--> Abstract: Amplitude Anomalies, Condensed Sections and Sequence Boundaries, Green Canyon Area, Gulf of Mexico, by R. W. Keach, II, R. K. Woidneck, and C. L. Blankenship; #91012 (1992).

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ABSTRACT: Amplitude Anomalies, Condensed Sections and Sequence Boundaries, Green Canyon Area, Gulf of Mexico

KEACH, R. WILLIAM, II, R. KEITH WOIDNECK, and CYNTHIA L. BLANKENSHIP, BP Exploration, Houston, TX

The Green Canyon 92 #1 well targeted a lower Pleistocene amplitude anomaly interpreted to represent a hydrocarbon-bearing sand at the base of a ponded, intraslope fan system. Well results indicate that shale-shale interfaces at the top of condensed sections can produce seismic amplitude anomalies comparable to those from hydrocarbon-bearing sands. The target anomaly was not caused by a hydrocarbon-bearing sand, but rather occurs at the boundary between a high acoustic impedance silty-shale over a low acoustic impedance clay-rich shale. The prediction of a fan system was correct; however, the seismic expressions of the various facies were not as expected. A low reflectivity zone above the target anomaly, expected to represent a shale sequence, actually contained abundant (247 net feet) sand.

In the GC 92/136 area, acoustic impedance values for shales (silty and clay-rich) and sands overlap considerably. Silty shales have the highest acoustic impedance values of all lithologies. Strong negative reflection coefficients result when silty shales overlie the clay-rich shales that often occur at or near sequence tops. The low acoustic impedance, clay-rich shales typically exhibit high neutron porosity and abundant nannofossils and are interpreted to represent condensed sections. Thus, in this area, condensed sections are "soft" streaks rather than "hard" streaks and often serve as the major seismic mapping surfaces in the Pliocene-Pleistocene.

 

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