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

Abstract

At most localities in southeastern Utah the Wingate Formation is a sandstone that exhibits generally uniform mineralogy and appearance. Along the southwest-dipping flank of the Lisbon Valley Anticline, however, substantial differences occur in the spatial distribution of authigenic kaolinite and hematite that permit differentiation of three outcrop areas. The northern and southern portions of this flank of the anticline are white (bleached). The bulk of the remaining, centrally located, Wingate outcrops are uniformly red (unbleached) because of hematite/clay coatings on constituent grains. Bleached rocks lack these grain coatings. In addition, most of the quartz grains in the bleached rocks have overgrowths. Compared to the other areas, the northern bleached rocks have three to five times more authigenic kaolinite, half the porosity, twice as many altered feldspars, and one-third as much mixed-layer clay. Both northern and southern bleached rocks contain hematite pseudomorphs after pyrite, and the northern rocks also contain hematite after siderite. In terms of clay content, the southern bleached rocks are more similar to unbleached rocks than to northern bleached rocks. Stratigraphy, primary depositional features, and petrography all demonstrate that post-depositional alteration resulted in the observed differences. Bleaching was driven by acidic and reducing fluids that infiltrated the strata, precipitating kaolinite and, locally, 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 less active and voluminous, resulting in less alteration than in the north. Late-stage oxidizing conditions locally converted siderite and pyrite to hematite. A probable source of the acidic/reducing fluids is leakage from the geographically associated sour petroleum reservoir at Lisbon Valley oil field.