Complex reservoir geometries in a deep water clastic sequence, Gryphon Field, UKCS: injection structures, geological modelling and reservoir simulation

The Balder Formation sandstone reservoir of the Gryphon Field was deposited during the late Palcocene-early Eocene, While the background sedimentation comprised pelagic shales and tuffs, sediment gravity flows transported sands from the shelf area in the west. The Crawford Ridge acted as a barrier to sediment transport in the east, causing the sands to be deposited predominantly as massive sand bodies as flow velocities decreased. During burial, sand remobilisation and injection occurred, possibly localised around faults. Cored intervals show a range of injection structures from thin cm-scale dykes to more massive metre-scale chaotic intrusive features. The crosscutting nature of the injection Structures. as observed in core and in seismic sections, accounts for the high degree of reservoir connectivity observed in the field. A study was undertaken with the objective of constructing a 3D geological model for dynamic reservoir simulation in order to evaluate future development activities in the field as previous, purely deterministic models had proven unsuitable, The complex depositional and post-depositional history of the Gryphon Field reservoir has resulted in a system for which there is no published analogue to aid in 3D reservoir modelling. Preliminary stochastic models based on analogues chosen to reflect the reservoir sand body geometries did not match the field production history. The fact that both the deterministic and fully stochastic models were unsuitable suggested that a novel approach was required to model this field, Subsequent models constructed using a combination of stochastic and deterministic models have proven successful in matching the production history of the field and evaluating infill opportunities. (C) 2002 Elsevier Science Ltd. All rights reserved.