Comparison of Sandstone Reservoir Geometry in a Valley-Fill Depositional Sequence and a Deltaic Progradational Sequence, Middle Jurassic, Yorkshire, England

R. Eschard, C. Ravenne, P. Houel

The geostatistic simulations elaborated by the reservoir geology group of the IFP and the Geostatistic Center of the Ecole des Mines (HERESI GROUP) are based on the interpretation of representative outcrops of littoral to deltaic deposits. The methodology includes some detailed studies of large outcrops in terms of reservoir geology, a computation of the geological and petrophysical parameters, the conditional simulation of reservoir geometry, and their internal heterogeneities by geostatistic methods.

Two sites, Cloughton and Ravenscar, have been selected along the Yorkshire coast in the Middle Jurassic siliciclastic series. The Ravenscar Group series (a stratigraphic and sedimentologic equivalent of the Brent Formation in the North Sea) is well exposed in wide, extended cliffs. The two-dimensional geometry of the reservoirs is reconstructed from photomosaics and surveying. Their three-dimensional architecture is controlled by core drills located just behind the studied cliffs. The wells are full-cored wire-line log (gamma ray and dipmeter) recorded, and porosity and permeability measurements are made every 20 cm in reservoir levels.

On each site, a typical depositional sequence has been detailed. The reservoir geometry is controlled by the relative sea level variations, a global low subsidence rate, and clastic sediment supply variations. At Cloughton, a complex fluvial-estuarine valley-fill sequence is described. The entrenchment of the valley corresponds to an initial relative sea level fall. It is followed by a progressive infill of the valley. Its vertical evolution reflects a progressive abandonment of the whole fluvial-estuarine system, possibly related to a low-amplitude relative sea level rise. At Ravenscar, a progradational sequence of a delta corresponds to an initial relative sea level rise followed by a highstand sea level period.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.