that type vertical section was deposited during a certain time period in Earth history. Wherever other rocks can be demonstrated to have been deposited during the same time period, then they are assigned to the same time-rock unit – so time-rock units are established in reference to some “type section” C. Ultimately we try to relate time-rock units to the absolute time before present when they were deposited. This is mostly done through radiometric dating. III. TIME STRATIGRAPHIC RECORD A. Time correlation = establishing time equivalence or isochronousness. It can be accomplished on a local scale by physical methods 1)Key beds – widespread rock unit that is easily recognized deposited instantaneously. • Bentonites -volcanic ash beds. -also radiometric dates • Concretion horizons (work in Cretaceous) • Burrowed chalk beds (work in Cretaceous) • Distinctive turbidites • Tsunami deposits? • CO3 bed in a clastic marine sequence (clastic machine turned off all at one time?) • In anoxic basin deposits we have diatom and coccolith layers. • Anything that is the former surface of deposition at an instant in time. To think about this, do assignment on time correlation 1. Which locality was up the slope during deposition? 2. For the transgression, which direction was the shoreline migrating? Volcanic ash Volcanic ash 2) Position in a transgressive – regressive cycle Scale is regional Remember – HERE IS WHAT YOU SEE! thickness horizontal distance Then you establish lithostratigraphic correlation between the stratigraphic sections thickness horizontal distance Then you draw the bisection line of the transgressiveregressive sequence thickness horizontal distance Or a surface representing a giant lateral jump in position of facies (facies dislocation) thickness horizontal distance 3)Varves in lake sediments or varves in basinal evaporites – Permian Castile Formation evaporite has 174,890 varves in 300 meters of sediment Two cores from the Castile separated by many miles Wiley online Stratigraphic Maps I (and cross sections) How we represent and communicate stratigraphic data I Remember that stratigraphic cross sections are vertical planes that have the structural complications removed A. Lithostratigraphic cross section 1. X axis is horizontal distance 2. Y axis is actual thickness of strata 3. Hung from a datum which is straight across and sections can be above, below or both above and below it Hung on wrong datum S Hung right datum Shoup, 2017 4. Contrast stratigraphic cross sections with structural cross sections, which are hung from elevations and show the deformation Shwe 2A Shwe 3 Shwe 6 N Shwe 4 S G 3.2 G 5.2 3.2A 5.2C 5.2B 5.2A PL SB G 3.2 horizon stratigraphic datum A stratigraphic cross section hung on the G3.2 stratigraphic marker It makes good sense in understanding the distribution of the reservoir units and shows flow units and baffles and connections across units It does not make sense in terms of the Gas-water contact! Shoup, 2017 Shwe 2A Shwe 3 N Shwe 6 S The structural cross section of the same reservoir hung on elevation -3000 m Shwe 4 -3000 m -3100 m 5.2C 5.2A 5.2C 100 m -3200 m 1000 m GWC GWC Gas Wet Ah! Now the gas-water contact mostly makes sense Shoup, 2017 4-1 4-2 3-1 3-7 1900 1900 2000 3-8 3-5 3-3 3-2 1900 1900 1900 1900 2000 2000 2000 2000 2000 1900 1900 2000 2000 2100 2100 2100 2100 2100 2100 2100 2100 2200 2200 2200 2200 2200 2200 2200 2200 2300 2300 2300 2300 2300 2300 2300 2300 2400 2400 2400 2400 2400 2400 2400 2400 2500 2500 2500 2500 2500 2500 2500 2500 2600 2600 2600 2600 2600 2600 2600 2600 2700 2700 2700 2700 2700 2700 2700 2700 2800 2800 2800 2800 2800 2800 2800 2800 2900 2900 2900 2900 2900 2900 2900 2900 3000 3000 3000 3000 3000 3000 3000 3000 3100 Shale P.O. 3100 3100 3100 3100 3100 3100 3100 3200 3200 3200 3200 3300 3300 3300 3400 3400 3400 3500 3500 3500 3200 11 3200 7 8 9 10 12 320t0act Irregula32r00 O/W Con 3300 3300 3300 3300 3400 3400 3400 3400 3500 3500 3500 3500 3600 3600 3600 3600 3700 3700 3800 3800 3700 3800 13 3700 3800 14 3300 Submarine Canyon 3900 4000 3900 Structural Cross-section Oil-water contact does not make sense Shoup, 2017 4-1 1900 2000 7 2100 8 2300 2200 2600 2700 2800 10 2900 3000 3100 11 3200 3300 12 3400 3500 3600 13 14 2000 2100 2500 3700 2300 2400 2500 2600 2700 2800 2900 3-8 3-5 3-3 3100 3200 3300 3400 3500 3600 3900 3800 3-2 1900 