Incorporating Rock Property Constraints And Geological Insights Into Seismic Inversion For Reservoir Porosity And Net to Gross Volumes

Experience shows that inversion results are strongly dependent on the low frequency background model derived from the wells in the study area, so an industry trend towards fewer wells and more reliance on seismic data prompts the question of how to address the associated degradation and uncertainty of inversion results. We will show how to use petrophysical analysis and geological insights to reduce the number of inversion parameters by creating a grid of virtual elastic property profiles or “pseudo wells” to supplement the real wells in the background model. The algorithms used to generate elastic properties at the pseudo wells were derived from core analysis and petrophysical modeling. We observed usable correlations between the sedimentologist’s core-based sand fraction log and both core-based reservoir properties and the gamma ray log. This allowed us to use the gamma ray alone to estimate all the key reservoir properties — porosity, permeability and water saturation. Density and compressional and shear sonic logs were reproduced from the gamma ray log and the associated reservoir properties using familiar equations, linking elastic and reservoir properties. Virtual wells were created by drawing a reservoir sand quality profile that was then transformed to density and compressional and shear velocity for modeling synthetic seismic gathers. The profiles were compared with the 3D seismic gathers at the same location, and optimized by improving the match while honoring the geologist’s views on allowable reservoir property profiles and stratigraphy. The resulting network of real and virtual wells was used as a constraint in the seismic inversion and produced a result consistent with petrophysical data and geological opinion.