ABSTRACT: Depositional Setting of Lowstand Carbonates in the Midland Basin-BC (Canyon) Field, W. Texas

SALLER, ARTHUR H., Unocal Science & Technology, Brea, CA, M. J. FRANKFORTER and S. A. BOYD, Unocal Oil & Gas, Midland, TX, and G. A. LIVESAY, Unocal Canada, Calgary, Alberta, Canada

The reservoir at BC (Canyon) field is a limestone deposited during a lowstand of sea level in Middle Pennsylvanian (Canyon) time. BC (Canyon) field is located in the Midland basin east of the Central Basin platform, south of the Horseshoe atoll and west of the Eastern shelf, which were depositional highs and sites of carbonate sedimentation during Pennsylvanian highstands of sea level. A lack of widespread basinal clastics allowed carbonate factories to develop in the basin during lowstands of sea level. Porosity occurs in ooid grainstones and phylloid bafflestones deposited on local highs. Canyon limestones in the reservoir interval can be divided into three parts: lower, middle, and upper. The lower part (30 ft thick) contains bryozoa-phylloid mounds that formed depositional highs w ich controlled subsequent Canyon sedimentation. The middle part of the Canyon (50 ft thick) contains six ooid-rich, shallowing-upward parasequences capped by subaerial exposure surfaces. These parasequences become thinner upward. The upper part of the Canyon (60 ft thick) contains three parasequences of phylloid wackestone and boundstone. The upper interval is overlain by crinoidal grainstones, wackestones, dark organic-rich mudstones, and lithoclastic conglomerates that were deposited as sea level rose and the carbonate factory drowned. Porous ooid grainstones on depositional highs pass offstructure into tight fossiliferous wackestones and packstones. Porous phylloid bafflestones on depositional highs pass laterally into tight organic-rich mudstones and lithoclastic conglomerates. Phyll id bafflestones preferentially occur in thicker parasequences (20-30 ft thick), whereas ooid grainstones preferentially occur in thinner parasequences (5-15 ft thick). Most porosity is secondary, created during subaerial exposure shortly after deposition.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)