Summary

The upper St. Louis Limestone, an important reservoir interval in southwestern Kansas, contains a significant mixed carbonate-siliclastic eolianite facies (up to 80 to 90% of total unit thickness). Reservoir intervals within the St. Louis are confined to relatively thin subtidal ooid grainstones intervals that form stratigraphic traps within the eolianite facies. Critical to exploration and development for St. Louis reservoirs is an understanding of the spatial and stratigraphic distribution of marine facies within the predominantly eolianite sequence. Cores through the St. Louis and overlying St. Genevieve limestones display features (e.g. paleocaliche and in situ eolianite breccias) at the top of the eolianites that are interpreted as long term exposure surfaces (i.e., sequence boundaries of various orders). Distinctive spectral gamma ray response, consisting of relative uranium enrichment at these surfaces, gradually decreasing downwards, provides a criterion for recognition of subaerial exposure surfaces in the absence of core. Overlying the subtidal grainstone shoals ia a muddy carbonate and shale facies with a relatively sharp contact, interpreted as representing a drowning surface separating shoal from muddy-open shelf facies. This surface has an equally distinctive spectral gamma ray response with a relative enrichment of potassium. Recognition of subaerial exposure surfaces and associated drowning surfaces using the spectral gamma-ray response calibrated with core observations is used to develop a sequence-stratigraphic and depositional interpretation for the upper St. Louis mixed eolianite and marine rocks in southwest Kansas.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah