Anomalous Elastic Properties of Mudrocks Bounding Reservoirs with Highconcentrations of Naturally Occurring CO2

Very high concentrations of CO2 have been encountered as carbonic acid in hydrocarbon reservoirs in the Greater Sarawak Basin. Anomalous features in 3D seismic data are found in areas with high CO2 concentrations. These features appear as halos around reservoirs, cutting across stratigraphy, indicating a hardening of the non-reservoir rocks within the envelope of the halo. Elastic log data from wells that pass through and adjacent to these seismic anomalies indicate that mudrocks within the anomalies have higher densities and velocities than would be predicted from locally derived compaction trends. Combinable magnetic resonance measurements indicate that the anomalous properties are the result of lower than-expected capillary-bound microporosities. It is proposed that carbonic acid in the reservoir fluids diffuses into the bounding rocks, causing a loss of porosity. The amount of porosity lost depends on the clay content of the mudrock and the initial level of compaction, with shallower, more clay-rich shales able to lose more porosity. The anomalous seismic signatures result from a sharp transition at the diagenetic front between normal and altered rocks. The alteration can significantly change the amplitude variation with offset response of the reservoirs and therefore the ability to correctly predict fluid phase and reservoir quality.