Abstract

Whether addition of oil to sandstones slows, stops, or has no influence upon pressure solution and quartz cementation has long been disputed, despite having major implications for petroleum exploration and appraisal strategies. To elucidate the effect of addition of oil to compaction, pressure solution, and cementation processes, this study utilizes an experimental approach simulating isochemical, volumetric compaction using granular halite in the presence of variable brine–oil mixtures. Each experiment, at 500 kPa effective stress, lasted 48 hours and involved repeated measurements of volumetric strain. The lithified products of experiments were examined using SEM techniques. After 30 hours, approximately 4% volumetric strain occurs when the brine–oil ratio is 100:0 (pure brine). Fractures are rare in samples from experiments undertaken with pure brine so that pressure solution, and concomitant cementation, must be the compaction mechanism. When the brine–oil ratio is decreased from 100:0 to 40:60, the volumetric strain remains about 4% but there are more fractures, as quantified from SEM image analysis. These observations suggest that, although pressure solution has occurred, some of the volumetric strain is the result of fracturing, implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine. When the brine–oil ratio is decreased further from 40:60 to 5:95, the volumetric strain increases up to 7.3%; SEM image analysis reveals that the strain is predominantly due to grain fracturing. This change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses. Brine-coated grain contacts undergo pressure solution but oil-coated grain contacts experience increasing stress up to the point of failure, as the surrounding grain pack compacts. When the brine–oil ratio is decreased to less than 5:95, volumetric strain decreased to about 1% with few induced fractures and little pressure solution. Compaction is negligible when the brine–oil ratio is 0:100 (only oil in the pores) and fractures are negligible. Halite is more water-soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz. Since even halite has its mechanism of volumetric strain affected when the brine–oil ratio is less than 40:60, quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling. This experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil.