Depositional facies controls on reservoir rock typing and heterogeneity: Devonian siliciclastic succession, south central Ghadames basin, Libya

Abstract

The Devonian sedimentary succession hosts the most potential reservoir targets in Ghadames Basin, Northwest Libya. This succession consists of a complex pattern of siliciclastic facies deposited during successive episodes of sea level fall and rise, thereby leading to a large scale heterogeneity in sandstone facies connectivity and distribution. The present study integrates core sedimentology, wireline logs, and routine and special core analysis to establish a framework for predicting the distribution of the best Devonian reservoir rock types (RRTs). A lowstand amalgamated fluvial sandstone bars only exist in the Lower Devonian Tadrart Formation; the other Devonian rock units consist of transgressive and highstand tidally-influenced estuarine and deltaic facies. The pore system characteristics are controlled by initial depositional attributes where open, megapores with entry displacement pressures <10 psi occur in the quartzose, high energy fluvial sandstone bars, delta distributary channels and mouth bars. These facies are classified as RRTI and are mostly associated with lowstand and late highstand facies. In contrast, the other RRTs display tight pore system with elevated irreducible water content >20%. RRTI rocks account for greater than 90% of the fluid flow capacity in the Devonian reservoirs. A link between the facies depositional setting, RRT distribution, vertical amalgamation and lateral compartmentalization of flow conduits, barriers and baffles zones has been proposed, thereby promoting the predictability of the best reservoir targets.