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 ...
The Southern North Sea has been a prolific source of gas to the UK economy over the last 30 years. As many of the producing fields are now becoming depleted, attention has switched to exploration in more marginal areas of the basin. In order to evaluate suitable structures in these areas, an understanding of the complex structural and thermal history of the Southern North Sea Basin is required. Particular emphasis must be placed upon the prediction of source rock maturity, the timing of generation and migration of hydrocarbons, and the thermal controls exerted on potential reservoir sand units.Over large parts of the basin the Carboniferous source rocks are not at their maximum depth of burial at present day. A variety of techniques utilizing sonic velocity, vitrinite reflectance and apatite fission track analysis have been used to quantify the magnitude of uplift that the Carboniferous section has undergone through geological time. With the exception of apatite fission track analysis, these techniques provide only an estimate of 'net uplift', i.e. the amount of uplift relative to maximum depth of burial or maximum temperature. They record neither the timing and effects of individual uplift events nor the timing of maximum burial prior to net uplift. In order to accurately reconstruct the burial and thermal history of the basin these techniques must be integrated with a detailed knowledge of the geological history of the Southern North Sea.The use of these techniques in isolation, together with a lack of appreciation of the limitations of the data has in the past resulted in spurious results, leading to criticism and mistrust of the techniques. This paper attempts to show that when correctly applied and in appropriate geological circumstances, each analytical technique can provide useful data for burial and thermal history reconstruction. Examples are shown to highlight the effects of incomplete data interpretation and the impact of inaccurate determination of present day temperatures. Particular attention is paid to determining the burial and thermal history of the economically important Carboniferous source and Permian reservoir units, and the interpretation of uplift data is restricted to the post-Zechstein section.
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