SANDSTONE RESERVOIRS OF MORROWAN AGE AT THE NORTHERN MARGIN OF THE ARKOMA BASIN, NORTHWEST ARKANSAS

Morrowan (Pennsylvanian) sandstone units produce natural gas from shallow depths along the northern margin of the Arkoma Basin in northwest Arkansas. The reservoir units are within intervals assigned to the Hale Formation and are productive in the White Oak, Batson and Clarksville fields. The Hale Formation is widely exposed on the Arkoma shelf in the Boston Mountains immediately north of those fields allowing outcrop access to the lithic variations that effect reservoir quality. The Prairie Grove Member of the Hale Formation on the Arkoma shelf is composed in part of a siliciclastic and carbonate succession characterized by abundant cross-stratification and oolitic zones. These high-energy deposits are punctuated by thin intervals of carbonate mudstone within which carbonate buildups are common. In addition to carbonate mud, the buildups are composed of an organic framework of bryozoans, primitive corals, brachiopods and crinoids. Sandstone intervals adjacent to the buildups contain abundant skeletal fragments and pervasive carbonate cement formed by the dissolution of skeletal fragments. Porosity values are low. Sandstone intervals lateral to but remote from the buildups have small amounts of quartz cement and little calcite. Petrographic porosity ranges to 20 percent and reservoir quality is good. A close and mappable relationship exists between buildup trends and reservoir quality.