Abstract:

The continuous discovery of giant hydrocarbon reservoirs in continental rift basins has highlighted bioclastic dolomite reservoirs as a promising new exploration frontier. This study integrates petrography,geochemistry,and in-situ isotopic techniques to investigate the genesis and pore evolution of bioclastic dolomite reservoirs in the Member 2 of Shahejie Formation in eastern Shijiutuo uplift of Bohai Bay Basin. The results reveal the following: (1)Three sets of bioclastic dolomite reservoirs are developed,and four types of replacive dolomites are identified—micritic envelope dolomite(ME),micritic-fine crystalline matrix dolomite(MD),early fibrous dolomite cement(CD₁),and late mosaic dolomite cement(CD₂). (2)Two dolomitization mechanisms are proposed: reflux and burial dolomitization,elucidating the genesis of different dolomite types. ME,MD,and early CD₁ formed under hypersaline reflux conditions in hot,semi-saline,lacustrine shallow shoals. (3)In the high-temperature,closed diagenetic environment of the shallow burial stage,organic acids generated from kerogen thermal decomposition promoted the dissolution-reprecipitation of MD and CD₁ to form CD₂. Concurrently,pore fluids rich in Mg²⁺ released by compaction,together with retained connate water,facilitated CD₂ cementation,filling intergranular pores. (4)During the synsedimentary stage,meteoric freshwater leaching selectively dissolved aragonite and high-Mg calcite,forming early secondary pores such as moldic and intrabioclastic cavities. In the middle to late burial stage,abundant organic acid production and hydrocarbon charging led to non-selective dissolution and microfracture development. However,reservoir space remains dominated by primary intergranular pores and early secondary pores. By coupling in-situ isotopic temperature reconstruction with multi-stage dolomitization mechanisms,this study refines previous models focused solely on reflux and mixing-zone dolomitization and provides a theoretical framework and predictive approach for reservoir quality enhancement and efficient hydrocarbon development in bioclastic dolomite reservoirs.

Key words: bioclastic dolomite, contributing factors analysis, pore evolution, geochemical analysis, the Member 2 of Shahejie Formation, Bohai Bay Basin

摘要：

陆相断陷盆地中持续发现的巨型油气藏揭示生屑云岩储集层已成为重要勘探新领域。结合岩相学、地球化学和原位微区同位素技术,聚焦渤海湾盆地石臼坨凸起东部古近系沙河街组二段(沙二段,Es2)生屑云岩储集层,揭示了生屑云岩白云石化成因及其孔隙演化规律。研究表明: (1)沙二段发育3套生屑云岩储集层,确定了4种类型的交代白云石: 泥晶包膜(ME)、泥晶—细晶基质白云石(MD)、早期纤柱状白云石(CD₁)与晚期镶嵌状白云石(CD₂);(2)提出渗透回流和埋藏作用2种白云石化成因,揭示不同类型白云石的形成机理,ME、MD和早期CD₁产状白云石发育于干热咸水、滨湖浅滩环境下的高盐卤水渗透回流作用; (3)浅埋藏期高温封闭成岩环境中,有机质热解产生的有机酸驱动MD和CD₁溶解—再沉淀形成CD₂;同时成岩压实释放的富Mg²⁺孔隙水与封存同生水协同参与CD₂胶结,充填粒间孔隙; (4)同生期大气淡水淋滤通过选择性溶蚀作用,生成铸模孔和生物体腔孔等早期次生孔隙, 中—深埋藏后期大量有机酸生成与油气充注引发非选择性溶蚀和微裂缝发育,但储集空间仍以原生粒间孔和早期次生孔为主。本研究将微区同位素温度重建与多期白云石化机理耦合,完善了以往回流渗透与混合水白云石化作用的偏颇观点,为生屑云岩储集层改造与油气藏高效开发提供了理论依据和预测思路。

关键词: 生屑云岩, 成因分析, 孔隙演化, 地球化学分析, 沙二段, 渤海湾盆地

CLC Number: 

• P571