The Lower Silurian Sayabec Formation in Northern Gasp6: Carbonate Diagenesis and Reservoir Potential

BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 49, NO. 2 (JUNE, 2001), R 282-298 The Lower Silurian Sayabec Formation in northern Gasp6: carbonate diagenesis and reservoir potential DENIS LAVOIE AND GUOXIANG CHI Geological Survey of Canada (Quebec) Centre Gdoscientifique de Qudbec 880 Chemin Sainte-Foy, C.P. 7500 Qudbec, QC G1V 4C7 ABSTRACT The Lower Silurian Sayabec Formation represents a peritidal-dominated carbonate ramp that developed at the northern edge of the post-Taconian Gasp6 successor basin. In the Late Silurian, during the Salinic disturbance, the Sayabec ramp was subaefially exposed locally. This could have lead to the formation of economically significant secondary dissolution porosity. A detailed diagenetic study of the Sayabec Formation was carried out at selected localities along the Northern Outcrop Belt in the Gasp6 Peninsula, where the Salinic unconformity and hydrothermal alteration of the carbonate facies have been documented. The diagenetic history consists of initial minor marine diagenesis (marine cements in boundstones and neptunian dykes) followed by pervasive burial diagenesis that resulted in the emplacement of various pore- and fracture-filling calcite cements, due to the mixing of basinal brines and hydrothermal fluids. Late Silurian tectonic exhumation of the lithifled carbonate ramp is recorded locally in meteoric-cement-filled fractures that were dissolution-enhanced after early burial. The significance of this event in generating porosity was relatively minor. Preserved porosity is observed where limestone facies and calcite cements were completely replaced by hydrothermal saddle dolomite. However, the porous dolostone is of geographically limited extent. The hydrothermal event is mostly recorded in high-temperature calcite cements that occlude burial fractures. R£StrM~ La Formation silurienne infrrieure de Sayabec reprrsente une rampe ~t carbonates ~ dominance prfitidale qui bordait la limite nord du bassin successeur post-taconien de la Gasprsie. Cette rampe fut, au Silufien tardif, localement exposre ~t des conditions sub-arfiennes (Discordance Salinique) menant h la formation d'une porosit6 secondaire de dissolution possiblement 6conomiquement importante. Une 6tude diagrnrtique drtaillre du Sayabec fut menre pour des sites reprrsentatifs le long de la Bande du Nord en Gasprsie o0 la discordance Salinique est bien documentre et o0 6gale- ment une altrration hydrothermale des facibs h carbonates est connue. L'histoire diagrnrtique consiste en une diagen~se marine rnineure (ciments marins dans des bioconstructions et dykes neptuniens), suivie d'un syst~me diagrnrtique d'enfouissement enregistr6 dans les grnrrations de calcite de rem- plissage de pores et de fractures reprrsentant localement un mrlange de saumures de bassin et de fluides hydrother- maux. L'exhumation tectonique au Silurien tardif de la rampe h carbonates drjh lithifire fut localement enregistrre grace ~ la prrsence d'un 6pisode de fracturation/dissolution ciment6 par des calcites mrtroriques qui a suivi un enfouissement initial. L'importance de cet 6vfnement pour le drveloppement de porosit6 fut mineure. Une porosit6 actuelle s'observe localement due ~ un remplacement total des faci~scalcaires et des ciments calcitiques par une dolomite baroque d'origine hydrothermale. Cette derni~re poss~de cependant une extension grographique limitre. L'rvrnement hydrothermal fut principalement enregistr6 dans des ciments de calcite de hautes temprratures colmatant les fractures d'enfouissement. Traduit par les auteurs. 282 THE LOWER SILURIAN SAYABEC FORMATION 283 INTRODUCTION This paper aims to improve our understanding of the diagenetic evolution of the Sayabec Formation in the The Paleozoic sequence in the Acadian Gasp6 Belt (Bourque Silurian-Devonian Northern Outcrop Belt (sensu Bourque et et al., 1995) of the northern Appalachian Orogen (Bourque et al., 1995) of the Gasp6 Peninsula (Fig. 1). In particular, and as al., 2001a, Figs. l, 2, this issue) contains few shallow-water car- observed for the La Vieille Formation in southern Gasp6 bonates. The notable exception to this is the occurrence of reef (Lavoie and Bourque, 1993), the possible diagenetic effects of and carbonate complexes that developed during the Late the Salinic disturbance on porosity will be addressed. As a Llandoverian-Wenlockian and the Ludlovian-Lochkovian. whole, this paper aims to document the diagenetic history and Limestone facies of the former, including microbial laminites, fluid evolution recorded in cement in order to evaluate the stromatolitic limestones, biohermal and biostromal bound- hydrocarbon reservoir potential of the northern segment of the stones, well-bedded packstones and grainstones, and muddy Gasp6 