Origin and migration model of natural gas in L gas field, eastern slope of Yinggehai Sag, China

Based on the chemical and stable carbon isotopic composition of natural gas and light hydrocarbons, along with regional geological data, the genetic type, origin and migration of natural gases in the L lithologic gas field, the eastern slope of Yinggehai Sag were investigated. The results show that these gases have a considerable variation in chemical composition, with 33.6%-91.5% hydrocarbon, 0.5%-62.2% CO2, and dryness coefficients ranging from 0.94 to 0.99. The alkane gases are characterized by delta C-13(1) values of -40.71 parts per thousand - -27.40 parts per thousand, delta C-13(2) values of -27.27 parts per thousand - -20.26 parts per thousand, and the isoparaffin contents accounting for 55%-73% of the total C-5-C-7 light hydrocarbons. These data indicate that the natural gases belong to the coal-type gas and are mainly derived from the Miocene terrigenous organic-rich source rocks. When the CO2 contents are greater than 10%, the delta C-13(CO2) values are -9.04 parts per thousand to -0.95 parts per thousand and the associated helium has a He-3/He-4 value of 7.78x10(-8), suggesting that the CO2 here is crustal origin and inorganic and mainly sourced from the thermal decomposition of calcareous mudstone and carbonate in deep strata. The gas migrated in three ways, i.e., migration of gas from the Miocene source rock to the reservoirs nearby; vertical migration of highly mature gas from deeper Meishan and Sanya Formations source rock through concealed faults; and lateral migration along permeable sandbodies. The relatively large pressure difference between the source and reservoir is the key driving force for the vertical and lateral migration of gas. Short-distance migration and effective source - reservoir match control the gas distribution.