Traditionally, interwell correlation for most reservoir sequences is achieved by using a combination of seismic, wireline log, biostratigraphical and sedimentological data. However, these techniques cannot always produce the required resolution for detailed stratigraphical modelling, particularly when dealing with barren fluvial sequences. Furthermore, the occurrence of thick monotonous successions of sandstones and mudstones, with repetitive wireline log characteristics and no prominent seismic reflectors, often makes correlating such sequences diffi- cult. To refine the stratigraphy of such successions, palaeomag- netism (Hauger et al. 1994), heavy-mineral analysis (Morton 1985, 1991, Morton & Hallsworth 1994, Mange-Rajetzky 1995, Morton & Hurst 1995) and isotopic techniques (Mearns 1988, 1989) are often utilized, although their success is often ham- pered by poor sample quality, insufficient sample volume, or dependency upon a specific lithology. Consequently, a method is needed that can generate independent stratigraphical frame- works for barren well sections (and fossiliferous successions) and is not restricted by sample type. Such a method is chemical stratigraphy, or as it is more commonly known, chemostratigra- phy. This paper presents the results of a chemostratigraphical investigation on the virtually barren continental mudstones and sandstones that make up the Upper Carboniferous Schooner For- mation (or "Barren Red Measures") from the Silver Pit Basin, southern North Sea. These deposits form an ideal subject on which to test chemostratigraphy, as the sandstones represent sig- nificant gas plays, but their interwell correlation has proved notoriously difficult. Geochemical data are acquired from the mudstones and sandstones, and those data related to detrital minerals are used to erect a correlative chemostratigraphical zonation for the Schooner Formation. Furthermore, integration of these data with sedimentological information shows how the depositional environments, weathering styles and pedogenic pro- cesses have affected sediment geochemistry. Pearce et al. (2005) incorporate the data presented herein into a multidisciplinary study that utilizes palynology, clay-mineral data, heavy-mineral data and Sm-Nd isotope data to produce a new stratigraphical zonation and provenance model for the Schooner Formation. Summary The virtually barren, mainly redbed sequences of the Schooner Formation (Upper Carboniferous) of the southern North Sea rep- resent an important gas reservoir, although well placement and reservoir development are hampered by the lack of a reliable stratigraphical framework. Chemostratigraphy has enabled the Schooner Formation encountered in well 44/21-3 to be divided into three chemostratigraphical units (S1, S2 and S3) and eleven sub-units (S1a-f, S2a-b and S3a-c), based on variations in mudstone and sandstone geochemical data acquired from core samples and cuttings. The geochemical characteristics of the units can be related to variations in mineralogy, depositional environment, climate and provenance. The units are correlated sub- regionally and the correlation is used to corroborate seismic correlations. A high-resolution chemostratigraphical zonation and correlation based on sub-units redefines models of reservoir architecture and enhances reservoir development in the nearby Schooner field.

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).