泥页岩沉积特征与生烃能力——以东营凹陷沙河街组三段中亚段—沙河街组四段上亚段为例

泥页岩的沉积环境与形成方式都将控制其矿物、有机质特征并影响岩石的生烃潜力。选取东营凹陷古近系沙河街组泥页岩,开展岩石薄片观察、X射线衍射和热解等检测,研究岩石的矿物学特征和有机质特征,解析各类泥页岩成因与生烃能力的差异。研究发现,研究区的块状、纹层状和含石膏构造泥页岩中构造特征各异,矿物成分(碎屑、黏土、碳酸盐矿物和石膏类矿物等)含量变化大,有机质特征也各不相同。综合分析矿物与有机质的特征及组合关系,发现细纹层页岩和断续纹层页岩是以生物作用为主导形成的沉积产物,生烃能力最强;宽纹层页岩和均匀块状泥岩属化学沉积作用的产物,具有较强的生烃能力;含碎屑块状泥岩主要通过机械作用沉积而成,含石膏构造泥岩则在蒸发环境下通过化学沉淀形成,两者的生烃能力较弱。比较东营凹陷沙河街组三段中亚段—沙河街组四段上亚段的泥页岩类型,发现沙河街组三段中亚段和沙河街组四段纯下次亚段以机械和化学作用形成的含碎屑块状和含石膏构造泥岩为主,导致2个层段的生烃能力较弱;沙河街组三段下亚段和沙河街组四段纯上次亚段以生物或化学沉积作用形成的纹层状页岩为主,导致2个层段的生烃能力较强。因此,关注泥页岩矿物和有机质组合关系,从机械、化学和生物等沉积方式差异的角度探讨各类泥页岩的成因和生烃能力的变化,能够为非常规油气勘探提供新的思路。

Sedimentary characteristics and hydrocarbon generation potential of mudstone and shale: a case study of Middle Submember of Member 3 and Upper Submember of Member 4 in Shahejie Formation in Dongying sag

Sedimentary environment and formation mode of mudstone and shale are able to control its mineral and organic matter characteristics as well as influence the hydrocarbon generation potential of rocks. The Paleogene mudstone and shale in Shanghejie Formation, Dongying sag was selected to carry out rock thin-section observation, X-ray diffusion, thermal decomposition and other detection methods, so as to study the mineral and organic matter characteristics of rocks as well as analyze the differences in genesis and hydrocarbon generation potential of various mudstone and shales. Results show that massive, layered and gypsum-bearing mudstone and shale have diversified laminated characteristics in the study area, where the content of mineral components (including clastics, clay, carbonate and gypsum minerals) show great changes and organic matter characteristics have also been diversified. Through comprehensive analysis on mineral and organic matter characteristics as well as their assemblage relationships, it was found that the fine-laminated shale and discontinuously-laminated shale were the products mainly formed by biological deposition with the highest hydrocarbon generation potential. The wide-laminated shale and homogenous-massive mudstone were the products of chemical deposition with higher hydrocarbon generation potential. The clastic-massive mudstone was mainly formed by mechanic deposition, while the gypsum-bearing mudstone was generated by chemical deposition in an evaporitic environment, and both showed weaker hydrocarbon generation potential. Comparing the mudstone and shale types in the Middle Submember of Member 3 and the Upper Submember of Member 4 of Shahejie Formation, Dongying sag, it was detected that the Middle Submember of Member 3 and Chunxia Submember of Member 4 were dominated by clastic-massive and gypsum-bearing mudstone and shale formed due to mechanic and chemical effect, leading to weaker hydrocarbon generation potential. The Lower Submember of Member 3 and Chunshang Submember of Member 4 of Shahejie Formation, Dongying sag were dominated by laminated mudstone and shale formed due to biological or chemical deposition, leading to higher hydrocarbon generation potential. Therefore, the focus on mineral-organic matter assemblage relationships of mudstone and shale as well as the changes in the genesis and hydrocarbon generation potential of various mudstone and shales from the perspectives of mechanic, chemical and biological deposition modes is able to provide a new idea for unconventional hydrocarbon exploration.