REGIONAL STRATIGRAPHIC AND SEDIMENTARY FACIES ANALYSES, BARNETT SHALE, FORT WORTH BASIN, TEXAS

摘要

In June 2006, the Barnett Shale in the Fort Worth basin was the top producing gas field of its type in the world with a daily gas production rate of 1.97 Bcf. Barnett Shale gas in place reportedly exceeds 25 Tcf. Despite the importance of this resource and numerous past studies of the Barnett Shale, causes of regional and local differences in gas production rates are poorly understood. To evaluate control on Barnett shale gas production, we used well logs from the Fort Worth basin to analyze the structure and stratigraphy of the Barnett Shale and adjacent formations.The Barnett Shale is a 50- to 1,100-ft thick formation comprised of multiple, thin, clay-, quartz-, carbonate-, organic- and phosphate-rich strata that result from distinctive depositional environments and episodes in a deep-water basin. A northeast-trending depocenter persisted in the northeast part of the Fort Worth basin throughout Barnett deposition, as evidenced by presence of anomalously thick strata in that area. In the northeast part of the Fort Worth Basin, where the Forestburg Ls. is present in the depocenter, the Barnett Shale is divided into upper and lower members. In a detailed stratigraphic analysis, we identified 10 stratigraphic sequences in the lower Barnett Shale and 3 sequences in the upper Barnett Shale. Then, on the basis of well log patterns, lithology, and petrophysical properties, we upscaled these 13 Barnett Shale sequences into four reservoir units and mapped their thicknesses. These reservoir units will be used to assess effects of reservoir properties on well performance.Reservoir Unit 1, at the base of the lower Barnett Shale is composed primarily of phosphate- and organic-rich shales and mudstones and ranges from 10 to 200 ft thick. Reservoir Unit 1 has a serrate gamma ray well log response. Commonly, Reservoir Unit 2 is marly carbonate zone in the middle of the lower Barnett shale. It ranges from 20 to 300 ft thick and has a uniform, medium-high gamma ray log response. Reservoir Unit 3, the upper unit of the lower Barnett Shale is composed of multiple, stacked, thin (15 to 30-ft thick) sequences that coarsen upward from basal, phosphatic shale (gamma ray spikes) to dolomitic mudstone and detrital silt. Gross thickness of Reservoir Unit 3 is 0 to 500 ft. Reservoir Unit 4, the upper Barnett Shale, is composed laminated, siliceous mudstone with interbedded shell-rich carbonate layers and phosphatic hardgrounds that result in laterally persistent hot streaks (high gamma ray spikes). Reservoir Unit 4 ranges from 0 to more than 100 ft thick. Preliminary evaluation of production data from vertical wells indicates that Reservoir Units 3 and 4 are most commonly perforated, whereas Reservoir Unit 2 is sometimes perforated and Reservoir Unit 1 is rarely perforated.