Seismic Expression and Sedimentologic Characteristics of a Permian (Wolfcampian) Carbonate Submarine Fan, Midland Basin, West Texas
Seismically mounded and channeled allochthonous carbonates of Late Wolfcamp age form small stratigraphic traps in the eastern Midland Basin, Texas. These reservoirs occur in three areally restricted seismic-facies packages confined to the lower slope and basin and were deposited during a lowstand in relative sea level. The producing reservoir is interpreted to be a carbonate lowstand submarine fan, based on external and internal mounded seismic signature, channeled seismic character, scoured lower boundary, allochthonous carbonate conglomerates, breccias and grainstones derived from the shelf edge, and deposition on a regional (extrabasinal) sequence boundary with well-documented subaerial exposure. The well-log response in the fan interval shows the unit to be thickly bedded, with a sharp base and low API gamma signature. Petroleum production comes mainly from primary and solution-enhanced primary porosity in carbonate grain-rich matrix and clasts.
Distribution of the unit suggests that the sediments were pointsourced from the Eastern Shelf and developed into a series of bifurcating channels on the basin floor. Wells that produce from the Wolfcamp line up along these channels, indicating that there is a strong stratigraphic control on the reservoir facies. Seismic modeling suggests that the fan unit is confined to channels or large-scale scours in updip and downdip positions.
One-and two-dimensional seismic modeling indicates that thickness and porosity are the principal controls on the seismic response of the submarine fan. Three different seismic signatures were observed: fan thickness of 30–40 m (90–120 feet) result in an amplitude anomaly on the basal reflection; where the fan is 40–50 m (130–160 feet) thick there is a cycle broadening of the basal reflection; and if the fan is greater than 45 m (150 feet thick, an extra cycle develops above the basal reflection. Zones of lower impedance developed within the fan generally decrease the minimum thickness at which these effects are observed.
KeywordsDebris Flow Seismic Response Highstand System Tract Platform Margin Basal Reflection
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