Clumped isotope record of individual limestone fabrics: A potential method to constrain the timing of oil migration

Abstract

This study applied clumped isotope analyses to investigate how different limestone components (larger skeletal grains and enclosing matrix) and cements may have varying degrees of susceptibility to recrystallization during progressive burial. The results also provide new constraints on the temperatures of recrystallization and cementation, the nature of the waters involved, the timing of the diagenetic events, and the effect of oil emplacement on inhibiting diagenetic processes. We used clumped isotope measurements from core samples in two giant oilfields, together with petrography and well-constrained thermal histories, to study the reactivity of different limestone fabrics and whether the displacement of pore water by oil affected the recrystallization processes. We recognize seafloor micritization, cementation, and recrystallization as distinct diagenetic processes. The results indicate that skeletal grains record burial temperatures of 60 °C to72°C and enclosing coarse-blocky calcite cements record temperatures of 85 °C to 86 °C. Oxygen isotopes suggest that both processes involve high water/rock ratios. Burial histories together with carbonate Δ47 values are used to estimate that the skeletal grains recrystallized from 99 to 70 Ma and the cements formed at around 54 Ma. By contrast, the matrix shows temperatures of 68 °C to 90 °C, consistent with continuous recrystallization at low water/rock ratios from 90 to 50 Ma. The large skeletal grains thus tend to recrystallize early, but are less reactive during subsequent burial because of larger crystal size, whereas the more finely crystalline and reactive matrix more faithfully records maximum burial conditions. The lower temperature recorded in the matrix relative to the present reservoir temperatures is interpreted as reflecting stoppage or inhibition of recrystallization by oil emplacement.