Utopia Algal-Bank Complex and Its Potential in Shallow Gas Exploration in South-Central Kansas: ABSTRACT

Abstract

The Utopia Limestone (Upper Pennsylvanian) was deposited as a series of algal banks forming a narrow arcuate band approximately 25 mi (40 km) long on the shelf margin of Sedgwick basin in south-central Kansas. Utopia banks may attain a maximum thickness of 70 ft (21 m) and are encased in a dark gray shale, creating excellent reservoirs for hydrocarbon accumulation. Having porosities of up to 22%, the Utopia produces economic quantities of gas from depths of 2,000 to 2,500 ft (610 to 762 m). The Utopia contains three distinct facies: (1) biomicrite, the core facies; (2) calcilutite, an Osagia-coated biosparite, the mound facies; and (3) an oolitic facies associated with the mound. The facies distribution was controlled by basin geometry and the variations in the subtidal and intertidal environments of the shelf. Sedimentation rate approximately equaled subsidence rate, maintaining the position of the bank tops within the shallow intertidal range. The banks are slightly asymmetrical and thicken basinward. The initial mound accumulation began with the deposition of a biomicritic mound core, in a quiet-water environment. The major constituents of the calcilutite facies are green algae (Osagia and Epimastopora), along with a wide assortment of shallow-water biota. The ool te shoal facies formed contemporaneously beside the mound facies. Subaerial exposure and subsequence leaching of the banks during successive marine transgressions created the intraparticle and oomoldic porosity. Locally, dolomitization of the algal fragments occurred. Results of this study indicate that further exploration in this area and basinward should prove profitable. End_of_Article - Last_Page 637------------