Stratigraphic Principles

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Stratigraphic Principles

• Types of Stratigraphy
• Focus on Modern Stratigraphy is Sequence-based
• History of Sequence Stratigraphy
• Terminology of Sequence Stratigraphy
• Examples from Permian Basin

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Stenos Laws

• It was recognized in the 1600's that in a
  sedimentary sequence, the older beds are on the
  bottom, and the younger beds are on the top. This
  has come to be called the Principle of
  Superposition. You can visualize how this occurs
  if you imagine a stack of newspapers in the
  corner of a room. Every day you put another
  newspaper on the pile. After several weeks have
  passed, you have a considerable stack of
  newspapers, and the oldest ones will be on the
  bottom of the pile and the most recent ones will
  be on the top. This fairly obvious, but very
  important fact about layering was first noted by
  Nicholaus Steno, and is the first of three
  principles which have come to be known as Steno's
  Laws.
• Steno's second law is the Principle of Original
  Horizontality, which states that sediments are
  deposited in flat, horizontal layers. We can
  recognize this easily if we consider a
  sedimentary environment such as the sea floor or
  the bottom of a lake. Any storm or flood bringing
  sediment to these environments will deposit it in
  a flat layer on the bottom because of the
  sedimentary particles settling under the
  influence of gravity. As a result, a flat,
  horizontal layer of sediment will be deposited.
• Steno's third law is the Principle of Original
  Lateral Continuity. If we consider again the
  sediment being deposited on the seafloor, the
  sediment will not only be deposited in a flat
  layer, it will be a layer that extends for a
  considerable distance in all directions. In other
  words, the layer is laterally continuous.

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Types of Stratigraphy

• Classical branches
• Lithostratigraphy
• Chronostratigraphy
• Biostratigraphy
• Newer subdisciplines
• Seismic stratigraphy
• Cyclostratigraphy
• Magnetostratigraphy
• Chemostratigraphy
• Integration of disciplines in Sequence
  Stratigraphy

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Terminology of Lithostratigraphic Units

• Supergroup
• Group
• Formation
• Member
• Bedset
• Bed
• Types of unconformities
• Angular unconformity
• Disconformity
• Paraconformity
• Nonconformity
• hiatus

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History of Sequence Stratigraphy Unconformities

• James Hutton (1726-1797) Father of Modern
  Geology
• First described unconformity at Siccar Point in
  northern Scotland
• In a paper entitled Theory of The Earth (1785)
  Hutton recognized the importance of
  unconformities
• Also recognized stratigraphic nature of igneous
  rocks and believed came from molten material
  within Earth

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History of Sequence Stratigraphy
Uniformitarianism
Principles of Geology (1833)
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History of Sequence Stratigraphy

• William Smith measured and described outcrop
  stratigraphy during excavation of transport
  canals in England beginning in 1791

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History of Sequence Stratigraphy Facies

• Johannes Walther (1892) proposed what is now
  known as Walthers Law of Facies Succession

"Facies adjacent to one another in a continuous
vertical sequence also accumulated adjacent to
one another laterally".
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History of Sequence Stratigraphy
Time-significant surfaces
1909 Eliot Blackwelder published use of
unconformities as time markers introduced
concept of time represented by surfaces
(erosional removal and sedimentary hiatus)
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History of Sequence Stratigraphy Cyclicity
1916 Amadeus Grabau proposed the pulsation
theory.
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History of Sequence Stratigraphy Base Level
1917 Joseph Barrell stated the most fundamental
events in geologic history--the time-space
distribution of deposition and non-deposition
the alternating rise and fall of Base-level.
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What Really Happened New Concepts from New
Technology
1951 John L. Rich proposes the concept of
clinoforms
recognition of seismic reflection geometries
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History of Sequence Stratigraphy Time
Stratigraphy
1958 Harry Wheeler produced first
chronostratigraphic chart
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History of Sequence Stratigraphy Time
Stratigraphy
1963 Lawrence Sloss recognized 6 major sequences
in North America controlled by eustatic sea level
changes
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Official Birth of Sequence Stratigraphy
1977 Peter Vail and Robert Mitchum co-ordinated
the publishing of AAPG Memoir 26 based on the
assumption that a seismic relection surface
represents a time line
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A Definition of Sequence Stratigraphy

