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Predictive Models for the Distribution of Effective Net Reservoir Intervals in Ephemeral Fluvial Successions: A Case Study from the Permian Organ Rock Formation, SE Utah, USA

Cain, Stephen A.1; Mountney, Nigel P.2
1 Earth Sciences, Keele University, Keele, United Kingdom.
2 Earth and Environment, University of Leeds, Leeds, United Kingdom.

Fluvial successions deposited in semi-arid climatic regimes form numerous important hydrocarbon reservoir intervals, including parts of the Permian Skagerrak Formation of the Central North Sea and the Triassic Ormskirk Sandstone of the East Irish Sea Basin. The characterization of such successions in outcrop is based primarily on the recognition of facies indicative of a progressive downstream decrease in i) fluvial discharge, ii) channel depth and width, iii) lateral and vertical connectivity of channel-fill elements, iv) evidence for channelized flow, and a systematic increase in i) evidence for sheet flood deposition, ii) eolian and/or playa deposits and iii) channel bifurcation. However, despite these criteria having been applied previously to a variety of outcrop successions, there is still no unifying facies model that adequately accounts for the stratigraphic architectural relationships expected for such systems, based on the varied styles of fluvial activity known from modern examples. Moreover, criteria for distinguishing these systems in the subsurface are poorly defined and the adoption of an inappropriate model can dramatically change predictions of net reservoir geometry.

The Permian Organ Rock Formation of the Paradox foreland basin, southern Utah, represents the deposits of a terminal fluvial system that experienced contemporaneous fluvial and eolian deposition. Nine facies associations are recognized and occur as architectural elements representative of: 1) proximal, multi-storey channel-fill complexes 2) medial, multi-lateral channel-fill complexes with locally derived rip-up clasts, 3) medial, internally sourced channel-fill complexes with extensive evidence for localized sediment reworking, 4) laterally amalgamated braided complexes, 5) sheet flood sandstones, 6) overbank deposits, 7) calcrete paleosols, 8) eolian dune complexes and 9) eolian sand sheet deposits. The geometry and inter-relationship of these elements is highly complex and reflects the interplay of a range of small-scale autogenic processes, that are themselves moderated by longer-term allogenic controls on climate, sediment supply and accommodation creation. Together, these factors determine the distribution of higher porosity-permeability units within the succession at a range of spatial scales. These relationships are expressed as a series of generic architectural models that can be used as predictive tools when applied to similar subsurface reservoir successions.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009