Impact on Hydrocarbon Retention of Sedimentary Fracturing and Faulting Around Salt Domes and Salt Sheets in Gulf Coast

S. Cao, I. Lerche, J. J. O'Brien

Emplacement of diapiric masses can have a profound effect on the structure of the formations overlying the diapir and on the formations being intruded. This influence may be manifested in many ways, such as the deformation of these formations to accommodate the intrusion, development of faulting in the supradomal formations, modification of the depositional pattern resulting in thinning of beds as one approaches the dome, and the development of a rim syncline at depth. These features may, in turn, also impact the further development of the salt diapir and may also create an environment favorable for the trapping of hydrocarbons.

In this paper, we present a model of the initiation of faulting in the formations overlying a salt diapir that is being emplaced at depth. A proposed technique to estimate the stress induced in the formations surrounding the uprising dome is based on analysis of the deformation induced in these formations and is expected to be valid for small to moderate levels of deformation. To underscore the physical processes that are operating, we invoke a model of the deformation of the sedimentary formations surrounding a salt dome; however, this analysis may also be extended to the situation where the deformation of the formations is estimated from the available observational data. Having specified the deformation of the sedimentary formations, we then infer the stresses that generated these d formations through application of the theory of elasticity. In this way, we can model the state of stress in the formations surrounding a salt dome during the time in which the dome is being emplaced.

In addition, the failure of sediments as a consequence of lateral motion of a salt sheet is examined. Variations in behavior are caused by salt thickness, depth of emplacement, speed of motion in relation to sediment deposition rate, and increases of sedimentary elastic constants with depth. Mohr's criteria for failure is typically satisfied two to five thicknesses ahead of the moving salt sheet as well as about a salt thickness or two to the sides of the salt. Failure angles range from acute to obtuse, and a pronounced asymmetry in pattern development is apparent above and below the salt. Inferences are discussed for fractured reservoirs above and below the salt, for detachment surfaces and normal and antithetic faulting, as well as for fracturing acting as an aid to ease the salt mo ion. The results obtained impact on the understanding of salt sheets as areas of prospective hydrocarbon accumulations.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.