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Core Description, Markov Chain Analysis and AVO Modeling of a Lagoon/Tidal to Fluvial Transition Zone in the Cretaceous Straight Cliffs Formation, Southern Utah

Abstract

Tidal successions are comprised of a complex mixture of fluvial, tidal, and marine lithofacies. Subtle changes in the transition from tidal to fluvial deposits make lithofacies differentiation from subsurface wireline log and seismic reflection data problematic. Because tide-dominated and tidally-influenced reservoirs account for a significant portion of petroleum reserves, forward seismic reflection modeling coupled with a predictive facies model framework derived from rock physics measurements on cores and outcrops can lead to invaluable insights for interpreting subsurface data. The Cretaceous John Henry Member (Straight Cliffs Formation), located in the Kaiparowits Plateau of southern Utah, reveals excellent outcrops of fluvial and tidally-influenced deposits, and offers an opportunity to improve our understanding of wireline log interpretation and seismic imaging in similar subsurface petroleum reservoirs. The focus of this study is a 240 m core that captures a progradational succession from shoreface through tidally-influenced lagoon to fluvial deposits. The full spectrum of lithofacies stacking patterns present in the core is captured with a Markov Chain analysis. Benchtop measurements were performed on 1 inch core plugs (57 total; 25 from the coastal plain succession and 32 from the tidal succession) to obtain physical rock properties (Vp, Vs, density, permeability and porosity) for each lithofacies. The rock properties show a wide range of values as a direct result of the highly heterolithic nature of these deposits. Although measurements from different lithofacies are overlapping, we observe a slight offset between fluvial and tidal rock properties. To test our ability to observe the tidal to fluvial transition with seismic imaging, average rock properties for each lithofacies were used to generate synthetic seismic reflection models for different expressions of upward-fining packages documented in the core. This investigation elucidates variations in amplitude versus offset responses as a function of variable tidal influence. The number of overlapping values highlights the complications associated with interpreting these deposits in subsurface data. However, the modeling shows promise in differentiating the end member packages, along with a gradational trend for intermediate packages consistent with the sedimentology of transitioning from more marine influence to more terrestrial influence.