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Interpretation of four-component seismic data in a gas cloud area of the central Gulf of Mexico

Englehart Energy, Houston, Texas, U.S.

Santi Randazzo, Allen Bertagne and Bill Cafarelli

PGS, Houston, Texas, U.S.

Corresponding author: T. Englehart, tom@englehartenergy.com

Corresponding author: A. Bertagne, allen.bertagne@pgs.com

The first 20% of the full text of this article appears below.

Although the Gulf of Mexico is a mature producing basin, additional reserves continue to be discovered via application of new geophysical technologies. Contributing to current record success rates are 3-D seismic data, powerful workstations with interpretation software, direct hydrocarbon indicators, AVO, and prestack depth migration. Recently, marine four-component (4-C) methods involving simultaneous recording of compressional (P-wave) and converted shear (C-wave) data have started to gain acceptance (see TLE's October 1999 special section). Although applications of 4-C technology are numerous, the best known are where upward seepage of gas from hydrocarbon accumulations results in a "cloud" that absorbs P-wave energy and causes significant velocity-related distortions. The combination of that and other negative effects can obscure the deeper subsurface image, making traditional interpretation difficult.

This paper focuses on a field on the southern central part of the offshore Gulf of Mexico shelf that exhibits gas cloud problems. The following discussion covers the geology of the field and summarizes results from three key wells that were drilled in 1998 and early 1999 (two producers and one dry hole). It next describes the analysis and interpretation of the combined P-wave and C-wave data resulting from four-component acquisition and closes with comments concerning the future of such applications in the Gulf of Mexico.

	Background

 
The field, a NE/SW structure in South Marsh Island Blocks 141 and 144, consists of faulted highs related to deep-seated salt movement (Figure 1). It was discovered in 1984 by Marathon, whose A platform ultimately produced 52 billion ft³ of gas from four Pleistocene inner shelf sandstones at depths of 2100–6400 ft subsea. More than half of that production was from the deepest E sand reservoir. Data from five wildcats and 10 development wells provided excellent subsurface control with reliable sand correlations . . . [Full Text of this Article]