ABSTRACT: Recognition of Subaerial Exposure and Flooding Surfaces in Carbonate-Siliciclastic Eolianites and Marine Carbonate Sequences in Southwestern Kansas
Recent work on the St. Louis Limestone in southwestern Kansas has demonstrated that these units contain a significant eolian facies component (up to 80-90% of total unit thickness). Reservoir intervals within the St. Louis are confined to relatively thin subtidal grainstones that, in turn, are capped by a muddy carbonate and shale facies. Critical to exploration and development for these grain-shoal reservoirs is an understanding of their spatial and stratigraphic distribution. Core through the St. Louis and St. Genevieve limestones has been examined and features (e.g., paleocaliche, and in-situ eolianite breccias) have been recognized at the top of the eolianites. These surfaces are interpreted as long-term exposure surfaces (i.e., sequence boundaries of various orders). Instinctive pectral gamma-ray response, consisting of relative uranium enrichment, provides additional evidence for relatively long-term exposure and caliche formation at the top of the eolian facies, and provides a criterion for recognition of subaerial erosion surfaces in the absence of core.
The contact between the subtidal grainstone shoals and the overlying muddy carbonate and shale facies is relatively sharp and is interpreted as representing a flooding surface separating shoal from muddy-open shelf facies. The contact commonly is represented by a thin conglomerate of reworked shoal facies, and has been interpreted as either a subaerial unconformity or a submarine hardground. The surface has a distinctive spectral gamma-ray response with a relative enrichment of potassium and thorium. This response is significantly different from the spectral response from the subaerial surface at the top of the eolian facies.
In the St. Louis Limestone, the subtidal carbonate grainstone reservoir intervals consist of primary interparticle porosity. Initial geologic models have viewed the reservoir zone as a single blanket-like layer with small structures and relatively random changes in facies thickness resulting from such processes as migrating shoals. However, a detailed integrated analysis of facies, well logs, and reservoir data within the framework of high-resolution sequence stratigraphy has resulted in a new model of St. Louis reservoir geometry. Each reservoir unit was deposited in a high-energy upper shoreface environment made up of oolitic and skeletal (crinoids, bryozoans) grains. Correlations based on log criteria tied to core data suggest that these reservoir units represent transgressive depo its ("keep-up deposits") at the base of individual higher order sequences deposited during fluctuations in relative sea level. The reservoir facies onlap and pinch out against a relatively long-term exposure surface at the top of the underlying nonporous sandy limestones (eolianites), and interfinger and downlap basinward with muddy, skeletal-rich carbonates deposited in muddy open-shelf environments. Continued rise of sea level led to the flooding of the shoal facies and deposition of deeper shelf shaly wackestone ("give-up deposits") over the grainstones forming the reservoir top seal.
Development of St. Louis reservoirs emphasizes the opportunities and challenges created by the heterogeneity and time-transgressive nature of the porous grainstone facies (i.e., multiple flow units related to minor higher order fluctuations in relative sea level). Extension and exploration concentrates on recognizing and mapping the updip pinch outs of porous grainstone facies related to maximum landward extent of paleoshorelines caused by relative major fluctuations in relative sea level. These shoal facies build-ups and pinch outs should form viable exploration targets as they cross structural elements formed or reactivated during the Pennsylvanian.
AAPG Search and Discovery Article #90991©1993 AAPG Mid-Continent Section Meeting, Amarillo, Texas, October 10-12, 1993.