Miocene marine-shelf deposits typically found in the Gulf Coast basin are composed of thin (0.1-10.0 cm), horizontally bedded or cross-stratified quartzarenites to subarkoses, laminated silty clays up to 50 cm thick, and bioturbated admixtures of these 2 end members. Some of the coarser grained sand units may contain appreciable quantities (up to 50%) of shell fragments. These lithotypes exert a significant control on the diagenetic mineral products and amount of secondary porosity observed in specimens that have been subjected to temperatures in excess of 120°C. Low-magnesian calcite, maximum microcline, high albite, and a kaolin mineral (possibly dickite) are the major diagenetic products in the sandstones. A regular mixed-layered illite/smectite (rectorite) is dom nant in the clay-rich materials. Secondary porosity is most common in those rocks that originally contained numerous shell fragments. Quartz overgrowths are ubiquitous. The diagenetic differences are striking when the closeness of the sand and clay association is considered. The thin clay seams may have obtained small quantities of potassium from some of the associated sands. The sands illustrate considerable reaction with connate fluids, during the albitization process. Kaolinitic minerals are most abundant in the sand with the highest original porosity. The original composition and diagenetic products define the optimum conditions for hydrocarbon migration.

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