Application of Sequence Stratigraphy in Production Geology and 3D Reservoir Modeling

John Howell and Stephen Flint: STRAT Group, University of Liverpool, U.K.

Abstract

Static and dynamic 3-D reservoir models have improved considerably over the last decade but intelligent upscaling from geomodel to simulation model is still a major challenge. The routine application of high-resolution sequence stratigraphy, not just in initial correlation but also right through the modeling workflow optimizes geological 'knowledge' while still allowing the necessary grid block reduction. Although subtle spatial variations of permeability are important, it is essential to capture permeability extremes. Fortuitously, key stratal surfaces such as flooding surfaces and sequence boundaries usually define these permeability extremes. In shallow marine reservoirs, flooding surfaces (and updip, time correlative coals) form robust model zone boundaries because (a) they are approximate time lines, (b) they are vertical permeability barriers and (c) they bound reservoir flow units that commonly clinoform in both depositional dip and (in river-dominated systems) in depositional strike. Within this deterministic framework, facies (thus permeability) can be populated accurately using stochastic approaches, as long as the analogue geometrical dataset is sampled in terms of the correct systems tract. For example, parasequences in transgressive systems tracts have different geometries and internal reservoir properties than those in late highstand or falling stage systems tracts. Similar concepts are useful in deep marine reservoir; basin floor fans are commonly built by higher frequency sandy growth phases, vertically separated both statically and dynamically by shaly units that represent transgressive and early highstand basin condensation and sand storage on the coeval shelf. These units are also robust model zone boundaries and provide a deterministic framework within which to populate deepwater facies associations. In fluvial reservoirs, ranges of channel aspect ratios conform (with some overlap) to systems tracts, allowing good database matching between outcrop analogue and subsurface asset. In addition, facies-controlled reservoir quality is predictably different between transgressive and high stand systems tract fluvial deposits. Sequence stratigraphy is therefore at the heart of scientifically rigorous, fit-for-purpose 3-D reservoir models.