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Abstract

3-D surveys have proven to be a powerful structural risk reducer for oilmen in West Central Texas. While the biggest structures were found using single-fold and 2-D common depth point (CDP) shooting, the success rate for stepout Mississippian and Canyon reefs drilled on 3-D surveys has increased substantially in this area: 3-D surveys have gained wide acceptance by the oil community in North and West Central Texas.

Whereas 2-D has been relatively ineffective in stratigraphic situations, this paper will showcase a 3-D survey which was used as a tool to pinpoint a shallow 1500-ft (460-m) meandering channel in the upper Pennsylvanian Cisco system.

Regional studies report evidence of a lowstand of sea-level for this formation, locally called the King, which occurred near the end of deposition (Brown et al., 1990). The 3-D data appear to demonstrate the presence of another relative lowstand of sea level, which occurred early during deposition of this formation.

Significant detail can apparently be seen in these data, including point bars, levees, and overbank deposits. The data appear to correlate with all logs that penetrate the 3-D volume: i.e., where a well penetrates the channel location depicted in the seismic image, the logs show evidence of a channel.

This 3-D survey was designed for objectives at 3000–6000 ft (425–1830 m). Because of this, the fold at the shallow fluvial unit is quite low (three- to four-fold). Additionally, the calculated bandwidth of the data is extremely high, in the 200 Hz and greater range. These data were acquired with an I/O II system and deep-hole dynamite, illustrating the increase in resolution available with modern 24-bit systems.

Because an oil show was reported in this zone during drilling for a deeper objective, a well was drilled to test a point bar identified in the 3-D data. Although no oil was found, an excellent point bar was encountered, which further authenticated the seismic survey's ability to provide stratigraphic detail about this shallow fluvial unit. This is significant, because most of the oil remaining to be found in this and other areas will come from stratigraphic traps.

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