Rudolf Bertrand

Chitinozoan, graptolite, and scolecodont reflectance as an alternative to vitrinite and pyrobitumen reflectance in Ordovician and Silurian strata, Anticosti Island, Quebec, Canada

Because of the absence of vitrinite in lower Paleozoic sediments, the reflectance of pyrobitumen is used as an indicator of thermal maturation. A new technique of preparing organic matter permits discrimination among types of zooclasts and solid bitumen, including pyrobitumen, and leads to more precise reflectance-histograms. Studies of organic matter from Ordovician-Silurian strata of Anticosti Island show that reflectance of zooclasts is different from that of solid bitumen, and that each group of the following zooclasts, chitinozoans, graptolites, and scolecodonts, has a specific reflectance evolution path with increasing depth. On a logarithmic scale, the paths of the three groups of zooclasts are linear, highly correlated, and converging. However, reflectance of solid bitumen is not linear throughout the range studied; solid bitumen becomes linear only where the reflectance is greater than 0.75%. Comparing the relations between zooclasts and solid bitumen with the vitrinite-solid bitumen correlation in the literature suggests that a hypothetical vitrinite evolution line in the Anticosti Island basin should be subparallel with that of chitinozoans and graptolites, but 0.4-0.8% lower, in the 1-2% range. However, scolecodonts and vitrinite should have the same reflectance at 1%, but scolecodont reflectance should be slightly (0.2%) higher for a vitrinite reflectance of 2%.

Correlations among the reflectances of vitrinite, chitinozoans, graptolites and scolecodonts

Abstract Paleozoic marine sequences sampled in outcrops and wells of northeastern Gaspe Peninsula (Quebec Appalachians) yield kerogen composed in part of vitrinite and zooclasts (chitinozoans, scolecodonts and graptolites) on which reflectance measurements were made. The comparison between means of random measurements show that reflectance of chitinozoans is similar to that of telinite (vitrinite with a cellular texture) and that the reflectance of scolecodonts is significantly lower than that of telinite in the lower range of studied values. At higher reflectivities differences decrease but, at the end of the condensate gas zone reflectancea are not yet identical. As shown by previous studies, reflectance of graptolites is slightly less than that of telinite. Pyrobitumen reflectance is related to the lithology of the host-rock and no generalized correlation can be established between the reflectance of pyrobitumen and that of vitrinite.

Hydrothermal dolomitization in the Lower Ordovician Romaine Formation of the Anticosti Basin: significance for hydrocarbon exploration

This study deals with the diagenetic evolution of dolostones of the Romaine Formation, from cores and outcrops on Anticosti and Mingan islands, and reports evidence for hydrocarbon migration in secondary porosity generated by high temperature / hydrothermal alteration. Four types of calcite (grouped into early (C1) and late (C2) assemblages) and six types of dolomite (grouped into 3 different (D1, D2 and D3) assemblages) are distinguished. According to fluid-inclusion and stable C-O isotope data, the early calcite group, which consists of sedimentary micrite, replacement microspar and early pore-filling calcite records near surface diagenetic environments. Early and pervasive replacement dolomites (RD1 and RD2 in the first dolomite (D1) assemblage) were formed during early burial, and are not related to porosity generation. Later replacement dolomites (RD3 and RD4 in the second dolomite (D2) assemblage) and pore-filling dolomite cements (PD1 and PD2 in the third dolomite (D3) assemblage) as well as minor sphalerite were formed from warm, saline fluids, which were likely hydrothermal in origin. Vuggy porosity was produced through brecciation and fracturing, and through some dissolution of the D1 dolomite assemblage by these hydrothermal fluids. Intercrystalline porosity was also created during formation of the D2 dolomite assemblage. The vugs were partly occluded by late-stage pore filling dolomite and calcite cements, pyrite and barite. Bitumen droplets in vugs together with hydrocarbon inclusions recorded in C2 calcite and in contemporaneous barite indicate a hydrocarbon migration event after the porositygenerating processes. The brecciation / leaching of a precursor dolomite host is uncommon in hydrothermal dolomite hydrocarbon fields, but is recognized in the adjacent coeval pool in Newfoundland. This type of alteration in the Ordovician carbonates with the presence of a rich hydrocarbon source rock and favourable maturation are key elements for the on-going exploration efforts in this basin.

