Geologic Controls on the Elastic and Petrophysical Properties of Clastic Rocks

Florez, Juan-Mauricio¹, Gary Mavko² (1) iReservoir.com, Littleton, CO (2) Stanford University, Stanford, CA

The elastic properties of sedimentary rocks vary according to their internal texture, fabric, and composition. Lithofacies is the concept that summarizes these rock descriptors. Any lithofacies results from depositional or diagenetic processes. Several studies have analyzed the effect that changes in lithofacies have on elastic properties of clastic rocks. We present a summary of these results emphasizing their relationship with geologic processes.

Depositional processes determine internal fabric, grain size, angularity, packing, clay content, and sorting. In unconsolidated sandstones, changes in grain size, clay content, sorting and packing have a significant impact on both permeability and porosity of clastic rocks. Laboratory studies and log data demonstrate that most of these changes have a relatively small impact on the elastic properties of clastic rocks and are characterized by gentle slopes in the velocity-porosity plane. These slopes can be approximated by the modified Hashin-Strikman lower bound (MHSLB), which can be used to separate the effect of sorting from the effect of mechanical compaction.

Diagenetic processes reduce porosity and conspicuously increase the elastic properties of clastic rocks. The slope of the data in the velocity-porosity plane shows this concomitant change, and varies according to the diagenetic process. Incipient cementation and pressure solution significantly increase velocity without a drastic change in porosity. Mechanical compaction, in contrast, presents gentle to moderate slopes showing that there is an important porosity reduction associated with the change in velocity. Incipient cementation can inhibit compaction. After incipient cementation, precipitation of additional cement produces intermediate slopes in the velocity-porosity plane.