三水盆地气态烃的地球化学特征和成因

GEOCHEMICAL CHARACTERISTICS AND ORIGIN OF GASEOUS HYDROCARBON IN SANSHUI BASIN

杜建国 徐永昌

Du Jianguo Xu Yongchang

本文根据三水盆地天然气的组分和碳、氢同位素特征,探讨了气态烃的成因。下第三系(土布)心组含500多米的生油岩,干酪根以Ⅱ型为主。富烃天然气含不等量的 N、He、CO,不含 HS;干湿指数变化范围为2.1—196;iC/nC值为0.42—0.87;δC和δD值分别为-61.0至-44.6‰和-237至-174‰,δC值在-33.4‰到-30.2‰之间。烃类气体属热解成因的油型气与次生生物成因气。

The origin of gaseous hydrocarbon is discussed on the basis of geochemical featuresof natural gas in Sanshui Basin,Guangdong Province.The Cretaceous-Eogene Systems inthe basin have a thickness greater than 7600m.The Buxin Formation(Eb)contains morethan 500m thick source rocks in which kerogen is mainly types Ⅱ and Ⅲ.Vitrinite reflec-tivity(R)is between 1.2% and 1.9%.Eleven samples of natural gas have been analyzedfor gas components and isotopic composition of carbon and hydrogen.The results showthat geochemical characteristics of ignitable gases are:1.hydrocarbon-rich natural gascontains N,He,O,but not Hs;2.the value of dry-wet indices (C/ (C+C))has the range of 2.1-196 which can be easily separated into two groups:>18 and <8;3. iC/nC ratios Change between 0.42 and 0.87;4.the value of δC and δC(PDB) ranges from -61‰ to -44‰ and -33‰ to -30‰ respectively;5.δDC value(SMOW) has the range of -237‰ to -174‰ which reflects the genetic process ofmethane and salinity of sedimentary water.Dry gas found in the highest part of structureis distributed in the area in which oil field water is of NaHCO type with a lower intensityof mineralization (25-50 g/l).The carbon and hydrogen isotope composition(δC=-60‰ and δD=-236‰--174‰) of methane in dry gas are identical with those of bio-genetical methan.Wet gas (C/ (C+C)<8)in company with crude oil is rich inC(δC=-48‰--44‰,δC=-33‰--30‰).The difference between δC and δCchanges from 14.7‰ to 15.2‰.The isotopic compositions of wet gas conform tothose of oil-type gas from Zhongyuan and Sichuan Oilfields.Wet gas is found in the areathat has NaSO type of oilfield water with a higher intensity of mineralization (50—100g/1).All of the geochemical evidences confirm that dry gas in Sanshui Basin is generatedby microbe altering organic matter (including oil and gas) after the period of oil genera-tion,wet gas originated from Eb source rocks.

10.11743/ogg19900311