Mississippian chat and tripolite zones in Osage County, Oklahoma: Paleokarst interpretation based on 3D seismic and well logs

Abstract

Mississippian paleokarst chat and tripolitic chert (tripolite) zones associated with the Mississippian Lime have been hydrocarbon exploration targets in Osage County for many decades. Chat is residual chert, either in place or transported, weathered out of chert-bearing Mississippian Limestone that was eroded at the Mississippian-Pennsylvanian unconformity. Thus, the formation of chat is an epigene paleokarst process. Tripolite occurs as a highly porous, silica-rich interval within the Mississippi Lime. It is formed by in-place alteration of limestone by silica-rich surface waters or deep-seated hydrothermal fluids, making tripolite formation a mixed or hypogene paleokarst process. Here, we have studied chat and tripolite by seismic analysis calibrated by well control with full-wave sonic and density log data. We identify that chat and tripolite show clear separation from Mississippian Lime log-based acoustic impedance and [Formula: see text]/[Formula: see text], but there is no meaningful separation of chat from tripolite, and they both exhibit total porosities greater than 20% with evidence of fracture porosity. We find that the sonic-based normal incidence wedge model for chat bounded above by the Pennsylvanian Shale and below by the Mississippian Lime indicate that two seismic expressions are plausible: first, a strong negative amplitude when the chat thickness is above the tuning thickness (for this survey it is 56 ft) and, second, a weak positive or negative amplitude associated with the small impedance contrast between chat and overlying Pennsylvanian Shale. Our analysis suggests that the traditional chat “strong response” and a new “dim-out” exploration strategy may be usefully applied in Osage County. We show that the tripolite response is consistently a negative amplitude event that strengthens with increasing tripolite thickness. We provide an interpretive framework for characterizing chat and tripolite zones associated with the Mississippian Lime in the U.S. midcontinent, which may apply to regions around the world.