Based on outcrop studies and borehole data, six bedded lithofacies and two reef types are recognized within the Much Wenlock Limestone Formation of the English Midlands and Welsh Borderland. The lithofacies are interpreted to represent a series of carbonate shelf environments extending from below storm wave-base to well above fair weather wave-base. In common with many other shallow marine carbonate depositional systems, the principal controls on lithofacies development were hydrodynamic energy, the supply of fine clastic sediment, and patterns of colonization of the sea floor by organisms. Reef distribution was probably controlled by the nature of the substrate, water circulation, and rate of siliciclastic sedimentation. A depositional model is proposed which incorporates biostratigraphical evidence suggesting that the formation youngs to the west on the northern part of the shelf. Deposition of the Much Wenlock Limestone Formation there began in the West Midlands, where 12 m of microbial limestone were lain down in a mid-shelf setting during a local regression. The remainder of the shelf was dominated by low energy siliciclastic deposition at that time. The West Midlands then returned to somewhat deeper water, lower energy deposition, the resulting impure calcareous muds becoming diagenetically changed into the nodular limestone lithofacies. That lithofacies is commonly overlain successively by the interbedded limestone and silty mudstone lithofacies, and then the crinoidal grainstone lithofacies. This vertical lithofacies sequence is uniform over the entire northern part of the shelf, reflecting a gradual decrease in water depth. The crinoidal grainstone lithofacies was deposited as a wave-influenced carbonate sandbody which prograded from east to west. Lithofacies sequences on the southern shelf are laterally impersistent, probably due to greater tectonic instability and topographical variablity. † Author for correspondence: a.t.thomas@bham.ac.uk younging from east to west in the northern part of its outcrop area. We here combine these biostratigraphical data with lithostratigraphy and facies analysis, in order to infer the depositional environments of the Much Wenlock Limestone Formation, and their development through time. ,, siliciclastic mudstones nodular limestones interbedded limestones and silty mudstones crinoidal grainstones thick bedded oncolite-rich lithofacies NBM MWLF Figure 7. Relationship of lithostratigraphical and biostratigraphical units from Wenlock Edge and West Midlands. LQLM = Lower Quarried Limestone Member; NBM = Nodular Beds Member; UQLM = Upper Quarried Limestone Member; LEB = Lower Elton Beds. See text for details.

The Much Wenlock Limestone Formation of the West Midlands was deposited in a mid-shelf setting and is divisible into three members; the Upper and Lower Quarried Limestone members being separated by the more argillaceous Nodular Limestone Member. Oncoids, composed predominantly of micritic fabrics with Rothpletzella and Girvanella, occur commonly in the Lower Quarried Limestone Member. These oncoids vary from subspherical bodies up to 5 mm in diameter to forms with a highly irregular and branched upper surface which reach 70 mm across. Each form is indicative of a different depositional environment, which is also reflected in the sediment enclosing the oncoid. Equidimensional oncoids in peloidal packstones were formed by continuous rolling, whereas the larger, branched forms enclosed in loosely packed wackestones developed in quieter conditions below wave base. The distribution of oncoid morphotypes in the Lower Quarried Limestone Member shows that small variations in relative sea level were superimposed on the overall middle to late Wenlock regressive episode during which the Much Wenlock Limestone Formation was deposited. The uniformity of the formation throughout the West Midlands indicates that the sea floor was essentially planar over a large area. Vertical variation of oncoid morphology within the Lower Quarried Limestone Member can be traced throughout the area, allowing accurate correlation of relative sea-level variations.

