Petroacoustic Modelling of Heterolithic Sandstone Reservoirs: A Novel Approach to Gassmann Modelling Incorporating Sedimentological Constraints and NMR Porosity data

Heterolithic or 'shaly' sandstone reservoirs constitute a significant proportion of hydrocarbon resources. Petroacoustic models (a combination of petrophysics and rock physics) enhance the ability to extract reservoir properties from seismic data, providing a connection between seismic and fine-scale rock properties. By incorporating sedimentological observations these models can be better constrained and improved. Petroacoustic modelling is complicated by the unpredictable effects of clay minerals and clay-sized particles on geophysical properties. Such effects are responsible for erroneous results when models developed for "clean" reservoirs - such as Gassmann's equation (Gassmann, 1951) - are applied to heterolithic sandstone reservoirs. Gassmann's equation is arguably the most popular petroacoustic modelling technique in the hydrocarbon industry and is used to model elastic effects of changing reservoir fluid saturations. Successful implementation of Gassmann's equation requires well-constrained drained rock frame properties, which in heterolithic sandstones are heavily influenced by reservoir sedimentology, particularly clay distribution. The prevalent approach to categorising clay distribution is based on the Thomas - Stieber model (Thomas & Stieber, 1975), this approach is inconsistent with current understanding of 'shaly sand' sedimentology and omits properties such as sorting and grain size. The novel approach presented here demonstrates that characterising reservoir sedimentology constitutes an important modelling phase. As well as incorporating sedimentological constraints, this novel approach also aims to improve drained frame moduli estimates through more careful consideration of Gassmann's model assumptions and limitations. A key assumption of Gassmann's equation is a pore space in total communication with movable fluids. This assumption is often violated by conventional applications in heterolithic sandstone reservoirs where effective porosity, which includes a capillary bound water porosity component, is used. The novel approach attempts to better address this assumption through incorporation of NMR porosity data which distinguishes between bound water and free (movable) fluid components of porosity. The simplistic approach to heterolithic sandstone sedimentology, with poor linkage between petrophysical and sedimentological analyses and ignorance of model caveats, compounds petroacoustic modelling issues. This research uses a single well dataset comprising a log suite including NMR and OBMI data, together with extensive core data including core-NMR, SEM images and detailed sedimentological analysis. Integration of log and core data enables better insight to the key sedimentological properties influencing reservoir elastic properties. This approach improves understanding of key sedimentological properties affecting acoustic propagation in heterolithic sandstones and in turn provides better models for describing these important reservoirs. This contributes to enhanced seismic data interpretation of reservoir properties, including fluid saturations, during exploration and development phases.