ペルシャ湾南部地域における石油集積機構に関する考察 (その1)

石油地質および関連学説の概要

抄録

The Arabian-Zagros sedimentary basin, renowned as a huge oil pool-bearing field, is composed of a large quantity of carbonate strata associated with a considerably limited argillaceous strata.
During the past quarter century, various studies on the mechanisms of hydrocarbon generation and accumulation in the Persian Gulf area have been carried out. As the result, the following two different opinions concerning the oil generation and migration have been raised: A) catagenetic origin and long distance migration, and B) diagenetic origin and short distance migration. However, at present, the opinions and points of these two groups are independent each other and there are no sufficient discussions between them.
The basic conception of the opinion (A) is that a possible source sediment should be placed to a deep burial depth through a sufficient geological time to generate petroleum via so-called “Kerogen”. This is a conception derived mostly from the studies on the argillaceous rocks. On the other hand, if we pay special attention to the organic and inorganic characteristics of carbonate rocks, particularly its efficient capability of hydrocarbon generation and the very likely possibility of generating hydrocarbons which directly generated from the organic matter of carbonate's own without via “Kerogen stage” as pointed out by TAGUCHI (1989), it is possible to accept the conception of (B) rather than (A).
Based on the such previous studies and the geological evidences obtained from the actual exploration and exploitation in the studied area, conceivable characteristics for the oil production in this area should be summarized as follows. Namely, (1)extremely high volume of carbonate sediments in comparison with argillaceous sediments, (2)a difficulty of large scale vertical migration due to extensive distribution of dense evaporite (anhydrite, in particular) and (3)the existence of many depletion type oil fields which may have been formed by deterioration of reservoir in the down flank area during the course of the structural growth by high saturation of formation water, etc.
Consequently, it is inferred that most of oil in the southern part of the Persian Gulf area might have originated from organic matter of carbonate rocks and accumulated in the favorable structures at the relatively early diagenetic stage through short primary migration pass.
In future, more detailed geochemical studies on the carbonate rocks (especially pure carbonates) will be required to solve the diversities among those opinions mentioned above.
(This abstract covers the contents of this Part 1 and Part 2 which is to follow. The subtitle of Part 2 is-“Noticeable points on the carbonate source rocks and the specific characters of the accumulation mechanism”-).