• A framework of genetically related stratigraphic
  facies geometries and their bounding surfaces
  used to determine depositional setting
• Subdivision interpretation of sedimentary
  record using a framework surfaces seen in
  outcrops, well logs, 2-D and 3-D seismic.
• Include Surfaces of erosion non-deposition
  (sequence boundaries), systems tracts
• Flooding (trangressive surfaces TS /or maximum
  flooding surfaces mfs) require that relative
  sea-level is known
• This framework used to predict the extent of
  sedimentary facies geometry, lithologic
  character, grain size, sorting reservoir
  quality

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A Simpler, More Useful Definition of Sequence
Stratigraphy
A method to impose the dimension of time on the
relationships of rock units in space (area and
depth) To do this we first describe
facies Then we observe the
geometric relationships among
facies
By understanding how rock units are related in
time and space, we can better interpret how they
are connected as oil and gas reservoirs
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How is Facies Used In Sequence Stratigraphy

• Facies the total textural, compositional and
  structural characteristics of a sedimentary
  deposit resulting from accumulation and
  modification in a particular environment
• Grain size, sorting, rounding
• Lithology
• Sedimentary structures
• Bedding type
• In other words, the simplest description of the
  genetically related vertical and lateral
  succession

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The Problem of Preservation Potential
Sedimentary facies models are useful to explain
spatial relationships of co-eval facies in the
modern
But what will be preserved in the stratigraphic
record?
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Original Sequence Stratigraphic Approach (seismic
stratigraphy) was based on recognition of
unconformity-bound sequences using geometry and
termination patterns of seismic reflectors.
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A critical assumption of the seismic
stratigraphic approach, illustrated in this
diagram from Vail et al (1977), is that seismic
reflectors follow time surfaces rather than
facies impedance boundaries. Note the regional
scale of this illustration.
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The Classic Slug Exxon Model
SB sequence boundary LST lowstand systems
tract TST transgressive systems tract HST
highstand systems tract SMW shelf-margin
wedge Mfs maximum flooding surface Lsw
lowstand wedge Sf slope fan Bf basin-floor fan
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Sequence Boundary

• The unconformity or correlative conformity that
  bounds a sequence
• Not always a major physical feature
• Not ever exposure surface is a sequence boundary!
• Commonly (but not always) represents a
  significant change in stratal arrangements and
  therefore reservoir properites

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Maximum Flooding Surface

• Surface that marks the turn-around from
  landward-stepping to seaward stepping strata
• Farther out on platform coincides with the
  downlap surface (depending on the degree of
  condensation of clinoform toes)
• Recognition of the MFS is important for
  separating TST and HST, which in turn is
  important for other stratigraphic analysis, but
  on the platform top (where 99 of carbonate
  reservoir facies occur) this can be difficult to
  pin down precisely. Dont get hung up on this.
  Try to pick it as closely as possible, knowing
  that your colleague will disagree in order to
  appear enlightened.

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Transgressive Systems Tracts

• Bounded below by underlying sequence boundary and
  above by maximum flooding surface
• Generally more mounded in geometry
• Sets of high-frequency cycles show upward
  thickening and upward deepening trends
• Typically less grainstone prone, more diverse
  skeletal assemblages

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Highstand Systems Tracts

• Bounded below by maximum flooding surface and
  above by overlying sequence boundary
• Generally shingled or offlapping (clinoformal)
  stratal geometry
• Sets of high-frequency cycles show upward
  thinning and upward shallowing trends
• Typically grainstone prone, less diverse skeletal
  assemblages

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Terminology of Stratigraphic Hierarchies
Composite sequence
High-frequency sequence
High-frequency cycle
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Transgressive sequence set retrogradational
sequence set Highstand sequence set
progradational sequence set Lowstand sequence set
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Can we do sequence stratigraphy in carbonates
with just logs? Does anyone here work for
Schlumberger?
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Indicator facies analysis Walthers Law Model
development
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