Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

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Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 22 Stratigraphic Nomenclature,
  Geophysical Logs

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North American Stratigraphic Code

• written by a committee of geologists referenced
  as -
• North American Commission on Stratigraphic
  Nomenclature (NACOSN, 1983)
• published in the American Association of
  Petroleum Geologists (AAPG) Bulletin, v. 67, no.
  5, p. 841-875

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North American Stratigraphic Code

• Formal Units recognized are
• Lithostratigraphic (content, properties, or
  physical limits)
• Lithodemic (intrusive or highly deformed)
• Magnetostratigraphic
• Biostratigraphic
• Pedostratigraphic
• Allostratigraphic
• Geologic-Time
• Chronostratigraphic
• Geochronologic, Polarity-Chronostratigraphic,
  Polarity-Chronologic, Diachronic, Geochronometric

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International Stratigraphic Guide

• also written by a committee, but published twice
  with a different lead author each time -
• Hollis D. Hedberg (1976)
• Amos Salvador (1994)
• published by The International Union of
  Geological Sciences and The Geological Society of
  America

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International Stratigraphic Guide

• Formal Units recognized are
• Lithostratigraphic (based on observable
  lithologic properties)
• Unconformity-bounded Units (otherwise known as
  allostratigraphic)
• Biostratigraphic
• Magnetostratigraphic
• Chronostratigraphic

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Formation

• a mass of rock or sediment which can be
  identified by
• lithological characteristics (ideally uniform)
• stratigraphic position
• mappable at the surface, or
• traceable in the subsurface

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Group, Supergroup

• Several formations may be combined into a Group
  (an example would be the Chuar Group)
• Several groups may be combined into a Supergroup
  (an example would be the Grand Canyon Supergroup)

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Member, Bed

• Formations may be subdivided into Members which
  have
• limited lateral extent, but are
• consistently found within a single formation
• Beds represent the smallest subdivision
• need not be formally named
• should always be numbered and characterized in
  the field on a graphic log

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Geophysical Logs

• supplement the information that can be obtained
  from drilling, sampling, and testing rocks with
  subsurface borings
• provide continuous analog or digital records for
  physical properties of the rock matrix and the
  fluids it contains

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Borehole Geophysics

• earliest measurements down a borehole were of
  temperature -
• 1869 Lord Kelvin
• 1897 Hallock
• 1918 Van Orstrand
• of USGS with resolution of 0.01 C

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Borehole Geophysics

• electrical resistivity logs were first made by
  the Schlumberger brothers in France in 1927
• sonic logs were not made until after World War II
  (1946)
• nuclear logs (gamma, gamma-gamma, neutron, etc..)
  were also first made during the 1940s

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Geophysical Logs

• Caliper Log - borehole diameter
• Gamma Log - the amount of natural gamma radiation
  emitted by the rocks surrounding a borehole.
  Potassium feldspar, micas, and beds of shale or
  clay that contain them have higher gamma readings
  because of 40K and Uranium-series isotope decays.
• Temperature Log - the water temperature in the
  borehole.
• Fluid-resistivity Log - the electric resistivity
  of water in the borehole. Identifies
  water-bearing zones and vertical flow in the
  borehole.

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Gamma Log

• p. 297 in Nichols
• High K content results in high gamma reading
• most logs are not this sharp or "square"

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Suite of geophysical logs in a sedimentary-bedrock
aquifer, northern Virginia
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Ohm's Law

• r E/I
• where r resistance, in ohms
• E potential, in volts
• I current, in amperes

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Geophysical Logs

• Single-point Resistance Log - electrical
  resistance from points within the borehole to an
  electrical ground at land surface. Increases with
  grain size.
• Spontaneous Potential Log - potential developed
  between the borehole fluid and the surrounding
  rock and fluids
• Normal Resistivity Log - variably spaced
  electrodes on the logging probe, 16 inches for
  short-normal resistivity and 64 inches for
  long-normal resistivity

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Suite of geophysical logs in an alluvial-basin
aquifer, southern California
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Sonic Log

• travel time of an acoustic wave from transmitter
  to receiver through the fluid in the borehole and
  surrounding rocks
• most useful for consolidated materials penetrated
  by uncased, fluid-filled boreholes
• travel times decrease with rock porosity
• travel times increase with depth and with
  increases in rock hardness or cementation

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Sonic Log

• p. 297 in Nichols
• transit time increases to left
• velocity would increase to right

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(TRAVEL TIME!)
Sonic Log

• Borehole geophysicists often refer to travel time
  as "acoustic velocity"
• Higher for Brine than for Freshwater

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Velocity and "Transit Time"
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Porosity increase with Sonic Log Travel Time for
some rocks in Idaho. r2.87
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Neutron Log

• neutrons emitted from a probe return to be
  measured at a detector after interacting with
  rock penetrated by the borehole
• Hydrogen (i.e. water) causes neutrons to lose
  energy
• one of the best indicators of rock porosity

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Typical responses of neutron and caliper logs to
a sequence of sedimentary rocks Neutron
interactions highest in porous limestones and
sandstones. In general, neutron logs detect
porosity changes better than sonic logs
do. Neutron logs detect small changes in
porosity when porosity is minimal.
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Neutron detectors can be calibrated using rock
standards to obtain very precise quantitative
estimates of rock porosity.
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Geophysical Logs in combination can identify most
sedimentary rock types.
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Geophysical Logs can be used to assist
Lithostratigraphic Correlation
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Geophysical Logs can be used for Facies Analysis