ABSTRACT

Production from the Zelten field, Libya, is from the highly porous shelf limestones of the Zelten Member (“Main Pay”) of the Paleocene and lower Eocene Ruaga Limestone. Fifteen facies are recognized, mapped, and predicted. Seven of the facies comprise the larger part of the Zelten Member. These include miliolid-foraminiferal micrite, argillaceous bryozoan/echinoid micrite, argillaceous-molluscan micrite, coralgal micrite, Discocyclina-foraminiferal calcarenite, foraminiferal calcarenite and micrite, and Discocyclina-foraminiferal micrite.

In the Zelten field secondary porosity is recorded as much as 40 percent; this porosity is related to the original depositional fabric of the sediment and is, therefore, facies controlled. Porosity is highest in the coralgal micrite and Discocyclina-foraminiferal calcarenite, which together form a northwest-southeast trend across the northern part of the field, and in the foraminiferal calcarenite and micrite. The rocks of these three facies primarily are grain supported with a micrite matrix in which the primary carbonate mud porosity was preserved because of the lack of compaction. Subsequent leaching by ground water through these porous zones probably enlarged the primary mud porosity and greatly altered the original carbonate texture. Porosity is lowest in the miliolid-foraminiferal micrite and argillaceous bryozoan/echinoid micrite facies, both of which are blanketlike in distribution over the top of the field and ore the caprock for the reservoir. Porosity also is low in the argillaceous molluscanmicritic facies that forms a lens-shaped body in the southern part of the field southwest of the corolgol-micrite and Discocyclina-foraminiferal-calcarenite facies.

A thick dolomite in the lower part of the Zelten Member is restricted to the area south of the trend formed by the coralgal micrite and Discocyclina-foraminiferal calcarenite. Thus, these sediments probably were deposited in a lagoonal area more susceptible to dolomitization. The coincidence of the dolomite and its off-structure position, however, leaves open the possibility for structural control.

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