Abstract:

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(E_1-2b)contains morethan 500m thick source rocks in which kerogen is mainly types Ⅱ and Ⅲ.Vitrinite reflec-tivity(R_o)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 N₂,He,_CO₂,but not H₂s;2.the value of dry-wet indices (C₁/ (C₂+C₃))has the range of 2.1-196 which can be easily separated into two groups:>18 and <8;3. iC₄/nC₄ ratios Change between 0.42 and 0.87;4.the value of δ¹³C₁ and δ¹³C₂(PDB) ranges from -61‰ to -44‰ and -33‰ to -30‰ respectively;5.δDC₁ 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 NaHCO₃ type with a lower intensityof mineralization (25-50 g/l).The carbon and hydrogen isotope composition(δ¹³C=-60‰ and δD=-236‰--174‰) of methane in dry gas are identical with those of bio-genetical methan.Wet gas (C₁/ (C₂+C₃)<8)in company with crude oil is rich in¹³C(δ¹³C₁=-48‰--44‰,δ¹³C₂=-33‰--30‰).The difference between δ¹³C₂ and δ¹³C₁changes 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 Na₂SO₄ 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 E_1-2b source rocks.