Research into the diagenetic alteration of sandstones and carbonates began in earnest in the 1970s and has been successfully applied to petroleum exploration and development of conventional reservoirs: progressing from a qualitative understanding of evolution of reservoir quality to today’s advanced computer models to quantify and predict reservoir quality ahead of drilling. The current industry focus on unconventional shale (mudstone) reservoirs has provided a wealth of new data and techniques to begin meaningful study of the potential impacts of diagenesis on mudstone reservoirs, much of which has yet to be fully realized. We believe that this field provides many research challenges, which if successfully addressed, will significantly improve understanding and prediction of reservoir and source rock quality in unconventional mudstone oil and gas plays.

The petroleum industry, academia, and government agencies are currently performing many studies to better understand the geologic processes involved during burial of mudstones and their evolution to form petroleum reservoirs, seals, and source rocks. Until recently, most researchers investigating mudstones concentrated their research efforts toward understanding (a) hydrocarbon generation and expulsion, (b) seal capacity, and (c) overpressure generation. Most data used to support these investigations were derived from organic geochemistry, relatively low magnification optical petrography, and bulk rock characterizations. Notably lacking from these past studies are the characterization and evaluation of the potential impact of mudstone diagenesis other than mechanical compaction.

Application of new analytical techniques, particularly with scanning electron microscopy (SEM), has facilitated the investigation of mudstone properties down to the nanometer scale (Camp et al., 2013; Olson, 2016). SEM observations of a variety of mudstones (shale reservoirs) have revealed the presence of authigenic minerals (cements and grain replacements) and have captured various stages of the transformation of organic matter during petroleum generation, including secondary porosity development within organic matter. This new research has led to an improved understanding of organic and inorganic diagenesis in mudstones that is required to better predict shale reservoir quality and heterogeneity.

This Memoir is an outgrowth of the joint SEPM-AAPG Mudstone Diagenesis Hedberg Research Conference held in Santa Fe, New Mexico, USA during October 16–19, 2016. The purpose of this conference was to bring together leading experts from industry, academia, and government to foster the free exchange of new ideas on the controls and impact of diagenesis on mudstone source rocks, reservoirs, and seals. A total of 88 scientists attended the conference, representing 11 U.S. states, and five countries (Argentina, Australia, Canada, Japan, and the United Kingdom). A summary of the conference is available from the SEPM website link to past conferences (https://www.sepm.org/MudstoneConference).

This Memoir is a continuation of the Mudstone Diagenesis Conference’s goal to identify new research and techniques that can be applied to improve unconventional shale reservoir exploration and development, and to ensure that learnings and best practices from other areas of mudstone science are utilized effectively. This book is intended for geologists and research scientists specializing in petroleum geology, organic and inorganic geochemistry, mudstone petrography, and rock mechanics.

The material presented in this volume represents new original research solicited from the Santa Fe Mudstone Diagenesis Conference attendees and other interested scientists. The chapters in this book cover a broad range of topics and are organized into five general categories: (1) new tools and techniques, (2) inorganic diagenesis, (3) mechanical properties, (4) organic diagenesis, and (5) applications and case studies.

The fine-grained nature and generally opaque character of carbonaceous mudstones have led to the development and adaptation of specialized analytical techniques to characterize mudstone hydrocarbon reservoirs. Many of these techniques, such as x-ray diffraction (XRD), optical organic petrography, crushed rock core analyses, mercury porosimetry, Argon-ion milling, focused ion beam-scanning electron microscopy, and micro x-ray computed tomography have become standardized and fairly routine methods and thus are not further explored here. For a recent summary of digital imaging technology and applications, the reader is referred to Olson (2016).

Several presentations during the Santa Fe conference described the considerable controversy regarding the description and nomenclature of organic matter as reported by geochemists as well as optical and SEM petrologists. Chapter 1 describes a new correlative light and electron microscopic technique that combines the well-established identification of organic matter (macerals) using optical microscopy with identical regions of interest imaged by SEM to reduce the ambiguity inherent with SEM images. Chapter 2 describes a new technique for characterizing organic matter and mineral content of carbonaceous mudstones using high-resolution laser scanning confocal microscopy and micro-Fourier infrared imaging.

Mechanical compaction has long been recognized as a dominant process responsible for reducing porosity in mudstone. Until recently, little study had been devoted to understanding recrystallization and formation of authigenic minerals and cements. Authigenic microquartz has been observed in many mudstones, prompting questions regarding the source of silica and cement paragenesis. Chapter 3 focuses on possible biogenic and abiogenic sources of silica for quartz authigenesis, and, most important, the temperatures under which quartz precipitates over time in siliceous mudstones. Relatively early quartz cementation arises from biogenic opal, whereas later generations of quartz have been thought to potentially form from silica derived from clay–mineral transformations (smectite–illite). However, the general lack of petrographic evidence for significant volumes of late quartz cements in mudstones raises important questions about the nature of elemental transfers during clay diagenesis. Silica cementation is also the focus of Chapter 4, which documents the striking similarities between authigenic microquartz in the Cretaceous Mowry Formation of the northern Rocky Mountains and the Devonian Woodford Shale of West Texas. Drawing comparisons even further, the authors note the fine-scale similarities to silicification documented in late Precambrian mudstones and speculate that silicification in Phanerozoic units may likewise be influenced by microbial processes and the presence of siliceous allochems.

