Sedimentologic and diagenetic controls on petrophysical properties in the Middle Dhruma Formation (Jurassic), Saudi Arabia

Abstract

Carbonate outcrops of the Jurassic Middle Dhruma Formation (D5 and D6 units), located southwest of Riyadh, Saudi Arabia, were investigated to characterize controls on potential reservoir quality. Porosity and permeability of the outcrop lithologies are dependent upon sedimentary facies and post-depositional modification. Both field and laboratory data were integrated to identify lithofacies types and depositional environments. Standard petrography and scanning electron microscopy were used to identify and characterize diagenetic features. Integration of sedimentologic descriptions and petrographic data yielded identification of ten lithofacies between the D5 and D6 units. Based on lithologic affinities, the ten lithofacies were grouped into three related lithofacies associations. The depositional setting included a shallow restricted lagoon, back shoal, and shoal complex environments, with a majority occupying shallow-lagoon environments in the inner shelf of a carbonate ramp. Depositional texture imparted a strong, first-order control on porosity-permeability trends. This observation led to a texture-based classification for petrophysical comparisons: grainy texture (grainstones), mixed texture (packstones and rudstones), and micritic texture (wackestones and mudstones). Overall, samples with similar porosity values in the grainy textures have higher permeability than the mixed and micritic texture rocks.Diagenesis, namely micritization and cementation, also controlled reservoir quality in outcrops and subsurface units of the Dhruma Formation. A comparison between the D5 and D6 outcrops in Saudi Arabia and other equivalent outcrops and reservoirs in the region revealed a similarity in their depositional environments, bulk density, and porosity.The findings of this study can be beneficial for comparison and proper reservoir characterization and formation evaluation of local and regional reservoirs and their future exploration plans.