What do geochemical data tell us about carbonate diagenesis? An example from Lower Cretaceous carbonates Guillermina Sagasti Langhorne “Taury” Smith Gregor Eberli Peter Swart

(from Azpiritxaga, 2001) Formation MAASTR COLON TURONIAN CENOMANIAN MARACA LA LUNA CAMPANIAN.SANT-CON. SOCUY ALBIAN LISURE PICHE Mb. MACHQ. TIBU RIO NEGRO APONAPON BARREMIAN APTIAN Age Ø Ø Ø Cogollo Group Maracaibo Basin (Venezuela)

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

 Best reservoir occur in Mollusk-rich packstone facies from Apon and Maraca Formations Cogollo Reservoirs  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

Leaching and late calcite cementation are critical factors Cogollo Reservoirs

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) Meteoric b) Burial c) Hydrothermal

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. 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

Available data  δ 13 C and δ 18 O from limestones and dolomites (bulk-rock and cements)  87 Sr/ 86 Sr from limestones and dolomites (bulk-rock and cements)  Fluid inclusion from calcite and dolomite cements  Isotopic and fluid inclusion data from previous reports

δ 18 O of Cretaceous seawater Aptian/Albian δ 18 O in the seawater was about -2 Seawater limestones will have δ 18 O signatures around -2 Seawater dolomites will be ~3 units heavier than limestones (+1)

δ 13 C δ 18 O δ 18 O and δ 13 C from Cogollo carbonates δ 18 O δ 13 C +1.62

δ 13 C δ 18 O δ 18 O and δ 13 C from Cogollo carbonates δ 18 O δ 13 C δ 18 O δ 13 C δ 18 O δ 13 C δ 18 O δ 13 C δ 18 O δ 13 C +0.64

Fracture dolomite Dolomite matrix and cement Matrix dolomite δ 18 O δ 13 C δ 18 O δ 13 C +3.14

Calcite cement δ 18 O δ 13 C Mould-cement

Calcite cement δ 18 O δ 13 C Fracture calcite δ 18 O δ 13 C Vug cement

Seawater Sr isotope through time to

87 Sr/ 86 Sr 18 O 18 O- 87 Sr/ 86 Sr 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 Sea water Continental signature (radiogenic Sr) δ 18 O and 87/86 Sr isotopes

Plane light photomicrograph of vug-filling sparry calcite, which is cut by multiple generations of microcracks containing fluid inclusions 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%

Proposed paragenetic sequence

Key Findings  In the Cogollo Group δ 18 O and 8 7/86 Sr values show the whole spectrum of data, from marine to burial and warm fluid signatures  Positive values of δ 13 C indicate that most diagenetic fluids are not meteoric  δ 18 O and 87/86 Sr values from matrix dolomite points to a near surface origin with seawater as the main dolomitizing fluid  Depleted δ 18 O values in calcite and dolomite cements document fluids that were somehow warmer than seawater (heated by the regional geothermal gradient and/or hydrothermal)

 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 Key Findings