Mudstone sedimentologists and petrographers are often drawn by the allure of “fools’ gold” (pyrite) that is commonly associated with reducing depositional environments and organic-rich mudstones. The observed iron sulfides may take on various forms, including individual crystals, isolated and aggregated framboids, concretions, and a replacement of preserved plant and animal fossils. Chapter 5 compares contrasting redox conditions during early diagenesis interpreted from the size and abundance of pyrite framboids observed in the Devonian Marcellus and Ordovician Point Pleasant mudstone hydrocarbon reservoirs in the western Appalachian Basin. Chapter 6 documents multiple generations of authigenic pyrite during burial of the Late Cambrian Alum Shale in southern Sweden, arguing for a later diagenetic, rather than syndepositional origin.

In the broader literature, mechanical aspects of diagenesis have generally received less attention than chemical aspects, perhaps because these processes mainly involve rearrangement of solids and pores and do not necessarily induce prominent changes in rock composition. Mechanical and chemical processes are often linked, of course, and the impacts of primary and diagenetic variations on mechanical rock properties have important practical ramifications for predicting fluid flow and deformation behavior.

One of the attributes for commercial hydrocarbon production from mudstone reservoirs is the ability to generate and propagate artificial fractures during hydraulic stimulation. Much of the past research has related various mechanical properties measured in the laboratory by triaxial compression tests of core plug samples to bulk composition measured by XRD or x-ray fluorescence. The controls of mudstone mineralogy have largely been attributed to depositional processes.

Microscopic examination of siliceous mudstones, in particular, have allowed for the interpretation of detrital silica as either terrestrial or intrabasinal (biogenic) in origin. New research further investigates the variation in fabric of the two end-member detrital silica sources, and related authigenic silica on mechanical behavior and porosity preservation, such as described in Chapter 7. Variation in mechanical heterogeneity across a range of scale in the Cretaceous Mancos Shale (western U.S.) is reported in Chapter 8. The results show that both primary depositional fabric (grain size and sorting) and diagenetic features (cements) are correlated with large variations in mechanical response. Chapter 9 presents results from deformation experiments and SEM imaging that address the impact of rock fabric on the nano-scale distribution of failure. Using contrasting examples of carbonate-cemented and silica-cemented mudstones, the study documented localization of failure along organic matter laminae that potentially enhances the transfer of fluids from pores hosted by organic matter to larger flow channels, and also demonstrated that the style of cementation (carbonate versus silica) impacts the character of pores developed during deformation.

A unique characteristic of petroleum source rocks and mudstone hydrocarbon reservoirs is the presence of significant amounts of organic matter, ranging from 1wt. % to more than 15wt. %. Several SEM studies previously published have described the microstructural characteristics of various forms of organic matter, including nanometer-scale pores. These studies have contributed greatly to the understanding of the physical changes of organic matter associated with the thermal alteration of kerogen to form hydrocarbons during burial diagenesis. Chapter 10 investigates the evolution of the transformation of kerogen in the Upper Devonian New Albany Shale in the Illinois Basin. This study combines SEM petrography with micro-Fourier transform infrared spectroscopy to provide a physico-chemical characterization of organic matter over a range of thermal maturities. Chapter 11 reports an optical and SEM study of gilsonite collected from a vertical fracture in the Green River oil shale in the Uinta Basin, Utah. The gilsonite (solid bitumen) is thought to be an early hydrocarbon generation product that may be analogous with pre-oil generation solid bitumen commonly observed in thermally immature unconventional shale reservoirs. The study documented that, unlike pyrobitumen observed in thermally mature shale samples, the gilsonite contained no significant amounts of nanoporosity, and concluded that pre-oil solid bitumen in shale reservoirs is an unlikely candidate for significant hydrocarbon storage.

The last section of the book provides examples of how observations of mudstone diagenesis can be applied for shale reservoir exploration and development. Chapter 12 describes a new model to predict porosity prior to drilling in unconventional shale reservoirs that includes the evolution of porosity developed in secondary, void-filling organic matter (“cement”) based on extensive SEM petrographic studies of a wide variety of productive U.S. shale reservoirs. Chapter 13 addresses the diagenetic complexity observed in the Wolfcamp Shale (Permian) of West Texas. Cementation, grain replacement, and fracture-filling by an assemblage of carbonate, silicate, sulfate, and sulfide minerals takes place over a protracted history that extends across conditions ranging from the sea floor to deep burial conditions characterized by warm saline fluids. Finally, Chapter 14, contrasts the diagenesis of interbedded organic-rich marls and recrystallized tight limestones in the Eagle Ford Formation of South Texas. Although most of the hydrocarbons are stored within the marls, the better performing horizontal wells are those completed within thin-bedded, more brittle limestones, resulting in better near-wellbore stimulated fracture complexity and connectivity with natural vertical fractures.