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Charles L. Vavra1, James A. Lorsong2
(1) ARCO Technology & Operations Services, Plano, TX
(2) ARCO El-Djazair Company, Plano, TX

Abstract: Pore-level controls on reservoir quality and petrophysical properties in the Rhourde El Baguel Field, Algeria

Successful redevelopment of producing fields requires a thorough understanding of the controls, distribution and variability of reservoir quality and heterogeneity. The Cambrian sandstone reservoir of the Rhourde El Baguel Field, Algeria, displays exceptionally variable and unusual reservoir properties due to the interplay of initial composition, depositional environment, and diagenesis. Although all of the sandstones are very quartzose, the reservoir comprises two major rock types with strikingly different petrophysical properties.

The upper part of the reservoir consists of well winnowed, shallow marine sandstones. Reservoir quality is controlled by grain size, sorting, bioturbation and fracturing. Variable quartz cementation is strongly controlled by depositional textures, greatly amplifying reservoir heterogeneity at the millimeter- to centimeter-scale. Pore geometry is dominated by intercrystalline macropores, resulting in low porosity but fair permeability. Fracture propensity is high, and natural fractures significantly increase productivity.

The lower part of the reservoir consists of more poorly sorted, fluvial sandstones that contained abundant feldspars and lithic fragments at deposition. These reactive grains were subsequently altered to clay and/or dissolved. Reservoir quality is controlled by the abundance and distribution of moldic pores and intraparticle micropores associated with dissolved and altered grains, respectively. Although pore bodies are large (dissolved grains), pore throats are small, resulting in fair porosity, low permeability and high capillarity. Fracture propensity is low.

In the middle part of the reservoir, the two major rock types are interbedded on centimeter- to meter-scale. Reservoir heterogeneity in this interval is extreme.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana