Understanding the Eocene Reservoirs of the Forth Field, UKCS Block 9/23b

Abstract

Forth Field reservoir sands are of Early Eocene age, and were deposited in a basinal marine setting. The discovery well penetrated alternating sands and muds underlain by 123 m of homogeneous, structureless sands. A conventional depositional model suggested that this stratigraphy was the result of turbidite deposition on a retreating submarine fan. A fundamental problem arose in that the sand envelope defined on seismic data suggested a confined area of sand occurrence which could not have been stable as a mound of unconsolidated sediment on the seafloor.

A multi-disciplinary approach to field appraisal was initiated, involving detailed interpretation of reprocessed seismic data in association with a comprehensive appraisal drilling program, rigorous sedimentology, detailed biostratigraphy and an innovative log correlation technique. The present reservoir geometry has resulted from a unique combination of the depositional mechanism and post-depositional disturbance. Rapid lateral thinning of the sand is due to deposition within steep-sided erosional scours associated with shelf-edge canyons, which confined and focused high density turbidite flows. Alternating sands and muds overlying the main reservoir formed due to the explosion of the sand body during the expulsion of excess pore water.

Understanding the unusual geometry of the Forth reservoirs is crucial to future development plans. Individual sand bodies will behave as isolated pools with excellent productivity but limited aquifer support. Maximum recovery from these reservoirs will depend critically on optimum placement of production and water injection wells.