The Mungaroo Formation in the Gorgon Field is a stratigraphically complex fluvial system of Triassic age. It is also a major hydrocarbon reservoir, therefore understanding its internal stratigraphic architecture is of paramount importance to exploitation of its reserves. Here, the technique of chemostratigraphy is used to construct a correlation framework for the Mungaroo Formation of the Gorgon Field. Chemostratigraphy is a tool that employs variations in inorganic whole rock geochemistry to enable the characterisation and subsequent correlation of sediments. For this study, a total of 1,514 cuttings and core samples from eight wells in the Gorgon Field have been analysed. Using data derived from both claystone and sandstone lithologies, the Mungaroo Formation is divided into nine chemostratigraphic packages, 22 geochemical units and 19 sand units. Additionally, three surfaces identified as time lines (T1–T3) are geochemically defined. Changes in values of Ga/Rb and Al2O3/(CaO+ MgO+K2O+Na2O) indicate that during deposition of the Mungaroo Formation, the paleoclimate became warmer and wetter, resulting in increasingly intense hydrolytic weathering. Steps in the values of these ratios allow three surfaces to be identified (T1–T3), at which there is a marked and sustained change in the paleoclimate. These three surfaces represent time lines that provide a quasi-chronostratigraphic framework for the formation. Values of Cr/Al2O3, Cr/Na2O and Nb/Al2O3 are related to changes in sediment provenance and indicate that during deposition of the Mungaroo Formation the provenance became more mafic and less intermediate. It is variations in paleoclimate and provenance modelled from the geochemical data that allows the packages, units and sand units to be characterised and correlated. The chemostratigraphic correlation is more detailed than is available from other stratigraphic techniques. Although in most instances the lithostratigraphic correlation of sand units based on wireline log correlation matches the one defined using chemostratigraphy, there are some significant differences between the two that influence reservoir models and gas production.

Principles of chemostratigraphic characterization and correlation employing whole-rock inorganic chemical data and heavy mineral grain counts are applied to the frontier Bowser and Sustut basins. Methodologies commonly used with well samples in mature petroleum provinces can be applied to field samples, providing a vital and practical link between the earliest frontier investigations and more advanced hydrocarbon exploration.The major stratigraphic divisions of the basins, the Bowser Lake and Sustut groups, have markedly different indications of sedimentary provenance from heavy mineral analysis, and are readily differentiated geochemically. Variations in key elements are related directly to the provenance indications identified by heavy minerals.

The Triassic Argilo-Gréseux Inférieur Formation (TAG-I) is one of the principal hydrocarbon reservoirs in the Berkine Basin of Algeria. Sedimentological studies have shown that it exhibits marked spatial and temporal facies variations on both a local field scale and a regional basinal scale. This variability, combined with a lack of diagnostic flora and fauna, makes regional correlation within the unit difficult. In turn, the lack of a consistent regional stratigraphic framework hampers the comparison of the various correlation schemes devised by operators in the basin. Contrasting the TAG-I in Blocks 402 and 405a exemplifies the problems encountered when attempting regionally to define a correlation framework for the interval. Between these two blocks, a distance of approximately 200 km, there are marked changes in the style of deposition from sand-dominated, proximal fluvial systems in the SW (Block 405a, MLN, MLC, KMD and MLNW fields) to a more distal, more clay-prone system in the NE (Block 402, ROD/BRSE/BSFN, SFNE and BSF fields). A chemostratigraphic study of the TAG-I in these two blocks has allowed a four-fold correlation framework to be defined, where each chemostratigraphic package has distinctive geochemical features. Chemostratigraphic Package 10, the oldest unit, lies above the Hercynian Unconformity, but beneath a geochemically identifiable hiatal surface. Chemostratigraphic Package 20 lies above the hiatal surface but is separated from the overlying packages by a mineralogical change identifiable in both claystone and sandstone geochemistry. Chemostratigraphic Packages 30 and 40 are chemically somewhat similar, but are separated by a regional event interpreted as a period of dolocrete and lacustrine development. By combining the geochemical differentiation of the units and recognition of their stratal boundaries, it is possible to define a correlation for the TAG-I between Blocks 402 and 405a. The proposed correlation between the two blocks suggests that the northern parts of Block 405a may have been occupied by a spur or subsidiary channel from the main SW–NE-trending fluvial system, resulting in one of the chemically defined packages being demonstrably absent in the MLNW, MLN, KMD and MLC fields when compared with the other areas of the study.