Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Solving the St. Louis Formation Reservoir Puzzle in Kansas: Application of a Regional Carbonate Cyclostratigraphic and Diagenetic Reservoir Model for Exploring in Mature Areas
Hammes and James Lakings
A cycle stratigraphic and diagenetic framework for the Mississippian St. Louis Formation in southwest Kansas was established to explain reservoir heterogeneities, trap configurations, and depositional and diagenetic patterns. Cyclicity during St. Louis time was driven by high-frequency, low-amplitude, sea-level fluctuations associated with the beginning of glaciation of Gondwanaland. Four fourth-order cycles punctuate the stratigraphic record of the St. Louis Formation. Generally, each upward-shallowing cycle starts with tidal-influenced, siliciclastic-rich grainstones (transgressive systems tract) overlain by lagoonal wackestones (maximum flooding surface), which are capped by skeletal ooid grainstones (highstand system tract). Skeletal grainstones and localized, dolomitized packstones and wackestones form the main reservoir compartments in most St. Louis fields. Identifying and correctly correlating these individual cycles contributed to the success of several discoveries and delineation of a waterflood in one field in southwest Kansas. Associated diagenetic environments are characterized by marine, meteoric phreatic, and shallow burial cements, which occlude primary pore spaces. Ooid grainstones, deposited in the center of paleohighs, exhibit a greater degree of cementation than the skeletal grainstones related to higher abundance of unstable aragonite mineralogy of predominantly ooids. In contrast, skeletal grainstones, located around the periphery of paleohighs, are mostly low-Magnesium calcite grains, which were stable under near-surface conditions and preserved significant primary porosity. Deeper burial dissolution affected ooid grainstones in creating microporosity in ooids and solution-enlarged pores in skeletal grainstones, thus enhancing porosity. Drilling slightly off the paleostructural high resulted in a successful well with good porosity in skeletal grainstones.