What%20do%20geochemical%20data%20tell%20us%20about%20carbonate%20diagenesis?%20An%20example%20from%20Lower%20Cretaceous%20carbonates

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What do geochemical data tell us about carbonate
diagenesis?An example from Lower Cretaceous
carbonates
Guillermina SagastiLanghorne Taury
SmithGregor EberliPeter Swart
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Cogollo GroupMaracaibo Basin (Venezuela)
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(from Azpiritxaga, 2001)
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Cogollo Group
Is a fractured carbonate reservoir in
whichpermeability and porosity are difficult to
predict
Maraca Fm Continuous sheet-like reservoir layer
with variable porosity
Apon Fm Isolated mounds and amalgamated mound
complexes Disconnected, heterogeneous porosity
Taken from Shell Venezuela SA
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Cogollo Reservoirs

• Best reservoir occur in Mollusk-rich packstone
  facies from Apon and Maraca Formations

• Porosity in these beds is enhanced by early
  (fabric selective) and later (non-fabric
  selective) leaching

• Late calcite cementation, however, reduces
  reservoir quality of these rocks

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Cogollo Reservoirs
Leaching and late calcite cementation are
critical factors
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Strategy
Understanding of how leaching and post-leaching
cementation occurred is essential to predict
porosity distribution and enhance reservoir
development
Possible origin of fluidsa) Meteoricb)
Burialc) Hydrothermal
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Strategy
A robust petrographic study is critical for the
identification and characterization of diagenetic
processes (i.e. cementation, neomorphism,
dissolution, and compaction). Petrographic
attributes, however, are not able to
unequivocally identify the diagenetic environment
and nature of the fluids.
An integrated petrographical-geochemical approach
is essential to capture the paragenesis of a
reservoir rock and to develop a comprehensive
diagenetic model
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Available data

• d13C and d18O from limestones and
  dolomites(bulk-rock and cements)
• 87Sr/86Sr from limestones and dolomites(bulk-rock
  and cements)
• Fluid inclusion from calcite and dolomite cements
• Isotopic and fluid inclusion data from previous
  reports

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d18O of Cretaceous seawater
Aptian/Albian d18O in the seawater was about
-2 Seawater limestones will have d18O signatures
around -2Seawater dolomites will be 3 units
heavier than limestones (1)
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d18O and d13C from Cogollo carbonates
d18O
d18O -4.37 d13C 1.62
d13C
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d18O and d13C from Cogollo carbonates
d18O
d13C
d18O -0.59 d13C 2.84
d18O -6.90 d13C 3.14
d18O -6.25 d13C 1.61
d18O -7.21 d13C 0.67
d18O -6.365 d13C 0.64
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Dolomite matrix and cement
Matrix dolomite
d18O -0.59 d13C 2.84
Fracture dolomite
d18O -6.90 d13C 3.14
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Calcite cement
Mould-cement
d18O -6.25 d13C 1.61
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Calcite cement
Fracture calcite
Vug cement
d18O -6.65 d13C 0.64
d18O -7.21 d13C 0.67
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(No Transcript)
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Seawater Sr isotope through time
0.7071 to 0.7077
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d18O and 87/86Sr isotopes
87Sr/86Sr
Continental signature (radiogenic Sr)
Sea water
18O
18O-87Sr/86Sr cross-plots show evidences that
diagenesis was (is) and ongoing process that
started in the marine realm, continued during
burial and was overprinted by warm fluids
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Fluid inclusions

• Fluid inclusions in saddle dolomites show
  homogenization temperatures between 85?C and
  103?C and salinities between 6.2 and 7.5 wt
• Fluid inclusion from vug cements show
  homogenization temperatures between 49?C and
  85.5?C and salinities between 3.6 and 6.6 wt

Plane light photomicrograph of vug-filling sparry
calcite, which is cut by multiple generations of
microcracks containing fluid inclusions
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Proposed paragenetic sequence
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Key Findings

• In the Cogollo Group d18O and 87/86Sr values show
  the whole spectrum of data, from marine to burial
  and warm fluid signatures
• Positive values of d13C indicate that most
  diagenetic fluids are not meteoric
• d18O and 87/86Sr values from matrix dolomite
  points to a near surface origin with seawater as
  the main dolomitizing fluid
• Depleted d18O values in calcite and dolomite
  cements document fluids that were somehow warmer
  than seawater (heated by the regional geothermal
  gradient and/or hydrothermal)

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Key Findings

• Radiogenic Sr signatures in calcite and (some)
  dolomite cements indicate that the warm fluids
  interacted with clastic or basement rocks
• Fluid inclusions provide evidence of warm brines
  circulating through the system
• Leaching by warm fluids that invaded the
  formation along deep-rooted faults potentially
  increase reservoir quality in vicinity of faults
• The preservation of diverse geochemical
  signatures evidences the incapability of a
  particular diagenetic process to completely
  remove the geochemical signatures of previous
  events, and gives us the opportunity of
  reconstructing the diagenetic story