--> --> Depositional Model for Shale Gas Deposits of the Besa River Formation in Liard Basin, British Columbia

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Depositional Model for Shale Gas Deposits of the Besa River Formation in Liard Basin, British Columbia

Abstract

The Middle Devonian to Middle Mississippian Besa River Fm is a predominantly shale succession that was deposited in Liard Basin and now outcrops along the eastern slopes of the Rocky Mountains in northeast British Columbia. It contains the deep-water basinal equivalents to carbonate and shale developed to the east in the subsurface of the Western Canada Sedimentary Basin. In the outcrop belt of westernmost Liard Basin, the Besa River Fm is about 300 m thick whereas the correlative carbonate and shale are over 2000 m thick near the Bovie structure. In central Liard Basin, the upper Besa River Fm contains a gas-saturated, silica- and organic-rich section equivalent to the Exshaw Fm and an underlying unit informally referred to as the Patry member. The Exshaw and Patry succession has recently been the focus of shale-gas exploration, with the initial development well producing over 6.5 BCF (184 × 106m3) of natural gas since 2011. Re-Os systematics of shale in upper part of the organic-rich zone, together with U-Pb dating of an associated thin tuff define an age at the Devono-Mississippian boundary and corroborates correlation with the Exshaw Fm. The underlying organic-rich Patry sequence is developed in central and western Liard Basin and is considerably thicker along its eastern limit at the Kotcho shelf edge, where it defines a north to north-easterly trending section that is over 200 m thick. The proposed depositional model for this succession incorporates a regional transgression near the end of Late Devonian Kotcho deposition that is accompanied by anoxic bottom waters. Maximum flooding occurred during deposition of the lower Exshaw. It is postulated that a west-side-down normal fault, similar in orientation to the Bovie structure, created the accommodation space for accumulation of a thicker section of organic-rich rocks. This model would suggest that the extensional tectonics related to the Bovie structure were initiated during the Late Devonian. The Exshaw Fm thickens along the northwestern margin of the Peace River Embayment (PRE), an extensional feature coeval with the Carboniferous Liard Basin. Although a thicker section of the Exhsaw Fm was tested along the northwest margin of the PRE, this unit has not been penetrated within the westernmost part of the PRE, particularly in the Fort St. John Graben system. Application of the model from Liard Basin would suggest the potential for a much thicker Exshaw section may exist in that area.