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

Roderick W. Tillman, William R. Almon

Diagenesis and physical and biologic 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 differs from the underlying unreworked facies; the reworked facies has a high chert and quartz content and low plagioclase feldspar content. The unreworked 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.

Chlorite is present in amounts averaging nearly twice as much in the 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 unreworked bar facies. Porosity and permeability calculated from core plugs in the reworked facies average 5.6% and 3.5 md respectively, whereas comparative values in the unreworked facies are 4.4% and 0.3 md. The percentages of illite and montmorillonite also differ significantly in the two facies, probably as a result of contrasting origins.

Results of this study are based on interpretation of three slabbed and polished cores (116 to 150 ft; 35 to 46 m) from the Spearhead Ranch field. Scanning-electron microscope and X-ray diffraction analyses and petrographic laboratory studies were carried out on samples from the cores.

Four "stacked," upward-coarsening sequences, up to 70 ft (21 m) 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 unreworked facies sandstones yields nearly perfect separation (97%) of the two facies.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado