Detrital zircons in sandstones of the Statfjord Formation (late Triassic-early Jurassic) in the northern North Sea have been dated with the sensitive high resolution ion microprobe (SHRIMP). These sandstones lack detailed biostratigraphic control, but display distinct variations in heavy mineral assemblages which facilitate a three-fold subdivision of the sequence. The lower and middle zones (heavy mineral Zones A and B) have similar characteristics, both displaying a marked upward decline in garnet abundance which has proved difficult to attribute unambiguously to change in either provenance or sedimentation history. The base of the highest zone (heavy mineral Zone C) is marked by changes in several heavy mineral indicators attributable to a shift in ultimate provenance compared with the underlying zones.
The detrital zircon ages are a direct fingerprint of the sediment source terrains, and so resolve ambiguities in interpreting provenance from heavy mineral abundances alone. The age spectra indicate that both low-garnet and high-garnet parts of Zone A have the same ultimate provenance. This strongly suggests that garnet removal was not a function of changing provenance, but took place during the sedimentation process, probably by weathering during periods of prolonged alluvial storage. The source of Zone A is not represented in any presently exposed landmass: the required combination of Caledonian granitoids intruding or in immediate proximity to high-grade Archaean-sourced metasediments is likely to have been either to the west, in the northern part of the Shetland Platform, or to the north, in what is now the Norwegian Sea. Zircons in Zone C have a distinctly different age spectrum consistent with a source in the Western Gneiss Region of southern Scandinavia, where there is the required combination of Proterozoic protoliths and Caledonian reworking.
Heavy mineral data provide an independent basis for subdivision and correlation of the Statfjord-Nansen reservoir sequence in the Brent Field, UK North Sea. Because of its continental fluvial depositional environment, this sequence lacks a well-defined biostratigraphically-based correlation framework. Detailed well correlations are difficult because of the heterolithic nature of the reservoir and the lack of chronostratigraphically significant markers. Three major heavy mineral zones and a number of subzones have been established. These can be confidently correlated in cored sections across the Brent Field. The correlations made using the heavy mineral scheme generally parallel the boundaries between reservoir units, giving confidence to the current reservoir zonation and indicating that it has a sound geological basis. However, the mineralogical zonation of a crestal, faulted well, 211/ 29-D44 (BD44), differs significantly from the reservoir zonation. This illustrates the difficulties inherent in establishing field-wide correlations without support of independent correlation schemes.
Sequence stratigraphic concepts have long been used to integrate core and well log data with 3D seismic data to establish predictive reservoir models. This paper documents a new technique to systematically evaluate large scale log patterns in order to identify key chronostratigraphic surfaces with confidence. Linked closely to 3D seismic structural and stratigraphic interpretations, this technique has been applied in several reservoir modelling studies in the Niger Delta. Special emphasis has been given to the influence of growth faults on reservoir development. The resulting tectonosedimentary framework has led to semi-quantitative predictions about sediment geometry and distribution which were subsequently utilized to constrain 3D models of the reservoirs. Such 3D models improve the quality and success of appraisal drilling as well as field development planning.
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