Exploring trends in microcrack properties of sedimentary rocks: An audit of dry-core velocity-stress measurements

Rock physics models are being used increasingly to link fluid and mechanical defor- mation parameters for dynamic elastic modelling. In this paper, we explore the input parameters of an analytic stress dependent rock physics model. To do this, we invert for the stress dependent microcrack parameters of over 150 sedimentary rock velocity– stress core measurements taken from a literature survey. The inversion scheme is based on a microstructural effective medium formulation defined by either a second–rank crack density tensor (scalar crack model) or both a second– and fourth–rank crack density tensor (joint inversion model). The inversion results are then used to explore and predict the stress dependent elastic behavior of various sedimentary rock litholo- gies using an analytic microstructural rock physics model via the initial model input parameters: initial crack aspect ratio and initial crack density. Estimates of initial crack aspect ratio are consistent between most lithologies with a mean of 0.0004, but for shales differ by up to several times in magnitude with a mean of 0.001. Estimates of initial aspect ratio are relatively insensitive to the inversion method, though the scalar crack inversion become less reliable at low values of normal to tangential crack compliance ratio (BN/BT ). Initial crack density is sensitive to the degree of damage as well as the inversion procedure. An important implication of this paper is that the fourth–rank crack density term is not necessarily negligible for most sedimentary rocks and evaluation of this term or BN/BT is necessary for accurate prediction of initial crack density. This is especially important since recent studies have suggested that the ratio BN/BT can be used as an indicator of crack fluid content.