Geological Controls on the Nora Coalbed Methane Field, Southwest Virginia

John F. Sinclair

Since 1988, 215 wells have been drilled to an average depth of 2200 feet in the Nora Coalbed Methane Field, which is located along the eastern margin of the Pine Mountain Thrust Sheet in Dickenson, Russell, Wise and Buchanan Counties, Virginia.

Stratigraphically, the coals in the Nora Field are Lower Pennsylvanian in age and are associated with a deltaic depositional setting. The coals occur within three distinct stratigraphic intervals, which are from oldest to youngest, the Pocahontas, Lee and Norton Formations. Multiple thin seams with a combined thickness from 120 to 150 inches are targeted in areas where economic production rates are controlled by gas content, rank, structural setting and hydrology.

Gas contents range from 250 to 450 scf/t and are associated with high-volatile A, medium and low volatile bituminous coals. Gas contents and rank decrease from east to west. Production rates in the eastern portion of the Nora Field are typically higher than those to the west.

Subtle structural closures, which have enhanced permeabilities within the coals, were mapped using 1200 shallow coal corehole and conventional gas well control points. In conjunction with the structure map, aeromagnetic surveys and drainage analysis interpretations indicate significant fracturing within the Pine Mountain Thrust Sheet. NE to SW trending anticlines have been broken into numerous smaller closures due to the right lateral movement of the Russell Fork Shear Fault and other right lateral faults which have been identified to the west of the Russell Fork Fault.

Salinity maps of the produced waters from the CBM wells indicate recharge is occurring along these right lateral faults. The recharge and discharge patterns not only indicate areas of higher permeability, but also creates a trapping mechanism which keeps the coals from desorbing.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995