No abstract

Deep-water sandstones of the Early Cretaceous Britannia reservoir are rich in ‘muddy’ material, with the development of unusual ‘slurry-flow’ deposits ( sensu Lowe & Guy 2000 ), including banded facies. The banding comprises couplets of pale sandstone containing microporous detrital chlorite and other clays, retaining substantial porosity, and dark sandstone in which biotite (now altered to chlorite) has promoted quartz pressure solution that has largely destroyed porosity. The main source of the abundant chlorite and biotite is the Jurassic Forties Igneous Province, underlying and surrounding the Britannia Field. Altered alkali basalts and other lithologies here are known to be rich in both minerals. Grain-size distributions have been examined using image analysis. The banded facies are generally finer grained than the high-density turbidite and unbanded slurry-flow sandstones, and may be a distal equivalent. Dark bands in some cases contain no more fine clays than associated light bands, indicating that Lowe & Guy’s model for dark-band formation, invoking a cohesive sublayer, is incomplete. Gelation of chlorite-rich clays within each banded couplet is proposed as an alternative mechanism. These microporous pore-filling chlorites have restricted the precipitation of quartz overgrowths and other non-porous cements, although their presence inhibits permeability.

Idealized facies of bottom current deposits (contourites) have been established for fine-grained contourite drifts in modern deep-marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine-grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non-existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo-Mediterranean Outflow Water. Foraminifera assemblages were used for ageconstraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain-size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo-Mediterranean Outflow Water, which is mainly driven by precession and millennial-scale climate variations. Tides substantially modulated the palaeo-Mediterranean Outflow Water on a sub-annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process-based interpretation of deep-water bottom current deposits.

Gl obal recoverable resources of heavy oil and oil sands have been assessed by CNPC using a geology-based assessment method combined with the traditional volumetric method, spatial interpolation method, parametric-probability method etc. The most favourable areas for exploration have been selected in accordance with a comprehensive scoring system. The results show: (1) For geological resources, CNPC estimate 991.18 billion tonnes of heavy oil and 501.26 billion tonnes of oil sands globally, of which technically recoverable resources of heavy oil and oil sands comprise 126.74 billion tonnes and 64.13 billion tonnes respectively. More than 80% of the resources occur within Tertiary and Cretaceous reservoirs distributed across 69 heavy-oil basins and 32 oil-sands basins. 99% of recoverable resources of heavy oil and oil sands occur within foreland basins, passive continental-margin basins and cratonic basins. (2) Since residual hydrocarbon resources remain following large-scale hydrocarbon migration and destruction, heavy oil and oil sands are characterized most commonly by late hydrocarbon accumulation, the same basin types and source-reservoir conditions as for conventional hydrocarbon resources, shallow burial depth and stratabound reservoirs. (3) Three accumulation models are recognised, depending on basin type: degradation along slope; destruction by uplift; and migration along faults. (4) In addition to mature exploration regions such as Canada and Venezuela, the Volga-Ural Basin and the Pre-Caspian Basin are less well-explored and have good potential for oil-sand discoveries, and it is predicted that the Middle East will be an important region for heavy-oil development.

A study of the Carboniferous sequences in some 93 wells from onshore England to onshore Netherlands has provided data on variations of depositional styles, sandbody dimensions and petrographic composition, both through time and, in the Upper Carboniferous, across the basin. Three separate sources of sediment input are recognized: a dominant, and petrographically fairly uniform, north to southflowing sediment distribution system which shows a general southerly decrease in channel sandbody size and abundance; a less important source on the Anglo-Brabant Massif; and a probable third source in SE Holland.Through time, there was a progressive increase in petrographic maturity, while channel sandbody percentages decrease to a minimum in the Westphalian B and then begin to increase again through the Westphalian C and D. Compared to the UK onshore, there is a close similarity between the Dinantian successions on the Mid North Sea High and in the Northumberland Basin and marked contrasts in sedimentation styles between the offshore Red Beds sequences in the centre of the basin and those seen onshore in the English Midlands. Regional mapping of channel sandbody statistics for different stratigraphic intervals could guide future exploration efforts towards areas with greater reservoir potential.The nature of the Carboniferous sequences and their distribution beneath the Southern North Sea have been reviewed by Leeder and Hardman (1990) and Besly (1990). These reviews are very valuable but are, of necessity, rather generalized. The increasing interest in Carboniferous sequences as exploration targets requires a more systematic evaluation of the nature and dimensions of Carboniferous sandbodies and this paper describes results of a study which attempted to fulfil that requirement.The principal aims of the study were: (1) to identify from what sources and through what distribution systems sediment was introduced; (2) to establish the main depositional styles which operated through the Carboniferous; (3) to assess how the distribution of these styles changed through time; (4) to distinguish different types of sandbodies present and to assess the likely dimensions of these potential reservoirs through both direct measurement of thickness and comparison with empirical analogue data from outcrop.The study was based on some 93 wells, extending from East Gelderland in the Netherlands to Tyneside, northeast England (Fig. 1). Most of the wells were for hydrocarbon exploration, although those in East Gelderland were drilled as part of a coal exploration programme. Eleven of the wells used in the study were made available by the operators on a confidential basis. The remainder were released wells, the distribution of which is shown in Fig. 1.For each well, geophysical logs were examined and operators' biostratigraphic reports were assessed. In addition, all available core was re-described and sandstones sampled for petrographic examination which was undertaken primarily to identify differences in provenance. Consequently, the material ...