重質・超重質油鉱床の成因と分布状況，および原始埋蔵量と可採量

抄録

Most heavy and extra-heavy oils originate from conventional oils which have been subsequently degraded in shallow and porous reservoirs by one or several of the following processes: biodegradation, water washing, loss of volatiles, and inorganic oxidation. These processes result in a decrease of the light ends of the crude oil, and also of the alkanes and low molecular weight alkylbenzenes, and an increase of the more resistant polyaromatics, resins, and asphaltenes. Furthermore, at certain conditions, sulfur extracted from sulfate by anaerobic bacteria may react with hydrocarbons to increase the sulfur content. The extent of degradation is associated with depth, proximity to aerial contact, and salinity of formation waters. Excellent examples are provided by the two major provinces of heavy and extra-heavy oils, i.e., Eastern Venezuela and Western Canada. These provinces are situated in foreland-type basins. The basin frequently offer a large volume of mature source rocks. Then, the existence of a wide homocline slope, with a tremendous drainage area, and the occurrence of conductors, such as sandstone layers formed in a paleodelta system or major unconformity surface, favor a long-distance migration from the deep part to the end part of the basin. A wide drainage area is an essential condition for gathering a huge amount of oil into a single accumulation. Finally, degradation itself seems to be an important factor for trapping very large quantities of oil. The R/P of recoverable conventional oil is estimated as c.a. 64.5 years from the recoverable reserves and the additional quantities of conventional oil, which were estimated in 2002 by JPDA. Oil-in-place of heavy and extra-heavy oils are 2,044.3 billion barrels and 4,145.4 billion barrels, respectively, estimated by UNDP project in 1999. In addition, heavy oil of 717 billion barrels and extra-heavy oil of 724 billion barrels are considered as potential recovery. These figures correspond to the R/P c.a. 24.5 and c.a. 24.7 years, respectively. Therefore, total R/P of recoverable oil resource, conventional oil plus unconventional oil, exceeds 100 years.