Petrographic observations of rock composition and texture continue to provide fundamental data on reservoir quality controls. However, an ongoing problem in diagenetic studies is the complex compositional and textural heterogeneities that exist at all scales. One way to evaluate the role those heterogeneities play in overall reservoir quality predictions is to analyze very small samples, and there have been several improvements in these types of techniques over the past several years.
Both laser and ion microprobes are now used to perform in-situ isotopic analyses of samples down to 30-40 microns diameter, approaching the dimensions of silt-sized grains, or single quartz overgrowths (e.g. Sharp, 1992). Successful studies include the analysis of oxygen isotopes in quartz described above (McKeever et al., 1994), and sulfur isotopes in pyrite (Fallick et al., 1991). More news at the small end of the size spectrum comes from the use of new microscopic techniques that allow scientists to study the surfaces of fine-grained materials such as clays, and actually observe mineral reactions at ambient temperatures and pressures. For example, recent advances in Transmission Electron Microscopy (TEM) techniques have led to significant improvements in the description of the crystal structure of clays, including layer stacking sequences of individual grains (e.g. Peacor, 1992). In addition, Baker et al. (1993) have now used an environmental Scanning Electron Microscope (SEM) to study reactions involving chlorite, illite/smectite, calcite, and fluids commonly injected into wellbores during production as a means of assessing the potential for reservoir damage. Both Blum and Eberl (1992) and Nagy et al. (in press) used the Scanning Force Microscope (SFM) to measure the dimensions of individual fibrous illite particles. Nagy et al. observed a significant correlation between depth, temperature, and illite-particle dimension that also correlates with K/Ar age. They used that information to develop preliminary reaction rate laws for fibrous illite growth. One other advance in the area of small-sample analysis is the ability to quantitatively analyze the chemistry of fluids within single fluid inclusions (e.g. Kihle, 1993; Horita et al., 1991; and Guilhaumou et al., 1990). There are also a number of new techniques for absolute age analysis of single crystals, as described in the following section.
On a slightly different topic, a number of advances in the field
of geothermometry and geochronometry have also been made over the
past few years. Application of these tools to diagenetic studies
has become increasingly frequent, brought about largely by a desire
to study diagenesis in the context of basin evolution as a whole,
and a desire to identify the most likely sources of diagenetic
cements. One approach that has already seen significant commercial
application is K/Ar dating of diagenetic illite (for recent papers,
see Ziegler et al., 1994; Hogg et al., 1993; Clauer et al., 1992;
Hamilton et al., 1992; Barnes et al., 1992; and Pevear, 1992, among
others). Both fibrous illite and mixed-layer illite/smectite have
been shown to be reliable clocks at temperatures below about
250
C, a property that makes them potentially attractive
tools for reconstructing burial history and calculating the time of
cement precipitation over much the temperature range typically found
in sedimentary basins.
The isotopic ages of fibrous illite have also been of
particular interest as a means of placing constraints on the time
of hydrocarbon migration. It is generally assumed that when
hydrocarbons are emplaced, the pore waters involved in diagenetic
reactions are displaced, and the rates of further diagenetic
reactions are significantly inhibited. Thus the isotopic age of
fibrous illite in a hydrocarbon-bearing zone can theoretically be
used to place an upper limit on the time of hydrocarbon migration.
However, problems associated with: (1) elimination of detrital
contaminants, i.e. clay grains present in the sediment at
deposition, and (2) interpretation of the timing and mechanisms of
illite growth, have resulted in significant speculation regarding
the interpretation of measured isotopic ages. Several techniques
under development will help to eliminate some of these
uncertainties. One of the more promising is the application of
Ar/
Ar dating to clay minerals in sandstones. This technique
should enable researchers to separate Ar released at lower
temperatures, and potentially carried by diagenetic components,
from Ar released at higher temperatures and potentially carried by
detrital components. A major technical problem with 
Ar/
Ar
dating has been the loss of 
Ar due to recoil from small samples.
Some recent progress in this area has been demonstrated by Foland
et al. (1992), among others. But Awwiller (submitted) calculates
that most recoiled 
Ar does not end up in the grain that
originally contained the 
K from which it formed, suggesting that
complete resolution of the problem will be difficult. As a means
of getting around this problem, Reynolds (1993) has developed
techniques for identifying different illite polytypes from analysis
of X-ray diffraction patterns, and recognized that conventional
separates used in dating may actually contain multiple types of
illite. Observations of this sort should help us to interpret the
measured illite ages. Finally, substantial progress has been made
in our understanding of reaction kinetics, which should provide
additional constraints on the time-temperature range of illite
formation (e.g. Nagy et al., in press, Huang et al., 1993; Small et
al., 1992; Velde and Vasseur, 1993; and Elliot et al., 1991).
The recent literature also contains an interesting variety of
other studies in which isotopes have been used to infer the
evolution of diagenetic fluids, thereby allowing researchers to
date diagenetic events. For example, Halliday and others (1991)
have placed constraints on models of crustal fluid flow by using
U/Pb, and Pb/Pb isotopes to date individual carbonate minerals, and
by using Rb/Sr, Sm/Nd, and K/Ar methods to date a number of clays,
sphalerite, apatite, and other minerals in clastic rocks. At the
more recent end of the time scale, Banner et al. (1994) have used
U-series isotopes to study the evolution of pore fluids in
Pleistocene carbonates from Barbados. Applications of rare gas and
Cl isotopes were mentioned in the above discussions. Wood and
Boles (1991) used carbon and strontium isotopes as evidence for
episodic emplacement of cements in the San Joaquin valley. The
Sr/
Sr ratio of present-day porewaters, measured in residual
pore salts, has also been used with some success to define the
evolution of closed and open pore-water systems and identify
hydrologic barriers (Smalley et al., 1992).
The potential of chlorites as geothermometers has been
discussed since the first descriptions of chlorite polytypes and
compositional variations in the early 1960's. However, recent
papers by Walker (1993) and de Caritat et al. (1993) have concluded
that both composition and polytype appear to be sensitive to a
variety of parameters other than temperature, and should be used
only with caution. In contrast to these studies, Ryan and Reynolds
(in press) have had some new success with the percentage of
interstratified 7Å layers in chlorites. They observed that in Gulf
Coast chlorites, the 7Å layers decrease from 20% at depths of 1-2
km to 
3% at depths of 4-5 km, and they are currently in the
process of trying to identify to possible transformation
mechanisms.
Chemical geothermometers, based on the concentrations of silica, and proportions of sodium, potassium, lithium, calcium, magnesium and other elements have been successfully used to estimate subsurface temperatures of hot springs and geothermal wells (e.g. Kharaka and Mariner, 1989). However, extensive experience in sampling and analysis of Gulf Coast waters recently led Land and Macpherson (1992b) to conclude that geothermometers based on mineral-water equilibria are of limited use in most sedimentary basins. They observed that metastable detrital phases persist in even the deepest wells, and that equilibria between stable phases is not commonly achieved, leading them to conclude that the concentrations of most components are governed by kinetics, rather than equilibrium. No doubt this debate will continue as pore fluids in older, more mature basins begin to be comprehensively evaluated.
Finally, it has been recognized for several years that diagenetic magnetic phases such as magnetite and pyrrhotite record the direction of the earth's magnetic field at the time they are precipitated, thus enabling researchers to date the diagenetic event responsible for their precipitation. The last several years have seen several new papers in this field, and the results are promising. Current research is now focused on obtaining a better understanding of the mechanisms of remagnetization and their relationship to specific fluid-flow events such as hydrocarbon migration (e.g. Elmore et al., 1993; Reynolds et al., 1993; and Suk et al., 1993).