Diagenetic Alteration of Wingate Formation: Possible Indications of Hydrocarbon Microseepage, Lisbon Valley, Utah

Ira S. Merin, Donald B. Segal

The bulk mineralogy and character of the Wingate formation is uniform, but substantial differences occur in the spatial distribution of authigenic kaolinite and hematite to permit differentiation of three areas: northern bleached, central unbleached, and southern bleached. Bleached rocks are gray/white due to a lack of hematite/clay coatings as constituent grains, most of which have overgrowths. These coatings are present in unbleached rocks, making them red. Compared to the other areas, the northern bleached rocks have 3 to 5 times more authigenic kaolinite, half the porosity, twice as many altered feldspars, and one-third as much mixed-layer clay. Bleached rocks contain hematite pseudomorphs after pyrite, and the northern rocks also contain hematite after siderite. In g neral, the southern bleached rocks are more similar to unbleached rocks than to northern bleached rocks.

Stratigraphy, primary depositional features, and sedimentary petrography demonstrate that bleached rocks were initially the same as the unbleached rocks and that differences are due to postdepositional authigenic processes. Bleaching was driven by acidic/reducing fluids that entered the strata soon after burial, resulting in precipitation of kaolinite and dissolution and local replacement of hematite by pyrite and siderite. An area between the two bleached zones remained unaltered. Fluids either remained in the southern area for a shorter period of time or were weaker, resulting in less alteration than in the north. The prime candidate for a source of these fluids is leakage from the geographically associated sour petroleum reservoir at Lisbon Valley oil field.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.