Abstract

Sea-level changes and synsedimentary tectonics affected the development of three potential hydrocarbon reservoir intervals in the Silurian–lowermost Devonian part of the Gaspé 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 (Ss) 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 Gaspé Belt. Comparison of the Gaspé relative sea-level curve with eustatic curves, proposed for the same time interval, indicates that the Gaspé 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 Gaspé Belt, which is the most promising area for hydrocarbon reservoirs in the Gaspé 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.

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