Standardization of Solid Bitumen Reflectance to Vitrinite in Some Paleozoic Sequences of Canada

Abstract The reflectance (R0) of solid bitumen in Ordovician to Devonian sequences in the Quebec Appalachians and in the Mackenzie area of northern Canada is standardized to the R0 of vitrinite. The migrabitumen, having autochthonous characteristics, is the best solid bitumen type to evaluate the thermal maturation from its R0 This migrabitumen is generally found as irregular high-reflecting areas, forming part of low-reflecting organic matter frameworks, called protobitumen, and appearing as amorphous bodies in transmitted light. The R0 difference between migrabitumen and protobitumen disappears with increasing maturation. Because vitrinite is missing in pre-Devonian sequences, the R0 of solid bitumens is standardized through the R- of zooclasts. The R0 of migrabitumen increases similarly to those of zooclasts and vitrinite with maturation, but no universal calibration between the R0of migrabitumen and vitrinite can be suggested. Scales of calibration are specific for different lithologies and basins. Th...

Sedimentologic, Diagenetic and Tectonic Evolution of the Saint-Flavien Gas Reservoir at the Structural Front of the Quebec Appalachians

ABSTRACT The Beekmantown Group (Lower Ordovician) of the Saint-Flavien reservoir has produced 162x106 m3 (5.7 bcf) of natural gas between 1980 and 1994. The conversion of the field into gas storage was initiated in 1992 and the pool became operational in 1998. Integration of structural and sedimentologic features, carbonate and organic matter petrography and geochemistry for 13 drill holes is used to define a tectonic-sedimentologic-diagenetic model for porosity evolution in these reservoir dolostones. The Beekmantown Group consists of numerous fifth-order shallowing-upward cycles 1.0 to 7.0 m thick (average of 2.4 m). Each cycle consists of a basal shale deposited during the initial flooding of the platform which was subsequently covered by a shoaling succession of subtidal and intertidal limestones to intertidal dolostones. Early dolomitization has produced intercrystalline porosity and preserved some moldic pores in the intertidal facies. Near surface, post-dolomite karstification has created vugs that were subsequently filled by early marine calcite fibrous cement based on the 18O and 13C ratios of calcite. Early burial elements consist of horizontal stylolites, pyrite and sphalerite. Late migrated bitumen was thermally altered or vaporized as native coke under deep burial conditions exceeding 240°C, partly due to overthrusting of Appalachian nappes. Under these conditions, breccias and fractures were generated and subsequently filled with K-feldspar, quartz, illite, and xenomorphic and poikilotopic calcite. The 18O of the poikilotopic calcite and homogenization temperature of coeval fluid inclusions indicate formation under high temperatures (Th about 260°C). Horizontal shear zones and marcasite-rich vertical stylolites were produced during folding and thrusting. Dissolution has preferentially affected late fracture-filling calcite and generated most of the actual porosity during or soon after the Taconian Orogeny. The relationship between the occurrence of smectite and this type of porosity indicates the low temperature condition of this dissolution (T <100°C). Porosity in the Saint-Flavien reservoir has been mostly produced by fracture-controlled, late to post-Taconian dissolution of early to late calcite in the intertidal dolomitic slightly porous facies at the top of rhythmic cycles that compose the Beekmantown Group. End_Page 126------------------------

Maturation thermique des roches mères dans les bassins des basses-terres du Saint-Laurent et dans quelques buttes témoins au sud-est du Bouclier canadien

