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Guillermina Sagasti Langhorne “Taury” Smith Gregor Eberli Peter Swart

What do geochemical data tell us about carbonate diagenesis? An example from Lower Cretaceous carbonates. Guillermina Sagasti Langhorne “Taury” Smith Gregor Eberli Peter Swart. Age. Formation. COLON. MAASTR. CAMPANIAN. SOCUY. .SANT-CON. TURONIAN. LA LUNA. CENOMANIAN. MARACA. ALBIAN.

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Guillermina Sagasti Langhorne “Taury” Smith Gregor Eberli Peter Swart

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  1. What do geochemical data tell us about carbonate diagenesis?An example from Lower Cretaceous carbonates Guillermina SagastiLanghorne “Taury” SmithGregor EberliPeter Swart

  2. Age Formation COLON MAASTR CAMPANIAN SOCUY .SANT-CON. TURONIAN LA LUNA CENOMANIAN MARACA ALBIAN LISURE A P O N PICHE Mb. MACHQ. APTIAN TIBU BARREMIAN RIO NEGRO Cogollo GroupMaracaibo Basin (Venezuela) Ø Ø Ø (from Azpiritxaga, 2001)

  3. POROSITY good intermediate poor NOT TO SCALE Cogollo Group Is a fractured carbonate reservoir in whichpermeability and porosity are difficult to predict Apon Fm Isolated mounds and amalgamated mound complexes Disconnected, heterogeneous porosity Maraca Fm Continuous sheet-like reservoir layer with variable porosity Taken from Shell Venezuela SA

  4. 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

  5. Cogollo Reservoirs Leaching and late calcite cementation are critical factors

  6. Strategy Understanding of how leaching and post-leaching cementation occurred is essential to predict porosity distribution and enhance reservoir development Possible origin of fluids:a) Meteoricb) Burialc) Hydrothermal

  7. 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

  8. Available data • δ13C and δ18O 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

  9. δ18O of Cretaceous seawater Aptian/Albian δ18O in the seawater was about -2 Seawater limestones will have δ18O signatures around -2Seawater dolomites will be ~3 units heavier than limestones (+1)

  10. δ18O and δ13C from Cogollo carbonates δ18O δ18O -4.37 δ13C +1.62 δ13C

  11. δ18O and δ13C from Cogollo carbonates δ18O δ13C δ18O -0.59 δ13C +2.84 δ18O -6.90 δ13C +3.14 δ18O -6.25 δ13C +1.61 δ18O -7.21 δ13C +0.67 δ18O -6.365 δ13C +0.64

  12. Dolomite matrix and cement Matrix dolomite δ18O -0.59 δ13C +2.84 Fracture dolomite δ18O -6.90 δ13C +3.14

  13. Calcite cement Mould-cement δ18O -6.25 δ13C +1.61

  14. Calcite cement Fracture calcite Vug cement δ18O -6.65 δ13C +0.64 δ18O -7.21 δ13C +0.67

  15. Seawater Sr isotope through time 0.7071 to 0.7077

  16. δ18O 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

  17. 9 mm 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

  18. Proposed paragenetic sequence

  19. Key Findings • In the Cogollo Group δ18O and 87/86Sr values show the whole spectrum of data, from marine to burial and warm fluid signatures • Positive values of δ13C indicate that most diagenetic fluids are not meteoric • δ18O and 87/86Sr values from matrix dolomite points to a near surface origin with seawater as the main dolomitizing fluid • Depleted δ18O values in calcite and dolomite cements document fluids that were somehow warmer than seawater (heated by the regional geothermal gradient and/or hydrothermal)

  20. 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

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