Defining a Steeply-Dipping Salt Flank in Mississippi with a New High-Certainty 3D Method

摘要

Salt-related structures are common along the U.S. Gulf Coast. Many conventional reservoirs (including giant fields) are related to these structures as they create pathways and traps for hydrocarbons. Salt domes and some isolated salt bodies commonly have steeply inclined and complex boundaries. Steep boundaries are difficult to image directly with the surface seismic method because reflections from these boundaries tend to be down going and not recorded with receivers at or near the surface. Such steep boundaries are commonly marked by the absence of amplitude on a seismic image or the termination of sub horizontal features. Sedimentary layers are generally dragged upward during salt movement and may reach dips up to 80-85 degrees at the contact with the salt body. Commercial hydrocarbon accumulations have been found in the cap rock of salt domes as in the famous case at Spindletop, but, as documented by many commercial finds, more hydrocarbon accumulation is to be expected along the flanks of the dome at greater depths. Exploring for such traps has been hampered by the inability of the surface seismic method to image the steeply dipping boundaries. We introduce a new method that is a significant improvement compared to traditional data processing in obtaining an image of the flank of a salt body. We use a standard vertical seismic profile (VSP) array tool and require no rigid interconnects or gyroscope, as is the case with conventional VSP salt proximity surveying. The principal method is a two-way vector reverse time migration (RTM) based approach complemented by an energy-scattering method that images the sediments that juxtapose the flank as well as increases the limits of the salt flank image. Each method is a new vector-based approach to analyzing three-component borehole seismic data. The results obtained from this 2013 VSP survey have been confirmed with real salt entry points and are used to enhance the processing and interpretation of the surface seismic data and reduce future drilling risk around the Midway Dome.