Abstract:

This study investigates the shale of the upper sub-member of Member 4 of Shahejie Formation in the Minfeng subsag,Dongying sag,integrating core and thin-section observations with elemental geochemistry to reconstruct paleoenvironmental conditions. An innovative multi-method analytical framework was employed—including scanning electron microscopy coupled with cathodoluminescence(SEM-CL),focused ion beam-scanning electron microscopy(FIB-SEM),laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS),stable isotope(C,O,Sr)analysis,fluid inclusion microthermometry,and U-Pb geochronology—to systematically characterize diagenetic mineral evolution and elucidate the reservoir formation mechanism. Results indicate that deposition of the target interval occurred under an arid to semi-humid climate,within a fine-grained,brackish-deep lacustrine setting characterized by strong redox stratification. High-frequency vertical cyclicity is evident,reflecting a progressive facies transition from carbonate-rich mixed shale → mixed shale → feldspar-rich mixed shale. Total organic carbon(TOC)ranges predominantly between 1.8wt% and 6.0 wt%,with abundant organic matter accumulation attributed primarily to high sedimentation rates and robust terrigenous and/or endogenous organic input—rather than prolonged bottom-water anoxia alone. Granular calcite and fibrous calcite exhibit distinct geochemical signatures: divergent rare earth element(REE)patterns,contrasting δ¹³C and δ¹⁸O values,and significantly different U-Pb ages. Notably,columnar-to-fibrous calcite laminae oriented near-horizontally yield a broad U-Pb age spectrum(31.2±4.3 Ma to 25.1±4.2 Ma),spanning the full early-to-middle diagenetic interval. Illite comprises 80%~100% of the clay mineral assemblage;its formation is dominantly controlled by montmorillonite-to-illite transformation,while feldspar dissolution represents a major silica source for authigenic quartz precipitation. Authigenic quartz is ubiquitously observed within argillaceous laminae and calcite laminae. Under overpressured conditions,co-precipitation of authigenic quartz and recrystallized calcite effectively buffered porosity loss during cementation—thereby preserving critical pore space and facilitating subsequent pore enhancement via acidic fluid dissolution during later diagenesis. This interplay constitutes a key reservoir-quality preservation and improvement mechanism in deeply buried lacustrine shales.

Key words: fine-grained sedimentary environment, diagenetic evolution, mixed shale, Minfeng subsag, Dongying sag

摘要：

以济阳坳陷东营凹陷民丰洼陷沙河街组四段上亚段(简称“沙四上亚段”)页岩为研究对象,利用岩心观察、薄片观察和元素分析等方法恢复沉积古环境,创新融合SEM-CL、FIB-SEM、LA-ICP-MS、同位素、包裹体及U-Pb定年等方法,明确页岩典型成岩矿物演化特征和成储机制。结果表明: 民丰洼陷沙四上亚段页岩沉积期属于干旱—半湿润气候,总体为咸水、深湖、强还原的细粒沉积环境,纵向高频旋回性特征显著,呈现由碳酸盐质混合页岩占主导到三端元矿物均匀混合页岩、再到长英质混合页岩占比增加的变化特点; TOC集中在1.8%~6%,高沉积速率、强物源供给使有机质大量富集而不完全依赖长期缺氧保存环境; 颗粒状方解石的稀土元素分配特征、碳氧同位素值以及U-Pb年龄等与纤维状方解石存在较明显差异,总体上,近水平方向柱纤状方解石纹层表现出较宽幅的年龄跨度(31.2±4.3~25.1±4.2 Ma),对应早成岩—中成岩的完整演化阶段; 自生石英在泥质纹层和方解石纹层中普遍发育,蒙脱石向伊利石转化(伊利石含量集中在80%~100%)、长石等硅酸盐矿物溶解释放的硅质是页岩自生石英主要的硅质来源; 在超压背景下,自生石英与重结晶方解石协同维持孔隙空间,缓解了因过度胶结对孔隙保存的破坏作用,为成岩体系偏酸性流体的溶蚀增孔提供了有利成储条件。

关键词: 细粒沉积环境, 成岩演化, 混合页岩, 民丰洼陷, 东营凹陷

CLC Number: 

• P618.130.2

• TE122.2