Microfacies of Carbonate Rocks

A rapid evolution in our understanding of carbonate rocks was triggered in the mid- to late 50’s by the discovery of carbonate reservoirs in various parts of the world, followed by intensive research of recent carbonates. Modern and ancient carbonate environments, diagenetic processes and facies models were studied between about 1955 and 1965. During the late 60’s and the 70’s microfacies became an essential part of facies analysis and paleoenvironmental interpretation of limestones. The increasing importance of limestones and dolomites as reservoir rocks and the use of thin-section fossils in subdividing carbonate platforms gave substantial impetus to the progress of microfacies research. The 1980’s were characterized by an increased application of geochemical techniques with the object of developing predictive models for carbonate diagenesis and porosity. The subsequent period of time has seen the rapid development of sequence stratigraphy which now plays a major role in the characterization of the geometry and reservoir potential of carbonate rocks.

Knowing where modern carbonates occur, what they are composed of, and what their controls are is essential for evaluating microfacies data. The objectives of this chapter are to summarize the settings and environments in which carbonate sediments are formed and to document which classifications are used for differentiating these environments. Emphasis is placed on the definitions of terms.

This chapter summarizes field work studies, sampling strategies and laboratoy methods relevant to microfacies analysis. A precise field record of geological and paleontological data as well as sampling strategies that consider the vertical and lateral variations in these data are vital for the success of microfacies studies. Laboratory methods study microscopic features and mineralogical and geochemical data.

The purpose of this chapter is to outline and discuss the criteria used in the description of matrix and grains. Microfacies criteria reflected by fabrics and quantitative microfacies data are discussed in Chap. 5 and 6.

The previous chapters of this book introduced various aspects of recognizing qualitative microfacies criteria and their signifcance, whereas this chapter will treat quantitative criteria. The first part describes selected principles of grain-size analysis and demonstrates the potential of grain size in the context of microfacies studies. The second part deals with methods of frequency analysis and their use in microfacies-based interpretations. The last part discusses the evaluation of microfacies by multivariate statistical analyses.

The preceding chapters provided information on the diagnostic criteria for ‘depositional microfacies’. This chapter deals with ‘diagenetic microfacies’ that reflects changes during lithification and rock history (Pl. 28). Diagenesis refers to physical, chemical and biological processes. The understanding of these processes and their products has high economic importance, because diagenetic criteria account for many of the petrophysical properties of carbonate rocks and determine their value as reservoir rocks and use in industry. Diagenetic studies require a combination of various methods, including standard optical petrography, cathodoluminescence, SEM observations, stable isotope analyses and rare element composition. The following text concentrates on diagenetic features that can be studied in thin sections. Comprehensive reviews of carbonate diagenesis can be found in textbooks listed under Basics in the reference list at the end of this chapter.

There are too many papers that simply conclude that the carbonate rocks of a given study can be arranged according to the classification of one or more authors. All too often these workers have failed to recognize that a classification is simply a tool for organizing information, not the source of a conclusion (Blatt et al. 1972).

Most limestones are directly or indirectly influenced and controlled by biological processes. The present chapter deals with the formation of carbonates by microbes and benthic encrusting organisms, and with the destructive role of micro- and macroborers. Knowledge of constructive and degrading processes is essential in evaluating carbonate budgets.

Many students of carbonate rocks are bewildered and sometimes frustrated by the morphological and microstructural diversity of skeletal grains and are satisfied by distinguishing major fossil groups. The present chapter will hopefully demonstrate that more detailed identifications of thin-section fossils are not so difficult and that identifications at lower systematic levels can provide fresh insights into the environmental controls on depositional processes. However, keep in mind, that thin-section fossils present only a part of the biota present in the rock and that thin-section diversity is not equivalent to biotic diversity (see Sect. 6.2.1.4).

Microfacies studies aim for the recognition of overall patterns that reflect the history of carbonate rocks, by means of a thorough examination of their sedimentological and paleontological characteristics. The previous chapters have shown which thin-section criteria are used in describing the microfacies of limestones. The evaluation of microfacies in the context of facies interpretation requires a synopsis of microfacies data observed in various samples into a microfacies type (MFT). Microfacies types and facies associations are fundamental to the development of models for carbonate sedimentation.

This chapter provides an overview of microfacies data that assists in recognizing environmental parameters that control carbonate deposition and the distribution of organisms. Section 12.1 summarizes paleoenvironmental proxies. Specific attention is given to paleoclimate-indicative criteria (Sect. 12.2) and to paleo-depth criteria (Sect. 12.3). The last section of this chapter deals with microfacies data that allow seismic events to be recognized.

To understand the formation and diagenesis of carbonate rocks mineralogical and geochemical data derived from the study of non-carbonate constituents, trace elements, and stable isotopes must be integrated. Other indicators for depositional and diagenetic conditions are organic matter and organic carbon.

Preceding chapters of part 2 of this book were focused on defining microfacies types, recognizing paleoenvironmental constraints and using an integrated approach to facies analysis. Chapter 14 introduces carbonate facies models, underlines the importance of environment-controlled distribution patterns of organisms, and gives an overview of Standard Microfacies Types and their application to the facies analysis of limestones. The latter topic is expanded in the case studies described in Chap. 15.

This chapter is an overview of tools for translating microfacies data into depositional settings. Using case studies of non-marine and marine carbonate rocks, we focus on the diagnostic criteria for recognizing the facies characteristics needed in basin analysis studies. Microfacies is an important element of basin analysis, but must be applied along with all the other field and laboratory observations available for reconstructing depositional conditions. This is exemplified by Fig. 15.1.

Having learned how microfacies can be integrated into facies models (Chap. 14) and how microfacies can be translated into depositional settings (Chap. 15), the following chapter summarizes the potential of microfacies for recognizing and interpreting major depositional constraints. These constraints are manifested by characteristic sedimentation patterns and require the application of specific facies analysis techniques.

Reservoir rocks are characterized by petrophysical properties favoring the capacity for the migration and storage of fluids, gas and minerals. Microfacies analysis has been strongly triggered by the exploration and development of hydrocarbon reservoirs and source rocks in limestone and dolomites and is still of interest in understanding depositional controls, facies heterogeneities and reservoir property distribution.

The industrial use of carbonate rocks depends on physical and chemical properties. These properties are the result of the sedimentary and diagenetic history of the rocks. In many cases, the ‘primary (depositional) facies’ influencing the course of diagenesis are important controls for specific physical-chemical rock criteria. Microfacies analysis assists in understanding these controls. The study of causal relationships between microfacies data and specific rock properties will be an important tool of future work. This chapter demonstrates some examples illuminating the potential of applied facies analysis in this field.

Archaeologists are faced with the problem of the provenance of materials used in ancient buildings, sculptures or ceramics. Archaeometric methods applied to source studies include analyses of element distributions, stable isotope patterns and studies of the mineralogical and petrographical composition (Herz 1987; Riederer 1987; Herz and Waelkens 1988; Gibson and Woods 1990; Walsh 1990; Waelkens et al. 1992; Orton 1993; Rapp and Gifford 1995; Herz and Garrison 1998; Pollard 1999). ‘Geoarchaeology’ (Rapp and Gifford 1998) is an exciting and promising interdisciplinary approach.