PRELIMINARY STUDY ON GEOCHEMICAL SIGNIFICATION OF TETRACYCLIC TERANES IN SEDIMENTS AND CRUDE OILS

Abstract: 17,21-secohopanes(C₍₂₄₎-C₍₂₇₎) and 8,14-secohopanes(C₂₇-C₃₀) are two types of tetracyclic terpanes which were found recently. They have also been found when we studied the biomarker hydrocarbons of Mesozoic, Cenozoic crude oils and source rocks in Shaanganning Basin( Shaanxi-Gansu-Ningxia ) and Baise Basin(Guangxi Province) by using GC-MS. This paper deals with the geochemical signification of these tetracyclic terpanes by combining the geochemical analytic data in terrestrial basins mentioned above. It is very interesting for the distribution of 17,21-secohopanes(C₂₄-C₂₇) series in the Early Tertiary source rocks in Baise Basin, Guangzi Province. They only appreaed in coals and source rocks which is immature and the type of organic matter belonging to Ⅳ and Ⅱ, but they are absent in I type's and Ⅱ_A type's source rocks. It is suggested that the distribution phenomenon is related to the microbial degradation which happened in the special period of Tertiary in this basin by the synthetic analysis of several geochemical data. These tetracyclic terpanes are passibled the products of biodegradation by broken E circle of precusor hopenes, and successively reducing into relative alkanes by geochemical action in the early diagenesis. This type of tetracyclic terpanes play an important marker part in geochemical charater in the oil/rock correlation of Baise Basin. The origin of 8,14-secohopane(C₂₇-C₃₀) series is mainly related to heating action, they were derived from hopane type of pentacyclic triterpanes by broken C circle in latter It is suggested that the richness of 8,14-secohopanes can be used to reflect the maturation of sedimentary organic matter from our researching results in Shaanganning basin. We have come to the conclusion that C₃₀-secohopane/4-methyl-C₂₀ sterane can be used as a new molecular parameter reflecting the mature level of sedimentary organic matter according to our researching results, the ratio increasing with the mature level of organic matters. Moreover, the M/E414 mass chromatogram reflecting the richness of C₃₀-secohopane and 4-methyl-C₂₉ sterane can also be used as the "fingerpriter fig." which can be used for oil/rock correlation. It is suggested that vast appearance of 8,14-secohopanes may be related with higher heat action, because it's absent for 8,14-secohopanes in the Early Tertiary crude oils and source rocks of Baise Basin, but there are more contents of 8,14-secohopanes in the Mesozoic crude oils and source rocks which had higher mature level in Shaanganning Basin. Therefore, 8,14-secohopane series perhaps is more suitable for studying the mature level of sedimentary organic matter which is relative older in geological age.