Petroacoustic Modelling of Heterolithic Sandstone Reservoirs: Constraints from Sedimentological Observations

Heterolithic sandstone or ‘shaly sandstone’ reservoirs are increasingly important. Petroacoustic models (a combination of petrophysics and rock physics) enhance the ability to extract reservoir properties from seismic data, providing a translation between seismic and fine-scale rock properties. Through incorporating sedimentological observations these models can be constrained and significantly improved.

Attempts to petroacoustically model heterolithic sandstones are complicated by the unpredictable effects of clay minerals and clay-sized particles on geophysical properties. Such effects are responsible for erroneous results when petroacoustic models for “clean” reservoirs (e.g. Gassmann, 1951), are applied to ‘shaly’ sandstones. Gassmann’s equation computes the elastic effect of changing reservoir fluid saturations; but a number of key assumptions are violated when applied to heterolithic sandstones. 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 (1975). This approach ignores current understanding of ‘shaly sand’ sedimentology and disregards other properties, such as sorting and grain size. This simplistic approach to heterolithic sandstone sedimentology, as well as poor linkage between petrophysical and sedimentological analyses, compounds petroacoustic modelling issues.

Potential improvements can be made though sedimentological constraints. This research uses a single well dataset that comprises a log suite including NMR and OBMI data, together with extensive core data including core-NMR, SEM images and detailed sedimentological analysis. The integration of log and core data enables a better insight to the key sedimentological properties influencing reservoir elastic properties. This approach can improve understanding of key sedimentological properties affecting acoustic propagation in heterolithic sandstones. This in turn provides better models for describing these important reservoirs, contributing to enhanced seismic data interpretation of reservoir properties, including fluid saturations, during exploration and development phases.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California