Predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics

Abstract

Abstract Based on the recently developed petrophysical classification of siliciclastics, which takes into account the amount of the volumetric clay content C and textural position of clay, it is shown that acoustic velocities can be fairly accurate tools in predicting lithology, porosity, and ultimately, transport properties of these rocks. Four major petrophysical groups of carbonate- and organic-poor siliciclastics are distinguished: (1) clean arenites (C < 2 percent), (2) arenites and arkoses (C = 2-15 percent), (3) wackes (C = 15-35 percent), and (4) shales (C > 35 percent). The compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients. This allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average (Wyllie) equation or its improved modification (Raymer equations), both of which neglect textural factors, or recently proposed relations based on the critical porosity concept. The transforms proposed display fundamental trends subject to only a second-order regional effects, such as details of mineralogy, grain size distribution, and authigenic clay development. These trends primarily reflect the processes of chemical diagenesis, including pressure solution, cementation, and mineral phase transformation. The processes of lithification of unconsolidated sediments by physical compaction and initial cementation are characterized by a steeper slope of the velocity-porosity transform because of a more pronounced velocity increase compared to the porosity reduction at this stage. The use of the V P /V S ratio versus velocity relation for lithology prediction is limited compared to the V P versus porosity plots; however, if both porosity and lithology are unknown, the velocity ratio can still be used for discriminating between predominantly grain-supported reservoir rocks (clean arenite, arenite and arkose) and clay matrix-supported (wacke, shale) rocks. Finally, a strong correlation between porosity and permeability of clean arenites is weakened somewhat in arenites. Nonetheless, even in the latter case, an order of magnitude accuracy in permeability assessment based on porosity can be achieved.