Module 1

• Sedimentary rocks are classified into two main groups based on their origin:

  1. Clastic Sedimentary Rocks:

  • Formed as a result of the weathering or fragmentation of pre-existing rocks
  • Divided into coarse-grained conglomerates, medium-grained sandstones, and fine-grained siltstones, mudstones, and shales
  • Clastics are further subdivided by mineralogy, reflecting the parent rock

  2. Chemical or Biochemical Sedimentary Rocks:

  • Formed as a result of chemical processes
  • Primary carbonate deposition results from the precipitation and deposits formed Calcite, the most abundant mineral chemically or biochemically precipitated in the oceans
• Five types of sedimentary rocks important in hydrocarbon production:

  • Sandstones: clastic rocks composed mainly of sand size particles set in a matrix of silt or clay, united by a cementing material
  • Carbonates: sediments formed by a mineral compound characterized by a fundamental anionic structure of CO3-2, examples include calcite and aragonite
  • Limestones: sedimentary rocks consisting chiefly of the mineral calcite, with or without magnesium carbonate, widely distributed and important
  • Dolomite: composed of more than 90% mineral dolomite and less than 10% mineral calcite
  • Shale: detrital sedimentary rock formed by the consolidation of fine-grained material including clay, mud, and silt, typically porous and contain hydrocarbons
• Evaporites:

  • Important to petroleum exploration as they make excellent cap rocks and generate traps
  • Composed of minerals that precipitated from saline solutions concentrated by evaporation
  • General sequence of precipitation: calcite, gypsum or anhydrite, halite, and finally bittern salts
• Sedimentary Structures:

  • Important attributes of sedimentary rocks, occurring on upper and lower surfaces of beds and within beds
  • Used to deduce processes and conditions of deposition, directions of currents depositing sediments
  • Classified into primary structures (mechanical) and secondary structures (chemical and organic)
• Mud cracks are formed in clayey sediments due to prolonged exposure to the atmosphere and are also known as shrinkage cracks or sun cracks
• Presence of mud cracks indicates that the sediment was exposed at the surface shortly after deposition, as drying of the sediments would not occur beneath a body of water
• Raindrop imprints are irregular, small crater-shaped depressions with slightly raised edges seen on fine-grained soft sediment surfaces
• Raindrop imprints are good indicators of subaerial exposure but are not exclusive to arid climates
• Sole structures are preserved on the base of a bed which is sharply differentiated lithologically from the bed below and are found most commonly in turbidite deposits
• Flutes are elongated depressions that form on the bottom of the body of water as the current erodes
• Flute marks are a characteristic structure of turbidites and are excellent indicators of current direction and tops/bottoms of beds
• Secondary structures are formed sometime after deposition, products of chemical action or biological activity contemporaneous with sedimentation or shortly thereafter
• Chemical structures, a type of secondary structure, include concretions, nodules, and geodes
• Concretions are spherical to elliptical bodies of diverse chemical nature that grow around a nucleus, such as a fragment of silica or phosphate
• Geodes are spherical to sub-spherical hollow shells of chalcedony with internally lined minerals like quartz or aragonite
• Solution action, another chemical structure, results from solution activity commonly in calcareous rocks and forms features like caves, stalactites, and stalagmites
• Stylolites are serrated surfaces within a rock mass along which mineral material has been removed by interstratal solution dissolution circulating under pressure
• Dendrites are branch-like structures formed along fissures in the rock by percolating mineral solution, common on the surface of sedimentary rocks, especially limestone
• Organic structures, another type of secondary structure, include tracks and trails, burrow marks, stromatolites, coral reefs, and casts and molds
• Tracks and trails are markings indicative of the activity of organisms over soft sediments, like dinosaur footprints or bird tracks
• Burrow marks are remnants of burrows and tunnels excavated by worms and other marine organisms that live on the bottom of the sea
• Stromatolites are laminated structures produced by the finer sediment trapping usually by an algal mat, common in limestones of all ages
• Coral reefs are dome to elongate, massive to bedded forms that accumulate largely as a result of organic buildups within otherwise horizontal or nearly flat-lying strata
• Casts and molds are formed when organisms buried in sediment slowly decay, leaving a hollow space that contains an exact imprint of the organism's shape and size
• Sedimentary environments are areas where sediment is capable of accumulating, possessing distinctive physical, chemical, and biological characteristics that allow for specific kinds of deposits
• Continental environments include alluvial fans, fluvial systems, lakes, deserts, and areas adjacent to or covered by glaciers
• Alluvial fans are fan-shaped deposits formed at the base of mountains, typically coarse, poorly sorted gravel and sand
• Fluvial environments include braided and meandering river and stream systems, with channel deposits consisting of coarse, rounded gravel and sand
• Lakes (lacustrine environments) may be large or small, shallow or deep, freshwater or saltwater, and filled with terrigenous, carbonate, or evaporite sediments
• Lakes in lacustrine environments can be diverse, varying in size, depth, and sediment content, including terrigenous, carbonate, or evaporite sediments
• In lakes, features like mudcracks, wave ripples, laminations, and varves may be present
• Fine sediment and organic matter settling in some lakes can produce laminated oil shales
• Deserts in aeolian environments are characterized by little to no rainfall, vast areas of sand dunes, well-sorted and rounded dune sands without gravel or clay, and common cross-bedding
• Swamps in paludal environments are areas of standing water with trees where decaying plant matter accumulates to form peat, which may eventually become coal
• Transitional environments, like deltas, beaches, barrier islands, lagoons, salt marshes, and tidal flats, are found at or near where the land meets the sea
• Deltas are fan-shaped deposits of sediment formed where a river flows into a standing body of water
• Beaches and barrier islands are shoreline deposits exposed to wave energy, dominated by sand with marine fauna, and separated from the mainland by a lagoon
• Lagoons are bodies of water on the landward side of barrier islands, protected from ocean waves by the barrier islands, and contain finer sediment than beaches, usually silt and mud
• Tidal flats are periodically flooded and drained by tides, with low relief, meandering tidal channels, and may have laminated or rippled clay, silt, and fine sand deposited
• Marine environments include reefs, the continental shelf, slope, rise, and abyssal plain