SHALLOW GAS PRODUCTION IN NORTHWESTERN ARKANSAS AND THE CLIFTY-SYLAMORE PROBLEM

A shallow gas province has been recognized in Washington and Madison counties, northwestern Arkansas. This area has seen the development of more than a dozen small gas fields producing from reservoirs at the base of the Chattanooga Shale (Upper Devonian), within the Boone Limestone (Lower Mississippian) and from the Wedington Sandstone Member of the Fayetteville Shale (Upper Mississippian). The Wedington Sandstone has seen the most completions, and consequently, the greatest cumulative production, but the largest volume wells produce from the reservoir at the base of the Chattanooga Shale. In particular, the Arkansas Western Natural Gas Threlkeld #1, in Washington County, has produced more than 1 bcf of gas from a depth of only about 600 feet, and it is still on-line.

There is some question about the lithostratigraphic assignment of the reservoir. The name Sylamore applies to the basal sandstone marking the Upper Devonian transgression across the southern Ozarks, and has been used to designate the reservoir. As might be expected, it has a pervasive distribution, but in some outcrops, the Sylamore rests on a Middle Devonian sandstone that is equivalent to the Clifty Limestone. In those exposures, the Sylamore is represented by a thin, phosphatic, angular chert gravel-bearing quartzarenite resting on a significantly thicker, orthoquartzitic sandstone with a basal limestone pebble conglomerate and exhibits neither phosphate nor significant chert. Since production below the Chattanooga Shale is sporadic, even in favorable structural settings, it appears that a reservoir is present only when the Clifty sandstone is developed.