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Origin and Characterization of the Nanopore/Micropores Network in the Leonardian Clear Fork Reservoirs in the Goldsmith Field in Ector Co., Texas

Abstract

The Goldsmith Clear Fork field was discovered in 1946 and has produced over 300 million barrels of oil from a thick carbonate section dominated by anhydritic dolomite with lesser limestone. Reservoir quality is low to moderate with a mixed pore network of interparticle and moldic megapores along with intercrystalline nanopores and micropores. The AMOCO #234 Goldsmith core was selected as the type core to define the origin of dolonanopores and dolomicropores and their characteristics. In thin section these pores are seen occurring in patches of the matrix, in peloids and clasts, and in fossils. These occurrences are similar to some examples of micropores networks observed in limestones. As seen on the SEM using Ar-ion milled samples, the nano- and micropores occur between crystals of dolomite that are euhedral and generally range in size from <400 nanometers to 5 microns. The crystals are poorly sorted relative to size. In general the pores are triangular in shape because they are positioned between euhedral crystals. The pores range from 20 nm to a few microns. Submicron diagenetic illite flakes commonly form in the pores further subdividing the pore. The nano-to microsized dolomite formed by replacement of matrix and grains and by precipitation into pore spaces. The resulting pore networks are quite varied. As an example, where the dolomite replaced former calcite microrhombic grains with associated micropores, the nano- and micropores mimic the former calcite micropore network. This is interpreted as a replacement process of microcrystalline rhombic calcite. In some fine peloidal dolopackstones, the peloids are replaced by a dense network of dolomite crystals with some triangular nanopores still present, while in the interpeloidal pores, some nano- to microdolomite crystals precipitated. The interpeloidal pores are small but appear to form a fair connected pore network. The porosities in the studies core are generally less than 15% and permeabilites are generally less than 10 md with most values less than 1 md. The recognition of nano- and micropores is important because they can be a major contributor to total porosity, while adding little to permeability. They also may affect reservoir saturation in that the megapores may be saturated with hydrocarbon whereas the finer pores are filled with water. Therefore, the quantity of nano- and micropores must be taken into consideration when calculating flow rates and field-wide reserves.