Abstract

The Barnett Shale originated as a normal marine shelf deposit on the southwestern flank of the subsiding Southern Oklahoma aulacogen, probably due to the Middle or Late Mississippian collision of the North American plate with South America and/or North Africa. It is a hard, bituminous formation that is easily distinguished in drill cuttings or on electrical and porosity logs. Subsurface correlations allow the Barnett to be informally subdivided into four or five members which can be traced through most of Jack, Wise, and Montague counties. It unconformably overlies rocks of Early to Middle Ordovician age, and it is in turn — with a few local exceptions — conformably overlain by the shales and limestones of the Marble Falls Formation.

The growth of Mississippian pinnacle reefs was initiated with the inundation of North-Central Texas by the warm, shallow Barnett seas. The reef complexes are subdivisible into three constituent facies: the reef core, the reef flanks, and the interreef area. The reef cores are porous enough to serve as stratigraphic traps for oil and gas, and they have yielded excellent production in the northern part of the Fort Worth basin for several decades. The Barnett Shale, though itself unproductive, is believed to be the hydrocarbon source for the reef production.