The modern concept of ``diagenesis'' was introduced by Walther one hundred years ago to encompass the postdepositional processes that affect sediments from deposition to low-grade metamorphism. Since Walther (1893), diagenetic studies have been concerned with an extraordinary variety of chemical and physical interactions, each of which has the potential to significantly affect the final composition and texture of a sedimentary rock. The issues to which those studies have been applied are equally varied and include: (1) improved techniques to predict the producibility of hydrocarbon reservoirs, (2) improved techniques to predict subsurface fluid pressures, (3) better constraints on the timing of fluid flow events, (4) reconstruction of global climate change, and (5) evaluation of groundwater contamination. Commercial applications of diagenetic studies have traditionally focused on the producibility of hydrocarbon reservoirs. As such, a review of this topic serves as an effective illustration of how basic research has been commercialized, and a useful focus for the following dicussion. However, it is worth noting that decreased industrial research funding, coupled with increased government funding for environmental research have recently resulted in a gradual evolution of diagenetic publications away from the producibility of hydrocarbon reservoirs toward the producibility of uncontaminated groundwater and other environmental issues. Despite the shift in emphasis, many of the basic research issues remain unchanged whether one is focused on deep hydrocarbon reservoirs or shallow aquifers.
In examining the recent literature related to hydrocarbon reservoir producibility, one can see that our basic concept of ``reservoir quality'' has changed dramatically over the past few years. Ten years ago, most publications focused on the prediction of reservoir porosity. In contrast, many of today's publications address the more difficult question of whether there is adequate permeability distributed in such a way as to allow successful oil and gas production from a number of potential reservoirs. This evolution is driven largely by the fact that worldwide oil and gas operations are now more often conducted in higher cost, higher risk environments in which a simple prediction of porosity does not provide adequate information to evaluate potential economic success. This transition has been accompanied by a trend toward larger multi-disciplinary studies in which diagenesis experts are linked with experts in stratigraphy, reservoir engineering, petrophysics, seismic interpretation and geostatistics, among others, to address complex issues related to reservoir management.
Coupled with changes in our basic concept of reservoir quality is an evolution in our understanding of basic controls on both porosity and permeability. We know that porosity is created, preserved, or destroyed by a combination of: (1) physical (mechanical) compaction of grains, and (2) mass transfer processes that supply and remove components required for cementation, grain dissolution, and mineral transformations. In my opinion, controversies surrounding the mechanisms of physical compaction have been largely resolved, save possibly the role of fluid pressure in inhibiting compaction (for recent papers see Lundegard, 1992; Pittman and Larese 1991, and Houseknecht, 1987). Major remaining issues revolve around the controls on chemical reactions and cement precipitation.
Researchers have long since identified most of the potential controls on cementation. However, we are often unsuccessful at quantitative predictions of cement volume and distribution within a single reservoir. This lack of success results largely from our frequent inability to predict complex reactions between rocks and fluids in compositionally heterogeneous systems that can also undergo rapid changes in temperature, pressure and fluid chemistry. As a means of approaching the problem, the decade of the 1980's saw a significant effort to develop integrated, process-driven computer models to explain observed patterns of cementation. Unfortunately, most models were unable to reproduce the complexities of real geologic systems. However, they did help bring attention to those complexities. The result has been a renewed emphasis on improved observations, better integration of empiricism and theory, and ongoing development of new technologies that will eventually provide the key information needed to resolve particular controversies.
To elaborate on these observations, I have chosen to focus the following discussion on two key areas. The first relates to major controversies that have been the subject of multiple papers over the past few years. In most cases, these controversies have arisen from attempts to explain why some deeply buried sandstones have high intergranular porosities, and consequently high permeabilities, while others are completely cemented. The second area of emphasis relates to new analytical techniques and tools that are being developed to address these controversies, as well as other issues. These topics undoubtedly reflect my own interests and expertise, and another researcher might have selected a different set of topics.
Omitted from discussion are a large number of recently published papers not covered by the selected topics. In addition, because much of the basic research in diagenesis has gone on outside the United States, the reference list is not restricted solely to U. S. sources. To examine broader issues of diagenesis not covered in this review, the reader is referred to: (1) Geothermometry and Geochronology using Clay Minerals (Eslinger and Glassman, 1993), (2) Prediction of Reservoir Quality Through Chemical Modeling (Meshrie and Ortoleva, 1990), (3) Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstones (Houseknecht and Pittman, 1992); (4) Diagenesis and Basin Development (Horbury and Robinson, 1993); (5) North Sea Formation Waters: Implications for Diagenesis and Production Chemistry (Smalley and Warren, 1994); (6) Proceedings of the 7th International Symposium on Water-Rock Interaction (Kharaka and Maest, 1992), and (7) Mineralogical Association of Canada Short Course in Burial Diagenesis (Hutcheon, 1989)