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Recent studies of fluid inclusions in diagenetic cements have attempted to determine paleosubsurface temperatures. Three sets of observations are necessary to make accurate interpretations: (1) detailed petrography to establish the relative time of formation of the inclusions, (2) careful analysis of the burial and tectonic history of the host rocks to relate the diagenetic paragenesis to the geologic history of the basin, and finally, (3) analysis of individual inclusions for homogenization and final melting temperatures, and for chemical composition to define the PVT properties of the trapped fluids.
Once these observations are complete, 2 major limitations on the temperature interpretation remain. First is the assumption that the inclusions have not altered in composition or volume since entrapment. Recently published work shows that inclusions can re-equilibrate, but the extent that this affects most observations in sediments is unknown. Second, we must independently determine a "paleopressure" during inclusion formation, and we must know whether this pressure was hydrostatic or approached lithostatic. Data from both hydrocarbon and aqueous fluid inclusions in core samples from the Mission Canyon formation, Williston basin, North Dakota, illustrate a method for independently determining both paleotemperature and paleopressure from a single set of fluid inclusion measurements. Th technique requires petrographic evidence for
simultaneous trapping of 2 immiscible fluids. Theoretical analysis of the PVT properties of coexisting immiscible fluids demonstrates that the isochores for the 2 different fluids must intersect at the temperature and pressure of entrapment of the inclusions. Calculation of the PVT properties of each fluid requires detailed chemical analyses of both fluids. Recent results from new analytical techniques, especially capillary column gas chromatography to analyze hydrocarbon inclusions and laser Raman spectroscopy to analyze gases in aqueous inclusions, demonstrate that this approach to paleotemperature studies can be widely applicable in sedimentary environments.
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