The Norphlet Sandstone is one of the productive hydrocarbon reservoirs in the Hatteras Pond field. Despite its great burial depth (-18,500 ft or -5,638 m), this sandstone retains fairly good porosity (10 to 14%), but has low permeability (0.1 to 5.0 md). Evaluation of pore-size measurements and pore types helps to explain the low permeability. Pore types are classified on time of generation (primary, secondary), size (megapore, mesopore, micropore), and mode of occurrence (intergranular, intragranular, micropore). Analyses of published data of permeability, porosity, and pore types indicate that intergranular pores produce the best permeability, and that intragranular and micropores provide low permeability. Microporosity is determined empirically as the difference betwee the porosity determined by porosimeter and in thin section.

Effective hydraulic pore radius (r_e) is defined as the radius of a straight capillary pipe having the same permeability as the rock under consideration. Most r_e values for Norphlet samples indicate the presence of micropores. Observation shows that pores of the water-bearing Norphlet are mostly intergranular and intragranular pores, whereas the hydrocarbon-bearing part has mostly micropores because of extensive illite cementation. Following decementation of carbonate and anhydrite, authigenic illite was precipitated in the sandstone and produced low permeability by two means: (1) by plugging throats of relatively large intergranular pores, and (2) by developing septa that subdivide large intergranular pores into small ones.

Principal diagenetic events in the Norphlet were (1) shallow cementation by anhydrite and calcite, (2) decementation, (3) dolomitization and illitization, (4) deep cementation, and (5) stylolitization. Decementation of anhydrite and calcite is probably related to hydrocarbon generation and accumulation and with briny formation water. Authigenic illite developed in secondary pores following decementation and largely controlled reservoir behavior. Differences in reservoir quality in the water-bearing and hydrocarbon-bearing zones are interpreted to be caused by different times of decementation and durations of illite development in the two zones.

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