Abstract Changes in the bulk inorganic geochemical composition of both sandstones and silty mudstones taken from conventional cores are used to subdivide the Lower Cretaceous Mannville Group and the overlying Colorado Group into chemostratigraphic packages and geochemical units. The chemostratigraphic packages are broadly equivalent to lithostratigraphic formations whereas the geochemical units are represent finer-scale informal stratigraphic subdivisions that occur within the formations. The chemostratigraphic packages are equivalent to the Lower Mannville Formation, the Upper Mannville Formation, the Basal Colorado Sandstone (an informal unit at the base of the Colorado Group), and the overlying Joli Fou Formation. The changes, which enable each lithostratigraphic unit, or chemostratigraphic package, to be geochemically fingerprinted, are primarily related to changes in mineralogy, which in turn appear to coincide with changes in sediment provenance and basin architecture. These chemostratigraphic packages, which are broadly equivalent to lithostratigraphic formations, can be considered to result from first-order changes in whole-rock geochemistry. The geochemical units are equivalent to incised-valley-fill sequences within the lithostratigraphic units, and here it is shown that the incised-valley-fill sequences of the Upper Mannville and Basal Colorado Sandstone each have a unique geochemical signature. Three incised-valley-fill sequences are present in the latter intervals, the oldest of which lie within the Upper Mannville Formation. Valleys at the base of the Basal Colorado Sandstone incise Upper Mannville deposits and are termed the lower Basal Colorado sandstones. The youngest valleys lie at the top of the Basal Colorado Sandstone and are termed the upper Basal Colorado sandstones. The sedimentary deposits in each of these incised valleys can be chemically distinguished from one another, even where a sandstone-on-sandstone contact is present in a single wellbore. The geochemical changes that enable each incised-valley fill to be recognized can be considered as second-order variations and are referred to as geochemical units. Stacked valleys observed in the Lower Mannville Formation have been the subject of previous chemostratigraphic work, and all of these have a unique geochemical signature and are also considered here to be geochemical units. The base of each incised-valley-fill succession, or geochemical unit, is a sequence boundary. By enabling clear identification of the geochemical unit above and below a sequence boundary, the technique of chemostratigraphy has direct application to aiding both lithostratigraphic and sequence stratigraphic correlations in low-accommodation basin settings. Application of Modern Stratigraphic Techniques: Theory and Case Histories SEPM Special Publication No. 94, Copyright © 2010 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-199-5, p. 93–107.

Abstract Important gas reservoirs occur in the Upper Carboniferous coal measures and red beds of the Southern North Sea. The thick red beds of the Boulton and Ketch formations are difficult to correlate, due to poor internal seismic definition, repetitive e-log signatures, and their barren nature. Although the underlying coal measures of the Westoe, Cleaver, and Caister Formations have better seismic resolution and contain palynomorphs, coals that die out laterally and the lack of diagnostic taxa over certain intervals contribute to their correlation being problematical. However, the application of chemostratigraphy to more than sixty wells from numerous fields in UK Quadrants 44 and 49, as well as from Dutch sector Blocks E, F, and K, allows the establishment of an independent, robust, detailed correlation framework for the aforesaid red beds and coal measures. Presented in this paper are correlative chemostratigraphic reference sections for the Caister, Westoe, Cleaver, Ketch, Boulton, and Step Graben formations. The chemostratigraphic zonations erected for these formations are based on variations in silty claystone geochemistry that can be tied to changes in provenance, climate, and depositional environment. In addition, the zonations are supported by stratigraphic changes in sandstone and coal geochemistry, the geochemical correlation of tonsteins and marine bands, and the recognition of different types of paleosol in the above formations. The chemostratigraphic correlation framework enables specific broad intervals (“packages”) to be correlated between fields and is also used to constrain seismic correlations with a view to highlighting potential exploration targets. Furthermore, the same framework allows much thinner intervals (“units” and “subunits”) to be correlated within fields: these smaller-scale correlations enhance reservoir correlations with respect to the development of fields such as Boulton, Schooner, Tyne, Ketch, and Topaz. In addition to using inorganic geochemical data to characterize and correlate sedimentological packages, data can also used to identify and correlate marker horizons and surfaces (tonsteins, coals, marine bands, major paleosols), which may be highly correlative low-diachrony surfaces, which greatly enhance the overall validity of the stratigraphic correlation scheme. Application of Modern Stratigraphic Techniques: Theory and Case Histories SEPM Special Publication No. 94, Copyright © 2010 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-199-5, p. 109–127.

Abstract Laterally extensive, thin, eustatically controlled, transgressive marine shale beds that occur within paralic sequences are generally regarded as reliable correlative markers. Such shale beds in the Carboniferous of NW Europe are referred to as marine bands and have been used extensively for stratigraphic correlations, particularly in the petroleum industry, where they are used to construct interwell correlations. True marine bands are represented by black anoxic shales (characterized by high U levels and high gamma API responses) that contain definitive ammonoid assemblages, i.e., demonstrably were deposited in a marine environment. However, not all black shales in the Carboniferous of NW Europe are the product of marine deposition, despite which they are still colloquially referred to as “marine bands” and are used for stratigraphic correlations. The problem of “marine band” recognition and correlation is exacerbated when dealing with well bores, where only wireline-log data and cuttings are available. This study demonstrates how inorganic geochemical data are used as a means to refine the identification of true marine bands and how these data can be used for enhanced stratigraphic correlations. “Marine-band chemostratigraphy” is established using core sections from the onshore Carboniferous Coal Measures sequences encountered in the West Midlands of England. Using variations in U, Mo, Zn, Cu, V, P 2 O 5 , Al 2 O 3 , Th, and Zr concentrations, a geochemically based facies classification scheme is erected, which allows the differentiation of mudstones deposited in marine, freshwater lacustrine, and floodplain environments, and which has been validated by palynological and sedimentological facies data. This scheme is successfully extended to a nearby well from which only cuttings are available. The general concept of marine-band chemostratigraphy can be applied to the sedimentary rocks deposited in any coastal-plain to marginal-marine setting. The methodology provides a robust technique for the identification and correlation of “marine bands” and also demonstrates the importance of inorganic geochemical data in the context of sequence stratigraphy. Application of Modern Stratigraphic Techniques: Theory and Case Histories SEPM Special Publication No. 94, Copyright © 2010 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-199-5, p. 221–238.