Reservoir characterization and potential of the old red sandstone around the inner moray firth, NE Scotland

Abstract

Old Red Sandstone deposition in North East Scotland occurred during Devonian times within the Orcadian Basin, a northeast-southwest elongated structure. The basin formed in an extensional tectonic regime that resulted from gravitational collapse of over thickened Caledonian crust. A half graben topography resulted with the faulting largely controlled by crustal heterogeneities inherited from the Caledonian period of mountain building. The ORS around the Inner Moray Firth is a red bed sequence deposited on the southern margin of the Orcadian Basin. The succession consists predominantly of braided fluvial sandstones, silts tones and mudstones, with lesser amounts of aeolian sandstones, evaporitic sabkha sandstones (only in the Upper ORS), and lacustrine mudstones and limestones (Middle ORS only). Lithologically the sandstones are coarse to fine grained, moderately to well sorted, and predominantly sublitharenite in composition. The sandstones show the following diagenetic sequence: (1) Eodiagenesis: formation of clay/Fe oxide rims and the dissolution of lithic fragments; (2) Mesodiagenesis: the precipitation of a blocky, irregularly distributed, calcite cement; and (3) Telodiagenesis: a major dissolution event following inversion during the late Carboniferous involving partial removal of the calcite cement, feldspars (predominantly plagioclase), and lithic fragments (mainly sedimentary and metamorphic) and an associated precipitation of kaolinite. Intergranular macroporosity is most abundant with lesser amounts of intragranular and microporosity. Porosity values are quite low (an average of 6% for both the Middle and the Upper ORS) and permeabilities are also poor (an average of 17 and 51mD respectively). Porosity reduction has occured mainly through cementation rather than compaction. The low permeabilities are thought to be due to low pore interconneclivity because of the patchy nature of the calcite cement, and to the presence of pore lining/filling kaolinite. Diagenesis has acted to largely overprint the primary permeability characteristics of the different lithofacies identified within the sandbodies. Some fractured samples however, had permeabilities of up to 1400mD similar to the situation in the Buchan Field ORS fluvial sandbody reservoir, where fracturing is the major control on reservoir characteristics. Reservoir heterogeneities occur on a variety of scales within the ORS and have a marked effect. At the microscale diagenetic heterogeneitites have reduced porosity and permeability to very low levels. Cross-bedding and other sedimentological structures exert directional anisotropics on the permeability. Additionally, the Middle ORS is separated into discrete segments from 10-100m thick by laterally extensive lacustrine deposits, resulting in there being virtually no vertical connectivity between sandbodies. The Upper ORS has a much higher sandbody connectivity with only Local horizons of thin, discontinuous fluvial mudstones which are scarce due to syndepositional erosion.