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Salt Tectonics and Effects on Sediment Structure and Gas Hydrate Occurrence in the Northwestern Gulf of Mexico from 2-D Multichannel Seismic Data

Lewis, Dan'l *1; Sager, Will 1; Nemazi, Leslie 2
(1) Oceanography, Texas A&M University, College Station, TX.
(2) Devon Energy, Houston, TX.

This study was undertaken to investigate mobile salt and its effect on fault structures and gas hydrate occurrence in the northwestern Gulf of Mexico. Due to mobile salt bodies, overlying sediments in the northern Gulf of Mexico are extensively deformed and faulted. We used industry 2-D multichannel seismic data to investigate the effects of the salt within an area of 53,625 km2 in the northwestern Gulf of Mexico. The western half is dominated mostly by a thick sedimentary wedge and isolated salt diapirs. Faults in this half of the study were long, deep and linear. They trended roughly parallel to the slope and represent faulting driven by gravitational sliding of the sediment wedge. The eastern half is dominated by a massive and nearly continuous salt sheet characterized by minibasins and a thin sedimentary section. Faulting in this part of the study is characterized by faults that extend to the top of salt as well as radial fault structures related to salt highs. The different styles of faulting and salt bodies can be attributed to the current volume of salt distributed across the region and to different sediment regimes and how these depositional regimes changed through time. We also found a sediment package with a unique upper surface. This package, labeled the deep, thick, gassy sediment package, or DTGSP, is usually found at depths >5s and in the NW corner of the survey area, between isolated salt diapirs. The DTGSP is highly folded at the top and is usually marked by gas chimney features. These chimneys are likely a conduit for gas or fluid movement and in some cases have radial fault structures around them. We searched for the occurrence of bottom simulating reflectors (BSRs). BSRs are widely accepted as a geophysical indicator of gas hydrate in seismic sections. We found a high occurrence of gas throughout the study area, as indicated by acoustic wipe out on the seismic lines. Non-traditional BSRs were also considered in our search; however, only limited occurrences of small, patchy, and isolated BSRs were found. We considered several factors for lack of BSRs including lack of source, fluid escapes and thermal instability due to the thermogenic properties of salt. The high volume of gas but lack of BSRs indicates an unfavorable thermal gradient due to the narrow GHSZ. We also considered the sediments may not be a good candidate for BSR creation as most are composed of low porosity mud within our modeled GHSZ.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California