Summary

Successful exploration within the El Garia Formation requires an understanding of its depositional history and pore network evolution to assist in estimating risk of encountering reservoir facies and finding adequate reservoir quality for economic production.

The mid-ramp El Garia Formation is comprised of bioturbated nummulitic sand- and gravel-grade packstones and grainstones which lack cross-bedding. The accumulation was deposited below fair-weather wave base and above storm wave base, at a water depth estimated to be between 30 to 60 meters (middle neritic). Although the thickness of the El Garia nummulite accumulation can exceed 100 meters, at time of deposition the slope across the accumulation dipped seaward and no significant depositional relief was present. There is no apparent strong subfacies organization within the accumulation, and relatively abrupt lateral and vertical variations in rock textures and nummulite A:B ratios appear to be common within the strike-oriented body.

Reservoir quality within El Garia nummulie facies is quite variable with most time packstones and grainstones having moderate to high porosity, but only poor to fair permeability. A large amount of ineffective porosity in the El Garia Formation is caused by abundant intraparticle porosity in nummulite living chambers and abundant microporosity in test walls. Permeability is controlled by the presence or absence of an intergranular pore network. The intergranular pore network is controlled mainly by depositional sorting and intergranular compaction. El Garia packstones and grainstones can form excellent hydrocarbon reservoirs, but there are pronounced lateral and vertical changes in reservoir quality.

Exploration should be based upon seismic structural mapping combined with prediction of facies belts and diagenesis from well control. Regional facies mapping combined with evaluation of syndepositional tectonic elements can predict the position of the mid-ramp facies belt. Depositional and diagenetic variability will always introduce a risk for encountering uneconomic reservoir quality even within the mid-ramp belt. Proximity of reservoir to mature source rock favors charge of the mid-ramp El Garia reservoirs over the narrower inner-ramp El Garia reservoirs and over the Ain Merhotta gastropod reservoirs, which also have a lower net to gross reservoir interval.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah