--> Carbonate Cementation in Lower Jurassic Jeanne D’Arc Formation, Terra Nova Oil Field, Newfoundland: Implications for Reservoir Quality

AAPG ACE 2018

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Carbonate Cementation in Lower Jurassic Jeanne D’Arc Formation, Terra Nova Oil Field, Newfoundland: Implications for Reservoir Quality

Abstract

Petrographic and in-situ (LA-ICPMS) geochemical analyses was carried out on cores of Late Jurassic Jeanne d’Arc Formation interval from wells C-09 (Graben region) and E-79 (Flank region) of the oilfield to understand control of diagenetic events on the reservoir quality of the formation.

Successively, early near surface dolomite (ankeritic and low-Fe dolomites) cementation, eogenetic dolomite cement dissolution, burial calcite cementation and dissolution are the major diagenetic events that controlled the reservoir quality of the formation sandstones. The constituent lithofacies of the formation range from basal coarse clastic – conglomeratic lithofacies (quartz sub-lithic arenite) (proximal fan delta) through fine to medium grained sandstone (quartz sub-lithic arenite) (incised valley braid bars) to organic rich shale (lacustrine). Lithic clasts are dominantly carbonates (limestone and dolomite) and shales. Iron contents of the dolomite cement grades from ankeritic dolomite cement (upper interval) to low-Fe dolomite (basal interval) of each well. Low Sr and Na; enriched Y (i.e. REEs) and apparent lack of clearly marine strata suggest that the dolomites are not of seawater origin. Instead, dolomite fluid is a diagenetic pore fluid of meteoric origin, with reactants derived from water-rock interaction with underlying Mesozoic Rankin Formation carbonate materials.

The origin and distribution of early dolomite cementation and other successive diagenetic events have a stratigraphic cycle control. Respective expulsion of pore fluid from shale sequences into the overlying sandstone unit led to formation of dolomite cement species. Infiltration of organic acid charged meteoric water into underlying dolomite cemented sandstones resulted in early dissolution; but the resulting porosity was occluded by re-precipitation of calcite concretions. The increase in abundance of relic corroded dolomite crystals with depth indicates that the early dissolution was most effective at the top of each stratigraphic cycle.

A decrease in Fe composition of the calcite cement with depth points to local derivation of reactants from local dolomite species; implying a stratigraphic control. Calcite cementation was succeeded by mesogenetic dissolution. The responsible acidic pore fluid attained its composition via removal of CO32-/HCO3- from the pore, as well as enhanced H+ contribution via silicate reaction in burial environment.

The stratigraphic control on the major diagenetic processes makes it possible to predict reservoir quality within the formation.