南祁连盆地木里坳陷油气来源分析:基于DK-9孔岩心样品热模拟实验

Source Analysis of Oil and Gas Indication in Muli Depression of South Qilian Basin:A Thermal Simulation Case Study of DK-9 Core

南祁连盆地木里坳陷部署的多个天然气水合物钻孔钻遇不同程度的水合物与油气显示伴生现象,指示该地区具有良好的油气勘探前景,有必要对已发现油气显示进行来源分析。由于水合物钻孔深度有限,针对DK-9孔4组油气显示样品,在开展现有烃源岩油源对比基础上,选取中侏罗统、上三叠统各5组代表性低熟烃源岩样品进行热模拟实验,模拟深部烃源岩生、排烃过程,将新生烃类再次与油气显示进行对比,进一步探究油气显示来源。结果显示,油气显示可分为两类(Ⅰ和Ⅱ),第Ⅰ类油气显示遭受生物降解作用,成熟度稍高,第Ⅱ类油气显示成熟度稍低;现有烃源岩主要分为三种类型(Ⅰ-Ⅲ),分别对应深度163.30~207.42 m、207.42~348.50 m、357.90~586.50 m。结合常规油源对比、热模拟实验与地质条件分析,最终推测第Ⅰ类油气显示主要与第Ⅰ类烃源岩同源;第Ⅱ类油气显示主要与第Ⅱ类烃源岩同源,此外可能还有第Ⅲ类烃源岩或更深层烃源岩的贡献,即其母质来源既与中侏罗统烃源岩有关又与上三叠统烃源岩有关。

Multiple gas hydrate boreholes in the Muli Depression(South Qilian Basin) have encountered varying coexistence of hydrates and oil-gas, indicating good potential for oil and gas exploration. It is thus necessary to perform source analysis on the discovered oil and gas indications. In this study, four groups of oil and gas samples from borehole DK-9 were studied. Due to the limited drilling depth, on the basis of oil source comparison of existing source rocks, thermal simulation was conducted on five representative lowly-mature source rocks(Middle Jurassic and Upper Triassic) to constrain the hydrocarbon generation process. The results were then compared with the oil and gas to further constrain their source. The results reveal that the oil and gas indications can be divided into two types:(Ⅰ) those with biodegradation and are slightly more mature, and(Ⅱ) those that are slightly less mature. The source rocks comprise mainly three types, corresponding to 163.30-207.42 m, 207.42-348.50 m, and 357.90-586.50 m depths, respectively. Considering also the conventional oil source comparison, the thermal simulation and the geological condition analysis, it is concluded that type I oil and gas is mainly homologous with type Ⅰ source rocks. Type Ⅱ oil and gas may have had the same origin as type Ⅱ source rocks, and may also be contributed by type Ⅲ source rocks and/or deeper source rocks, i.e., its parent material is related to both the Middle Jurassic and the Upper Triassic source rocks.