Abstract

Hitch field (T. 33 S., R. 34 W., Seward County, Kansas) produces from Upper Mississippian Chester sandstone, deposited in a narrow valley incised into Mississippian Ste. Genevieve and St. Louis limestones. The Hitch reservoir originally had a small gas cap, a 98-foot oil column, and two minor water legs. Midway within the oil column, 20-30 feet of otherwise porous sandstone is filled completely with solid bitumen. The tar mat composition is similar to produced oil but with concentrated heavy components, and probably formed by precipitation, rather than biodegradation.

Although historical core descriptions reported "dead oil stain," and a low-permeability layer was well known from routine core analyses and well tests, the complete occlusion of the pore system was not originally recognized. Core-cleaning procedures removed most of the bitumen, resulting in acceptable porosity and grain-density values, and conventional wireline-log calculations indicated porous sandstone at irreducible water saturation, as expected. The tar mat is readily apparent with UV fluorescence core photography, thin section and SEM petrography, and NMR wireline logs, but these techniques were not incorporated into early field studies.

The Hitch tar mat went unrecognized for 20 years, despite multiple cores and standard wireline-log suites. Many producing reservoirs in the Hugoton Embayment contain similar oils, at similar pressures and temperatures, implying that in situ precipitation of heavy organic material should not have been unique to Hitch. It is likely that other tar mats, probably in reservoirs with disappointing production performance, are waiting to be recognized.