Origin of carbonate cements in Ben Nevis sandstones of Jeanne d’Arc Basin: Insights from geochemistry

Abstract

The Ben Nevis sandstones in the Jeanne d’Arc Basin in offshore eastern Newfoundland have abundant carbonate cements, and are the primary hydrocarbon reservoirs of the White Rose Oilfield. A multiapproach of petrographic, carbon and oxygen isotopic, and in situ geochemical analyses were applied to investigate the origin of the carbonate cements in the Ben Nevis sandstones (quartzose to litho-quartzose, Well F-04). Petrographic examinations reveal five generations of cements that occur in concretions, which are (from earliest to latest), siderite, early calcite cement (C1), recrystallized fossil (Crf), sparry fossil-filling calcite (Cf), and late calcite cement (C2). The comparable carbon (3.6‰ VPDB) and oxygen isotopic (−1.3‰ VPDB) compositions of siderite to those of the bivalve shells (δ13C = 3.6 ± 0.6‰ VPDB and δ18O = −1.7 ± 0.5‰ VPDB, respectively) indicate an early precipitation from suboxic water near the sediment-seawater interface. The well-preserved intergranular volumes (IGVs up to 41%) and low δ18O (−3.3 ± 1.1‰ VPDB) as well as comparable δ13C values (3.6 ± 0.6‰ VPDB) of the early poikilotopic calcite (C1) argue for a pre-compaction precipitation at a shallow burial setting with carbon derived mainly from the dissolution of bioclasts. The later calcite cement (C2) precipitated during progressive burial with involvement of organic carbon as indicated by its depleted δ18O (−6.7 ± 2.0‰ VPDB) and δ13C (1.8 ± 1.3‰ VPDB) values, and the reduced IGVs (28.2%), as well as the consistent depletion of Sr with enrichment of Mn. The dissolution of aragonitic bioclasts, was likely the main source of Ca for calcite, which is consistent with the high Sr content of C1 (1369 ± 597 ppm), Crf (2005 ± 472 ppm), Cf (1354 ± 48 ppm) and C2 (1831 ± 696 ppm). The parallel shale normalized (REESN) patterns of the cements and the way they mimic, to some extent, that both of seawater and meteoric water, suggest that the diagenetic fluid is bracketed by those of the early Cretaceous marine and meteoric waters, as reflected by the estimated δ18OSW composition of parent fluid for C1. The consistent Eu anomalies of carbonate cements and the increasing δ18OSW composition of pore water during C2 precipitation reflect the evolution of the diagenetic fluids in siliciclastic intervals during progressive burial.The secondary porosity development as the main contributor to the present reservoir porosity seem to be closely related to the dissolution of calcite cements, especially in the lower part of the Ben Nevis interval. The meteoric water influx from the overlying unconformities may have induced the dissolution of calcite concretions.