The Sable and Abenaki Subbasins, within the Scotian Basin, have accumulated Mesozoic and Cenozoic rocks in excess of 12,000 m (40,000 ft). No major unconformity is recognized. The present temperatures therefore are believed to be the maximum, and threshold levels required for the generation of hydrocarbons appear to have been reached in the Cretaceous.
The organic carbon content is sufficient (1 to 3 percent) to have generated significant amounts of hydrocarbons, but the organic-matter type (dominantly terrestrial) and the time/temperature relationship appears responsible for the absence of good source rocks.
Organic-matter coloration and type, light gas analyses, and C15+ extract data all suggest the Sable and Abenaki subbasins are thermally immature. This has created a thick marginally mature zone above the mature level that is characterized by shows of gas, condensate, and light oils, a situation that does not conform to the usual hydrocarbon sequence of immature dry gas, mature liquids followed by over mature condensate and eventually dry gas again. Thus the occurrence of condensates, in particular, above the mature zone, is in contrast to the normal sequence of hydrocarbon occurrence observed in mature petroleum provinces.
The ability to predict source rock potential and the type of hydrocarbons that might occur is critical to the assessment of the hydrocarbon potential of sedimentary basins. This has encouraged the rapid development and use of hydrocarbon geochemistry over the past decade. In 1972, the Canada Dept. of Energy, Mines and Resources started analyzing samples for this purpose from wells drilled in the Canadian eastern offshore areas. One of the first areas to be studied was the Scotian Shelf, where 64 wells have been drilled to date.
This paper discusses the results and interpretation of analyses from nine wells: Union et ale Shell Ojibwa E-07 (43°46'22.44"N, 61°46'14.42"W); Shell Demascota G-32 (43°41'27.18"N, 60°49'54"W); Mobil Tetco Cohasset D-42 (43°51'06.52"N, 60°37' 13.89"W); Texaco Shell Intrepid L-80 (43°49' 35.78"N, 59°56' 43.52"W); Mobil Tetco Sable Island 4-H-58 (43 °57'27.2"N, 60°07'37.8"W); Mobil Tetco Texaco Citnalta I-59 (44°08'42.58"N, 59°37'32.11"W)i Mobil Tetco Bluenose G-47 (44°06' 20.5"N, 59°21' 27.5"W), Shell Sauk A-57 (44°16'05.70"N, 58°37'44.41"W), and Mobil Texaco Sachem D-76 (44° 35'12"N, 57°41 '58"W).
We have knowledge of four previous papers relating to the hydrocarbon geochemistry of the Scotian Shelf. Robbins and Rhodehamel11 from geothermal-gradient data suggested that the top of the "oil window" should occur in-this area at 2,400m (7,870 ft) and the maximum temperature of generation would be reached at about 6,080 m (19,942 ft).
Rashid and McAlaryl0 in a study of an individual salt dome concluded that the genesis of hydrocarbons can occur in shallow horizons as a result of anomalous heat flow effecting overlying sediments.
Cassou et al.4 discussed the maturation of organic matter in relation to subsidence curves generated from clay mineral diagenesis, temperature gradients, and reflectance characteristics in offshore wells drilled in eastern Canada. The paper does not deal with source rock potential.
