The Nechako Basin: New Insights into a Cordilleran Intermontane Basin

Ferri, Filippo¹, Janet Riddell¹, Lavern Stasiuk², Arthur Sweet², Paul O'Sullivan³ (1) BC Ministry of Energy, Mines and Petroleum Resources, Victoria, BC (2) Geological Survey of Canada, Calgary, AB (3) Apatite to Zircon Inc, Viola, ID

The Nechako Basin, one of several large Intermontane Basins found along the Canadian Cordillera, covers some 70,000 km2 and contains over 3000 m of Mesozoic and Cenozoic clastics. These represent overlap successions deposited after accretion of volcano-sedimentary terranes along the western edge of North America. Jurassic and Cretaceous sequences are composed of marine and fluvial-deltaic clastics whereas Cenozoic sediments represent fluvial and possibly lacustrine environments. Mesozoic stratigraphy is locally dominated by volcanics and succeeded by widespread Eocene and Miocene age flood basalts.

These rocks underwent mid-Cretaceous compression and Late Cretaceous to Paleogene transcurrent faulting. Strike-slip structures locally produced extensional basins filled with Cenozoic clastics.

Eight exploratory wells were drilled in the southern part of basin between 1960 and 1985. Correlation between wells is difficult due to facies changes, faulting and lack of stratigraphic control. Live oil and gas shows were encountered in several wells and porosity ranges from poor to good (15%). Initial subsurface source rock analysis suggests poor to moderate TOC values and a dominance of Type III kerogens.

New source rock analysis of correlative surface sections has recognized organic-rich Middle Jurassic horizons not encountered in the subsurface. New subsurface vitrinite reflectance data indicates oil to dry gas maturation levels. Characterization of oil inclusions in core from several of the wells is being attempted in hopes of understanding their origin. Current and new age data suggests that subsurface sequences are Cretaceous to Cenozoic in age and initial apatite fission track data indicates that basin uplift began in the Eocene.