Chemostratigraphy: a method to improve interwell correlation in barren sequences — a case study using onshore Duckmantian/Stephanian sequences (West Midlands, U.K.)

Abstract Chemostratigraphy has been applied to onshore Duckmantian/Stephanian successions encountered in outcrop and penetrated by two boreholes from the West Midlands (U.K.). These successions represent the onshore equivalents of the `Barren Red Measures' which are important hydrocarbon-bearing sequences in the Southern North Sea. Much is known about the onshore successions in terms of sedimentology, mineralogy and provenance and thus they provide the ideal test for the validity of chemostratigraphy as a stratigraphic tool. Reliable inorganic geochemical data have been acquired from geochemical analyses of core, sidewall core and cuttings samples, with 19 elements being determined. Stratigraphic variations in elemental concentrations are compared with known variations in the mineralogical data. The established lithostratigraphic units of the Duckmantian/Stephanian intervals can also be recognized from the geochemical data and by using these data can be subdivided further. This results in an independent chemostratigraphic correlation being established for the two boreholes, which has been assessed statistically by discriminant function analysis. From the geochemical and mineralogical data, distinct changes in provenance are identified within the Upper Carboniferous successions. The sediments of the Coal Measures were derived from a north westerly and westerly source (?Caledonian), whereas the Etruria Formation sediments came from the Wales Brabant Massif, the sediments having mixed Caledonian and Cadomian characteristics. Eventually these sediments were replaced by sediments from a southern Hercynian source (Halesowen and Salop Formations).