SALLER, ARTHUR H., Unocal S & T, Brea, CA, MATTHEW J. FRANKFORTER, Unocal International, Sugar Land, TX, and STACIE A. BOYD, Unocal Oil & Gas, Midland, TX

ABSTRACT: Depositional Setting for Lowstand Carbonates in BC (Canyon) Field, Howard County, Texas

The reservoir at BC (Canyon) field is shallow-marine limestones deposited in the Midland basin during lowstands of sea level in the Middle Pennsylvanian (Canyon). A lack of widespread basinal clastics allowed carbonate factories to develop in the basin during sea level lowstands. Porosity occurs in ooid grainstones and phylloid bafflestones on depositional highs. The Canyon section was divided into four parts labeled, from top downward, "A," "B", "C," and "D." The Canyon "A" and "D" are generally deeper water limestones and shales. Porous, shallow-marine limestones occur in the Canyon "B" and "C." The lower part of the Canyon "C" (30-ft thick) contains bryozoa-phylloid mounds which formed depositional highs that controlled subsequent sedimentation. The upper Canyon "C" (50-ft thick) c ntains six ooid-rich, shallowing-upward cycles capped by subaerial exposure surfaces on depositional highs. Porous ooid grainstones on depositional highs pass offstructure into tight fossiliferous wackestones and packstones. The overlying Canyon "B" (60-ft thick) contains three cycles of phylloid wackestones and boundstone on depositional highs. Porous phylloid bafflestones on depositional highs pass laterally into tight organic-rich mudstones, shales, and lithoclastic conglomerates. The Canyon "A" contains crinoidal grainstones, wackestones, dark organic-rich mudstones, shales, and lithoclastic conglomerates, which were deposited as sea level rose and the carbonate factory drowned. Most porosity is secondary, created during subaerial exposure shortly after deposition.

AAPG Search and Discovery Article #90994©1993 AAPG Southwest Section Meeting, Fort Worth, Texas, February 21-23, 1993.