Wettability and connate water saturation in hydrocarbon reservoirs with bitumen deposits

Oil and gas reservoirs of the Alberta foothills and N.E. British Columbia commonly have bitumen deposits lining pores. These deposits create surfaces which are partly hydrophobic. Some of these reservoirs have connate water saturations obtained from oil-base cores by Dean Stark extraction (OBCDS) which are much less than would be predicted using equilibrium mercury capillary pressure data for rocks of similar porosity. The hypothesis is presented that the high temperatures which thermally cracked a former oil phase to gas, leaving residues of pyrobitumen, may also have caused water-loss and partial dehydration of the reservoirs during escape of excess gases. Large amounts of pyrobitumen in some reservoirs provide evidence of filling with more than one charge of oil. The resulting partial hydrophobicity of these rocks may have inhibited the re-imbibition of water to former levels. In these circumstances, the common assumption that gas reservoirs are strongly water-wetted and the use of conventional capillary pressure methods may lead to erroneous conclusions about water saturation and, therefore, may lead to underestimations of hydrocarbon reserves.