Porosity evolution within high-resolution sequence stratigraphy and diagenesis framework: outcrop analog of the upper Jurassic Arab-D reservoir, Central Saudi Arabia

Abstract

The upper Jurassic Arab-D reservoir is considered as the most prolific reservoir in the Ghawar field in Saudi Arabia. Exposed strata equivalent to the Arab-D reservoir was investigated and evaluated to establish a relationship between lithofacies, sequence stratigraphy, diagenesis, and porosity evolution within the Arab-D reservoir analog. The study revealed eight lithofacies which interpreted to have been deposited in an open-marine lower-slope and upper-slope of a ramp platform, ramp-crest, distal to proximal lagoon and tidal flat environments. The diagenetic and paragenetic analysis showed early marine dissolution and cementation, followed by replacement of aragonite and high Mg calcite to low Mg calcite. Porosity enhancement started with shoaling-up of the system. This produces most of the moldic and intraporosity in the study area. Although the outcrop section showed a degree of burial compaction, there is no pronounced effect of compaction on porosity reduction. With extensive progradation of the proximal lagoon and inner ramp along with meteoric realm, near-surface dolomitization was formed and resulted in porosity increase. This was very clear at the top of each of the high-frequency sequences of the lower part of the outcrop. Later, the whole system had undergone fracturing which enhanced porosity dramatically. Although, the outcrop has little or no macro porosity due to sub aerial exposure and recent meteoric cementation, the paragenesis study provides a predictive porosity distribution model within a high-resolution sequence stratigraphy framework and its associated diagenetic events. This model could provide better understanding of porosity evolution and valuable guide for subsurface exploration.