Abstract: Regional Depositional Setting and Diagenesis of Larger Foraminifera Nummulite Deposits: Implications for Exploration and Development

LOUCKS, ROBERT, ARCO Exploration and Production Technology, Plano, TX 

Larger foraminifera nummulite deposits form excellent reservoirs, especially in the lower Eocene strata of North Africa. Successful exploration within these deposits requires an understanding of their depositional history and pore network evolution to assist in estimating risk of encountering reservoir facies and finding adequate reservoir quality for economic production. Nummulite deposits in Tunisia are an excellent example for demonstrating the characteristics of these rock types.

The mid-ramp El Garia Formation in Tunisia is comprised of bioturbated nummulitic sand- and gravel-grade packstones and grainstones. The accumulation was deposited below fair-weather wave base and above storm-wave base, at a water depth estimated to be between 30 and 60 m (middle neritic). There is no apparent strong subfacies organization within the accumulation, and relatively abrupt and seemingly random lateral and vertical variations in rock textures appear to be common within the strike-oriented body.

Nummulite deposits can form excellent hydrocarbon reservoirs, but there are pronounced lateral and vertical changes in reservoir quality. Lime packstones and grainstones have moderate to high porosity (15-25%), poor to good permeability (>0.01 to 1000 md). Abundant intraparticle porosity in nummulite living chambers and abundant microporosity in test walls produce a large amount of ineffective porosity. The amount of effective intergranular porosity controls permeability. The intergranular pore network is mainly a function of depositional sorting of nummulite grains, nummulite debris, and lime mud, and later burial mechanical and chemical compaction of grains.

Exploration should be based on 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, nummulite facies belt. Depositional and diagenetic variability will always introduce a risk for encountering uneconomic reservoir quality even within the thickest nummulite deposits. 

AAPG Search and Discovery Article #90912©1999-2000 AAPG International Distinguished Lectures