Fluid Inclusions and their Origin
When observed at room temperature using a transmitted light microscope, most fluid inclusions have a rather sharp outer boundary marking the edge of the inclusion cavity (Fig. 2.1). This is because of a significant difference in refractive index between inclusion fluids and their mineral hosts: most aqueous fluids have refractive indices between 1.33 and 1.45 whereas the minerals in which they are included have refractive indices from 1.43 to as high as 3.22. Hydrocarbon liquids, however, have refractive indices that may be similar to their mineral hosts (Burruss, 1981), and thus, are not all easily visible. The inclusion cavity generally contains a large amount of bright, clear liquid (Fig. 2.1A, D, E) and some may contain a small dark bubble of vapor or gas (although any liquid-to-vapor ratio is possible) that is dark because of internal reflection (Fig. 2.1D). However, as shown in Figure 2.1E, bubbles in flat inclusions may not appear that dark. Though most liquids appear colorless, some hydrocarbon liquids may have colors ranging from reddish-brown to yellow.
Inclusions smaller than 1 µm currently are not possible to study because of microscope optical limitations. The sizes of most inclusions readily studied in diagenetic phases are about 2 to 7 µm in longest dimension. For the most part, coarsely crystalline diagenetic minerals contain more workable-sized inclusions than fine-grained minerals, and smaller inclusions are typically much more abundant than larger inclusions in diagenetic phases. Because of the small size of the inclusions, petrographic study requires a good microscope properly adjusted,
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.