Abstract

Mississippian Reef Production Models of the Fort Worth Basin/Bend Arch

Dan Steward
Republic Energy

Industry’s search for Mississippian oil accumulations in North Texas began in the late 1930’s as operators tested deeper strat sections down the eastern flank of the Bend and southern flank of the Red River Arches. One of the early discoveries was the Brownsville reef in northern Stephens County in September 1943. Texas Pacific Coal and Oil Company used subsurface control from shallow Caddo production (Vickers, 1957) to pinpoint the location of the reef. Their success and that of other operators prompted increased application of 100% seismic methods to explore further east into the Fort Worth Basin. The ability to locate smaller reefs improved as geophysical exploration progressed from 100% data to multifold 2-D, then multi-line swath and finally to 3-D surveys.

Operators generally completed in reefs by topping them with the drill bit, setting production casing, perforating and acidizing. The assumption being that the reef might have a water leg and penetrating too deeply would result in premature watering out. Many of the reefs had oil shows at the Barnett Shale/reef contact and the reef models at the time suggested porosity distributed throughout the carbonate pinnacle. In the rare case of a well failing to establish commercial production the reefs were generally of a large enough size to drill additional wells and establish production. Improving seismic techniques allowed operators to identify and drill progressively smaller features, in addition the study of Waulsortion mounds brought about a greater understanding of these Mississippian reefs and their reservoir characteristics. As the smaller reefs were drilled, there was increasing incidence of dry holes or non- commercial wells.

In the mid 1980’s two events caused Mitchell Energy’s North Texas team to reevaluate the production model of the reefs, first the Ray Richey Miss. reef blowout in the Ora B. Jones well in Eastland County and secondly our experience with high relief small aerial extent ( ~40 acres) reefs in Jack County. Over the years our evolving model went from a carbonate reservoir dominated by fractures, to one reliant on debris aprons with fracturing and porosity on the reef flanks; however it was not until we had established commercial production in the Barnett Shale gas window that many of us on the team settled on a model of fracture halos in the oil mature Barnett associated with the reef. Our conclusion being that the actual contribution of the reef was to increase the amount of structural fracturing rather than being the primary site for oil storage.

AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013