Reservoir architecture analysis using floodbasin palaeosols : Statfjord Formation, Brent Field, northern North Sea
The Statfjord Formation, reaching a maximum thickness of 1,000 ft in the Brent Field area, comprises a highly heterolithic alluvial sandstone, siltstone, shale and mudstone succession (the Eriksson and Raude Member). The uppermost Statfjord Formation is, however, made up of a thin succession of medium-coarse grained shallow marine sandstones (Nansen Member).
Analysis of material from 11 cored wells, and wireline log suites from a further 61 non-cored wells has revealed a distinct pattern of "sequence" development which is present throughout the Statfjord Formation of the Brent Field. A sparsely preserved fossil assemblage (pollen spores
etc. ) has precluded biostratigraphic correlation of the main Statfjord Formation fluvial suite. Furthermore, the positions of lithostratigraphic markers (e. g. the base of the Nansen Member) within a sedimentary succession frequently reflect variations in the spatial development of facies, rather than chronostratigraphically equivalent events. Thus, if derived from purely lithostratigraphically driven correlation, the reservoir geologists' perception of parameters essential for flow unit designation prior to field simulation studies (e. g. sandbody connectivity), are often poorly constrained.
Palaeosols are abundant within the fluvial Statfjord Formation succession, where they can be readily recognised in core. The palaeosol development is controlled by parent material, climate,
biological factor, topography and time. The Statfjord Formation palaeosols are classified into five groups in terms of soil maturity. Understanding of wireline log (i. e. GR, Sonic and CNL) responses of different palaeosols allows identification of the Statfjord Formation palaeosols in non-cored wells. Whole rock geochemical analyses reveal variations between different types of Statfjord Formation palaeosols, however it is difficult to distinguish the five groups of palaeosols purely on the basis of chemical compositional variations because the palaeosols were complicated by mixed parent material. This study has also resulted in subdivision and correlation of the fluvial Statfjord Formation reservoir in the Brent Field into a series of reservoir units which are identified on the basis of
their petrophysical and geochemical characteristics. Reservoir units are sequences which have distinctive geochemical compositions, and are recognisable on the basis of their petrophysical log response using a "Formation Lithology Factor" (FLF, defined by this study) based upon variations in sonic and compensated neutron log responses. Variations in geochemical
composition are interpreted as reflecting varying amounts of sediment input from different provenances. Variations in FLF for both sandstone and mudstone lithotypes can be used to
define reservoir units. This simple and novel technique may be applied to other sandstone suites in understanding reservoir connectivity and flow unit definition. High resolution reservoir correlation using palaeosols has allowed the interpretation of reservoir sandstone interconnectivity within the Statfjord Formation. The successful application of the
pedofacies model (Bown and Kraus, 1987) and pedofacies sequences (Kraus, 1987) identified in the Statfjord Formation of the Brent Field has proved that they can be applied to a wide rane of fluvial deposits. In summary, this study has applied a multidisciplinary approach to the problem of correlation using palaeosols as potential indicators of "channel proximity" within a floodbasin. Using these,
together with petrophysical, chemostratigraphic, heavy mineral data and field production data has allowed an integrated novel approach to be used in the interpretation of reservoir sandstone