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The molecular and carbon and hydrogen isotope compositions associated with the aerobic consumption of methane (C l), ethane (C 2) and propane (C 3) were determined from incubations of marine sediments collected from the seafloor surface of the gas- and oil-bearing Bohai Bay in eastern China. The fresh sediment samples were incubated aerobically at 28 °C and treated with three types of hydrocarbon gas mixtures as follows: mixture I (C 1 :C 2 :C 3 = 100:0:0), II (C 1 :C 2 :C 3 = 99.00:0.70:0.30) and III (C 1 :C 2 :C 3 = 79.96:10.04:10.00), corresponding to incubations i, ii and iii, respectively. All the hydrocarbons are of thermogenic origin, except C 1 (microbial) in incubation ii. The oxidation of C 1 -C 3 was observed in all three treatments, with variations in carbon isotope fractionation factors (ε C) of C 1 and average values of −10.5 ± 0.1, −5.7 ± 1.6, and −10.9 ± 2.9, respectively. The hydrogen isotope fractionation factors (ε H) of C 1 in the three incubations also display variation, with average values of −118.4 ± 3.0, −68.3 ± 15.0 and −56.3 ± 8.4, respectively. Experiments with thermogenic C 1 display greater carbon fractionation than those with microbial C 1. The ε C and ε H values for C 2 and C 3 were obtained only in incubation iii. Compared to C 1 , C 2 and C 3 exhibit weaker carbon isotope fractionation but stronger hydrogen isotope fractionation. The fractionation factors presented in this study can be applied to estimate the extent of C 1 -C 3 oxidation in natural environments, especially at seafloor and terrestrial seeps where aerobic microbes are commonly present. Our study has important implications in oil and gas exploration, with an alert for exercising caution when using the molecular and isotopic characteristics of hydrocarbon gases to evaluate the gas sources because these are susceptible to aerobic microbial oxidation. • Experimental evaluation of aerobic oxidation of methane (C l), ethane (C 2), propane (C 3). • C 1 -C 3 display different hydrogen and carbon isotope fractionation patterns. • Aerobic microbial oxidation can erase genetic information of hydrocarbon gas. [ABSTRACT FROM AUTHOR]

According to the preliminary geological data of gas hydrate bearing-sediments (GHBS) at site GMGS3-W19 in the third Chinese expedition to drill gas hydrates in 2015, a production model using three different recovery pressures was established to assess the production feasibility from both production potential and geomechanical response. The simulation results show that for this special Class 1 deposit, it is a little hard for gas production rate to reach the commercial extraction rate because the degree of hydrate dissociation is limited due to the low reservoir permeability and the permeable burdens. However, the free gas accumulating in the lower part of the GHBS can significantly increase gas-to-water ratio. It also generates many secondary hydrates in the GHBS at the same time. Decreasing the well pressure can be beneficial to gas recovery, but the recovery increase is not obvious. In term of geomechanical response of the reservoir during the gas recovery, the permeable burdens are conducive to reduction of the sediment deformation, though they don't facilitate the gas recovery rate. In addition, significant stress concentration is observed in the upper and lower edges of GHBS around the borehole during depressurization because of high pressure gradient, and the greater the well pressure drop, the more obvious the phenomenon. Yield failures and sand production easily take place in the edges. Therefore, in order to achieve the purpose of safe, efficient and long-term gas production, a balance between the production pressure and reservoir stability should be reached at the hydrate site. The production pressure difference and sand production must be carefully controlled and the high stress concentration zones need strengthening or sand control treatment during gas production. Besides, the sensitivity analyses show that the hydrate saturation heterogeneity can affect the production potential and geomechanical response to some extent, especially the water extraction rate and the effective stress distribution and evolution. Increasing GHBS and its underlying free gas formation permeabilities can enhance the gas production potential, but it probably introduces geomechanical risks to gas recovery operations. [ABSTRACT FROM AUTHOR]