ABSTRACT: Depositional Controls on Reservoir Quality in Isolated Greenhouse Carbonate Shoals and Swells, Lower Cretaceous, Bab Basin, Middle East

Kerans, Charles¹ and F. Jerry Lucia²
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX
(2) The University of Texas at Austin, Austin, TX

The Lower Cretaceous Shuaiba Formation of the Bab intrashelf basin in the Middle East contains a suite of isolated shoals developed on salt structures. These Shuaiba reservoirs record a complex interaction of salt-driven differential subsidence and deposition but are distinct from their Tertiary counterparts in having a low-amplitude - high-frequency eustatic signal. This low eustatic amplitude produces topographically subtle structures with a more layer-cake stratigraphic framework and less frequent meteoric exposure. Stratigraphic heterogeneities in these greenhouse shoals are produced by (1) windward-leeward depositional asymmetry and (2) a depositional cyclicity driven by fluctuations in oxygenation rather than water depth changes.

The Shuaiba reservoir at Idd el Shargi is typical of this class of salt-controlled isolated platform/shoal reservoirs in the Bab Basin. The stratigraphic framework is constrained by facies and sequence interpretation of 14 cored wells and log-calculated rock-fabric facies from 60 wells with conventional log suites. The reservoir is developed in the upper third of the Shuaiba Formation that is dominated by sub-fair-weather-wave-base Orbitolina wackestones to mud-dominated packstones. The uppermost two high-frequency cycles are late-highstand facies associations that build into fair-weather wave-base and show preferential development of higher energy coral-skeletal facies on the northwestern side of the structure. These maps bear a striking resemblance to Holocene carbonate facies maps from active salt structures in the Persian Gulf. Earlier models that stressed meteoric leaching on the crest of the structure do not match observed distributions of enhanced reservoir quality and cannot be justified from core or thin-section observations and are inconsistent with the low-amplitude greenhouse eustatic regime. The windward-leeward depositional asymmetry model of reservoir quality explains similar distributions of reservoir quality in related salt-cored isolated platforms of Shuaiba age in the Bab Basin.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia