CARTWRIGHT, JOSEPH, HOWARD JOHNSON, LIDIA LONERGAN, RICHARD JOLLY, STEPHEN MOLYNEUX, and NICK LEE, Imperial College, London, and MICHAEL COPE, Fina Exploration Ltd.

Abstract: Post-depositional Remobilisation of Deepwater Massive Sandstone Reservoirs: Impact on Reservoir Modelling and Prediction

Modelling of the numerous productive Paleogene deepwater sandstone reservoirs found in the North Sea is invariably based on primary depositional geometries. However, evidence from more isolated sandbodies in fields such as Balder, Forth/Harding, Gryphon, and Alba shows that original depositional geometries can be substantially modified by subsurface remobilisation of the sands in a fluidised state under high pore fluid pressures.We have found evidence of considerable reconfiguration of primary depositional geometries by various remobilisation processes, including clastic injection into overlying seal formations, and the interaction with polygonal fault systems.

Here we review the well and seismic evidence for post-depositional remobilisation processes in an attempt to assess their impact on reservoir geometry and prediction.We also review field evidence from analogue examples of injection structures in order to contrain likely remobilisation mechanisms and products, and in order to set a theoretical framework for future studies.

We illustrate our sub-surface review with case studies from UK Quads 15, 16 and 21.A wide spectrum of remobilisation features is observed spanning a size range from core to seismic scale. Most common are different forms of clastic injection structure (dikes and sills), but equally important are structures associated with the propagation of polygonal fault systems during early burial. The scale of many of the clastic intrusions suggests that these structures are important in predicting reservoir geometry and performance. They have the potential to enhance production in some cases, and pose significant production problems in other cases.

We conducted field studies of analogue examples of reservoir and sub-reservoir scales of clastic intrusion in the Santa Cruz region of California, and in Western Ireland. The main results of these field studies and literature review are:

1. Stress state, burial depth, fluid pressure and the nature of the sedimentary host rock are crucial in controlling intrusion styles, geometry and scale.
2. Seismicity, tectonically driven changes to the stress field and excessive build up of in situ pore pressure are the most commonly cited explanations for the occurrence of clastic intrusions.
3.The addition of another fluid such as oil or gas is more rarely referred to as a triggering mechanism.
4. For clastic intrusions to occur, the source material for the intrusion must be uncemented and sealed such that an overpressure with a steep hydraulic gradient can be generated within the sand pocket.
5.The cohesivity of the host rock controls whether the intrusion is emplaced by one of two mechanisms: a) stoping (the incorporation of host rock material as rafts in the intrusion), or b) dilation, the forceful pushing apart of the host rock to create space. The result of the two processes is diverse styles of intrusive geometry.

On a more general note, we also conclude that the scale of the intrusive complex is governed by the stress and the depth at which the intrusion occurs. At shallow depths, within.a few meters of the surface, small irregular intrusions are generated, more commonly forming sills, whereas at depth dykes and sills form clastic intrusion networks. These features are illustrated using examples from Ireland, Santa Cruz and Panoche Hills, California.

Importantly, our work suggests that the scale of North Sea Paleogene intrusions require the presence of an external fluid pressure (e.g. gas) to drive the injection, and we present evidence to support this proposition from 3D seismic and isotopic studies.

In summary, we propose a number of factors that are important for the development of significant remobilisation of deep water reservoirs: wrench reactivation of basement faults, gas migration through fracture systems, the contemporaneous development of polygonal fault systems, and the presence of an overpressured, uncemented source sandstone. This important, but poorly documented, phenomenon is likely to impact hydrocarbon reservoir characterisation in numerous basins world-wide, including the North Sea, the Atlantic Margin, and the West African margin.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England