Shales--Their Sedimentology and Geology: ABSTRACT

Shale and mud form at least 60% of the world's sediments and have been deposited throughout geologic history. They occur in every major depositional basin. They are major source beds for hydrocarbons, hosts for metallic minerals, sources of ceramic materials, cause unstable foundation conditions, and produce soils for our food. Yet the study of shales has lagged far behind that of other sedimentary rocks. However, the following general observations can be made about these important rocks. More studies have been made of recent muds than of ancient shales. Clay mineralogy, geochemistry, and paleontology of muds and shales are better understood than their stratigraphy, petrology, and sedimentology. Most shales occur in marine sequences and are associated with deltaic depocenters. They are more commonly geosynclinal than cratonic, and are most commonly deposited in deep water as distal turbidites and pelagic muds. Cratonic shales occur as widespread, thin sheets distal from their source land. Sedimentology Most muds are transported in suspension and are commonly deposited as aggregates of floccule or fecal pellet origin. Floccule formation is sensitive to water chemistry--salinity, organic compounds, and turbulence. Mud aggregates are commonly deposited under higher hydraulic energy conditions than previously assumed for muds. Size analyses for mud and shales are generally useless for interpretation of depositional hydraulics. Vertical variations in bedding properties, texture, color, organic content, mineral composition, fossil content, and bioturbation are the most useful parameters for the deduction of depositional environments of shales and muds. These, integrated with basin geometry, hold the most promise for future studies. Stratigraphy Shales almost always have an internal stratigraphy that is well-expressed on wire-line geophysical well logs. Thin beds of high or low density, different lithologies and organic content, and fossil zones or concretions within shale sequences are commonly widespread and are good markers for internal stratigraphy. Most shales have a clinoform internal stratigraphy that can be related to basin geometry and can be observed in seismic profiles. Diagenesis Muds compact more than other sediments and hence expel more fluids. Pore water chemistry, mineralogy, thickness, and density all change as pore water is expelled. Water expulsion concentrates hydrocarbons and metals in interbedded porous reservoirs. Seismic response, heat flow, and our perception of original shape of shale bodies all change with burial compaction. Tectonics The concept of "lutokenesis"--mud makes its own tectonics--applies. Where deposition has been rapid, buried shales are overpressured and undercompacted. Undercompacted shales can form diapiric structures. Shales thin over rising structures and thicken into synclines and subsiding basins. In large-scale overthrusts, bedding-plane faults form fold trains; shear and flow occur mostly in shale units. Source Beds Shales are major source beds of hydrocarbons. Total organic content is sensitive to the original circulation in the basin, biogenic productivity, and influx of fine terrigenous and carbonate muds--a dilution factor. Preservation of organic matter is best in muddy basins because density stratification inhibits vertical mixing and oxidation--rapid burial inhibits biodegradation. Best indicators of thermal history of shaly basins are kerogen, conodonts, vitrain, and clay minerals. Relating Sedimentology of Shales to Resources Establish internal stratigraphy and facies distribution. Relate every resource variable to internal stratigraphy, and make an isopach map and relate facies distribution of basin. For gas, relate primary and secondary porosity (fracturing and permeability) to maturation, facies, and internal stratigraphy. For oil, correlate kerogen content to internal stratigraphy and maturation. For uranium, correlate kerogen to internal stratigraphy. End_of_Article - Last_Page 1682------------