Carbonate cementation in the Tithonian Jeanne d'Arc sandstone, Terra Nova Field, Newfoundland: Implications for reservoir quality evolution

Petrographic and in situ geochemical analyses were carried out on cores of the fluvial sandstone (4 mm; sublithic quartz arenite) dominated Tithonian Jeanne d'Arc Formation interval from wells C-09 and E-79 of the Terra Nova oilfield to understand diagenetic control on reservoir quality of the formation. Lithic clasts are dominantly carbonates and shales. Successively, early near-surface dolomite cementation, eogenetic dolomite dissolution, burial calcite cementation and its subsequent dissolution are the major diagenetic events that controlled the reservoir quality of the sandstone. Dolomite cement grades from mostly Fe-rich (FeCO3 ca 12 +/- 2 normalized mol.%) in the upper part of the formation to Fe-poor (<0.07 normalized mol.%) in the lower part of the formation in both wells. The dolomite and calcite cements generally have low Sr and Na; enriched Y and low delta O-18 and delta C-13 values relative to typical marine carbonates. Strontium is enriched in the Fe-poor (higher Mg) dolomite relative to the Fe-rich phase. Mineral liberation analyses of shale samples reveal common Fe-chlorite and minor glauconite. Furthermore, considerable amounts of solutes in the early diagenetic pore fluid were likely derived from water-rock interaction with underlying Oxfordian Rankin Formation marine carbonate that was also exposed in the watershed of the study area at the time. Together, these suggest that the dolomites were formed from an early diagenetic bicarbonate-rich pore fluid of mixed meteoric and seawater origin. The origin and distribution of early dolomite cementation and other successive diagenetic events have a depositional cycle control. Episodic post-deposition transgression of seawater and/or compactional fluid expulsion from shales into overlying sandstones led to formation of dolomite cement. Subsequent infiltration of organic acid-charged meteoric-water into underlying dolomite-cemented sandstones resulted in early dissolution. This porosity was mostly occluded by re-precipitation of calcite. Increase in the abundance of relic corroded dolomite crystals with depth in each cycle indicates that early dissolution was most effective at the top of the cycles. Iron depletion in the calcite cement with depth through the formation points to a local derivation of reactants from in situ dolomite species; implying a stratigraphic control. Calcite cementation was succeeded by continuous mesogenetic dissolution concomitant mechanical compaction. The responsible low-pH pore fluid attained its composition via enhanced H+ contribution from silicate burial diagenesis. The control of depositional settings on the major diagenetic processes makes it a key contribution to region-wide reservoir quality prediction for the formation.

Automated summary

The study indicates that the sandstone reservoir of the Tithonian Jeanne d'Arc Formation in the Terra Nova oilfield underwent early dolomite cementation and subsequent diagenetic events, playing a significant role in controlling reservoir quality. The diagenetic processes were influenced by depositional settings, with cycles of early dolomite cementation and subsequent events controlling the sandstone porosity structure and quality.