The Evolution of Carbonate Porosity in a Diagenetic-Environment Framework
When carbonate and quartzose clastic pore systems are compared (Table 1), it is obvious that while quartzose clastic pore systems are controlled to a large extent by primary textures, the basic controls on the ultimate porosity of limestones are dominantly diagenetic in nature. As indicated previously, these post-depositional influences place definite restraints on utilization of classic sedimentological models and require expansion of exploration strategy to include the diagenetic evolution of the rocks during burial history. To be truly effective in a carbonate terraine, the exploration geologist must be fully aware of the basic characteristics of limestone porosity and the nature and extent of the diagenetic controls over this porosity. This seminar is designed to outline the basic characteristics of limestone pore systems, to describe the broad realms of diagenetic activity critical to an understanding of the evolution of these pore systems, and to delineate and evaluate the tools currently available to the geologist seeking favorable reservoir rocks in a carbonate terraine.
These remarks are not intended to exclude the geologist concerned with the exploitation of proven reservoirs in carbonate sequences, but only to emphasize the need for a new awareness in exploration concepts. The need for the exploitation geologist to integrate diagenetic constraints into production concepts has long been recognized, and continues to be of prime concern.
This material is in the form of a progress report, since the fruits of some fifteen years of intensive research into the basic physical and chemical characteristics of the limestone — water system
Figures & Tables
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.