A layer of permafrost, approximately 500m thick, lies immediately beneath the surface of Arctic Alaska’s Central North Slope. This frozen layer of unusually high seismic velocity overlies more than twenty producing fields containing over 60 Billion in place barrels of light and heavy oil. Multiple reservoir levels ranging in depth from 1000m to 3000m are nominally imaged using conventional surface 3D seismic technologies. Natural arctic surface features and man-made sub-surface activities produce local bodies of unfrozen sediment with anomalously low velocity in the surrounding high velocity permafrost. Abrupt lateral and vertical velocity variations detrimentally affect the conventional surface seismic image at reservoir depths, thereby negatively impacting our ability to describe the reservoirs and maximize production of this vast resource.
BP-Alaska, as operator of many of these fields, is motivated to apply optimal seismic acquisition and processing techniques to minimize permafrost-related image distortions. We are therefore developing a permafrost velocity characterization of both the offshore and onshore environments on the Arctic Central North Slope to guide the appraisal of available geophysical techniques. Our characterization is underpinned by: 1) physical properties of frozen and unfrozen sediments taken from the literature, 2) regional and local morphologies of the permafrost based on well log measurements, 3) the application of numerous geophysical techniques for the widespread measurement of shallow (upper 600m) velocities, and 4) a dynamic model for thawing of the permafrost caused by either naturally occurring or man-made circumstances to help predict the extent, shape, and depth of local velocity anomalies.
BP-Alaska is making progress in understanding the permafrost velocity field on the Central North Slope; but gaps in our ability to characterize a large portion of the permafrost interval still persist. Some techniques show promise but may not be economic in a brown field business environment located in an arctic acquisition environment.