Post depositional modifications (diagenesis) of sandstones play an important role in controlling hydrocarbon discovery and production. Diagenesis can effect the ways in which geologists and engineers interpret, evaluate and develop sandstone reservoirs. In particular, diagenesis has a significant effect on several topics of importance to the oil industry including: 1) Petrophysics, 2) Fluid System Compositions, 3) Drill Stem Testing, 4) Well Production, and 5) Enhanced Recovery.

Diagenesis can control log shapes, leading to erroneous interpretations of depositional environment. It is important that the impact of diagenesis on log shape be understood so that accurate assessment of depositional environment can be made. Depositional environment governs reservoir trend, shape, and at times dimensions and quality. It is, therefore, of importance to formation evaluation and field development. The importance of this control is heightened because of our necessary dependence on logs.

Post-depositional addition of various cements in the pore system, significantly effect gamma-ray log shape, porosity calculations from neutron and density logs, and also Sw calculations from Archie's equation resulting in pessimistic interpretations of water saturations.

Drilling, completion and enhanced recovery fluids react with natural (authigenic) cements in rock pores and these reactions can effect the quality and production of any reservoir. This is particularly true in sandstones of West Texas, where a wide variety of natural cements are present. In some locations, sandstones are sensitive to fresh or sodium-based drilling and completion fluids because of the presence of the swelling clays illite-smectite or smectite. In other areas these clays are absent and no swelling problems exist.

Certain areas are characterized by sensitivity to either HCl or HF acids (whether used in mud clean-up or stimulation) due to the presence of specific sulfides or oxides in the pore systems. The sensitivities of sandstones to waters and acids are not uniform across West Texas, but vary from area to area and formation to formation. Thus drilling and completion fluids should be designed on a local not regional scale.

Diagenesis can have a significant effect on DST results. The presence of authigenic cements often causes poor DST's. There are instances when DST results should be ignored and well completion encouraged. Poor DST's in the West Texas sandstones result from any or all of the following: a) Secondary porosity, b) Silica overgrowths, and c) Illitic minerals. Thus, poor DST results should be used to condemn a well only after the diagenesis of the sands is understood.

Both the rate of production decline and the ultimate productivity of any well are linked to diagenesis. Rapid initial production declines are not always caused by the presence of migrating fines. At times, rapid decline rates are due to the type of pore system dominant in the sands. Clay stabilizing treatments (for purposes of preventing or arresting production declines) can at times be worthless, and even damaging to the formation, if they are applied without a detailed knowledge of the pore system and its morphology.

Nothing is more important to the success of EOR than a detailed understanding of the pore systems of a sand. The number and locations of injectors, the composition of the flood fluid, and the treatment of injection and production wells should be based on an understanding of the morphology of the pore system. At least 3 distinct varieties of pore systems occur in the sandstones of West Texas, 1) Pore systems dominated by fractures, 2) Pore systems dominated by large, irregularly-shaped secondary pores, and 3) Pore systems lined with cements of variable composition. The success of any enhanced recovery project will be dependent, to a large extent, upon the compatability of project design and pore system morphology.

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