The Origin of Shallow Gas in Dongying Depression, Bohai Bay Basin, East China: Implication for Shallow Petroleum Exploration
Shallow gas reservoirs in Dongying depression have largely been encountered in traps along the margin of the depression. However, the origin of these shallow gases is still not well understood. A suite of free gases, solution gases and associated oils from different depths(from 1060 to 3035m) in Dongying depression have been analyzed for the molecular compositions and the isotopic values to compare the features of gases at different depths and study the formation mechanism of the shallow gases. The shallow gases display clear geochemical evidence of alternation by biodegradation, with high dryness (C1/C2+ from 20 to 4785), C2/C3 ratio (from 0.49~19.5), high nitrogen content (from 0.70% to 7.42%), and variable carbon dioxide content (from 0.01% to 11.18%) compared with deep buried gases. The carbon isotopic compositions of wet gas components (C2~C4) in shallow gases are heavier than those in deep, while the carbon isotopic signatures of methane are variably heavier or lighter. The gases contain CO2 with δ13C values from -4‰~11.9%, however these 13C rich CO2 are not from magmatic activity with the evidence of helium isotopic composition. The geochemical characteristics of oils associated with these gases show that these oils also suffered biodegradation at different degree. Relatively high dryness, 13C-enriched CO2 and D-enriched methane provide direct evidence for secondary microbial gas and indicate the generation of biogenic methane through CO2 reduction under anoxic condition. The degree of CO2 conversion controls the carbon isotopic composition of methane. Secondary microbial gas is a by-product of formation of heavy oil by anaerobic biodegradation. These gases are closely associated with heavy oils, occurring as gas caps with, or in separate gas pools above or updip from, the heavy oil pools. The secondary microbial gas has a very important implication for biodegraded heavy oil and thus the undiscovered heavy oil under the discovered shallow gas will be the target in the future exploration.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017