Source-rock seismic-velocity models: Gassmann versus Backus

摘要

Source rocks are described by a porous transverselyisotropic medium composed of illite and organic matter(kerogen, oil, and gas). The bulk modulus of the oil/gas mixtureis calculated by using a model of patchy saturation.Then, the moduli of the kerogen/fluid mixture are obtainedwith the Kuster and Toks?z model, assuming that oil is theinclusion in a kerogen matrix. To obtain the seismic velocitiesof the shale, we used Backus averaging and Gassmannequations generalized to the anisotropic case with a solidporeinfill. In the latter case, the dry-rock elastic constantsare calculated with a generalization of Krief equations tothe anisotropic case. We considered 11 samples of theBakken-shale data set, with a kerogen pore infill. TheBackus model provides lower and upper bounds of the velocities,whereas the Krief/Gassmann model provides a goodmatch to the data. Alternatively, we obtain the dry-rock elasticmoduli by using the inverse Gassmann equation, insteadof using Krief equations. Four cases out of 11 yielded physicallyunstable results. We also considered samples of theNorth Sea Kimmeridge shale. In this case, Backus performedas well as the Krief/Gassmann model. If there isgas and oil in the shale, we found that the wave velocitiesare relatively constant when the amount of kerogen is keptconstant. Varying kerogen content implies significant velocitychanges versus fluid (oil) saturation.