Middle and Upper Devonian subsurface rocks in six counties in northwestern West Virginia can be divided into formal units of organic-rich black shales overlain by inorganic gray and greenish-gray shales. The section thickens west-to-east from 2,000 to 4,000 feet, including some fine sandstones and siltstones in the upper part.
Drilling since 1980 has discovered several productive intervals in this thick sequence. Generally, younger siltstones and shales down to the Huron Shale produce oil and gas; below the Huron, gas dominates. In map view, three broad oil and gas plays can be related to facies changes and to the Burning Springs Anticline. West of this structure, both organic-rich and inorganic shales produce gas; on and immediately east of the anticline, transitional facies (including black and gray shales, and younger siltstones) produce oil and gas; and farther east, siltstone bundles produce gas and some oil. Thus, completion-zone thickness ranges between 30-2,000 feet, with varied completion techniques.
Under a current Gas Research Institute contract, we are seeking possible relations among stratigraphy, completion intervals and techniques, initial potential (IP), and production. Preliminary studies in three counties indicate that gas IPs are highest when three or more formations are completed using nitrogen-fracturing. In one first year production study, the average shale oil well east of the Burning Springs Anticline is short-lived, declining by 95% of IP after one year before leveling off. A similar first year production study for 3 adjacent counties indicated less rapid production study for 3 adjacent counties indicated less rapid decline of oil wells located along the Burning Springs Anticline.
Historically, gas producers in West Virginia have drilled Devonian shale wells in areas of known production, all in the western one-third of the State. In that area, the shale section is relatively thin, between 1,000 and 2,500 ft., and consists of two main lithologies: gray shale and black shale. Production generally was established by shooting either the upper black shale section (Huron) or the Huron and lower (Rhinestreet) black shales, plus the intervening gray shale.
Recently, however, deeper-pool tests in low Mississippian fields over and near the Hiring Springs Anticline have discovered both gas and oil at several depths in the shale section, in several counties (Figure 1). In this newly productive area, the shale section is thicker, between 2,000 and 4,000 ft., and consists of interbedded lithologies that include fine-grained sandstones, siltstones, and both gray and black shales. Furthermore, production has been established by completing various combinations of lithologies over intervals ranging from 30 to 2,000 ft., utilizing several completion techniques, including nitrogen, carbon dioxide, and foam fracs. As measured by initial potentials (IPs) reported on completion forms, results range from poor to exceptional.
The broad differences in number of formations completed, thickness of zones completed, completion technique, and results, lead to some questioned: What zones and what techniques yield the best wells? Also, because of the vast number of wells that have been drilled and completed in the area, how can we best handle these diverse data for a large number of wells.
To answer these questions, a three-county area of high current activity was selected to study the relations among stratigraphy, completion techniques, and initial potential.