Kansas Geological Survey, Subsurface Geology 12, p. 57-58
Robert L. Brenner
University of Iowa
Basin analysis has its roots in the first half of the twentieth century when tectonic settings were shown to control the compositions of rock associations. The importance of diagenesis became apparent as petroleum-exploration efforts expanded into deep basins and into relatively unstable potential reservoir-rock facies. Research dealing with the role of diagenesis in basin analysis is proceeding along several lines, including: 1) determination of the effects of provenance on sandstone mineralogy and on diagenetic style, 2) delineation of the relationships between depositional settings and sandstone-reservoir characteristics, and 3) construction of integrated diagenetic models that can be used to predict rock properties.
Diagenetic styles are controlled by eight interactive variables: 1) sediment compositions, 2) temperature histories, 3) rates of subsidence (pressure histories), 4) age (time that sediments have been exposed to other variables), 5) sediment textures, 6) sediment-body structures (internal architectures), 7) sediment-body external geometries, and 8) fluid-chemistry histories. Each of these variables is affected by variations in the other variables, and each is an integral part of any basin analysis. Thus, diagenesis studies are intimately intertwined with basin analysis.
Tectonic and paleogeographic settings determine the primary compositions of both chemical and siliciclastic sediments. Siliciclastic provenances are reflected by the mineralogy of sandstones. The source or sources of sediment in sandstone units within genetic sequences and the contribution of each source need to be evaluated in terms of quantitative effects on the various diagenetic styles observed. Diagenetic studies are currently underway in the Pennsylvanian of the midcontinent and the Cretaceous of the Western Interior. Composition and reservoir properties of sandstone units in these two realms were controlled by provenance, sedimentary processes, and diagenetic alterations. These interrelated parameters were affected by sea-level fluctuations in both realms.
Pennsylvanian sandstones are being studied by using a combination of petrographic and geochemical techniques. These include light microscopic examination, cathodoluminescence (CL), SEM, TEM, and x-ray microanalysis techniques. CL analysis is used to help segregate mineral grains into different compositional classes. Microanalytic techniques are used to identify and quantify elemental differences. These analyses have indicated that some quartz arenites are diagenetic in origin, having been deposited as subarkoses that subsequently underwent feldspar dissolution. Core samples from Iowa are being used to study the origins of mineral grains, such as feldspars and micas, by subjecting them to rubidium/strontium (Rb/Sr) isotopic analysis in order to match isotopic ratios with potential siliciclastic sources. Matches made in this manner are used with sedimentologic data to reconstruct sediment-transport pathways between sources and basin-facies tracts, and to relate mineral grains in sandstones to potential siliciclastic sources.
Similar analyses of Cretaceous sandstones, such as the Parkman and Shannon formations of Wyoming, have shown that reservoir properties can be predicted by relating diagenetic styles to tectonic and sedimentary settings. For example, marine sandstones of the Parkman deltaic-shelf system, which interfinger with marine shales, have early diagenetic chlorite coatings on quartz grains that retarded later mineral precipitation resulting in the preservation of reservoir properties. Sandstones in more proximal positions that are not in contact with the shales lack these coatings and were later pervasively cemented with silica. Nonmarine sandstones in this system that were exposed to acidic waters in an organic-rich fluvial setting have kaolinite pore fillings probably derived from dissolution of feldspars and micas.
Once paleoenvironmental interpretations are made and cyclic sedimentary sequences are delineated, diagenetic styles can be interpreted with respect to cyclic changes in sea level. Early diagenetic processes are evaluated in terms of chemical reactions that would take place between original sediment pore waters inferred to be present in each environment and both inorganic (mineralogical) and organic components of sands and surrounding muds. Distributions and concentrations of ionic components in sediment pore fluids are critical variables that must be considered in any diagenetic model. For example, some Pennsylvanian fluvial sandstones in the midcontinent show evidence of early feldspar dissolution. These phenomena resulted from the circulation of low pH, organic-rich waters through a moderately open hydrologic system. As burial conditions vary and reactions take place, dissolution, precipitation, and mineral alterations proceed at rates determined by temperatures, resident times, and ionic concentrations.
Petrographic characteristics are correlated statistically with paleodepositional settings determined from the mapping and cross sectioning of outcrop, well-core, and geophysical well-log data. Paleoenvironmental interpretations are made by integrating all stratigraphic, sedimentologic, and paleontologic data available for the basin being studied. Genesis and delineation of cyclic sedimentary sequences allow diagenetic styles to be interpreted with respect to cyclic changes in sea level. Early diagenetic processes are evaluated in terms of chemical reactions that would take place between original sediment pore waters inferred to be present in each environment and both inorganic (mineralogical) and organic components of sands and surrounding muds. Distributions and concentrations of ionic components in sediment pore fluids are critical variables that must be considered in any diagenetic model. As burial conditions vary and reactions take place, dissolution, precipitation, and mineral alterations proceed at rates determined by these conditions and ionic concentrations.
With the use of modern settings as partial analogs, sedimentologic and petrographic data can be used to reconstruct sandstone architectures and original fluid chemistries. Subsidence histories and sea-level fluctuations are then used to reconstruct geochemical histories of each sandstone body within a time-temperature basin-setting framework. These geochemical and hydrogeological reconstructions are required to establish geochemical schemes for each paleodepositional setting. As more insight is gained into the relationships between tectonic settings, depositional settings, and diagenetic styles, geohistory analysis and backstripping methods will play increasingly important roles in basin analysis. The volumes of data and complexities of relationships between data sets will require computer-based data-handling systems.
Diagenetic modeling is an integrative process because each of the variables interacts with the other variables in a complex manner so that no single aspect can be accurately isolated. Artificial intelligence in the form of expert systems is ideally suited for this type of modeling because of its abilities to integrate large amounts of data from many sources and to iteratively calculate the simultaneous influences of multiple processes upon a sedimentary system during successive time intervals. Once models are generated for sandstones from various paleodepositional settings and cyclic positions, they can be used to predict reservoir qualities, such as porosity and permeability. A complete basin analysis designed to produce models for predictive purposes must include diagenesis because alteration of rock properties can vary from nil to complete recrystalization, removal, or replacement during the history of a basin.
Kansas Geological Survey
Comments to firstname.lastname@example.org
Web version May 11, 2010. Original publication date 1989.