Application of Borehole Images to Three-Dimensional Geometric Modeling of Eolian Sandstone Reservoirs, Permian Rotliegende, North Sea

ABSTRACT High-resolution electrical borehole images of the Permian Rotliegende Sandstone (North Sea) clearly reveal a vertical succession of eolian and fluvial bedding facies. Porosities, which are found to be primarily a function of these facies, show a hierarchy of reservoir heterogeneities related to alternating depositional mechanisms. Dips and azimuths were measured in interdune layers, bounding surfaces, and cross-strata using, in addition to electrical images, full-circumference acoustic borehole images and dipmeter results. The vertical sequence and geometry of the bedding elements indicate composite cross-bedding formed by downwind migration of superimposed bed forms alternating with interdune sections containing only few single bed forms. Foreset direction scatter decreases significantly with increasing set thickness. Statistical modeling of the thickness and foreset direction of cross-bedded sets is shown to constrain the three-dimensional geometry of the sand bodies. At least two size populations are identified, one with maximum cross-bed thicknesses from 5 to 20 ft (1.5 to 6 m) and another with a thickness of 30 ft (9 m). Estimated widths and lengths of cross-bedded sets are 100 and 200 times the thicknesses. Crescentic (semi-elliptic) bed forms with oblique migration account best for the observed azimuth data; the dune crescents probably were laterally linked into a ridge. From bed-form orientation and migration angle, a local paleowind direction from the northeast is suggested, fitting well the Permian paleo-tradewind regime. A few cross-beds in the central part of the sequence indicate wind-parallel migration and generally thicker bottomset portions; these factors are interpreted as evidence for stronger, less variable winds in the central erg. The results statistically predict distances at which wells are not connected by highly porous layers. An average preferred drainage direction for the reservoir as well as local drainage direction for each cross-bed is obtained. Extent and thickness of the cross-beds may also serve as input for average horizontal and vertical permeability as well as radial inflow performance estimates.