A Glitch in the Matrix: How Particle Properties and Diagenesis Complicate Hybrid-Event Bed Interpretations in Carbonate Slope Deposits

Resedimented carbonates in slope and basin-floor positions constitute a significant portion of the carbonate stratigraphic record, host important archives of paleo-environmental change, and are major targets for the exploration and production of hydrocarbons—however, carbonate slope-basin deposits are poorly understood compared to their siliciclastic counterparts. Understanding depositional process is a very useful predictive tool for defining stratigraphic architecture, reservoir connectivity, reservoir quality, and geomechanical anisotropy, and in siliciclastic slope-basin systems, these processes are inferred from grain-size grading and sorting and sedimentary structures (e.g., Bouma sequence or hybrid event bed, or HEB). In carbonate systems, grain size is a more complex issue, with grain size, shape, and density extremely variable among different carbonate settings and through geologic time (e.g., Mississippian crinoids vs. Cretaceous rudists). The other major difficulty is ascertaining what constitutes “matrix” in carbonates, because diagenesis can strongly alter the original depositional fabric. This is particularly problematic for deep-marine deposits, where the amount and type of muddy ‘matrix’ is important for depositional-process interpretations and associated predictive 3D architecture (e.g., HEBs in lobe-fringe settings). Another challenge is that HEBs commonly have vertically variable mud content that form geomechanical boundaries that do not coincide with the event-bed boundary—this can lead to erroneous relationships between bed thickness and fracture dimensions. This study quantifies clast variability in carbonate and mixed-lithology HEBs from several outcrop and subsurface locales, and provides criteria for interpreting and quantifying matrix proportion, even in diagenetically-altered deposits.