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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 12. (December)

First Page: 1925

Last Page: 1925

Title: Upper Devonian First Bradford Formation of Southwestern Pennsylvania: Environment of Deposition and Factors Affecting Gas Production: ABSTRACT

Author(s): Timothy M. Murin, Jack Donahue


Core descriptions and thin-section analyses from five wells in southwestern Pennsylvania, in addition to regional cross sections and subsurface maps, provide the basis for subdividing the Upper Devonian First Bradford formation into three distinct members. These include upper, middle, and lower sandstones. Although individual members vary in geometry and areal distribution, similar characteristics in sedimentation persisted over a distance of 125 km (78 mi) along strike and 25 km (16 mi) along dip. The sandstones are generally very fine to fine grained, exhibit upward coarsening, and are calcite cemented. Depositional structures range from planar bedding to cross-laminations, which include flaser, wavy, and lenticular stratification. Bioturbation and disseminated organic ebris are common.

Isopach maps show the sandstones to be strike-oriented linear ridges and/or pods 1-3 km (0.6-1.9 mi) wide and 3-20 km (1.9-12.4 mi) long. Thicknesses rarely exceed 10 m (33 ft). Regionally, sand percentage for the overall interval (approximately 46 m or 151 ft) decreases west and southwest of Clearfield County where only the lower sandstone is preserved in parts of Armstrong and Westmoreland Counties. The postulated depositional environment is a shallow marine, inner to mid-shelf bar facies. The sediments accumulated as a result of current reworking; in places, deposition was controlled by shelf topography. Sediments were probably derived from a deltaic complex in the vicinity of Clearfield County.

Gas production is confined to the stratigraphic limits of the clean sandstones and is locally enhanced by a high structural position. This "combination trapping" permits the segregation of gas and water and is evidenced by production in Westmoreland and Fayette Counties. Core porosities average 6.7%; permeabilities, with an effective overburden pressure of 1,000 psi (6,895 kPa), average 0.04 md. Production capabilities are ultimately affected by various diagenetic influences, which include compaction, cementation, authigenesis, replacement, and leaching.

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