Caradocian source rocks belonging to the Utica Group occur around and overlay the southeastern part of the Canadian Shield from the Ottawa embayment, in the southwestern part of St. Lawrence Lowlands, to the Anticosti Island basin. The kerogen of the source rocks is mainly made up of amorphous organic matter (transmitted light), resembling a solid bitumen framework in reflected light, and contains a small portion of zooclasts (graptolites and chitinozoans). The kerogen is often too mature to have preserved the structure of the original constituents, except those of graptolites and chitinozoans. Otherwise, UV fluorescent colonial alga and tasmanites are observed. In the eastern half of the St. Lawrence Lowlands, stratigraphic and tectonic burial is the main control of source rock thermal maturation. From an area located 50 km west of Quebec City, the estimated vitrinite reflectance (Ro-Std) of the source rock increases in both northeastern (Montmorency Falls) and southwestern (St. Maurice River) directions, from Ro-Std of 1.2 to 1.5 and 1.7%, respectively. The source rocks of the Ottawa embayment and Montreal area (western half of the basin) are overmature due to hydrothermal and igneous activities (2.3%<Ro-Std<2.8%).. In Chicoutimi and Lac Saint-Jean outliers, the source rocks show reflectances between 0.9 and 1.2%, slightly less than reflectance values in the Quebec City area (Ro-Std between 1.2 and 1.5%). On Anticosti Island, the thermal maturation is related to burial only and increases downdip of the inclined platform, from the northeast toward the southwest (Ro-Std between 0.9 and 2.1%). These observations suggest that the St. Lawrence platform and the southeastern Canadian Shield were buried by at least 3.5 km of Paleozoic strata prior to erosion to Grenville basement. This implies that most potential hydrocarbon fields are presumably also lost by erosion.

Diagenetic history and porosity evolution of Upper Carboniferous sandstones from the Spring Valley #1 well, Maritimes Basin, Canada–implications for reservoir development

Abstract Eighty-two core samples were collected from the Spring Valley #1 well which penetrates the Upper Carboniferous strata in the Late Devonian–Early Permian Maritimes Basin. The strata consist of alternating sandstones and mudstones deposited in a continental environment. The objective of this study is to characterize the relationship of sandstone porosity with depth, and to investigate the diagenetic processes related to the porosity evolution. Porosity values estimated from point counting range from 0% to 27.8%, but are mostly between 5% and 20%. Except samples that are significantly cemented by calcite, porosity values clearly decrease with depth. Two phases of calcite cement were distinguished based on Cathodoluminescence, with the early phase being largely dissolved and preserved as minor relicts in the later phase. Feldspar dissolution was extensive and contributed significantly to the development of secondary porosity. Quartz cementation was widespread and increased with depth. Fluid inclusions recorded in calcite and quartz cements indicate that interstitial fluids in the upper part of the stratigraphic column were dominated by waters with salinity lower than that of seawater, the middle part was first dominated by low-salinity waters, then invaded by brines, and the lower part was dominated by brines. Homogenization temperatures of fluid inclusions generally increase with depth and suggest a paleogeothermal gradient of 25 °C/km, which is broadly consistent with that indicated by vitrinite reflectance data. An erosion of 1.1–2.4 (mean 1.75) km of strata is inferred to have taken place above the stratigraphic column. δ 18 O values of calcite cements (mainly from the late phase) decrease with depth, implying increasing temperatures of formation, as also suggested by fluid-inclusion data. δ 13 C values of calcite cements range from −13.4‰ to −5.7‰, suggesting that organic matter was an important carbon source for calcite cements. A comparison of the porosity data with a theoretical compaction curve indicates that the upper and middle parts of the stratigraphic column show higher-than-normal porosity values, which are related to significant calcite and feldspar dissolution. Meteoric incursion and carboxylic acids generated from organic maturation were probably responsible for the abundant dissolution events.

