Understanding characteristics of the velocity trend of subsalt sediments and its spatial variation is essential to build a realistic earth model, both for subsalt exploration and appraisal in the deep-water Gulf of Mexico.
A study on velocity and pore pressure trends using over 300 wells in the central Gulf of Mexico revealed that subsalt velocity not only depends on variations in mineralogy and porosity, but also upon local stress state, or effective stress regime. Effective stress refers to the difference of overburden pressure and fluid pressure in the pore space. Both overburden and pore pressure are affected by salt bodies due to perturbation in the density profile and permeability barrier of the salt. Hence, the existence of salt, its scale, and sealing capacity also cause variations in velocity of the subsalt sediments. In addition, shale diapirs and carbonate rafts near the base of salt further add to the complexity of the velocity in the subsalt sediments.
Throughout most of the central Gulf of Mexico, at a depth beneath thick salt canopies, low- and near-constant velocity (as a function of depth) in the Upper Tertiary (Miocene) sediments are often observed. The low-velocity trend is caused by the lack of compaction due to reduction of effective stress with burial depth. Below the low-velocity sediment interval, velocity ramps up in the Lower Tertiary and Cretaceous sediments. Wells in the Tonga and Tahiti fields in the Green Canyon area in the northeast, wells in the Shenandoah, Yucatan, and Lewis prospects from the Walker Ridge area, and those in the Rickenbacker and Voss prospects from the Keathley Canyon area to the southwest are of this type. This pattern changes in wells further southeast, especially those southeast of the Sigsbee Escarpment, where the salt becomes less patchy and at a somewhat shallower depth.
AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016