Burial dolomitization and dissolution of Upper Jurassic Abenaki platform carbonates, Deep Panuke reservoir, Nova Scotia, Canada

Bulletin of Canadian Petroleum Geology

Knight

Lavoie

et al. 2009

The lower part of the St. George Group of western Newfoundland consists of Tremadocian shallow marine platform carbonates of the Boat Harbour (about 180 m thick) and the underlying Watts Bight (about 60 m thick) formations. In the Boat Harbour Formation, dolomitization is pervasive at the top of most shallowing-upward, metre-scale, peritidal hemicycles. Petrographic examination of the Boat Harbour Formation carbonates suggests that the succession has been affected by at least three phases of dolomitization, which influenced the final rock porosity. These phases have crystal-size ranges of about 4 to 40 µm (earliest dolomite D1), 50 to 150 µm (D2), and 300 µm to 20mm (saddle dolomite D3), respectively. They occur as both replacements and cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The occurrence of near-micritic size dolomites (about 4-40 µm) may suggest that dolomitization started at low temperatures during early stages of diagenesis. The lack of evaporite interbeds in the formation and the depleted δ 18 O values (-6.2±0.8‰ VPDB) as well as the low Sr contents (168±45ppm) of the earliest dolomites likely exclude a brine origin from evaporated seawater. The Sr/Ca molar ratios (0.0069 to 0.0017), calculated for the earliest dolomitizing fluid, suggest a mixture of marine and meteoric waters possibly in a mixing zone environment. The petrographic features and geochemical attributes of D2 and D3 phases, such as their depleted δ 18 O values (-6.9±1.5‰ and -8.3±0.9‰ VPDB, respectively) and Sr contents (177±76 and 117±33ppm, respectively), suggest that they were formed under relatively deeper burial conditions and possibly from hydrothermal fluids which is supported by homogenization temperatures (up to 135º C) and estimates of salinities (up to 23 wt. % NaCl) in the latest dolomites (D3). Based on visual estimates from thin sections, the porosity varies from <1% in most of the formation to about 10% in a dolomitized algal lime mudstone bed in the upper part of the formation, a few metres below the Boat Harbour Disconformity. Except for some vugs, the majority of pores are intercrystalline and associated with D2.The dolomitization events recorded by the Boat Harbour Formation carbonates resemble, in their succession and geochemical signatures, those previously documented in the Aguathuna Formation. However, significant differences are noted with the dolomitization events recorded in the Lower Ordovician Romaine Formation on Anticosti Island; these differences are tentatively associated with the very distinct tectonic settings of each case. . Dans la Formation de Boat Harbour, la dolomitisation est envahissante au sommet de la plupart des demi-cercles péritidaux de moindre profondeur vers le haut, à l'échelle métrique. L'analyse pétrographique des carbonates de la Formation de Boat Harbour suggère qu'au moins trois épisodes de dolomitisation ont agit sur la RÉSUMÉ

Section: Development Of Secondary Porosity Reservoir Potential and Rmentioning

confidence: 96%

Section: Petrographymentioning

confidence: 99%

Section: Development Of Secondary Porosity Reservoir Potential and Rmentioning

confidence: 99%

See 1 more Smart Citation

Origin of dolomites in the Boat Harbour Formation, St. George Group, in western Newfoundland, Canada: implications for porosity development

Bulletin of Canadian Petroleum Geology

Knight

Lavoie

et al. 2009

“…The preserved porosity in the Catoche Formation is mainly secondary and occurs in dolomites (Fig. 2), suggesting that it is directly related to the dolomitization process (e.g., Dravis, 1992;Esteban and Taberner, 2003;Wierzbicki et al, 2006;Conliffe et al, 2009;Azmy and Conliffe, 2010). Porosity is mostly of the intercrystalline variety (apparent long axes of pores w50e 600 mm) and mainly associated with D2 but also occasionally occurs among the subhedral crystals of D3.…”

Section: Variations In Porosity Distributionmentioning

confidence: 99%

Origin of Lower Ordovician dolomites in eastern Laurentia: Controls on porosity and implications from geochemistry

