Influence Of Diagenetic Reactions On Reservoir Properties Of The Neslen, Farrer, And Tuscher Formations, Uinta Basin, Utah

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INFLUENCE OF DIAGENETIC REACTIONS ON RESERVOIR PROPERTIES OF THE NESLEN, FARRER, AND TUSCHER PROPERTIES OF THE NESLEN, FARRER, AND TUSCHER FORMATIONS, UINTA BASIN, UTAH Abstract Thirty-eight core samples of alluvial sandstones and siltstones from the Neslen, Farrer, and Tuscher Formations of the Upper Cretaceous Mesaverde Group from the Southman Canyon field, southeastern Uinta Basin, Utah, were studied in thin section and by scanning electron microscope (SEM). Extensive diagenetic modifications of the reservoir rocks have occurred, some of which significantly improved reservoir properties. Although early carbonate cement and authigenic clays are common, later dissolution and leaching at depth have created significant amounts of secondary porosity. Successful exploration programs are facilitated by recognition of diagenetic factors which influence the reservoir. Introduction There is growing recognition that original lithologic characteristics of reservoir rocks, which are related to depositional history, and which establish a basic physical and chemical system for a potential reservoir, are commonly drastically modified by diagenetic processes. Numerous investigations of diagenetic alteration of sandstone clearly show the variety of ways in which diagenesis (including authigenesis) affects, enhancing or reducing, reservoir quality. The purpose of this paper is to discuss various diagenetic features paper is to discuss various diagenetic features and explain how they may influence reservoir properties. Diagenesis includes all the changes properties. Diagenesis includes all the changes that occur in the reservoir after deposition of the sediment. Mechanical compaction due to burial physically reduces available pare space and commonly promotes cementation by causing dissolution at grain contacts and reprecipitation of mineral matter in pore spaces. Secondary porosity, porosity formed by post-lithification dissolution, is now recognized post-lithification dissolution, is now recognized as an important aspect of many reservoirs throughout the world. Detrital framework grains and authigenic cement may be removed by solution, and the resultant secondary porosity can substantially low permeability coupled with moderate porosity is characteristic of rocks having significant secondary porosity. Authigenic minerals form by chemical precipitation from pore fluids due to changes in the precipitation from pore fluids due to changes in the physical chemistry of pore waters, and form after physical chemistry of pore waters, and form after sediments have been deposited (generally post lithification). The influence of both detrital and authigenic clay minerals on pore geometry and chemical sensitivity has been stressed in several recent investigations. Authigenic clays commonly coat detrital framework grains and line pores. Some of these clays are mechanically sensitive and, if dislodged by migrating fluids, can clog pores, reducing permeability. If iron-rich chlorite is present and exposed to acid treatment, it will dissolve and precipitate FE(OH)3 when the acid is exhausted, greatly reducing effective permeability. If the presence of iron-rich chlorite, permeability. If the presence of iron-rich chlorite, or other iron-bearing minerals, is recognized prior to acidizing, the treatment can be modified to keep the iron in solution. Thirty-eight samples of core material (sandstones and siltstones) adjacent to gas-producing units were examined in thin section, by X-ray diffraction, and with the scanning electron microscope (SEM). These samples, from the Southman Canyon field, represent a vertical interval of 336 m (1641–1977 m) below ground surface. Modal analyses were made by counting 300 points for each thin section. Selected modal measurements are summarized in Table 1. Thirty-six of the samples are moderately sorted, fine- to very fine grained sandstones. The remaining two samples are coarse siltstone and were studied but not included in Table 1. X-ray diffraction traces of bulk rock samples showed the presence of illite, kaolinite, and minor amounts of chlorite. No swelling clays were identified. PORE SPACE REDUCTION PORE SPACE REDUCTION Initial porosity of unconsolidated, fine-grained, moderately well-sorted sand varies, but probably averages about 40 percent. P. 77