Anatomy of a Parasequence: Quantitative Heterogeneity Analysis for Reservoir Modeling, Book Cliffs, Utah

O'BYRNE, CIARAN, and STEPHEN FLINT, University of Liverpool, Liverpool, U.K.

The Cretaceous Blackhawk Formation (Mesaverde Group), exposed in the Book Cliffs of southeast Utah, comprises a set of clastic parasequences dominated by shoreface sand bodies. Such sand bodies

form important hydrocarbon reservoirs in the Gulf Coast, the North Sea Brent Group, and other oil provinces worldwide.

The 45-50 m thick Grassy Member shoreface system comprises three foreward-stepping parasequences. Individual parasequences exhibit a systematic distribution of lithofacies and major changes in vertical log profile relative to depositional dip. Parasequence stacking can result in repetition of certain facies associations in vertical sequence, particularly in the up-dip section. Parasequence bounding surfaces here are often cemented and/or intensely bioturbated and correlate down-dip with a marine flooding surface. Each facies association within a parasequence has a characteristic shale distribution that facilitates correlation of internal heterogeneities on well logs. Three-dimensional mapping of individual parasequences provides guidance for subsurface correlation at common (0.5-1.0 k ) well spacings.

Shales are divisible into minor flooding surfaces (fieldwide Kv barriers) and several more restricted intraparasequence types, relating to predominantly autocyclic events. Correct identification of the specific shale types in vertical sequence utilizes outcrop dip-log, gamma ray, and core analysis; i.e., data obtainable in the downhole environment. These results allow more accurate prediction of internal heterogeneity geometries and distribution in shoreface reservoir sand bodies and should provide datasets for reservoir modeling and simulation.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)