Secondary Carbonate Porosity as Related to Early Tertiary Depositional Facies, Zelten Field, Libya
D. G. Bebout, Charles Pendexter, 1979. "Secondary Carbonate Porosity as Related to Early Tertiary Depositional Facies, Zelten Field, Libya", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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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 are the caprock for the reservoir. Porosity also is low in the argillaceous molluscan-micritic facies that forms a lens-shaped body in the southern part of the field southwest of the coralgal-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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Geology of Carbonate Porosity
In clastic situations, primary porositv is a direct function of texture and fabric, including size, sorting and shape (Fig. 1). Grain size, sorting, fabric, as well as sedimentary structures are related directly to sedimentary processes acting at the time of deposition (Fig. 1). Each depositional environment is characterized by a distinct suite of processes distributed across the active sediment water interface in a pattern unique for that environment (Fig.2). This suite of processes gives rise to a group of products, including sediment texture, fabric, and structures distributed across the active sediment water interface in a pattern unique for each depositional environment (Figs. 1 and 2). In a prograding or regressive situation, when sedimentation is taking place at the active sediment-water interface, a vertical sequence of sediments is formed which reflects, in an orderly fashion, from deepest at the base, to shallowest at the top, the progressive changes in texture, fabric and sedimentary structures resulting from the progressive changes in processes found along this interface from shallow to deep water (Fig. 3). Each sedimentary environment then, can be characterized by a unique vertical sequence of sediment textures, fabrics and sedimentary structures. It is this unique suite of characteristics that is commonly used for the identification of depositional environments in ancient rock sequences, and most importantly, is used to predict the presence and detailed distribution of the most porous (best sorted, coarsest) potential reservoir facies (Fig. 3).
In a regional setting, the recognition of distinct sedimentary environments and knowledge of logical lateral relationships is the keystone for prediction of the lateral extension or even presence of potential reservoir facies.