SupraSalt 3D Basin Modeling and Shallow Borehole Results
Authors: Hall Jean Sebastien , Mitchell Stephen Paul , Scherschel Craig , Greeley David F , Jeanjean Philippe 
Published: May 2013

A 3D basin model was created for the deepwater Tubular Bells and Kodiak fields development in the Mississippi Canyon Protraction Area of the Gulf of Mexico. The primary purpose of the model, which includes fluid flow simulation, was to enhance knowledge and understanding of the geologic controls on pore pressure in the shallow section between the seabed and the top of the allochthonous salt canopy. Specifically, geologists sought to quantify overpressures within the shallow sands, which have caused shallow water flow issues in several offset wells, and to map the various rock properties of the shallow sediments in support of future field development. Although a number of exploration and appraisal wells have been drilled below the salt canopy, measurements taken above the salt canopy are quite sparse. In order to calibrate the basin model, two sets of geotechnical boreholes were drilled between 750 ft and 1,000 ft below mudline. Wireline logs, cores, and piezoprobe measurements were collected. The resulting data indicate that, in this area, overpressure starts very close to the seabed. However, the overpressure is somewhat less than that experienced at the Mars Field in the Ursa Basin to the west. Finally, given the initial estimates of sedimentation rate, compaction, and permeability, the original 3D basin model's predicted pore pressures match the shallow borehole data quite well. Thus, there is a degree of confidence that the basin model's results, extrapolated away from well control, can be used by geotechnical staff in planning the field development of this area.

Introduction

Basin models allow geologists to maximize their understanding of an entire basin, including the shallow section's geohazards (e.g., shallow water flows due to overpressure) before the drilling process begins. Specifically, 2D and 3D models of an offshore basin, built before exploration and development wells are drilled, can significantly benefit upstream endeavors by providing characterization of sedimentary layers, pressures, and hydrocarbon migration in the subsurface. A geologic (sedimentary) basin is an area that, as a result of a variety of processes, experiences sedimentary infilling over long periods of time. A basin model, in turn, is essentially a geologic model through time that recreates the physicochemical processes beginning with the first sedimentary layer from 10s to 100s of millions of years ago to the present day.

" Recent work has focused on how sedimentation and common stratigraphic architectures couple to produce two- and three-dimensional flow fields. For example, if a permeable sand is rapidly loaded by a low-permeability mud of varying thickness, fluids flow laterally to regions of low overburden before they are expelled into the overlying sediment. This creates characteristic distributions of rock properties, fluid pressure, effective stress, temperature, and fluid chemistry in the aquifers and bounding mudstones. This simple process can cause slope instability near the seafloor; in the deeper subsurface, this process drives fluids through low-permeability strata to ultimately vent the seafloor " (IODP.org)

In 2010, geologists built a 3D basin model of the Tubular Bells and Kodiak fields of the Gulf of Mexico and then, in 2011, they drilled boreholes that had been placed strategically in pairs at two locations in the Kodiak field. The data collected was used to verify the accuracy of the original 3D basin model, thereby providing a significant contribution to plans for the fields' development. This paper provides the results from these efforts.