Abstract

Taking the inter-salt organic-rich shales in the third member of Paleogene Shahejie Formation (Es₃) of Dongpu sag in Bohai Bay Basin as an example, the origin of overpressure, development characteristics, formation and evolution mechanism, formation stages and geological significance on shale oil and gas of overpressure fractures in the inter-salt shale reservoir were investigated by means of thin section identification, scanning electron microscopy observation, analysis of fluid inclusions, logging data analysis, and formation pressure inversion. The results show that overpressure is universal in the salt-lake basin of Dongpu sag, and under-compaction caused by the sealing of salt-gypsum layer, pressurization due to hydrocarbon generation, transformation and dehydration of clay minerals, and fault sealing are the 4 main factors leading to the occurrence of overpressure. The overpressure fractures are small in scale, with an average length of 356.2 μm and an average underground opening of 11.6 μm. But they are densely developed, with an average surface density of 0.76 cm/cm². Moreover, they are often accompanied by oil and gas charging, and thus high in effectiveness. Overpressure fractures were mainly formed during two periods of large-scale oil and gas charging, approximately 25–30 Ma ago and 0–5 Ma ago. Inter-salt overpressure fractures play dual roles as the storage space and migration paths of shale oil and gas. They contribute 22.3% to the porosity of shale reservoir and 51.4% to the permeability. They can connect fracture systems of multiple scales, greatly improving the quality of shale reservoir. During the development of shale oil and gas, inter-salt overpressure fractures can affect the extension and morphology of hydraulic fractures, giving rise to complex and highly permeable volumetric fracture networks, improving hydraulic fracturing effect and enhancing shale oil and gas productivity.

中文翻译：

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盐间超压裂缝的形成及其对页岩油气的意义-以渤海湾盆地东圃凹陷古近系沙河街组第三段为例

摘要

取古近纪沙河街组第三段盐分间富含有机质页岩（ES ₃以渤海湾盆地东圃凹陷为例，通过盐分法研究了盐间页岩储层超压裂缝的页岩油气超压成因，发育特征，形成演化机理，形成阶段和地质意义。薄层识别，扫描电子显微镜观察，流体包裹体分析，测井数据分析和地层压力反演。结果表明，东圃凹陷盐湖盆地普遍存在超压，盐石膏层封闭，烃类生烃增压，粘土矿物转化脱水引起的超压实和断层封闭是其中的4。导致超压发生的主要因素。超压裂缝的规模很小，平均长度为356.2μm，平均地下开口为11.6μm。但是它们密度很高，平均表面密度为0.76 cm / cm²。而且，它们经常伴随着油气的充注，因此效率很高。过压裂缝主要是在大规模油气充注的两个时期形成的，大约在25-30 Ma之前和0-5 Ma之前。盐间超压裂缝在页岩油气的储藏空间和运移路径中起着双重作用。它们对页岩储层的孔隙度贡献了22.3％，对渗透率贡献了51.4％。它们可以连接多个规模的裂缝系统，从而大大提高了页岩储层的质量。在页岩油气开发过程中，盐间超压裂缝会影响水力裂缝的延伸和形态，形成复杂，高渗透性的体积裂缝网络，提高水力压裂效果，提高页岩油气产能。