Organic Facies and Petroleum Potential of Eastern North American Margin: ABSTRACT

Abstract

Sufficient data now exist to permit reconstruction of the organic facies for Late Jurassic and middle Cretaceous times when the potential for source rock deposition was greatest on the eastern margin of North America. Three distinct and one mixed organic facies have been mapped for these 2 geologic intervals: facies B (oil-prone) amorphous organic material plus alginite and exinite deposited under marine anoxic conditions, facies C (gas-prone) terrestrial plant debris deposited in mildly oxic environments, facies B-C (waxy oils and gas condensate) mixed facies, and facies D (nonsource) degraded and/or recycled organic material. Distribution of the Late Jurassic organic facies was greatly influenced by the relatively arid subtropical climate and the shelf-edge carbonate End_Page 289------------------------------ bank and reef that existed along most of the margin. Facies C and D are common on the shelf, C being particularly significant on and seaward of Late Jurassic deltas. Two submarine-fan complexes formed off present Nova Scotia and the Middle Atlantic states. Seasonal upwelling plus fan development produced mixed B-C and C facies along the margin. Facies D dominated the deep basin. The middle Cretaceous organic facies reflect profoundly changed conditions from those that existed in the Late Jurassic. The continent had a warm, humid, temperate-tropical climate producing abundant vegetation. Ancestral rivers carried large volumes of detrital plant material to the sea. Thus, facies C is virtually ubiquitous except for the inner shelf and isolated parts of the ocean basin. Active reefs and carbonate-bank development at the shelf break was limited to the low latitudes. In the early Cenomanian, a major transgression substantially broadened the shelf and upwelling probably occurred on the middle shelf. Facies B-C is postulated to exist on the upper slope, locally, and in the basin where anoxia prevailed. Exploration targets for oil are the buried carbonate bank and reef and related fore-reef talus deposits. Secondary objectives are interreef channels and their seaward extensions, the upper parts of submarine fans. The shelf and margin basins are dominated by facies C source rocks. In the Late Jurassic, however, locally ideal conditions permitted deposition of facies B source rocks. End_of_Article - Last_Page 290------------