Getting Started with Understanding Diagenetic Controls on Sandstone Reservoir Quality, A Compendium of Influential Papers

Compiled by Kitty L. Milliken

Department of Geological Sciences
Jackson School of Geosciences
University of Texas at Austin


Introduction

At deposition, the primary intergranular porosity of well-sorted sand is around 40 to 45 percent. From a reservoir quality perspective it would be wonderful if sandstones in the subsurface could retain this primary porosity, but, they don't, of course, because of a process known as diagenesis. Diagenesis is defined as the sum of all the processes that affect sedimentsfollowing deposition. At high temperatures, diagenesis grades into metamorphism; in another part of the rock cycle, diagenesis grades into weathering. Both mechanical and chemical processes, including biologic ones, are part of diagenesis, and diagenetically induced changes in rock properties can be profound. Not only can sediments be transformed into 'hammer-ringers', but massive changes in bulk chemistry canbe induced in the process.

A few comments on the history of diagenetic studies as related to the petroleum industry will give some perspective on the goals of this volume. In the late 1970s, technical advances led to a vast expansion of diagenetic discoveries and literature. One of these advances was a very humble invention, the application of colored impregnation media for highlighting the natural (as opposed to sectioning-induced) pores in rocks. Before colored 'epoxy' (mostly plastic resins, not actually epoxy) the pores in rocks were extremely obscure as seen in thin section. In plane-polarized light pores were clear and did not attract the eye any more than the average quartz grain (also clear). In cross-polarized light pores were black, and disappeared into the background along with any grains that happened to be at extinction (also black). And in either viewing mode one could not discriminate the real, natural pores from ones induced by plucking during the thin sectioning process, and so, the natural tendency was to just tune out the pores and focus on the crystals. Before the 1970s, few petrographic studies paid much attention to the porosity. But with the widespread application of color impregnation media, pores could finally take their rightful place among the grains and cements as fundamental rock components that could be measured, classified, and interpreted in terms of their historical context. The relevance to understanding reservoir properties was starkly obvious. Other technical advances were the widespread application of stable isotopic and electron microbeam techniques that allowed quantitative constraints to be placed on the timing and conditions of pore-modifying processes. Through the mid-1980s many companies maintained vibrant research groups dedicated to investigating sandstone diagenesis and funding flowed to universities in support of this research.

Two developments conspired to bring this period of flourishing research and discovery to an untimely end. First, the severe drop in the price of oil during the mid-1980s dampened enthusiasm for research across a broad swath of topics, diagenesis included. It has to be acknowledged that the impact of this situation on diagenetic research was exacerbated by the fact that models of truly predictive value had yet to emerge at that time, making it seem 'safe' to abandon this particular line of inquiry. Over the next decades the tremendous success of sequence stratigraphy and 3D seismic technologies absorbed much of the focus in industry, and academia, in large measure, followed suit with the curricula being offered to students. In fact, an understanding of large-scale stratigraphic variations in sand volumes is, indeed, largely sufficient information for making predictions about reservoir characteristics in sandstones that are relatively young, shallow, cool, and quartz-rich.

Today, improved drilling technologies and interest in unconventional resources combine to inspire drilling activity that, increasingly, brings petroleum geologists into contact with rocks that have been dramatically altered by diagenesis. Over the past 25 years, research in diagenesis, nurtured under the purview of a small (but dedicated!) community of researchers, has yielded new knowledge that gives us an understanding of diagenesis that has great practical value. Because of years of relative neglect in both industry and academia, unfortunately, many companies today are facing the need for this vital information without in-house staff trained in the basics of sandstone petrology. This compilation of papers endeavors to give the reader insights into those diagenetic processes that most significantly affect porosity and other petrophysical properties in sandstones. Very importantly, some of these processes are also predictable. Knowledge of diagenesis has value for reservoir quality risk assessment, and the papers assembled here provide a view to the fundamental scientific advances that have made this possible. These papers are chosen with the aim of giving readers who are unfamiliar with the field of diagenesis a quick primer on basic concepts that are widely applied today in reservoir quality assessment.