The Moab Anticline, east‐central Utah, is an exhumed hydrocarbon palaeo‐reservoir which was supplied by hydrocarbons that migrated from the Moab Fault up‐dip towards the crest of the structure beneath the regional seal of the Tidwell mudstone. Iron oxide reduction in porous, high permeability aeolian sandstones records the secondary migration of hydrocarbons, filling of traps against small sealing faults and spill pathways through the Middle Jurassic Entrada Sandstone. Hydrocarbons entered the Entrada Sandstone carrier system from bends and other leak points on the Moab Fault producing discrete zones of reduction that extend for up to 400 m from these leak points. They then migrated in focused stringers, 2–5 m in height, to produce accumulations on the crest of the anticline. Normal faults on the anticline were transient permeability barriers to hydrocarbon migration producing a series of small compartmentalized accumulations. Exsolution of CO2 as local fault seals were breached resulted in calcite cementation on the up‐dip side of faults. Field observations on the distribution of iron oxide reduction and calcite cements within the anticline indicate that the advancing reduction fronts were affected neither by individual slip bands in damage zones around faults nor by small faults with sand: sand juxtapositions. Faults with larger throws produced either sand: mudstone juxtapositions or sand: sand contacts and fault zones with shale smears. Shale‐smeared fault zones provided seals to the reducing fluid which filled the structural traps to spill points.

The structure and content of the Moab Fault zone are described for 37 transects across the fault zone where throws range from less than 100m to c. 960m. The 45km long fault trace intersects a sedimentary sequence containing a high proportion of sandstones with good reservoir properties, interspersed with numerous mudstone layers. Typically, the fault zone is bounded by two external slip zones with the fault zone components separated by up to nine internal slip zones. Fault zone components are tabular lenses of variably deformed sandstones and sandstone cataclasites and breccia, with a wide size range, usually enclosed in a matrix of shaley fault gouge containing mm to m scale entrained sandstone fragments. Neither fault zone structure nor content can be predicted by extrapolation over distances as little as 10m. Although variable in thickness, shaley gouge is always present except where the mudstone is

Abstra~. The Devonian lower Clair Group (c. 300-800 m thick) is dominantly composed of fluviatile sandstones and conglomerates, aeolian sandstones and minor floodplain and lacustrine shales. These facies are organized into stratigraphical cycles which have a threefold hierarchy: an unconformity bounded cycle representing first order retreat and advance of the fluvial drainage system; three subsidiary second order cycles bounded by minor unconformities, and numerous third order cycles. These stratigraphical cycles exhibit common characteristics: they are bounded by sharp, commonly erosive surfaces; they are composed of a fining-upwards succession overlain by a variably developed coarseningupward element; the fining-upward element records a progressive decrease in the fluvial component, and increasing proportion of aeolian, floodplain or lacustrine facies (this trend is reversed in the upper, coarsening-upward section). The grain size of the fluvial facies, and nature of the non-fluvial facies (aeolian vs floodplain/lacustrine) within any order of cycle is dependent on it's position within a lower order (larger scale) cycle.The cycles are areally extensive, and are present in all wells within the Clair Basin, although spatial changes in internal facies make-up occur. The second order cycles show progressive onlap followed by ofliap of the basin margin.These hierarchical cycles are interpreted to be the product of changes in the accommodation:supply ratio of the depositional system. An increasing accommodation:supply ratio provided space to preserve the record of waning fluvial influence, enhanced aeolian reworking, preserved floodplain fines or lacustrine mudrocks. In contrast, a reducing accommodation:supply ratio resulted in filling of accommodation space and sediment bypassing. Accommodation space variations were dictated by tectonic variations in strain rate, which caused incremental uplift of the basin margin and subsidence of the basin. Sediment supply changes were influenced by source area uplift, which changed sediment yield, and climate change which affected runoff. Climatic changes are detectable within the non-fluvial elements of the Clair Group by stratigraphic variations between aeolian, floodplain and lacustrine facies. These vary systematically within and between second order unconformity bounded units, suggesting a tectonic influence on climate.Within the second and third order cycles, those which were evolving towards, or were occurring within, semi-add to arid climatic conditions are commonly (but not exclusively) biased towards fining-upward successions, whilst the more humid systems show a better developed coarsening-upward element. This may be the result of the changing impact of discharge versus sediment supply variations when one of these parameters became suppressed or enhanced under varying climatic conditions.

Stratigraphical and geographical variations in the composition of pebble suites in Upper Jurassic and Lower Cretaceous rocks in southern Britain and northern France provide a means for studying both local and regional changes in tectonic conditions associated with basin development. Local variations in pebble suites provide evidence of major fault-associated uplift and erosion of intrabasinal and basin-margin highs during earliest Cretaceous times, and the subsequent post-faulting subsidence and marine onlap in mid-Cretaceous times.Regional provenance studies have demonstrated that the Upper Jurassic and Lower Cretaceous pebble suites of southern Britain and Normandy are separable into six assemblages.

SUMMARYMzia is a very large gas field, of Cretaceous age, located in the Ruvuma Basin within the Block 1 licence, offshore Tanzania (Figure 1).In this presentation we will discuss how our understanding of the field has evolved from the initial identification of the prospect on 2D seismic data through to appraisal well drilling, with a particular emphasis on how the depositional setting has influenced the unusual sedimentology of the field.