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Determining Oil-Expulsion Efficiencies of Source Rocks by Hydrous Pyrolysis


Amounts of oil expelled from and retained in a source rock are important attributes in evaluating unconventional-oil potential of tight-oil-shale plays. These attributes can be defined as the expulsion efficiency (mg expelled oil/g original total organic carbon (TOC)) of a source rock. Oil-expulsion efficiency can be determined by heating thermally immature source rocks in the presence of liquid water (i.e. hydrous pyrolysis) at temperatures between 350 and 365°C for 72 h. Unlike anhydrous closed-system (e.g. gold tube) and open-system (e.g. Rock Eval) pyrolysis methods, hydrous pyrolysis generates oil that is compositionally similar to natural crude oil and expels it by processes considered operative in the subsurface. Consequently, hydrous pyrolysis provides a means of determining oil-expulsion efficiencies and the rock properties that influence them. This study examines the expulsion efficiencies derived from hydrous pyrolysis of 15 diverse, thermally immature source rocks that range from Cambrian through Miocene age and contain Type-II or -IIS kerogen. These source rocks range in mineralogy from calcite-dominated marlstones and limestones to clay-mineral dominated mudstones. TOC content ranges from 4 to 24 wt% with porosities that range from 5 to 32%. Based on the hydrous-pyrolysis expulsion efficiencies measured on this diverse set of source rocks, the initial porosity of a source rock and to a lesser degree clay mineralogy have the biggest effect on expulsion efficiencies. TOC values greater than 4 wt% appear to have no significant or systematic effect on expulsion efficiencies. Carbonate content shows no significant or systematic influence on expulsion efficiencies, but high clay-mineral content, particularly smectite can reduce expulsion efficiencies by 88%. The effect of porosity is best observed in carbonate-rich source rocks where chalky marlstones with porosities of 32% can reduce expulsion efficiencies by 35%. Although various relationships between porosity and clay mineralogy can be constructed to predict expulsion efficiencies, the most effective approach is to directly determine expulsion efficiencies of a source rock by subjecting a representative sample to hydrous pyrolysis at 355°C for 72 h. In addition, a positive linear relationship between the hydrous-pyrolysis expulsion efficiency and Rock-Eval hydrogen index of an immature sample provides an insightful comparison with other source rocks.