Rock Property Inversion in Organic-Rich Shale: Uncertainties, Ambiguities, and Pitfalls

Abstract

Geophysical characterization of organic-rich shales involves remote estimation of the geological and geomechanical properties. Important geological properties are porosity, clay content (Vclay), and the total organic carbon (TOC). Important geomechanical properties are natural fractures and fracability (ease of hydraulically fracturing the rock). Apart from various engineering factors, the fracability depends on the following geological factors: in-situ stress (the vertical effective stress and the minimum and maximum horizontal effective stresses), pore pressure, brittleness, and rock strength. At well locations, we sometimes have the luxury of having enough measurements to completely characterize the shale properties of interest. However, we do not normally have enough measurements away from wells, and as a result the estimation of rock properties remains under-determined. On the other hand, the seismic measurements typically can be used to provide an estimate of the acoustic impedance (AI) and Vp/Vs ratio of the rock. Occasionally we may estimate density if seismic data have sufficient quality and enough offset coverage, and the anisotropic elastic properties with varying degrees of measurement errors. Due to the complexity of shale reservoirs, the relatively large number of rock properties, and the limited amount of independent information that we can measure at the field, the inversion for the rock properties often remains non-unique.