Diagenesis of Frontier Formation Offshore Bar Sandstones, Spearhead Ranch Field, Wyoming

Abstract Diagenesis and physical and biological reworking of the sandstones at Spearhead Ranch Field control the entrapment of oil. The major producing facies is at the top of a sandstone interpreted to be a reworked offshore marine bar. Mineralogically the reworked facies differ from the underlying non-reworked facies; the reworked facies is high in chert and quartz and low in plagioclase feldspar. The non-reworked bar facies is somewhat lower in chert and quartz and high in plagioclase. Diagenesis of the two mineralogic suites has contributed to preservation of a good reservoir in the reworked facies and destruction of the reservoir in the underlying facies. Nearly twice as much chlorite is present in the reworked facies as the non-reworked facies. The percent chlorite has a negative correlation (0.7) with quartz in the reworked reservoir facies. Apparently the growth of chlorite grain-coatings has impeded growth of quartz overgrowths and thereby maintained primary porosity and permeability in the reworked facies. Quartz overgrowths have almost completely eliminated porosity and permeability in the non-reworked bar facies. Porosity and permeability calculated from core plugs in the reworked facies average 5.6% and 3.5 md respectively, while comparative values in the non-reworked facies are 4.4% and 0.3 md. The percentages of illite and montmorillonite are also significantly different in the two facies probably resulting from contrasting origins. In the reworked bar facies below the topmost sandstone, calcite cement has in some areas degraded the potential reservoirs. Results of this study are based on interpretation of three slabbed and polished cores (116 to 150 feet in length) taken by Mountain Fuel (Nos. 2, 3, 4 Spearhead Ranch). Scanning electron microscope analyses, X-ray diffraction analyses and petrographic laboratory studies were carried out on samples from the cores. Four ‘stacked” upward-coarsening sequences, up to 70 feet thick, were interpreted from analyses of sedimentary structures and vertical sequences of lithologies and facies. Discriminant analysis of the X-ray diffraction results of 36 reworked and non-reworked facies sandstones yields nearly perfect separation (97%) of the two facies.