GAMMACERANE AS A BIOMARKER IN TERRESTRIAL CRUDE OIL AND SOURCE ROCKS

Gammacerane in crude oil and source rocks belongs to a pantacyclic triterpane series and occurs at the eluted peak between 17β(H), 21a(H)-30-homohopane 20R and 17α(H), 2lβ(H)-30-homomortane on m/e 191 mass chromatogram. It has not been found in marine crude oil and source rocks. This compound is one of the terrestrial markers of sedimentary facies. The study of crude oil and source rocks from terrestrial sediment in our country shows that gammacerane is distributed in the terrestrial crude oil and source rocks with different abundance, depending on the corresponding sedimentary environment, the type and maturity of organic matters, etc. Usually, a great abundance of gammacerane presents in the samples of a sapropelic kerogen or humic-sapropelic kerogen, existing in semideep or deep water sediments, especially in shales deposited in the salt lakes, and the less presents in the shallow water sediments of lake facies and swamp facies in sapropelic-humic kerogen. In the case of the same type of kerogen, the abundance of gammacerane decreases with the increase of the function of thermodynamics and the maturation of organic matters as well as the buried depth of crude oil and sedimentary rocks. According to the distribution of gammacerane in the sediments and the recent tetrahymanol as a precursor of gammacerane in the recent plants and animals, the author infers that the genesis of gammacerane is as follows: 1 ) It originates from the protozoan living in the terrestrial lake. 2) It originates from higher plants such as fern. The tetrahymanol in these plants is brought into the lakes by the surface water and groundwater, and deposited with other matters, The tetrahymanol would be more abundant with the increase of concentration of salt in the lake water, because aquatic algal, from which hopanoid mainly derived, decreased and the preservation of'the organic matters in the water or sediments improved. This is the reason for the highest concentration of gammacerane in the salt lake sediments. 3 ) It originates from the composition of some terpenoid in the continental plants. The author considers that gammacerane is a better indicator in petroleum geochemistry. It can be used to indicate sedimentary environment, type of kerogen, maturation of organic matters, and can be applied to indicating correlation between source rocks and crude oil with specific geological and geochemical conditions.