Abstract For several decades geologists have been applying field, petrographic, and geochemical methods to study the diagenesis of limestones, dolomites, evaporites and sandstones. The most successful studies have integrated field and petrographic work with various geochemical methods. For most applications, the value of any one of the most commonly applied techniques has often been limited; however, when applied together they have proven very useful. Careful petrography has been the most important and reliable component of diagenetic studies. Trace and minor element analysis of diagenetic phases is limited by poor knowledge of distribution coefficients, unknown applicability of distribution coefficients, or unknown pore-fluid chemistry. Interpretation of values of stable isotopes (∂ 13 C and ∂ 18 O) may be plagued by unknown temperature, pore fluid composition, water-rock ratio, or unknown fractionation factors for some systems. All of the foregoing methods are indirect methods of interpreting diagenetic history in that they are the result of diagenetic processes rather than samples of the diagenetic systems themselves. Such indirect approaches often yield data that can easily be misinterpreted. Fluid inclusions are fluid-filled vacuoles sealed within minerals. When trapped within diagenetic minerals, they provide the only direct means of examining the fluids present in ancient diagenetic environments. Fluid inclusions can be thought of as time capsules storing information about ancient temperatures, pressures, and fluid compositions. They may provide the following valuable information with simple petrographic observation, microthermometric analysis, or sophisticated geochemical analysis of inclusion contents.

Abstract 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,

Abstract This chapter considers the phase relations of simple systems applicable to aqueous and petroleum fluid inclusions in sedimentary environments. One cannot appreciate the potential power of fluid inclusion observations, or the inherent limitations of the technique of microthermometry, without a sound understanding of the phase equilibria and their representation as phase diagrams. The equilibrium phase relations provide the link between the laboratory measurements of temperatures at which phase transitions occur within inclusions when heated and cooled and the interpretations of the measurements. Without knowledge of the phase diagrams, one cannot be a careful, critical inclusionist because one will not know the appropriate phase transitions to look for, and one will not know the assumptions upon which interpretations of phase transitions are made. Even the most basic petrographic approach to the fluid inclusion technique begins with a strong foundation in phase equilibria. These fundamental principles are elucidated in this chapter by first evaluating the phase relations of the unary systems pure H 2 O and pure CH 4 . Then, the effects of adding NaCl to water are evaluated, followed by a treatment of the effects of adding CH 4 to water. Finally, a generalized treatment of the phase relations of petroleum fluids are considered. A more detailed presentation of phase behaviors that are observed within inclusions, as well as discussions of inherent observational and interpretative limitations are presented in Chapter 7.

Abstract The phases trapped within fluid inclusions and any phase changes that may subsequently be observed in the lab are dependent not only on the micrometer and submicrometer scale physical and chemical processes active during entrapment, but also on processes that proceed after entrapment. If fluid inclusions are to be useful, it is important to know the degree to which their chemical composition and density are representative of the bulk of the diagenetic fluid from which they were entrapped. The degree to which an inclusion's fluid is representative of the ancient diagenetic fluid involves an assessment of several important questions. First, for any fluid trapped within an inclusion, a question arises as to its similarity with the major, minor, trace element, and isotopic composition of the ancient pore fluid that existed prior to entrapment. Second, if more than one fluid phase were present in a diagenetic system during inclusion formation, a question arises as to whether the phases trapped within inclusions are representative, proportionately and compositionally, to those phases present in the pore fluid. Finally, one must question the potential of natural processes to cause changes in fluid inclusion location, shape, volume, and compositions after initial entrapment and during subsequent uplift and/or burial. This chapter attempts to answer these essential questions in two major parts. First, we will determine the degree to which fluid inclusions (formed from both homogeneous and heterogeneous fluid systems) might differ from the bulk of the diagenetic fluid during the entrapment process. Second, any changes resulting from

