The Stratigraphic Hierarchy: Framework for Integrated Basin Analysis at the Exploration and Production Scales- Paradox Basin Case Study
James L. Weber, J. F. Sarg, and J. M. Armentrout
Analysis of seismic, outcrop, core, and log data into a stratigraphic hierarchy for the Middle Pennsylvanian mixed carbonate, siliciclastic, and evaporite strata of the Paradox basin, southwest USA allows for an improved prediction of reservoir, source, and seal. Facies stacking patterns indicate that 5th-order depositional cycles (<= 100 k.y.), stack into 4th-order depositional sequences (~400 k.y.), each with lowstand, transgressive, and highstand systems tracts. This framework comprises the reservoir architecture of the giant Aneth field. Fourth-order sequences stack into seismically resolvable, biostratigraphically defined 3rd-order composite sequences (~1.5 m.y.). Secondary porosity is most pronounced in highstands beneath 3rd-order sequence boundaries. Five 3rd-o der sequences make up part of an Atokan-Missourian 2nd-order transgressive-regressive cycle. This framework is used to predict the regional distribution of reservoir, source, and seal.
A second-order relative rise in sea level during the lower three 3rd-order sequences (Atokan-E. Desmoinesian) resulted in a basin-rimming carbonate shelf that backsteps through time. During 3rd-order lowstands, basin restriction and drawdown resulted in accumulation of subaqueous evaporites. Third-order transgressions over vast evaporite deposits led to dissolution, elevated salinity, and high organic productivity. These source rocks are basinwide in extent. The ensuing turn-around and initiation of the 2nd-order fall in sea level resulted in rapid progradation of the carbonate shelf. Drawdown was sufficient during 4th-order lowstands to allow shallow subaqueous evaporites to accumulate in each 3rd-order sequence. World-class reservoirs formed in algal buildup and grainstone facies of the 3rd-order highstand above the 2nd-order turn- around. Seal facies overlie reservoirs and are composed of lowstand evaporites and transgressive carbonates.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California