Defining Sequence Stratigraphy, Facies, and Stratal Patterns in Mudrock Systems: Understanding the Controls, Methodologies, and Realities
Ruppel, Stephen C.; Rowe, Harry
Production response in mudrock reservoirs illustrates that the nature and distribution of depositional facies in these rocks are variable and complex. Although such heterogeneity is present in conventional systems, both our understanding of the processes and products responsible for this complexity in mudrock systems and the validity of conventional methods to characterize it need to be reconsidered. Traditional methods of core description and thin section petrography are inadequate for defining facies and stacking patterns in these rocks and cannot form a basis for interpreting wirelogs nor defining stratal architecture and continuity. Although such conventional approaches should be used where possible, they must be considered secondary to the information provided by chemostratigraphic methods.
We have collected comprehensive suites of chemostratigraphic data (including elemental concentrations, stable isotopes of carbon, oxygen, and nitrogen, and organic carbon abundances) from subsurface cores in mudrock reservoir systems ranging in age from Devonian to upper Cretaceous (e.g., Bakken, Woodford, Barnett, Wolfcamp, Leonard, Bone Spring, Pearsall, Haynesville, Eagle Ford) and used them to define high resolution variations in facies and the conditions under which these rocks were deposited.
Major elements define shifts in mineralogical assemblages (dominantly carbonate, quartz, and clay minerals) and allow facies stacking patterns to be defined with high precision and correlated to wireline log response. These data are thus fundamental to correlating facies and determining their continuity.
Redox sensitive trace elements define changes in bottom water chemistry. In systems like the upper Cretaceous Eagle Ford (but also in distal regions of Devonian, Mississippian, and Jurassic basins) where nutrient supply rather than eustasy and platform shedding control sediment flux and the application of sequence stratigraphic concepts is problematic, redox indices may be the best tools for temporal correlation.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013