Paleogeomorphology of the Sub-Pennsylvanian Unconformity of the Arbuckle Group (Cambrian-Lower Ordovician)

Kansas Geological Survey
Open-file Report 2001-55

Correlating Arbuckle Paleogemorphology to Hydrocarbon Production

Polygonal Karst (Cockpit)

Sapped Plateaus

Two different styles of erosional terrains were identified on the Sub-Pennsylvanian Arbuckle surface, a polygonal karst terrain (cockpit karst, shown above left) and plateau-like erosional features modified by ground-water sapping (above right). The two types of erosional terrains are end members and numerous intermediate examples exist. However, differences in oil productivity were recognized between cockpit terrains andplateau regions. Relatively small, irregularly shaped fields that occur in mature polygonal karst landscapes experience relatively low cumulative production per well (typically 50,000-100,000). Fields that occur on ground-water sapped platueas show significantly higher per well cumulative
hydrocarbon recovery (200,000-300,000).

Initial analysis of drill stem test data from relatively recent (1960-present) infill wells from the study area highlight other production differences in the two types of geomorphic terrains. In general, infill wells located in areas dominated by polygonal karst experienced relatively high final shut in pressures (1000-1200 psi). Infill wells located on sapped plateaus were characterized by significantly lower final shut in pressures (250-750 psi). One possible explanation for this might relate to the stratigraphic controls on reservoir development. Ground water sapped plateausare interpreted to have intra-formational shale layers or permeability barriers within the Arbuckle strata, responsible for directing ground water
flow down-dip. The same ground water flow barriers could also be responsible for reducing vertical permeability and aquifer support. The permeability barriers reduce the influence of a strong bottom water drive. The cockpit terrains characteristic of Arbuckle reservoirs formed by vertical ground water movement show strong pressure support from a bottom water drive.

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Last updated January 2002