则具有较高的绿藻和陆源高等植物输入,湖盆水体盐度的差异是影响蓝、绿藻繁盛的关键因素。③芦一和芦二段烃源岩具有特征的高丰度C30 αα-藿烷(受控于热演化程度),C30 αα/αβ有望成为判识成熟度的新指标。④吉南凹陷二叠系存在三种类别不同的原油,Ⅰ、Ⅱ类原油分别对应芦一和芦二段烃源岩,Ⅲ类原油显示非芦草沟组油源特征,主要来自井井子沟组上部泥岩。研究对吉南凹陷未来油气勘探与开发具有参考意义。Abstract: [Objective] In recent years, significant progress has been made in the exploration of conventional sandstone reservoirs in Jingjingzigou Formation, Jinan Depression, Junggar Basin, with tertiary reserves exceeding 100 million tons and excellent exploration prospect. Although the main source rock as Lucaogou Formation of Middle Permian in Jinan Depression has been realized based on former research, the study on geochemical characteristics of source rocks and oil source correlation have been rarely reported. [Methods] Based on comprehensive utilization of test data such as total organic carbon, rock pyrolysis and characteristics of biomarkers,The geochemical characteristics of source rocks in different intervals of Lucaogou Formation and oils source correlation of Middle Permian have been studied in this paper. [Results]The results show that:①Lucaogou Formation can be divided into three sections according to the characteristics of logging curves. P2l1 and P2l2 are excellent with high organic carbon content, hydrocarbon generation potential and soluble organic matter content, organic matter type asⅠ-Ⅱ1 and maturity as early to peak, while P2l2 is better. The main of P2l3 is "non-general" source rock withⅠ-Ⅱ1 organic matter type and high maturity affected by overlying.②General reduced environments with P2l1 as stratified saline water and P2l2 as fresh-brackish water, P2l3 is somewhere in between.beneficial to the preservation of organic matter are indicated by the molecular geochemical characteristics of source rocks. In terms of biological composition, it is proved that there are two characteristic hydrocarbon-generating parent materials in Lucaogou Formation, namely, green algae and cyanobacteria. The former evidence is a biomarker (C28 sterane) related to age, while the latter is the detection of β -carotene and medium-chain monomethyl alkane, Hydrocarbon-generating organisms in different intervals are different with P2l1 and P2l3 mainly composed of aquatic organisms such as bacteria and algae, and cyanobacteria dominate, while P2l2 of high input of aquatic plants(green algae) and terrestrial plants. Differences in water salinity in lake basins are the key factors that affect the bloom of cyanobacteria and green algae in different layers of source rocks.③C30αα- hopane was abundant in P2l1 and P2l2, but is lacking in the source rocks of P2l3, Based on the analysis of different isomerization parameters of sterane and hopane,C30αα/αβ may be a possibe maturity parameter.④the Permian crude oil in Jinan Depression can be divided into three types. Type Ⅰ and Ⅱcrude oils were correspond to the source rocks of P2l1 and P2l2 respectively, and Type Ⅲ crude oil is different to P2l source rocks, possibly correlated tomudstone of Jingjingzigou formation. The research has reference significance for future oil and gas exploration and development in Jinan sag.
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