Azomani

Marine and Petroleum Geology

Blamey

et al. 2013

Self Cite

a b s t r a c tThe Catoche Formation of the St. George Group in eastern Laurentia (western Newfoundland) consists of early Ordovician (Arenigian) shallow marine platform carbonates (w160 m thick), which were extensively dolomitized during the course of their diagenetic history. The dolomites occur as both replacement and pore-filling cements and are a major control on porosity distribution in the formation. The origin and diagenetic history of the Catoche dolomites at Daniel's Harbour (western Newfoundland) were analysed in comparison to equivalent successions at Port au Choix (PaC) and Port au Port Peninsula (PaP) to assay the reservoir potential of these dolomites in eastern Laurentia. Petrographic examination identified at least three generations of dolomites in the Catoche Formation, which are: (1) an early replacement sub-to eu-hedral micritic dolomite (<4 mme30 mm, D1), (2) eu-to sub-hedral dolomite (70 mme1 mm) often with cloudy cores and clear rims (D2), and (3) subhedral to anhedral saddle dolomite cement (200 mme3 mm, D3). The micritic dolomite (D1) exhibits a dull cathodoluminescence (CL) under cathodoluminoscope, whereas dolomite D2 exhibits consistent concentric CL zonation. Some subhedral crystals of D3 appear zoned both in plane polarized light and cathodoluminoscope, otherwise D3 exhibits a dull CL. Stoichiometric dolomite occurs in all three generations with D2 as the dominant dolomite by abundance.The low strontium (47 ± 25 ppm) content coupled with depleted d 18 O value of dolomitizing fluids (-10 to -11.2& VSMOW) and near-micritic grain size, suggests an early precipitation of dolomite D1 at low temperatures of near-surface conditions from solutions likely formed by mixing of early Ordovician sea and meteoric waters. In contrast, microthermometric measurements of primary two-phase fluid inclusions in dolomite D2 (homogenization temperatures of 102e168 o C with a salinity range of 19.8e25 eq wt% NaCl) and dolomite D3 (homogenization temperatures of 158e190 o C with a salinity range of 20.2e22.2 eq wt% NaCl), suggest that both dolomite generations were generated in mid to deep burial settings from high salinity, low temperature (<200 o C) hydrothermal fluids likely under suboxic conditions. This is consistent with the low Sr concentrations for D2 (36.4 ± 8 ppm) and D3 (38.7 ± 9 ppm), d 18 O values of dolomitizing fluids for D2 (þ2.1 to þ8.1&, VSMOW) and D3 (þ6 to þ8.1& VSMOW), coupled with Fe contents of D2 (1684 ± 1096 ppm) and D3 (1783.7 ± 618 ppm) as well as the respective Mn (D2 ¼ 131.2 ± 50 and D3 ¼ 197.5 ± 55 ppm) concentrations.SREE and shale normalized (REESN) values of Catoche carbonates indicate enrichment in rare earth element (REE) composition of the earliest calcite (C1) relative to those of Arenig seawater, whereas the REESN profiles of the dolomite generations mimic that of calcite C1. The Ce (Ce/Ce*)SN and La (La/La* ¼ Pr/Pr*)SN anomalies of the Catoche dolomites are consistent with precipitation in equilibrium with source fluids in slightly oxic to suboxic conditions whereas Eu...

“…Petrographic study of lower St. George Group carbonates on the Northern Peninsula indicates that secondary porosity is the dominant type and mainly is associated with dolomitization (e.g. Dravis, 1992;Esteban and Taberner, 2003;Wierzbicki et al, 2006;Azmy et al, 2009;Conliffe et al, 2009). Visual estimates suggest that porosity varies between <1 and 10%.…”

Section: Variations In Vertical and Lateral Distribution Of Porositymentioning

confidence: 99%

Dolomitization of the lower St. George Group on the Northern Peninsula in western Newfoundland: implications for lateral distribution of porosity

Bulletin of Canadian Petroleum Geology

Conliffe

2010

The Watts Bight Formation constitutes the lowermost part of the St. George Group in western Newfoundland. On the Northern Peninsula, it consists of Tremadocian (lower Ordovician) shallow marine platform carbonates (approximately 50 m thick). Dolomitization is extensive throughout the formation except for its topmost part. Petrographic examinations suggest that the succession was affected by at least three successive phases of dolomitization, which influenced secondary porosity. These phases have crystal-size ranges of approximately 4 to 50 µm (earliest sub-to euhedral dolomite D1), 50 to 200 µm (eu-to subhedral D2), and 300 µm to 3mm (anhedral saddle dolomite D3), respectively. They occur as replacement and/or pore-filling cements and exhibit dull (D1 and D3) to zoned (D2) luminescence under the cold cathodoluminoscope. The D2 phase is the dominant dolomite while the other two phases are rare.Microthermometric measurements of the primary two-phase fluid inclusions in D2 (homogenization temperatures up to 157 o C and salinity estimates up to 24.3 eq. wt% NaCl) suggest that it formed under relatively deep burial conditions. This is supported by the petrographic character (eu-to subhedral relatively big crystals) and geochemical composition (depleted δ 18 O mean value of -8.7±1.2‰ VPDB and low Sr contents of 68±30ppm) of the D2 phase. Thin-section examination suggests that porosity is dominantly intercrystalline and associated with the D2 phase. Visual estimates suggest that porosity varies from <1% in most of the formation to approximately 10% in two horizons, each approximately 2m thick, at approximately 10 m and 20 m from formation base. Correlations with the equivalent Watts Bight Formation section in the Isthmus Bay (300 km to South) reveal porous intervals at comparable stratigraphic levels. Also the geochemical results and microthermometric measurements suggest that D2 in the Northern Peninsula section was formed from hotter fluids under relatively more reducing conditions relative to their Isthmus Bay counterparts. The porous zones seem to be associated with fluctuations in sea-level marked by negative shifts in the δ 13 C profile. RÉSUMÉLa Formation de Watts Bight constitue l'unité la plus basse du Groupe de St.George dans l'ouest de Terre-Neuve. Dans la péninsule Great Northern, elle est composée de carbonates de plateaux marins (d'environ 50 m d'épaisseur) peu profonds du Trémadocien (Ordovicien inférieur). La dolomitisation est considérable dans toute la formation, à l'exception de sa partie la plus élevée. Les analyses pétrographiques donnent à penser qu'au moins trois épisodes de dolomitisation ont affecté la succession, ce qui a influencé la porosité secondaire. La taille des cristaux de ces épisodes de dolomitisation varie de 4 µm à 50 µm (dolomite D1 précoce de subautomorphe à automorphe), de 50 µm à 200 µm (D2 automorphe à subautomorphe) et de 300 µm à 3 mm (dolomite D3 en selle anédrique), respectivement. Ils se présentent sous forme de remplacement et/ou de cimentation d'espaces intergra...