Study On Natural Gas Origin And Effective Source Rock Of Nanpu Depression

Nanpu Depression is a representative hydrocarbon-rich depression in Bohai Bay Basin developed in Jurassic and shaped in Neogene. In recent years, exploration for beach structure of Nanpu Depression has made great progress, which found Nanpu-1, 2, 3, 4, 5 hydrocarbon-bearing structural belts with four sets of gas-bearing strata developed, i.e., Ordovician, Shahejie Formation, Dongying Formation, Guantao-Minghuazhen Formation. The origin of natural gas and distribution of effective source rocks have restricted further oil and gas exploration for beach structural belts, therefore, study on natural gas origin and effective source rock of Nanpu Depression is significant for guidance to the next-step oil and gas exploration.At present, high-productivity gas wells are mostly distributed in No.1 and No.2 structural belts, with major pay zones as Ordovician and Dongying Formation. The dry coefficient of natural gas is less than 0.95. The content of methane is 70%~90%, The content of ethane is 5%~10%, The content of Carbon dioxide and N2 is less than 2%. Nanpu Depression of gas is oil associated gas.Multiple strata of the beach structure in Nanpu Depression contain gas, carbon isotope of methane is -55‰~-30‰,δ13C1 value is -55‰~-30‰,δ13C1 is -30‰~-24‰, the equation of regression analyse that Ro is less 1.70. with organic matters of most amorphous solids and a few algainites. Natural gas is oil associated gas.Analysis on a large amount of mudstone samples proves that three effective source rocks were developed in Nanpu Depression, i.e., mudstones of Sha-3, Sha-1 and Dong-3 Members. Kerogen isⅡ, with TOC higher than 2%, Ro is 0.8%~1.75%, that can produce a part of gas. We analysised the biomarker of oil and sourse rock which came from the same reservoir, that comes to the conclusion of sha-3 member and sha-1 member were the main sourse rock of Nanpu Depression, the gas is accompanying gas with crude oil.