Depositional Facies and Diagenesis of Upper Pennsylvanian Carbonate Reef Reservoirs, Ropes West Field, Hockley County, Texas

T. Wingate, S. J. Mazzullo

Ropes and Ropes West fields are located in southeastern Hockley County, Texas, in the northern Midland basin. The fields produce oil from Upper Pennsylvanian reefs that lie seaward of the continuous shelf edge to the west and, accordingly, are regarded as foreshelf atolls similar to several fields along the Eastern shelf. Cores from three wells in Ropes West field were examined in order to understand controls on the depositional and diagenetic origin of the field.

The productive zone in the field consists of a shoaling-upward sequence of foram and phylloid algal wackestone to packstone capped by fusulinid grainstones. This section is abruptly overlain by dark shales that represent drowning of the atoll. Both the foram and phylloid algal wackestones comprise the reservoir facies in the field, whereas the fusulinid grainstones are nonporous. Petrographic examinations and stable oxygen and carbon isotope studies suggest that the pore system was created in the burial diagenetic environment. Pore types include foram and algal-blade molds (solution enlarged to form vugs), solution-enlarged fractures, and, locally, open stylolites. Partial occlusion of this porosity in the burial environment was affected by the precipitation of saddle dolomite and coa se crystalline calcite cements.

Comparison of Ropes West and Higgins Ranch field (Coke County) suggests similar styles of facies development and regional depositional setting. Comparison of diagenetic patterns between the two fields, however, is less clear. Whereas porosity in Higgins Ranch field appears to have resulted from subaerial exposure, such is not readily evident at Ropes West field. Regardless, inferred histories of sea level and/or subsidence changes are similar between the two fields and point to regional controls on facies patterns during the Late Pennsylvanian. Recognition of similar facies development in the Midland basin, by virtue of detailed sedimentologic studies, has the potential for creating new plays in this mature hydrocarbon basin.

AAPG Search and Discovery Article #91026©1989 AAPG Southwest Section, March 19-21, 1989, San Angelo, Texas.