Remoblization and Injection in Deepwater Depositional Systems-Implications for Reservoir Architecture and Prediction

Lidia Lonergan, Nick Lee, J.A. Cartwright, R.J.H. Jolly, and H.D. Johnson, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, London, UK

Abstract

Several productive Paleogene deep water sandstone reservoirs in the North Sea show evidence of having undergone major post-depositional remobilization and clastic injection, which can result in major disruption of primary reservoir distribution (e.g. Alba, Forth/Harding, Balder, Gryphon). We present case studies of deepwater sandstones from UK Quadrants 9, 15, 16, and 21 to illustrate the wide spectrum of remobilization features, which range from centimetres (e.g., core-scale) to hundreds of metres (e.g., seismic scale). Most common are clastic injection structures such as dikes and sills. Sills of massive sand, over 20m thick, have been identified. Intrusions associated with the propagation of syn- to early post-depositional, dewatering-related polygonal fault systems in adjacent deep water mudrocks are also common. The scale of the clastic intrusion and remobilisation has significant impact on reservoir architecture and production performance, including changes in (a) original depositional geometries; (b) reservoir properties; (c) connectivity, (d) top reservoir surface structure, (e) reservoir volumetrics, and (f) recovery/performance predictions.

There are two prerequisites for sandstone intrusions to form:(1) the source sediment must be uncemented, and (2) the ‘parent’ sand body must be sealed such that an overpressure with a steep hydraulic gradient can be generated. The seal on the overpressured sand body must then be breached for the sand to fluidize and inject. The stress state within the basin, burial depth, fluid pressure, and the nature of the sedimentary host rock all contribute to the final style, geometry and scale of intrusion. At shallow depths, within a few meters of the surface, small irregular intrusions are generated, more commonly forming sills; however, at greater depth, dykes and sills form clastic intrusion networks. Field examples from the Ordovician in Ireland, and Santa Cruz and Panoche Hills in California are used to illustrate the control of burial depth/stress on intrusion scale. The cohesivity of the host sediment, and the flow velocity of the intruding sands appears to control whether the intrusion is emplaced by stoping (the incorporation of host rock material as rafts in the intrusion), or dilation (the forceful pushing apart of the host rock to create space), resulting in diverse styles of intrusive geometry.

Earthquake induced liquefaction, tectonics, and build up of excess in-situ pore pressure are the most commonly cited explanations for the occurrence of clastic intrusions. However, our work suggests that the large-scale, ‘catastrophic’ sandstone intrusions within the North Sea Paleogene, which remobilized hundreds of cubic metres of sediment, probably require the presence of fluids migrating from deeper within the basin (e.g., gas charge) to drive the injection. Deep water sand bodies within the North Sea that appear most susceptible to remobilization occur in mud-dominated successions and include (1) narrow, elongate channel or gully-filled sands (i.e., non-leveed channel systems), and (2) isolated sand-rich mounds (e.g. ‘ponded’ sand bodies and terminal fan lobes). Sand bodies located above basinal faults, which periodically appear to have acted as vertical fluid escape pathways, were especially susceptible to remobilization. Sand remobilization may influence reservoir distribution in other mud-dominated, deep water depositional systems.