Peninsula. nodular limestones, have been described from various parts of the Gasp6 Peninsula and northern New Brunswick (Fig. 1) PALEOGEOGRAPHIC AND ENVIRONMENTAL SETTING (Hrroux, 1975; Bourque, 1975; Noble, 1976; Lee and Noble, In the Gasp6 Belt, the Sayabec and La Vieille carbonate 1977; Bourque and Lachambre, 1980; Desrochers, 1981; ramps encompass approximately 10,000 and 20,000 km 2, Lachambre, 1987; Lavoie, 1988; Lavoie et al., 1992). All these respectively. These ramps developed at the margin of the occurrences are within two formations, the Sayabec Formation Qu6bec Re-entrant and St. Lawrence Promontory (Bourque et in northern Gasp6 Peninsula and Mataprdia Valley, and the La al., 2001a, Fig. 5c, this issue). The Sayabec and La Vieille for- Vieille Formation in southern Gasp6 Peninsula and northern mations constitute a thin limestone unit in an otherwise thick New Brunswick. The Lower Silurian Sayabec and La Vieille Silurian siliciclastic succession (Bourque et al., 2001a, Fig. 2, formations represent the first occurrence of shallow-water lime- this issue). The Sayabec attains a thickness of nearly 320 m, stones in the Paleozoic succession of the northern segment of whereas the La Vieille at its thickest comprises only 180 m of the Appalachian Orogen (Bourque et al., 2001a, this issue). the 2500 to 4000 m thick Silurian succession of the northern They have been interpreted, on the basis of palinspastic recon- and southern parts of the Gasp6 Belt. Carbonate ramp develop- struction, as part of the same carbonate ramp that developed ment coincided with the peak of the first shallowing phase in along the margin of the Qurbec Re-entrant and St. Lawrence the Gasp~ Belt following the Taconian Orogeny (late Middle Promontory (Lavoie et al., 1992; Bourque et al., 1995; Bourque Ordovician) (Bourque et al., 1995; Bourque et al., 2001a, this et al., 2001a, Fig. 5c, this issue). Various aspects of the sedi- issue; Bourque, 2001, this issue). Carbonate ramps were mentology, paleobiological communities, and paleogeography deposited during two transgressive-regressive cycles of earliest of peritidal-dominated Sayabec/La Vieille ramps have been to late Wenlockian age (Lavoie et al., 1992; Bourque, 2001, published elsewhere (Bourque et al., 1986; Lavoie, 1988; Fig. 6, this issue). Both ramps were gently south-dipping and Desrochers and Bourque, 1989; Lavoie et al., 1992; Bourque et exhibited the same lateral facies zonation (Lavoie et al., 1992). al., 1995). Both units have similar facies and overall sequence Four parallel depositional belts are recognized: from nearshore evolution (Lavoie, 1988; Lavoie et al., 1992). to offshore (Fig. 2, Table 1) they are (1) a wide peritidal mud- The general diagenetic evolution of the Sayabec and La fiat, containing mostly microbial carbonates (laminites, stroma- Vieille formations is characterized by little marine cementation tolites, thrombolites, oncolites) and local elastics; (2) a narrow followed by burial diagenesis (Lavoie, 1988; Lavoie and knob reef rim built by a consortium of skeletal metazoans Bourque, 1993). A significant Upper Silurian (Pfidolian) mete- (corals, bryozoans, stromatoporoids), skeletal calcareous algae, oric diagenetic event has been recognized in the La Vieille and microbial communities; (3) a well-sorted lime sand belt Formation in southern Gasp6 Peninsula (Lavoie and Bourque, and; (4) a deeper water nodular mud belt. The ramps were 1993). This event, related to the combined effects of tectonic buried by an influx of deep-water siliciclastic sediments during uplift, relative sea-level lowstand, and erosion (Salinic uncon- a late Wenlockian transgression (Lavoie et al., 1992; Bourque, formity), led to the generation of significant secondary porosi- 2001, Fig. 6, this issue), but part of the carbonate ramps was ty that was filled ablruptly by meteoric calcite cement (Lavoie uplifted and eroded in earliest Pridolian time (Bourque et al., and Bourque, 1993). This event also affected the lower part of 1986; Lavoie and Bourque, 1993). the Ludlovian-Pridolian reef complex in southern Gasp6 (Bourque et al., 2001b, this issue). Although the Salinic uncon- THE SAYABEC FORMATION formity truncates the Sayabec Formation in northern Gasp6 In the Northern Outcrop Belt (NOB; Fig. 1), the Sayabec Peninsula (Lachambre, 1987; Lavoie et al., 1992), its signifi- Formation has been divided into four informal members (A to cance in porosity generation has not been documented previ- D, Table 1, Lavoie et al., 1992). These members correspond ously. The limited diagenetic information of previous studies of broadly to a major initial transgressive-regressive cycle fol- this segment of the Gasp6 Peninsula has focused only on the lowed by a transgressive event (Lavoie et al., 1992). In the cen- burial diagenetic

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