ABSTRACT: Shale-Filled Incised Channels in the Frio Formation, South Texas

NICE, DAVID E., MARTIN L. THERING, and KEITH R. MILLER, Exxon Co. USA, Corpus Christi, TX

The Mariposa South field, Brooks County, Texas, is a stratigraphic trap situated in a structural syncline. Seismic facies mapping, 2-D seismic modeling, and sequence stratigraphic concepts were integrated to develop a depositional model in which a shale-filled channel truncates older reservoir facies and defines the reservoir limits.

Two seismic facies are identified in the field area. Productive sandstone reservoir facies are characterized by high-amplitude, continuous seismic reflectors whereas shale channel-fill facies are characterized by low amplitude, discontinuous seismic reflectors. The shale-filled channel exhibits a concave upward cross-sectional geometry and truncates the high-amplitude, continuous reflectors. The shale-filled channel facies provides the lateral, updip seal for the reservoir. The 2-D seismic modeling supports this channel fill model.

All available well data have been incorporated into the model to develop a sequence stratigraphic framework. Until recently, the only successful penetration of reservoir facies was the discovery well, which is located outside the channel as defined by the distribution map. Within the channel boundaries, several wells penetrate the correlative interval, which is characterized by shale that exhibits a flat SP response. The shales differ from the usual silty shales of coastal plain origin that are more typical of the area and are interpreted to be marine. These marine shales are interpreted to have been deposited during a rapid transgression that followed the downcutting event which incised the channels.

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