Date of Award


Document Type



Carbonate cement formed during diagenesis of sedimentary rocks frequently entraps samples of diagenetic fluid as fluid inclusions, which record the pressure and temperature of the fluid at the time of formation. Homogenization of liquid-gas inclusions yields an estimate of the minimum temperature of inclusion and crystal formation; freezing point depressions of liquid inclusions can provide a close approximation of the salinity of the trapped liquid. Homogenization temperatures (T^) and final melting temperatures (Tf) were determined for 222 small (4-15 micron), primary and pseudosecondary, aqueous liquid-gas fluid inclusions in coarse (0.25-3.0 mm) poikilotopic calcite cement. The cement is from the moderately buried (2.4-3.3 km) Smackover Formation (Upper Jurassic, Oxfordian), which was sampled from wells in Walker Creek and Mount Vernon oil fields in southern Arkansas. T^ values, uncorrected for excess ambient pressures during inclusion entrapment, range from 58° to 158°C. This large range is interpreted as resulting largely from gradual precipitation of calcspar and entrapment of inclusions during changing temperature conditions. Depressed freezing temperatures (-15° to -32°) and low initial melting temperatures (-51° to -68°C) indicate that the fluid in the inclusions is a very saline, complex calcium chloride brine containing the equivalent of between 19 and 31 wt percent NaCl. The elevated T^ values, high fluid salinity and apparent presence of CaCl2 suggest precipitation of the calcite cement in the subsurface from a hot, concentrated calcium chloride brine.