Contrasting Styles of Eolian, Fluvial, and Lacustrine Sequence Development: United Kingdom Southern North Sea

M. L. Sweet: ExxonMobil Upstream Research Company, Houston, TX

Abstract

A sequence stratigraphic framework for the Permian, non-marine rocks of the southern North Sea was constructed by recognizing and correlating lacustrine transgressive surfaces, eolian sequence boundaries and fluvial erosion surfaces. These surfaces and the stacking patterns of the rocks they bound were found to have markedly different expressions in lacustrine-dominated areas towards the basin center as compared to the fluvially dominated systems at the basin margin.

In the more distal reaches of this depositional system the elevation of the water table exerted the main control on the development of surfaces and facies architecture. Here, during lake lowstands, laterally extensive eolian sequence boundaries recorded deflation down to the water table and the growth of eolian dune fields. These eolian deposits have excellent reservoir quality. Laterally extensive, lacustrine transgressive surfaces overlain by poor reservoir quality sabkha and lacustrine deposits recorded a shift to a wetter climate and rising lake levels. Fields in this part of the depositional system are characterized by sheet-like reservoir architecture with good lateral, but poor vertical connectivity.

In contrast, sequences nearer the basin margin were marked by locally deep fluvial incision. Incised valleys were filled with a complex mosaic of eolian, fluvial and lacustrine facies. Lateral variability is high which reduces lateral connectivity and predictability. These fluvial systems experienced diminishing competence during falling lake level. Eolian erosion and deposition characterize lake lowstand deposits. It was during the onset of lake transgression when gradients were steep and fluvial discharge was increasing that extensive fluvial incision occurred.