Abstract

The Lower Devonian Upper Gaspé Limestones consist of the Forillon, Shiphead and Indian Cove formations. The depositional basin and stratigraphic succession were controlled by synsedimentary extension faults. Three major paleogeographic domains are recognized: a northern proximal outer shelf, a central distal outer shelf, and a southern toe-of-slope. Numerous oil seeps occur in fractured limestones of the central domain, and limited hydrocarbon production has been recorded from these units.

Primary porosity occurs in the northern domain in the Indian Cove Formation brachiopod-rich facies. Local meteoric dissolution enhancement of porosity is the result of late influx of meteoric waters, as inferred from the nature of fracture-filling calcite cements. Porosity enhancement is restricted to the northern domain, but this area is barren of hydrocarbons.

Multiple episodes of burial fracturing, dissolution, and calcite cementation are found in the central and southern domains. Petrographic evidence indicates three distinct diagenetic events occurred before the development of vertical stylolites related to the Acadian Orogeny (Middle Devonian). Carbon (C) and oxygen (O) stable isotopes and fluid inclusion microthermometry of the fracture-filling calcite cements indicate significantly higher thermal conditions for the southern domain than the western sector of the central domain. Low thermal maturation and hydrocarbon occurrences in the eastern part of the central domain (e.g. the Mississippi Anticline area) have been documented. These observations are consistent with our new isotopic and fluid inclusion data, which suggests lower thermal conditions compared with adjacent areas. Highly fluorescent hydrocarbon fluid inclusions are present in fracture-filling calcite cement. API values average around 40°, and GC–MS analysis of a limited volume of decrepitates suggests a Devonian or older marine black shale source rock.

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