Abstract:

In the Well Pen-1 West sag within the central depression of the Junggar Basin, the third member of the Permian Fengcheng Formation (Feng-3 Member) hosts a deeply buried clastic reservoir system that represents a key target for hydrocarbon exploration and reserve enhancement in the region. This study integrates core observations, well logs, cast thin sections, scanning electron microscopy (SEM), and high-pressure mercury intrusion (HPMI) analyses to systematically compare the petrological characteristics, reservoir properties, pore types, and microscopic pore structures of sandstone-conglomerate and siltstone reservoirs in the Feng-3 Member. The aim is to clarify the mechanisms controlling reservoir quality differentiation among clastic reservoirs with different grain sizes. The results reveal that sandstone-conglomerate reservoirs were characterized by relatively high initial porosity. Compaction was a primary cause of pore loss, while some samples were further modified by intensive cementation. As a result, subsequent dissolution contributed only limited porosity enhancement, and the average present-day porosity is approximately 7.14%. In contrast, siltstone reservoirs had relatively lower initial porosity and experienced stronger compaction. However, owing to weaker cementation, they underwent intense organic-acid dissolution driven by hydrocarbon-generation overpressure during deep burial, resulting in the development of a well-connected secondary dissolution pore system. The average porosity increase resulting from dissolution reached 8.27%, and the present-day porosity commonly exceeds 10%, significantly higher than that of the contemporaneous sandstone-conglomerate reservoirs. Reservoir quality in the Fengcheng Formation Member 3 is jointly controlled by rock fabric and diagenetic processes. Variations in depositional hydrodynamics produced distinct grain sizes, matrix contents, and grain-support structures, which determined the initial pore architecture and subsequent diagenetic evolution pathways. Siltstone reservoirs evolved along a “weak cementation-strong dissolution” pathway, generating abundant secondary pore space and substantially improving reservoir quality, whereas sandstone-conglomerate reservoirs followed a “strong cementation-weak dissolution” pathway and experienced progressive reservoir tightening. These findings reveal the intrinsic mechanisms governing the differential evolution of deeply buried clastic reservoirs with different grain sizes, enhance current understanding of reservoir enhancement in fine-grained clastic rocks, and provide a theoretical basis for reservoir evaluation and hydrocarbon exploration in the Fengcheng Formation and other analogous deep tight clastic reservoirs.

Key words: Well Pen-1 West Sag, Fengcheng Formation, diagenesis, pore evolution, differential evolution mechanism

摘要：

准噶尔盆地中央坳陷盆1井西凹陷二叠系风城组三段发育一套深埋碎屑岩储层，是区域油气增储上产的重要勘探目标。本文综合运用岩心、测井资料，以及铸体薄片、扫描电镜、高压压汞等分析方法，对风三段砂砾岩和粉砂岩两类储层的岩石学特征、物性特征、储集空间与微观孔隙结构展开了系统对比研究，探讨不同粒级碎屑岩储层品质差异的形成机制。结果表明，砂砾岩储层初始孔隙度较高，压实作用是其孔隙损失的重要原因，同时部分样品受到强烈胶结作用改造，后期溶蚀增孔作用相对有限，最终孔隙度约为7.14%；粉砂岩储层初始孔隙度相对较低，并经历更强烈的压实作用，但由于胶结作用较弱，在生烃增压驱动下发生强烈有机酸溶蚀，形成较发育的次生溶蚀孔隙系统，平均增孔量达8.27%，最终孔隙度提高至10%以上，明显优于同期砂砾岩储层。研究区风三段碎屑岩储层发育受岩石组构与成岩作用耦合控制。沉积动力差异形成了不同的颗粒粒度、杂基含量及支撑结构，决定了储层初始孔隙格架及后续成岩演化路径；压实作用和早期胶结作用控制原生粒间孔保存程度和后期流体可进入性，而晚期有机酸溶蚀则是改善储层物性的关键建设性成岩作用。粉砂岩储层沿“弱胶结—强溶蚀”路径演化，形成丰富的次生储集空间，有效提高储层品质；砂砾岩储层则沿“强胶结—弱溶蚀”路径持续致密化。上述认识揭示了深埋条件下不同粒级碎屑岩储层差异演化的内在机制，拓展了对细粒储层成岩增储过程的认识，可为准噶尔盆地风城组及类似深层致密碎屑岩储层评价与勘探提供理论依据。

关键词: 盆1井西凹陷, 风城组, 成岩作用, 孔隙演化, 差异机制

CLC Number: 

• P618.13

• TE122.2