苏北盆地高邮凹陷古近系阜宁组二段(阜二段)页岩油勘探获得重大突破,是下一步增储上产的重要领域。页岩层系中天然裂缝发育复杂,其对页岩油富集和保存的影响认识不清,制约了下一步勘探部署。综合来自HY1、H101、HY7、S85X等井的岩心、录井、测井和镜下薄片、扫描电镜、大视域扫描电镜拼接(Maps)、微区矿物扫描(Qemscan)、冷冻岩心有机地球化学分析等资料,结合HY1HF、HY3HF等井的生产动态资料,系统分析了阜二段页岩层系中天然裂缝发育特征,进一步揭示了不同类型天然裂缝对页岩油富集和保存的影响。结果表明,阜二段页岩层系主要发育穿层构造裂缝和层控构造裂缝,以及层理缝和收缩缝等非构造裂缝。其中,穿层构造裂缝发育程度与岩性及构造部位密切相关;层控构造裂缝主要发育在白云石条带、砂质/白云质混合条带和顺层方解石脉等脆性层;层理缝主要为异常高压成因,发育集中在阜二段中下部Ⅲ~Ⅴ亚段。层理缝与顺层方解石脉、白云石条带和砂质/白云质混合条带中发育的层控构造-溶蚀缝洞既是页岩油的有利储集空间,也是高效渗流通道,有利于页岩油富集;穿层构造裂缝有效性较好,相对于页岩的基质更为富集页岩油(S₁),但其纵横向连通性较好,因而也是页岩油逸散的高效通道,对阜二段Ⅴ亚段“Ⅴ下”甜点层保存条件不利,对Ⅳ亚段甜点层影响较小,平面上与断层距离越小,断层规模越大,构造裂缝越发育且越不利于页岩油保存。

Significant breakthrough has been made in shale oil exploration in Member 2 of the Funing Formation in Gaoyou sag, Subei Basin, a key area for increasing petroleum reserve and production. The advancement of shale oil exploration in the study area, however, is hindered by a lack of understanding of the complex fracture system and its impact on shale oil enrichment and preservation. In this contribution, we systematically analyze the characteristics of nature fractures using core logging, thin section, scanning electron microscopy, and Maps and Qemscan methods; the impact of fracture types on shale oil enrichment and preservation are further revealed through organogeochemical analysis of frozen cores, combined with well production data. Structural fractures such as layer-perpendicular and layer-controlled fractures are widely developed in shales of Mmember 2 of the Funing Formation, as well as nonstructural fractures such as bedding fractures and compression fractures. The development of layer-perpendicular fractures is controlled by lithology and structural location, while layer-controlled fractures mainly develop in brittle layers such as dolomite bands, sand/dolomite mixed bands, and bedding calcite veins. The bedding fractures are mainly caused by abnormal high pressure and are concentrated in the middle and lower parts of the study area. The bedding factures and the layer-controlled fractures and caves are conducive to shale oil enrichment, as they are not only favorable storage spaces but also efficient flow channels. Layer-perpendicular fractures show higher effectiveness of shale oil (S₁) enrichment compared to shale matrix; however, they can also be efficient oil seepage channels due to vertical and horizontal connectivity. This causes unfavorable preservation condition at the sweet spot “lower Ⅴ” in subsegment Ⅴ, but has less impact on subsegment Ⅳ. In general, the smaller the planar distance from the fault and the larger the fault scale is, the more developed and unfavorable the structural fractures are for shale oil preservation.