Petrophysical Characterisation of Lower Palaeozoic Reservoirs of Saudi Arabia: An Insight into Factors Controlling Reservoir Quality

By

 Stephen Carney¹, Stephen Hill², Stephen G Franks¹

(1) Saudi Aramco, Dhahran, Saudi Arabia (2) iRES, Ellesmere port, England

 There is significant conventional and tight gas potential within the Lower Palaeozoic clastic reservoirs of Saudi Arabia and a number of prospective, sand-prone intervals have been identified in Silurian to Cambrian section. This interval is relatively under explored, but often has encouraging hydrocarbon shows and well tests. Using a very extensive database, the authors have identified the principal factors controlling reservoir quality. This has implications for future exploration in Saudi Arabia.

Sedimentological and petrological data were collected from core, logs, and outcrop in order to identify significant petrophysical trends and relationships and better understand the factors controlling reservoir quality. Core porosity, permeability, and capillary pressure trends have been identified and related to depositional facies and depth of burial.

The variable burial and temperature histories of the sandstones result in significant diagenetic control over porosity and permeability. Key factors detrimentally affecting reservoir quality include at least two phases of quartz cementation and pervasive illite cementation. In deeply buried sandstones (>12,000 feet), illite rich reservoirs are typified by dominant (ineffective) microporosity and low permeabilities (<0.1 md). Much higher permeabilities (locally >1000 md) are recorded in less deeply buried sandstones where pervasive illite cementation is not present. Burial history models and fluid inclusion data have been used to constrain the time of quartz cementation and relate it to time of hydrocarbon generation and migration.

Enhanced reservoir quality is encountered in areas where early diagenetic development of chlorite grain coatings has inhibited subsequent quartz overgrowth cementation, and where intergranular porosity has been supplemented by feldspar grain dissolution.