Application of sequence stratigraphy and trace fossil analysis to reservoir description: examples from the Jurassic of the North Sea

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Abstract
Recent sequence stratigraphic schemes proposed for the North Sea Jurassic have been developed primarily as exploration tools to predict reservoir distribution away from well control. As a result, most of these schemes are at the megasequence and depositional sequence scale. In field-scale studies where significant core is available, integration of trace fossil studies with other available correlation data (sedimentological, biostratigraphic, seismic, well log) can increase the precision and resolution of the sequence stratigraphic framework so that it is applicable to development and production problems. Trace fossils effectively provide an in situ record of environment and environmental change. They can be used to identify depositional facies and facies shifts, as indicators of salinity change, as direct indicators of stratal surfaces and condensed sections, and in evaluating the significance of palaeosols. This often allows stratal surfaces to be identified where other data fail (e.g. tracing marine flooding surfaces into the coastal plain and offshore shelf) and provides a way of tackling highly bioturbated shallow marine reservoirs. On a field scale, parasequence correlation is usually at a higher resolution than biostratigraphy and provides a deterministic approach to the evaluation of sandbody architecture and permeability barrier geometry. Such a sequence stratigraphic framework is crucial to (i) identifying field-wide permeability barriers (e.g. marine shales and cemented zones associated with marine flooding surfaces); (ii) understanding the stacking pattern, connectivity and anisotropy of reservoir sandbodies; and (iii) identifying horizons of incision (sequence boundaries) which may erode through units that would otherwise form field-wide permeability barriers. High-resolution, field-scale sequence stratigraphy thus provides a framework for prediction of reserves, reservoir simulation and field development planning. Case studies from (i) the Halten Terrace (Lower to Middle Jurassic); and (ii) the Central Graben area (Upper Jurassic), both from the Norwegian Continental Shelf, are presented to illustrate the use of high-resolution sequence stratigraphy in the stratigraphic description of hydrocarbon fields.

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