Secondary porosity is considered to be that which formed from modification of original carbonate sediment after deposition. Most secondary porosity actually depends upon the presence of a considerable amount of primary porosity which was necessary in order to allow fluid movement through the sediment for the development of secondary porosity.
Secondary porosity will be discussed here through the detailed examination of two relatively well-known ancient areas of carbonate deposition which are interpreted to have had very similar depositional settings: the Zelten Field of Libya and Lower Cretaceous Stuart City Trend of Texas. Both areas comprise grainstone, packstone, and framestone/boundstone facies which were deposited on the edge of a broad carbonate platform. However, secondary carbonate porosity is extremely high in the Zelten Field and is essentially lacking in the Stuart City Trend. Climate, carbonate components (matrix and allochems), regional and local depositional setting, carbonate facies (rock fabric), and burial history (structural setting) control the formation, preservation, or destruction of porosity. These parameters all play an important role in controlling the final porosity of a carbonate rock. In order to aid in comparing and contrasting the Stuart City Trend and the Zelten Field, papers are included here which summarize detailed studies of facies distribution, depositional environments, and porosity controls of both areas (Bebout, Schatzinger, and Loucks, 1977; Bebout and Pendexter, 1975).
Recent research on sandstone diagenesis over the past few years has resulted in the widespread recognition of development of secondary porosity in the shallow to deep subsurface (as deep as 10,000
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.