In order to develop an improved pore pressure prediction model for the overburden mudstones in the Rovuma Basin, offshore Mozambique, we apply Eaton's method to example well data from three exploration wells, which intersect Quaternary, Tertiary and Cretaceous sediments over depth intervals down to ∼3 km below seafloor. The predictive method only included the effects of mechanical compaction, which is a reasonable assumption for the low-temperature shallow sections. We found that Eaton's method applied to resistivity and acoustic log attributes works well and can be used to identify the mudstones that display over-pressured or normally pressured sections. The predicted pore pressures showed a good match to pore pressures in permeable formations. Using this calibration, we derived an improved pore-pressure prediction method for these wells and for the Rovuma Basin in general. The resulting model should give a good basis for future analysis of compaction processes and pore pressure in this basin.

Mudrock compaction trends from the Rovuma basin offshore Mozambique are compared with those of the Norwegian North Sea, the Gulf of Mexico and the Kutai basin offshore Indonesia. The comparison reveals that burial rates and timing of rifting are the dominant causes for the differences observed. The compaction trend for the Rovuma basin is broadly similar to the trends for the Kutai basin and the Gulf of Mexico, but very different from those for the Norwegian North Sea data, which show higher porosity and shallower onset of overpressure than those from the other three basins. The relationships for seismic velocities as a function of depth show strong similarities between the Rovuma and Gulf of Mexico basins.We then use these comparisons to make a general assessment of the capillary sealing potential of Cretaceous mudrocks in the Rovuma basin, using a mudstone permeability prediction function and a method for mapping permeability to threshold pressure, allowing estimation of maximum column heights for CO2 and CH4, with uncertainty ranges. Predicted CO2 column heights are slightly less than the equivalent CH4 column heights. The observed CH4 column height at one of the wells is significantly lower than that predicted from mudstone permeability, which is probably due to other factors such as fracturing or gas migration out of the structure. The comparison indicates generally good capillary sealing potential for the Rovuma basin Cretaceous shales and offers a general approach for assessing CO2 storage potential from hydrocarbon sealing datasets from multiple offshore basins.This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

Cretaceous shales from the emerging gas province of the Rovuma basin are examined using well‐log data from two exploration wells. The P‐wave acoustic impedance (AIp) data were estimated, and then brittleness and ductility were assessed within the framework of the Reuss–Voigt limits for acoustic impedance. We then predicted the shale consolidation using a weighting function (Wc) which varies between 0 for the case of grains in suspension (the Reuss limit) and 1 for the case of consolidated rock or cemented shale (the Voigt limit). At the Reuss limit, the formation AIp is highly sensitive to pressure while at the Voigt limit the formation AIp is insensitive to pressure. For the wells in this study, most data plot close to the lower bound with Wc < 0.5, hence showing a significant sensitivity to pressure. Although the Cretaceous shales in the Rovuma basin are dominated by a mechanical compaction regime, some onset of chemical compaction is observed. The rock‐physics analysis showed that the same shale in these two exploration wells had very distinctive and contrasting elastic properties. The deeper well interval clearly shows a ductile shale while the shallower well shows a range of ductile to brittle behaviour matching with the onset of chemical diagenesis at temperatures >58°C. These differences in the development and rate of chemical diagenesis in the same formation are likely due to contrasts in the rate of burial and have important implications for future seismic exploration studies.

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