Deep-Marine Hyperpycnal Sandstones and Implications for Exceptional Reservoir Quality Preservation

Abstract

Summary Deep marine hyperpycnal sandstones form prolific hydrocarbon reservoirs but remain poorly understood despite over 40 years of research. Predicting their geometry, composition and reservoir quality requires a thorough knowledge of the processes that formed them and the effects of diagenesis in the presence of brackish depositional pore waters. Dissolution of unstable grains (e.g. feldspars and volcanic material) and replacement by kaolinite and chlorite/smectite occurs more readily in the presence of brackish, acidic pore fluids. This is enhanced locally as compaction drives fluids through the aquifer. Pore lining chlorite cements can help to prevent chemical compaction of quartz grains and impede later quartz overgrowths, helping to preserve reservoir quality at depth. Commonly in hyperpycnal deposits, remnant pore fluids are of low salinity, resulting in anomalous low salinity DST results (e.g. Agat, NOCS). The salinity of the pore fluids soon after deposition can be quantified by measuring the isotopic composition of early carbonate cements, which may form strata bound or nodular baffles to flow within the aquifer. The influence and mobility of low salinity pore fluids during the early diagenesis of deep marine hyperpycnal deposits is a key subject for future research.