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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Hydrocarbon plays in the Silurian -- Devonian Gaspe Belt, Quebec Appalachians
Vol. 49 (2001), No. 2. (June), Pages 217-237

Sea Level, Synsedimentary Tectonics, and Reefs: Implications for Hydrocarbon Exploration in the Silurian-Lowermost Devonian Gaspe Belt, Quebec Appalachians

Pierre-Andre Bourque,

ABSTRACT

Sea-level changes and synsedimentary tectonics affected the development of three potential hydrocarbon reservoir intervals in the Silurian-lowermost Devonian part of the Gaspe Belt on the margin of Laurentia. A relative sea-level curve is based on sequence analysis of facies and distribution of benthic faunas. The succession recorded two major, low-order sedimentary cycles, each composed of a regressive-transgressive couplet and represented by shallowing- and deepening-upward sequences. The first shallowing episode (S1) covered the Rhuddanian-Sheinwoodian (Llandovery-early Wenlock) interval, whereas a rapid deepening episode (D1) followed during the Homerian (late Wenlock). A second shallowing episode (S2) started during the late Homerian, and ended in the late Ludfordian (late Ludlow) or earliest Pridolian, and was then followed by a latest Silurian-Early Devonian deepening episode (D2). Potential reservoir rock units developed mostly during sea-level lowstands or early transgressions in the Gaspe Belt. Comparison of the Gaspe relative sea-level curve with eustatic curves, proposed for the same time interval, indicates that the Gaspe curve was significantly influenced by local synsedimentary tectonics.

Salinic extensional tectonics, a predominantly Late Silurian-Early Devonian (Ludlovian-Pragian) event, resulted in block faulting and tilting along normal listric faults. Interaction between block faulting and eustatic sea-level fall at the end of the Ludlovian-early Pridolian exposed the highest parts of the blocks to subaerial erosion and diagenesis, and allowed reefs and reef complexes to settle at the margins of the blocks, or on erosional remnants. Salinic extensional tectonics may also have provided suitable pathways for hydrocarbon migration and charge at various times during the mid-Silurian to Early Devonian, particularly for hydrocarbons that may have been generated in the underlying Cambro-Ordovician shales. In the northeastern part of the Gaspe Belt, which is the most promising area for hydrocarbon reservoirs in the Gaspe Peninsula, block faulting and tilting have changed the fluid migration pattern from an updip-northeastward flow from the basin centre to basin margin during the Late Ordovician to early Late Silurian, to a potential updip-southwestward flow in each faulted block during the early Late Silurian. Basin tectonics may have played a significant role in driving hydrocarbon-rich fluids toward potential reservoirs, such as the Llandoverian Val-Brillant sandstone bodies, the Sayabec carbonate sands and knob reefs, or the West Point reefs.

RESUME

Les fluctuations du niveau marin et la tectonique synsedimentaire ont influence le developpement de trois horizons susceptibles de constituer des reservoirs petroliferes dans le Silurien - tout debut Devonien de la Ceinture de Gaspesie a la marge de Laurentia. Une courbe relative des niveaux marins a ete construite a partir de l' analyse sequentielle des facies et de la distribution des faunes benthiques. La succession a archive deux grands cycles principaux, chacun forme d' un couple regression-transgression et exprime par une sequence a bathymetrie decroissante, puis croissante. Le premier episode a bathymetrie decroissante (S1) couvre l' intervalle de temps Rhuddanien-Sheinwoodian (Llandovery-Wenlock precoce); il fut suivi par un rapide episode a bathymetrie croissante (D1) durant l' Homerien (Wenlock tardif). Un second episode a bathymetrie decroissante (S2) a debute a l' Homerien tardif pour culminer au Ludfordien (Ludlow tardif) ou au tout debut du Pridolien; il fut suivi par un episode a bathymetrie croissante (D2) a la toute fin du Silurien ou au debut du Devonien. Les unites de la Ceinture de Gaspe presentant un potentiel en reservoirs petroliferes se sont deposees durant

End_Page 217------------------------

un bas niveau marin ou le tout debut d'une transgression. La comparaison de la courbe relative des niveaux marins de Gaspesie avec des courbes eustatiques etablies pour le meme intervalle de temps montre que la courbe gaspesienne a ete influencee de facon importante par la tectonique synsedimentaire locale.

La tectonique d' extension salinique a predomine au Silurien tardif-Devonien precoce (Ludlovien-Praguien) et a produit des blocs failles bascules le long de failles normales listriques. L'interaction entre le basculement des blocs et la chute eustatique du niveau marin a la fin du Ludlovien-debut Pridolien a expose les parties hautes des blocs a une erosion et une diagenese aeriennes et permis l'implanatation de recifs ou de complexes recifaux en peripherie des blocs ou sur des hauts-fonds herites de l'erosion. La tectonique d'extension salinique peut aussi avoir produit une tuyauterie adequate pour la migration et l'emmagasinage d'hydrocarbures qui auraient pu etre generes dans les shales cambro-ordoviciens sous-jacents. Dans le nord-est de la Ceinture de Gaspesie, qui est le secteur le plus prometteur pour les reservoirs petroliferes, le basculement de blocs failles a modifie le patron de migration des fluides, d'un ecoulement amont-pendage vers le nord-est, i.e. du centre vers les marges du bassin, de l'Ordovicien tardif au debut du Silurien tardif, a un ecoulement amont-pendage vers le sud-ouest dans chacun des blocs. Cette inversion peut avoir joue un role important en dirigeant des fluides riches en hydrocarbures vers des reservoirs potentiels, tels les gres llandoveriens de Val-Brillant, les sables calcaires et les buttes recifales de Sayabec ou les recifs West Point.

Traduit par l'auteur.


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