--> ABSTRACT: An Organic Geochemical Assessment of New Shallow Overpressured Gas Play in Nova Scotia, Canada, by P. G. McMahon and D. C. Carter; #91022 (1989)

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An Organic Geochemical Assessment of New Shallow Overpressured Gas Play in Nova Scotia, Canada

P. G. McMahon, D. C. Carter

A recent diamond drilling program designed to examine a Carboniferous evaporite sequence in Nova Scotia encountered a significant over-pressured flow of natural gas which resulted in the termination of the drill hole at 683.05 m (2,241 ft). Prior to this occurrence, the Shubenacadie basin has been virtually unexplored in terms of hydrocarbon potential.

A source rock assessment revealed two possible origins for the gas: (1) lacustrine shales and mudstones of the initial basin fill and (2) carbonates found within a cyclic marine evaporite sequence. Although analytical data obtained from nearby basins have shown the lacustrine shales and mudstones to be of source rock quality, direct evidence from within the Shubenacadie basin has not been obtained due to the lack of exploration at the suspected depth of this formation (~ 1,200 m or ~ 3,900 ft). Regional source rocks studies indicate that the distribution of the organic-rich lacustrine facies is localized by paleotopographic control, and that they may not be present in the Shubenacadie basin. The carbonates are favored as the source beds due to their high total organic carbon content, organic matter type, evidence of formation fracturing from possible episodic "explosive" hydrocarbon expulsion, and structural/migration relationships. These factors, in conjunction with Rock-Eval pyrolysis results and the late mature to postmature organic thermal evolutionary state, are indicative of past oil expulsion. Thermal cracking of this oil and the existence of a very efficient evaporite seal would allow for a long-term maintenance of reservoired gas and the enigma of a gas associated with a nearby postmature depleted source rock.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.