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Gas of Biodegradation Origin and Their Pooling Characteristics in Shallow Reservoirs of Langgu Sag


Biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms via the way of CO2 reduction or acetate formation. It is estimated that biogenic gas accounts for approximately 20% of the world's natural gas resource. Despite the fact that C6-C9 and higher normal paraffins can be removed, bacterial alteration of subsurface reservoired hydrocarbon accumulations can extend into the range of the C1-C4 gas hydrocarbon, preferentially attacking the wet gas components in most instance. In recent years, a lot of small gas accumulations have been discovered in the shallow burial depths in the Langgu sag, located in the north part of Jizhong Depression of Bohai bay basin, especially in the areas of Guan and Liuquan. These gas reservoirs have the features of biogenetic gas such as high dryness coefficient(C1/ΣC1-5>0.95) and light stable carbon isotope (δ13C1 ranging from-50‰~-60‰). Therefore, much attention has been paid to the biogenic gas potential in this area and the study of their genetic type and occurrence. Based on chemical and stable carbon isotopic compositions, stable hydrogen isotope of methane of natural gas, however, it can be concluded comprehensively that the discovered biogenic gas reservoirs are dominated by secondary biologic modification gas reservoirs with mixed secondary biogenic gas from biodegradation of thermal oil/gas and residual thermal gas. The following evidences were obtained from detailed geochemical character analysis: the value of methane carbon isotopic composition is larger than the typical primary buried biogas of Qaidam basin, resulted from the mixed secondary biogenic gas and residual thermal gas; the heavy hydrocarbon gas content is high (∑C2+>0.05), especially the ethane, displaying the trace of thermogenic gas; iso-paraffin increases significantly due to biodegradation, which leads to the increase of i/n-C4; the carbon isotope sequence of many gas samples occur partial reverse in propane, indicating biodegradation trace; the δ13C1 and δD of methane to part samples fall in the region of CO2 reduction, a considerable part samples extend to transition zone, some even in the thermogenic region, displaying that secondary gas is formed on the background of the thermal genetic gas. Overall, biodegradation degree, preservation conditions, replenishment of thermal gas at later stage are the factors that complicate the characteristics of biodegradation gas.