Abstract To think of fluid inclusion analysis as a “black box technique” is a grave mistake. A fluid inclusion study is not something that one “just does” — it requires a more scientific methodology, first involving the formulation of specific questions along with conceivable hypotheses for possible outcomes. Such a philosophical approach is necessary because very commonly the fluid inclusion data that are required to answer a particular question may not be present in the rocks! So, a more structured approach may allow recognition of such a predicament at an early stage in a proposed fluid inclusion study, preventing the ordeal of wasting considerable effort in the collection of meaningless data. The authors know of many horror stories in which a research supervisor has told a student, consultant, or subordinate to “do a fluid inclusion study” on some set of samples. Of course, the doleful soul taking the orders assumes it must be possible because the boss told him to do it, and feels compelled to come up with data no matter what the fluid inclusion population looks like. Time and time again, these researchers spend months conducting a fluid inclusion study on material that may not have the fluid inclusions appropriate for answering a particular set of questions. For example, if the research supervisor wanted to know the temperature and salinity from which some authigenic mineral precipitated, the subordinate would feel compelled to find primary fluid inclusions in that mineral, whether they were present or not. The subordinate would deceive

Abstract This chapter presents practical methodologies for examining the petrographic characteristics of fluid inclusions in diagenetic minerals, together with a format for interpreting the diagenetic environment and thermal history of a sample from the petrography of the fluid inclusions alone! Accomplishing a study of fluid inclusion petrography is much like any other petrographic study in that there are certain necessities, and some amount of thought and effort is required before a sample can be studied: samples must be collected with respect to the problem at hand, samples must be prepared for microscopic observation, and a properly adjusted microscope must be available. The difference between standard thin section petrography and fluid inclusion petrography is that each of these mechanical steps must be carried-out with great care; otherwise, great barriers will impede even the most persevering scientist from obtaining useful information from the fluid inclusions. Luckily, the reader will be relieved to know that these mechanics have been solved to the extent that all that is required for their successful implementation is to take note of the information presented in this chapter. The next hurdle will be for an inclusionist to record the fluid inclusion petrography. This is not a trivial undertaking — remember what it was like when you took your first thin section petrography course! In a single thin section there can be thousands of crystals. The object is to be selective: a petrographer learns to answer a series of mental questions (e.g., minerals present, textures present, etc.) in order to

Abstract The determination of temperatures of phase changes within fluid inclusions during heating and cooling of samples is termed microthermometry. The technique is invaluable for discovering the temperatures at which minerals form, the thermal history a rock has experienced, and the compositions of the fluids that traversed a rock in its history. The fundamental principles upon which microthermometry is based are the principles of phase equilibria introduced in Chapter 3. And, as explained in Chapter 6, it is best to first spend a considerable amount of time and effort to characterize the fluid inclusion petrography prior to attempting microthermometry. This chapter elucidates useful philosophical approaches and important practical procedures that, if learned and employed, will ensure successful and efficient microthermometric studies. Also, phase changes for some simple but potentially applicable fluid systems common in the diagenetic realm will be described.

Abstract If readers have taken heed of our recommendations for an appropriate mental framework for selecting inclusions for microthermometry (see Chapter 7), then the method for proper presentation of data will be straightforward. Those who have not collected data with respect to petrographically related assemblages of inclusions (FIAs) have chosen an inappropriate methodology in which they must assume that any variability in their data is due to the vagaries of nature. Such an assumption leads to a philosophy that maintains that it is both appropriate and sufficient to portray variability to a reader by lumping all data into a few encompassing histograms, and then to interpret the data from statistically determined means and modes. It is very understandable that such logic seems reasonable, but the fallacy is the assumption that any variability is “natural” and to be expected. Consider the numerous natural cases where inclusions along a single microfracture or a single growth zone yield very consistent microthermometric results, or the cases in which data from synthetically produced inclusions vary less than a few tenths of a degree! For these groups of inclusions trapped at a certain P-T-X condition, there is, in fact, little “natural” variability. So, in contrast to what a geoscientist might initially think, the more appropriate philosophical viewpoint should be that data are expected to be consistent, but only if the following criteria are met: 1. The data come from a single petrographically related population (FIA) of inclusions that samples a single event.