The Impacts of Trap-to-Kitchen Axial Orientation and Natural Gas Sweeping on Migration Efficiency: A Multi-Field Case Study From the West of Shetlands (United Kingdom)

Abstract

The process of secondary migration, i.e., the movement of hydrocarbons from the source rock through a carrier system to a trap, is the least understood aspect of the petroleum system and yet has a substantial impact on hydrocarbon volume estimates. This process is numerically accounted for via a migration efficiency factor, which is defined here as the proportion of petroleum accumulated in a trap relative to the total amount of petroleum expelled within that trap's fetch area (assuming no limit on trap size). Previous studies have demonstrated a number of controls on migration efficiency, including length and intricacy of the migration path (e.g., Sluijk & Nederlof, 1984) and style of migration (vertical versus lateral) (e.g., Demaison & Huizinga, 1991), with typical basin-wide migration efficiencies averaging around 10% (Katz & Kahle, 1989). Here, we present a migration efficiency case study from the Clair field and surrounding structures located in the West of Shetlands (United Kingdom). Clair is a giant Paleozoic/basement oil field located along the Rona Ridge, with an estimated recoverable resource of 1100 MMbbls (IHS EDIN, 2016) sourced from the Kimmeridge Clay Fm. in the Foula and West Shetland basins. A 3D basin model constructed over the area of interest was used to generate thermal maturity and drainage area maps through time. These inputs, coupled with data-constrained source rock richness and net thickness estimates, were then maintained constant to estimate the migration efficiencies necessary to account for reported oil-in-place volumes for Clair and surrounding fields. We found that the migration efficiencies necessary to account for hydrocarbon volumes reported for Clair exceed the range of typical reported efficiencies. We attribute the high efficiency of this system to the parallel orientation of the Clair structure relative to basinal axes of the Foula and West Shetland source kitchens, which maximized the Clair field's access to available hydrocarbon volumes. We also consider the impact of the high maturity of the Kimmeridge Clay source rock in the basinal parts of the kitchen, which may have resulted in gas “sweeping” of the carrier system. This study provides a potential migration efficiency analog for similarly oriented fields, as well as highlights the need to estimate migration efficiencies on a field-by-field basis rather than relying upon a global mean value.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017