Abstract

Wells from the Wolf Flat Field in the Palo Duro Basin penetrated rocks deposited on an open marine carbonate shelf which has classic karst features. Mound, back-mound and oolitic facies typically comprise the vertical facies succession. Mounds developed next to the shelf margin and are composed of crinoids, bryozoans, corals, and algae. Surrounding these mounds are skeletal packstone/grainstones and also back-mound low-energy wackestones. Ooid grainstones were deposited in high energy passages onto the shelf and overlie beds of skeletal detritus, to form small shallowing-upward sequences (fifth-order).

Karsting was the result of a major sea level drop of at least 30+ meters (third-order) which exposed the Lower Cisco shelf margin to subaerial diagenesis. Karst features include vadose silts, collapse breccia, banded speleothem cements (both isopachous and pendant), solution pipes and solution vugs. Contact with the overlying basinal shale is irregular with shale infilling the upper leached vuggy porosity. Stratigraphically lower (5-6 meters), speleothem cements commonly fill vuggy porosity. This transition probably marks the change from an upper undersaturated zone of more intense leaching to an underlying saturated zone of precipitation.

Grain-rich mound and oolitic grainstone facies typically have extensively developed equidimensional vugs, and leached porosity may have been up to 60-70% (prior to porosity occlusion by speleothem cements and burial dolomite). Vugs formed during karsting as a result of undersaturated fluids migrating through the pervasively porous grain-rich matrix. Equidimensional shape of the vugs and associated isopachous cements suggest formation may have been in the meteoric phreatic environment. Grain-rich facies are more extensively dolomitized because of the well-developed vuggy porosity which served as permeable pathways for late-stage dolomitizing fluids. Some porosity and permeability is preserved within these vuggy grain-rich facies.

Mud-rich back-mound facies generally lack equidimensional vugs but have long (up to a meter) vertically oriented sediment and cement-filled voids. Some vertical voids are tubular which suggests rootlet origin, whereas others are planar which suggests a fracture origin. The vertical orientation with solution enlargement and the common pendant cements suggest formation within the meteoric vadose environment. Cavernous solution would most likely occur within this dense limestone where solution was concentrated along fracture or rootlet channels. More pervasively porous grain-rich facies developed smaller solution vugs (generally less than 10-12 centimeters) and would not be as likely to develop cavern size porosity. This concept is supported by observations from the Mullin #2 well which show collapse breccia up to 20 meters thick overlying the grain-rich mound facies. This is interpreted as a solution collapse which developed within the mud-rich facies. Thin grain-rich facies such as oolitic and skeletal shoals may have been incorporated within this collapse.

Stable isotope analyses support the interpretation of a karst event. The δ¹⁸O values consistently fell within the range of early meteoric diagenesis. The δ¹³C values are all negative and indicate input from organically-derived CO₂ near an exposure or karst surface. Increasing negative values for δ¹³C in an upward direction indicate increasing proximity to the actual exposure surface.

In summary, karsting appears to have been an important event effecting diagenesis of shelf-edge carbonates of the Palo Duro Basin. Primary depositional facies, in part, control the extent of the karstification and the subsequent diagenetic alteration.