ABSTRACT

The salt-floored basin (S-FB) concept provides a dynamic view of a passive margin geosyncline evolution. This concept incorporates tectonics created by dynamics of sediment wedge, and tectonics of both salt and basement. Exploration of interaction between these different tectonic suites may reveal heretofore unknown specific hydrocarbon exploration targets and up-grade areas for further investigation. Salt domes, ridges, and massifs move basinward at rates of one to ten cm/yr, causing fracturing, thermal anomalies, and under- and over-pressure horizons. Basin-spanning shelf-edge growth faults sole out in shale layers; migrating salt can create counter-regional and local growth faults that terminated in semi-plastic salt and may create aureoles of fractures in front of the advancing salt. Counter-regional normal faults may exist in the sub-salt section shearing intra- and inter-salt providing minute, plastic deformation. Abundant earthquake activity (some 150 events over the last 50 years) in the northern Gulf of Mexico are of intensities up to 4.5 to 5.5 (Richter scale) and at depths from 2 to 28 km. These earthquakes provide energies for continuing tectonic deformation along existent faults and fractures as well as the earthquake generating tectonics within the sediments themselves, along the sediment-basement contact, and within the basement itself.

Increasing fluid pressures eventually fracture calcite-filled shale top seals of deep, overpressured fluid compartments. With salt absent and/or prior to salt intrusion, topography can influence sedimentation. With time, thinning and draping of deposits can focus subsequent bending, flexing, and fracturing over basement highs. Uneven basement subsidence can be caused by varying sediment loads, local geothermal anomalies, ancestral faulting patterns from pre- and post-rifting and then subsequent fault reactivation (note historic seismicity) and possible magma intrusions. Uneven subsidence maintains basement-related tectonics. The intersection of several tectonic types, regardless of origin, may improve hydrocarbon migration in a shattered sediment wedge as through a colander, not a sieve.