ABSTRACT

The quality of sandstone reservoirs in the Tertiary-Pleistocene sequence of the Gulf Coast is affected by both original composition and diagenesis. Porosity and permeability are controlled by 1) original depositional matrix, 2) grain crushing, 3) solution and reprecipitation of carbonate shells, 4) authigenic clay pore-linings, 5) authigenic clay and carbonate pore-fills, and 6) development of secondary porosity. Factors 1 through 4 act as important controls on reservoir quality in Pliocene-Pleistocene offshore reservoirs. All six factors combine to control reservoir quality in pre-Pliocene sandstones.

The presence of diagenetic minerals in a sandstone can totally aIter the electric log response. A log may indicate that a sandstone is water saturated, and yet the well may produce water-free oil. This is commonly a result of the presence of authigenic clay pore-linings which bind water to the framework grains. Hydrocarbons may thus be seriously underestimated when a sandstone contains only a few percent authigenic clays. The acid or fresh water sensitivity of a sand may also be controlled by the diagenetic minerals. Diagenetic pore-linings receive the greatest exposure to drilling, treatment, and completion fluids, and greatly effect reservoir quality. A knowledge of both sandstone composition and diagenesis are essential in defining optimum exploration strategies, as well as drilling and stimulation treatments.

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