Paleotemperatures from Fluid Inclusions: Advances in Theory and Technique

Abstract

Fluid inclusions in diagenetic minerals can be used to determine paleotemperatures. 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 tie diagenesis to the geologic history of the basin, and, finally, 3) analysis of the phase behavior and chemical composition of individual inclusions to define the pressure-volume-temperature (PVT) properties of the trapped fluids. Once these observations are complete, two major limitations remain in the interpretation of paleotemperature. 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 to which that process affects most observations in sediments is unknown. Second, an independent measure of “paleopressure” during inclusion formation is necessary to distinguish whether the pressure was hydrostatic or approached lithostatic. Data from coexisting hydrocarbon and aqueous fluid inclusions in core samples from the Mission Canyon Limestone (Mississippian), Williston basin, North Dakota, illustrate a method for independently determining both paleotemperature and paleopressure from a single set of fluid inclusion measurements. The technique requires petrographic evidence for simultaneous trapping of two immiscible fluids. The PVT properties of such coexisting fluids require that the isochores for the two different fluids intersect at the temperature and pressure of entrapment of the inclusions. Calculation of the PVT properties of each fluid is based on 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 paleotemperature studies can be widely applicable in sedimentary environments