Cambrian-Ordovician Shales in the Humber Zone: Thermal Maturation and Source Rock Potential

ABSTRACT Thermal maturation and potential for hydrocarbon generation and pooling are evaluated in the Upper Cambrian Grosses-Roches Formation and the Lower Ordovician Riviere Ouelle Formation of the Humber Zone in Gaspe Peninsula. Five types of organic matter (OM) analyses are used: petrography of rock thin sections and of OM concentrates, reflectance analysis, programmed pyrolysis, and combustion analysis. All analyses indicate that these successions are overmature with respect to hydrocarbon generation, but do not reach the threshold of metamorphism. Source rocks in these formations are in the dry gas zone, and the previously generated oils are now altered into various types of migrabitumen or native coke. A regular increase in maturation is observed in the Riviere Ouelle Formation (Anse du Crapaud member) from the upper condensate zone to near the end of the dry gas zone. No such gradient is observed in the Grosses-Roches Formation. Our results suggest that the upper member of the Grosses-Roches Formation may have had initial fair hydrocarbon potential (some initial TOC values up to about 2 wt%, assuming Type II kerogen) and the D2 unit of the Anse du Crapaud member may have had initial good hydrocarbon potential (some initial TOC values up to about 3 wt%, assuming Type II kerogen). The OM petrographic composition of these fine-grained potential source rocks shows some similarities with those of the Late Ordovician Macasty (Anticosti Island) and Lower Ordovician Green Point (Western Newfoundland) formations. The OM petrographic composition of the A to B2 units of the Anse du Crapaud member suggests former gas storage. No producible hydrocarbons remain. The recognition of Upper Cambrian to Lower Ordovician oil source rocks down-dip of potential Ordovician dolostone reservoir rocks of the Anticosti and St. Lawrence platforms opens a new perspective for hydrocarbon exploration in eastern Quebec. End_Page 213------------------------

Genesis and controls of hydrothermal dolomitization in sandstones of the Appalachian thrust belt, Québec, Canada: Implications for associated galena-barite mineralization

Abstract The Appalachian thrust belt of eastern Quebec hosts six epigenetic barite-galena(-sphalerite) occurrences in interdigitated carbonate-cemented sandstones, carbonate-clast-bearing conglomerates and minor siltstones of the Upper Cambrian St. Damase Formation. Two of these, the St. Fabien and Cap Enrage deposits, consist of veins and disseminations enveloped by dolomitized zones, and contain ankerite as an essential gangue mineral. In contrast, the other occurrences consist exclusively of calcite-cemented veins, and are conspicuously devoid of dolomite. The domains of the St. Fabien and Cap Enrage deposits,are intensely fractured and contain abundant dolomite-barite-sulfide-cemented interconnected fractures and pores, whereas those of the other occurrences display relatively minor fractures and pores, which are only rarely interconnected. This difference in porosity leads to the conclusion that differences in paleo-permeability were the most likely reasons for the presence of dolomite and disseminated mineralization in the two deposits, and their absence from the vein-only occurrences. Mass-balance calculations relating altered and unaltered rock suggest that dolomitization was accompanied by large gains of CaO, MgO and CO 2 , and corresponding losses of SiO 2 and K 2 O under conditions of constant volume. The mass of silica removed from the altered sandstone at St. Fabien (14.5 g per 100 g of rock), when considered in the context of quartz solubility, implies a minimum water/rock ratio of 2635. A similar value (2500) is indicated by the content of lead of a representative sample of the St. Fabien deposit and typical concentrations of lead in Mississippi Valley-type fluids. The pH of the mineralizing/dolomitizing fluid was calculated from the solubility of dolomite and fluid-inclusion estimates of m Ca 2+ , m Mg 2+ and m CO 2 to be between 3.9 and 4.6. Fluid-inclusion data suggest that dolomitization and mineralization at St. Fabien (and Cap Enrage) were the products of interactions between local sedimentary rocks and minor metabasaltic dikes with hydrothermal brines. Detailed X-ray diffraction analysis of the phyllosilicates in the host rocks in and around the St. Fabien deposit (mainly sandstones and minor metabasalt) indicates that these brines could have acquired virtually all magnesium and all iron (± manganese) needed for dolomite precipitation as a result of the replacement of chlorite by illite. Since P - T conditions during deposition of the two types of mineralization were similar to each other, it is concluded that these conditions did not govern the formation of dolomite.