Rock Physics Patterns of Clastic Depositional Sequences

Juan-Mauricio Florez-Nino¹ and Gary Mavko^2
1 iReservoir.com, Highlands Ranch, CO
² Stanford University, Stanford, CA

Lithofacies successions from diverse depositional environments show distinctive patterns in various rock-physics planes (e.g. Vp-porosity, Vp-density). These patterns are the consequence of textural and compositional variations in the mm- to cm-scale depositional fabric, and coeval post-depositional processes like bioturbation. Four examples of decameter-scale lithofacies sequences are documented: (1) syntectonic fluvial deposits which show an inverted-V pattern indicative of mm- to cm-scale dispersed fabric in the mixed lithofacies, (2) mud-rich deep-water deposits that present linear trends associated with mm- to cm-scale horizontally laminated sand-clay mixtures, (3) sand-rich deep-water deposits with a pattern resulting from the scarcity of mixed lithofacies, and (4) shallow marine coarsening-upward sequences with evidence of both dispersed and horizontally laminated lithofacies.

The applicability these patterns to predict the seismic properties of larger sequences and away from well control depends on the vertical and lateral persistence of the lithofacies assemblage. Syntectonic fluvial deposits present good vertical persistence. In contrast, mud-rich deep water deposits show a high vertical variability of lithofacies.

Extrapolation of the patterns observed to similar depositional settings in other basins depends on the repeatability of the lithofacies assemblage and diagenesis. This study documents the similarity in the rock physics patterns of sand-rich deep-water deposits from offshore Gulf of Mexico and the North Sea.

Rock physics models provide the framework to understand the patterns of lithofacies successions in the rock-physics planes. The models explain the concomitant change in porosity and elastic properties of dispersed and laminar mixtures of sand and clay.