A Stochastic Model of Reservoir-Facies Distribution Within an Incised Valley Fill Deposited During an Interval of Episodic Sea-Level Rise: Upper Pleistocene–Holocene Strata of the Mobile Incised Valley System, Offshore Alabama
Authors: Bartek Louis R. , Cabote B. S. 
Published: April 2012

Approximately 2,125 km of high-resolution seismic reflection profiles was collected within a 900-km2 area of offshore Alabama, Mississippi, and Florida to determine how the nature of changes in the rate of Holocene sea-level rise (fast, slow, punctuated) impacts evolution of the stratigraphic architecture of an incised valley fill and deposition on associated interfluves on a passive continental margin with low storm and wave energy, microtidal range, and low to moderate sediment supply. These data were subjected to sequence stratigraphic and seismic facies analyses, and the products of this work provided input for the statistical analyses that were employed to produce the stochastic model. Quantifiable and statistically significant lateral seismic facies associations and vertical facies successions were identified within a chronostratigraphic framework for the incised valley and interfluves by using Repeated Measures ANOVA, Q-Mode Factor Analysis, and Binomial Markov process analysis.

The Oxygen Isotope Stage 2 lowstand led to bifurcation of the Mobile-Tensaw River system and yielded two distinct incised valleys, eastern and western. These major incised valleys have very different geometries and orientations relative to the paleo-shoreline, and they contain both similar and dissimilar quantifiable seismic facies assemblages, which vary as a result of lateral variations in shelf gradient, sediment supply, and subsidence.

The response to punctuated sea-level rise in the late Pleistocene and Holocene yielded three parasequences and associated transgression (retrogradation) of depositional systems within the incised valleys. The western incised valley received the majority of sediment input, as indicated by a thicker lowstand component and a fluvial-dominated lowstand delta at the shelf edge. The analyses revealed the presence of nine assemblages of distinctly different seismic facies and stratigraphic-surface composition. Analyses of vertical facies successions within each assemblage revealed that one of the most statistically significant vertical facies successions is from one of a large array of facies, upward to a flooding surface, and then from the flooding surface to a large array of facies. The fact that there are nine assemblages (and that the most significant vertical facies succession is upward toward a flooding surfce, comprising a wide variety of facies, and then from a flooding surface to a wide variety of facies) is a consequence of the interaction of sediment supply, basin subsidence, and most importantly the magnitude of the rapid rise in sea level. Variation in the magnitude and rate of rapid rises in sea level following stillstands or intervals of very slow sea-level rise results in rapid landward translations of seaward depositional systems over landward depositional systems. The rate of lateral translation of basinward facies over landward facies is so rapid that the vertical facies successions do not follow Walther’s law; therefore, virtually any landward depositional facies will lie under the transgressive surface and nearly any basin-ward facies may lie above the flooding surface. The net result is that fill of the incised valley is highly compartmentalized and contains many relatively small reservoir sand bodies that are sealed in impermeable, organic-rich muds. Ice sheets that are capable of rapid collapse, and therefore induce rapid sea-level rise, have been present in Antarctica since at least the mid-Miocene; therefore, successful exploration and production within reservoirs associated with incised-valley-fill deposits that are mid-Cenozoic and younger require use of stochastic models presented in this paper.

Our research shows that the facies architecture of the Mobile incised valley system has many counterparts throughout geologic history and supports the validity of using modern depositional systems as analogs to the architecture of ancient systems. Because of the similarity of the Mobile incised valley system to ancient systems, it is important to quantify facies distribution within the modern systems and to develop stochastic models of spatial variability of facies. These stochastic models can be used to assign probability of encountering a particular facies during oil and gas exploration of incised valley systems.