Abstract

Data from geologic and geochemical studies of the New Albany shale group indicate that a 19-county area of southeastern Illinois is a favorable area to explore for gas in Devonian shale. Although gas shows in the shales have been encountered in several wells drilled in this area, no attempts were made to complete or evaluate a shale gas well until 1979. In 1979, core samples from two Wayne County wells were obtained, permitting the first quantitative assessment of gas content of the shales in this area. Seventy core samples from the two wells were sealed in airtight canisters at the drilling site for off-gas analysis. The quantities of shale gas released from the core samples in a 34-day period ranged from 0. 16 to 2.40 cu ft HC/cu ft (0. 16 to 2.40 m3 HC/m3) of shale. This gas is richer in heavy hydrocarbons than is the average "pipeline gas". It has a calculated average BTU value of 1,240 Btu/cu ft (46 200 kJ/m3). Because the proportion of non-hydrocarbon gases is unknown, the calorific content of produced gas would necessarily be slightly lower. The gasbearing intervals generally correspond with high radioactivity intervals on gamma-ray logs. The quantities of shale gas in core samples from these wells are similar to those in gas-producing areas of Devonian shale in the Appalachian basin. However because the New Albany shale is much thinner than its Appalachian basin equivalents, the total gas resource is probably much smaller. Conventional rotary drilling with mud base drilling fluids likely causes extensive formation damage and may account for the paucity of gas shows and completion attempts in the Devonian shales; therefore. commercial production of shale gas in Illinois probably will require novel drilling and completion techniques not commonly used by local operators.

Introduction

U.S. production of gas from Devonian black shales began in 1821 with the drilling of a well near Fredonia, N.Y. In the ensuing 161 years, production from Devonian shales has extended into eastern Kentucky, southern and western West Virginia, and scattered areas of Pennsylvania, Ohio. New York, western Kentucky, and Indiana. Currently, about 9,600 wells produce gas from shale in the Appalachian basin. One giant gas field the Big Sandy field in eastern Kentucky, has produced more than 2 Tcf (5.6×10(10) m3) of gas from Devonian shale. The Devonian shales cover a broad area of the north-eastern U.S., extending westward into the Illinois basin, where they are called the New Albany shale group (Devonian Mississippian). Although lithologically similar to the Devonian shales of the Appalachian basin, the New Albany has produced gas in only a few small areas of Indiana and western Kentucky. No gas production is known from the New Albany in Illinois. Because of the great lateral extent and thickness of the shales, however, resource estimates of the total gas that may be trapped within them are vast. Producing gas from the shales has not been commercially attractive in the Illinois basin because of technological constraints (low production rates) and price regulations. If suitable technologies can be developed to recover the gas from these low-permeability reservoirs and thereby increase the productive capacity of wells, the Devonian shales might become an important source of natural gas in the future. In mid-1976, the Illinois State Geological Survey began a detailed study of the geology and geochemistry of the New Albany shale in Illinois to evaluate its potential as a source of hydrocarbons, especially gas.

SPEJ

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