Characterization and classification of the microporosity in the unconventional carbonate reservoirs: A case study from Hanifa Formation, Jafurah Basin, Saudi Arabia

Formation porosity is a key factor that exerts substantial control upon reserve estimates, and ultimately may affect the development viability of unconventional resources. Microporosity studies have largely focused on siliciclastic mudstones, with limited examples that assess porosity type and distribution in organic-rich calcareous mudstone successions. In this work, a comprehensive porosity characterization study was performed on the organic-rich carbonate-dominated Hanifa Formation of the Jafurah Basin, which is the largest unconventional basin in Saudi Arabia. An extensive experimental program, involving petrographical description, SEM analysis, mineralogical analysis using XRD and QEMSCAN, and geochemistry analysis was employed in order to characterize the samples and analyze the geological origins of the microporosity. The main constituent minerals in this study are calcite (87 wt%) and anhydrite (9 wt%), as well as <1 wt% quartz, illite, pyrite, and dolomite. Total organic carbon reaches up to 5.46 wt %, yielding 5.09 mg HC/g rock (S2), while the free hydrocarbon content (S1) is 5.36 mg/g rock. Type II kerogens characterize the rock samples, and the thermal maturity index is 1.28. The microporosity in Hanifa Formation can be classified into three groups, including framework, solid bitumen, and intraparticle pores. Framework porosity is observed as the main type of microporosity, with solid bitumen representing a subordinate component of the overall porosity. Sheltered pores are also encountered associated with coccoliths tests deposited within the pelagic sediments. Higher degrees of thermal maturity of these sedimentary rocks increase the storage space for the generated hydrocarbon hosted in the solid bitumen. This study introduces an under-explored category of unconventional resources, nominally those that have a carbonate content >85%. Fundamentally, the findings of this study demonstrate that the Hanifa Formation in the Jafurah Basin has the potential for gas exploration and recovery.