Practical Aspects of Geobarometry
Fluid inclusions provide a good tool to determine pressure at the time of fluid inclusion entrapment, but the ability to use this tool is extremely dependent on the fluid inclusion assemblage (FIA) preserved and the state of knowledge of the composition and P-V-T properties of the fluid system trapped in the inclusions. The best method for using inclusions to determine true pressure of entrapment requires that FIAs preserve a record of gas-water immiscibility. Without this petrographic evidence of immiscibility, guesses must be made about the temperature of fluid inclusion entrapment to estimate the pressure of entrapment. For some fluid inclusions, the best that one can accomplish reliably is to determine a minimum pressure of fluid inclusion entrapment, and there are several possible approaches. Many of the methods developed in the previous chapter to determine entrapment temperature have parallels for determining entrapment pressure. Another approach using the intersection of isochores from petroleum and water systems (Narr and Burruss, 1984) is not covered in this text because recent work suggests a need for careful reevaluation of its theoretical and practical basis (Burruss, 1992).
Figures & Tables
The past decade has revealed significant advantages to using fluid inclusions as a means of understanding the physical and chemical history of fluids in sedimentary basins, but it also has revealed important limitations which have required that a new approach must be employed to effectively use fluid inclusions. This book is divided into six sections: (1) what fluid inclusions are and what geologic history they are capable of recording; (2) basic phase equilibria that must be known to understand the behavior of pore fluids and fluid inclusions in nature; (3) the question of validity of using fluid inclusions as records of ancient diagenetic systems is dealt with in such a way that the questions commonly asked about the limitations of the technique are addressed; (4) hot to conduct a fluid inclusion study, a new petrographically based approach for conducting fluid inclusion research that is followed by methods that allow for the interpretation of compositions of pore fluids that existed in sedimentary rocks, and methods of geothermometry and geobarometry; (5) selected case histories that are designed specifically to give practice in evaluating fluid inclusion data from the diagenetic realm; and (6) a summary of the arsenal of analytical techniques that may be applied to fluid inclusions to develop additional constraints on fluid inclusion composition.