Exploration potential of the mature Southern North Sea basin margins: some unconventional plays based on alluvial and fluvial fan sedimentation models

Abstract

Basin margins are characterized by clastic, terrigenous accumulations which occur mostly in two morphologically similar but sedimentologically distinct depositional environments: alluvial and fluvial fans. Their differences in sedimentary processes result in distinct reservoir characters and subsurface modelling challenges. The ability to predict, recognize and discern these deposits in the subsurface is therefore the key for effective exploration and development of hydrocarbons. Sedimentary processes characterizing alluvial fan accumulation determine internal reservoir architecture and, ultimately, distribution of porosity and permeability, which however do not change substantially with the distance from apex to toe. Reservoir characteristics however, change considerably if debris-flow or sheet-flooding processes dominate. This distinction is crucial for prediction of reservoir quality, which in the case of a sheet-flood-dominated alluvial fan, can be several orders of magnitude better (higher lateral connectivity and vertical permeability) than a debris-flow dominated alluvial fan. Sedimentary processes on alluvial fans are controlled primarily by the nature of the bedrock and the colluvial deposits of the drainage basin. It is important therefore to understand the palaeogeography and geology of the original basin margin in order to predict reservoir characteristics of alluvial fan deposits. Fluvial fan depositional environments have river-like characteristics, especially in terms of geomorphic features and sedimentary processes. On average they are much larger in size then alluvial fans and internally more variable. Grain-size, texture and facies are controlled by sedimentary processes, which depend on the distance from source area. Proximal fluvial fan deposits may resemble those of alluvial fan in facies and architecture. However, the lower net:gross, lower lateral connectivity and size of sand bodies in the medial and distal area of fluvial fans, will require different geological concepts and therefore different exploration, appraisal and development approaches than alluvial fan reservoirs.Available data inferred from of the Southern North Sea hydrocarbon province indicate that, repeatedly over the geological history, the continental basin margins were characterized by fluvial depositional systems entering the subsiding sedimentary basins. At present, the industry experience in the Southern North Sea in targeting such reservoirs is very limited. However, successful exploration and production activity, especially in the Permian gas basin in The Netherlands and Britain, indicate that reservoirs genetically associated with fluvial fan development have good and commercially sound reservoir quality. Based on this experience and a large number of modern and ancient analogue studies, alluvial fan and fluvial fan ‘unconventional’ reservoirs may represent a high-potential target for the future exploration, especially considering the occurrence of stratigraphical traps both on the southern and northern margin of the Southern North Sea.