Salt Tectonics and its Effect on Sediment Structure and Gas Hydrate Occurrence in the Northwestern Gulf of Mexico from 2D Multichannel Seismic Data
Dan’l Lewis and William Sager
Department of Oceanography, Texas A&M University, MS 3146, College Station, Texas 77843–3146, U.S.A.
This study was undertaken to investigate mobile salt and its effect on fault structures and gas hydrate occurrence in the northwestern Gulf of Mexico. Industry 2D multichannel seismic data were used to investigate the effects of salt within an area of 7577 mi² (19,825 km²) on the Texas continental slope in the northwestern Gulf of Mexico. The western half of the study area is characterized by a thick sedimentary wedge and isolated salt diapirs whereas the eastern half is characterized by a massive and nearly continuous salt sheet topped by a thin sedimentary section. This difference in salt characteristics marks the edge of the continuous salt sheets of the central Gulf of Mexico and is likely a result of westward decline of original salt volume. Beneath the sedimentary wedge in the western part of the survey, an anomalous sedimentary package was found, that is described here as the diapiric, gassy sediment package (DGSP). The DGSP is highly folded at the top and is marked by tall, diapiric features. It may be either deformed shale or the toe of a complex thrust zone detaching the sedimentary wedge from deeper layers. The dataset was searched for the occurrence of bottom simulating reflectors (BSRs) because they are widely accepted as a geophysical indicator of gas trapped beneath gas hydrate deposits that are known to occur farther east in the Gulf. Although, many seismic signatures were found that suggest widespread occurrence of gas within the upper sediment column, few BSRs were found. Even considering non-traditional definitions of BSRs, only a few occurrences of patchy and isolated BSRs features were identified. The lack of traditional BSRs is likely the result of geologic conditions that make it difficult to recognize gas hydrate deposits. These factors include: (1) unfavorable layer geometries, (2) flow of warm brines from depth, (3) elevated geotherms due to the thermogenic properties of salt and its varying thickness, and (4) widespread low porosity and permeability sediments within the gas hydrate stability zone.
AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012