Rock Physics Modeling Guided by Depositional and Burial History in Low to Intermmediate Porosity Sandstones

Abstract

Prediction of reservoir properties as porosity (P), type of pore fluid (F) and lithology (L) from seismically derived properties as for instance the P-to-S velocity ratios (Vp/Vs) and P-wave acoustic impedances is crucial in quantitative seismic interpretation. The most common strategy is to cross plot the seismic parameters, obtained from data, within Rock Physics Templates (RPT) (Avseth et al., 2005) where certain projections of results from forward rock physics modelling have been embedded. Figure 1 shows one example where 29 synthetic data points have been plotted within an RPT made for consolidated siliciclastic rocks. The synthetic data points cover a span in porosity, gas-to-water saturation and lithology (clay-tosand ratio). The continuous lines define the shale line (upper left) and sand line (lower curve), where pore fluid is water and porosity increases from 0 to 0.3 from right to left for both lines. For various porosities along the water-filled clean sand line fluid substitution effects, gradually exchanging water with gas, are displayed. Studying the RPT we can see no specific trends in the data since all the before mentioned reservoir parameters vary. Only some few data points are captured by the clean sand projection. In this paper we present the concept of Inverse Rock Physics Modelling (IRPM) which aims to capture all combinations of the reservoir properties which correspond to a set of data points, given a specific underlying rock physics model. In Figure 2 the results from an IRPM are shown, where each data point now is associated with a numerical correlation function of the reservoir parameters. The approach thus gives the full suite of possible solutions representative for each data point, and thus more specifically reveals the non-uniqueness in estimation of PLF-parameters from seismic parameters