NATURAL GAS POTENTIAL OF PENNSYLVANIAN "TIGHT SANDSTONE" RESERVOIRS--FRONTAL OUACHITA THRUST BELT, WEST-CENTRAL ARKANSAS

This study examines the natural gas reservoir characteristics of Middle to Lower Pennsylvanian sandstones along the frontal Ouachita thrust belt of west-central Arkansas. As natural gas reserves continue to decline in the “fairway region” of Arkansas, new exploration ventures are focusing on the deeper and more structurally complex natural gas resources located proximal to the frontal Ouachita thrust belt. The reservoir characteristics and thermal maturity profiles of three exploration gas wells are examined along the frontal thrust belt and compared to conventional gas reservoirs within the fairway region located to the north. Well cuttings and cores were sampled from the wells of the Shell, C.M. Bettis No. 1-14; Arkla, Edwards “B” No. 1 1-6; and the Hunt, USA No. 1-1 located in Scott and Yell Counties, Arkansas. Vitrinite reflectance values from the Edwards and USA wells are very high and range from Ro = 5.6 to 6.7% with corresponding depths from 9,010 to 16,670 ft. Core plug samples from the Bettis well at depths of 9,280 to 11,073 ft are characterized by low porosity (0.2% - 5.1%) and extremely low permeability values (0.0002 to 0.0035 millidarcies (md)). The authors conclude that the primary factor in the elevated vitrinite reflectance measurements and lower permeability and porosity values for the three wells is due to their proximity to the core region of compressional deformation associated with the Benton Uplift in the Ouachita Mountain region. Heated orogenic fluids expelled during the Late Paleozoic Ouachita Orogeny would likely exert more influence on the heat maturation process than other contributing geologic factors. Orogenic fluids combined with increased pressure along the frontal thrust belt resulted in a higher degree of mechanical and chemical compaction, quartz overgrowth, dissolution of soluble materials, and increased quartz and clay ingrowths compared to conventional sandstone reservoirs in the fairway region. Low porosity and permeability characteristics coupled with increased drilling depths and structural anisotropy along the frontal thrust belt will likely result in higher exploration and development costs compared to the fairway region. Hydraulic and chemical stimulation procedures are necessary to enhance gas production from the fine-grained “tight sandstone” reservoirs.