Diagenetic Controls on Reservoir Quality and Generation–Accumulation Mismatch in the Rakopi Formation: Evidence From Petrography and Basin Modeling

ABSTRACT This study investigates the diagenetic controls on reservoir quality in the Rakopi Formation (Romney‐1 well) and their implications for hydrocarbon accumulation using an integrated petrographic and basin modeling approach. Thin‐section petrography, scanning electron microscopy (SEM), and x‐ray diffraction (XRD) analyses were employed to characterize pore systems, mineralogical composition, and diagenetic alterations. Petrographic observations indicate that the formation initially developed a primary intergranular pore system, which was progressively degraded during burial. Mechanical compaction reduced primary porosity through grain reorganization and increased packing density. Carbonate cementation, particularly siderite and ferroan dolomite, further occluded pore spaces, in many cases completely eliminating intergranular porosity. Authigenic kaolinite precipitation contributed to pore filling, resulting in a microporous system characterized by poorly connected pores. Despite measurable porosity, SEM analysis shows that pore networks are largely disconnected, with pore throats obstructed by cements and clay minerals. Consequently, effective porosity and permeability are severely limited, indicating that the reservoir is dominated by noneffective pore space. Basin modeling results indicate hydrocarbon generation during the Miocene (∼25–15 Ma), with peak generation at ∼20 Ma. Integration of modeling with petrographic data reveals that peak hydrocarbon generation coincided with advanced pore system degradation. This temporal relationship suggests a generation–accumulation mismatch, whereby hydrocarbons were generated in a system lacking effective porosity and connectivity, preventing accumulation. These findings highlight the critical role of diagenesis in controlling petroleum system effectiveness and emphasize the importance of integrating petrographic and basin modeling approaches in reservoir evaluation.