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3700 Datum = Formation 10 3-7 1900 1900 2000 2000 2100 2100 2200 2200 2300 2300 2400 2400 2500 2500 2600 2600 3000 3100 3200 3300 3400 2800 3500 2900 2900 3000 3000 3100 3100 3200 3200 3300 3400 2000 2000 2100 2100 2200 2200 2300 2300 2400 2400 2500 2500 2600 2600 2700 2700 2800 2800 2900 2900 3000 3000 3100 3100 3200 3200 3300 3300 3400 3400 3500 3500 3600 2000 2100 2200 2300 2400 2500 2600 2700 2800 3300 1900 1900 2700 2700 3000 3800 4000 3-1 1900 2200 2400 9 4-2 Submarine Canyon 3500 2800 2900 3000 3100 3200 3300 3400 3500 3700 3600 3800 3700 3900 3800 Stratigraphic Cross-section Ah! Here is why. It is the shape of the reservoir unit. Shoup, 2017 Beware of vertical exaggeration with all cross sections That includes seismic cross sections – They are vertically exaggerated 5 km Fracture Attribute (Controlled Beam) Processing Typical Aspect Ratio is 6 to 1 5 km Fracture Attribute (Controlled Beam) Processing Seismic Profiles are Cross Sections in TWT 5. Remember (from lab) that fence or panel diagrams are cross sections that give a 3D perspective B. Remember from lab another type of cross section is the time stratigraphic cross section (aka chronostratigraphic-cross section, -chart, or Wheeler diagram) 1. X axis is horizontal distance 2. Y axis is time or time stratigraphic units Stratigraphic cross section Chronostratigraphic cross section SEPM STRATA Stratigraphic Maps II (and cross sections) How we represent and communicate stratigraphic data II. Isopachous Contour Maps – Isopach maps. (Isopachous means equal thickness) A. Map view of distribution of thickness of a particular stratigraphic unit. Measured top to bottom (if horizontal, or normal to bedding; if one maps vertical thickness irrespective of the structural dip, then it is called an isochore map) B. Lines on an isopach map connect points where a formation, flow unit, or other lithologic unit is the same thickness. C. What does thickening mean on an isopach map? 1) A low that is being filled with sediment (topographic basin) 2) A basin that was filled to base level with more subsidence in the center (never deeper in the center; basin of accumulation) 3) Erosional remnant 4) Bump on the sea floor (bar, reef, platform) 5) If you are just mapping one facies, a facies change Margin of Shu’aiba carbonate platform, south Abu Dhabi; Pierson et al, 2010 Isopach and structure contour map superimposed Deepwater reservoir, offshore Niger Delta; (Don et al, 2020) What is the relationship between the anticline and the thickness of the reservoir unit? Isopach and structure contour map superimposed What is the relationship between the fault and the reservoir thickness? Deepwater reservoir, offshore Niger Delta; Don et al, 2020 D. What might the “O” isopach mean geologically. Two carbonate shoals on the edge of a Silurian Ramp, Illinois Basin D. What might the “O” isopach mean geologically. 1) Pinchout caused by facies change. 2.) Pinchout caused by nondeposition up against some topographic high (base level control) 3.) Post depositional erosional truncation of a stratigraphic unit 4) How do we tell them apart? For each, what do you expect to see in comparing a facies map to the isopach map? What would you see in a stratigraphic cross section? Cross sections to you show you what I mean. This is a variant on the isopach map called an isochore map that does not correct for the dip and only uses the vertical thickness (thick slope deposits – Wolfcampian of Permian Basin) III. Isolith maps – maps which show contour lines of equal proportional thicknesses of lithologies. – usually expressed as % thickness of sandstone or shale of a formation, or other strat. unit; or contouring total thickness of a particular rock type Net Sandstone Isolith Map, Black Warrior Basin, Alabama; Pashin, 1994 IV. Other contour maps which show areal variation of rock types, can be contoured on sand-shale ratio (by thickness), Ls-Sh. ratio or other ratios of rock types, or percentages or ratios of chemical compositions; most useful if only 2 lithologies are present. V. Facies maps – quantitatively or qualitatively map variation in rock types. But show an area of map pattern for each! Example of QUANTITATIVE KEY for a Facies Map >75% sandstone 50-75% sandstone and

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