Development And Environmental Effects Of Shallow Gas In The Yangtze Subaqueous Delta

River deltas are hotspots of carbon burial.Meanwhile,shallow gas（SG）is prevalent in these regions due to high organic carbon burial flux.SG is a common disaster-inducing factor,with its major component methane（CH₄）being a potent greenhouse gas.By far,the research on SG in coastal sediments mainly comes from some semi-enclosed water environments,while SG in open shelf deposits dominated by large rivers remains poorly understood.Due to the enhancement of anthropogenic activities in drainage basins,riverine sediment supplies have diminished in recent decades.Thus,the past decades have witnessed the transformation of many deltas from constructive to destructive phases.The impact of seabed erosion caused by sediment starvation on gas-charged deposits is unclear.Therefore,it is urgent to carry out systematic research on SG in the depositional systems of large rivers.The Yangtze River is deeply affected by anthropogenic impacts.Therefore,the Yangtze Delta is an ideal area to study SG in larger river delta systems and the influence of human activity on it.Based on a mass of geophysical data from the Yangtze subaqueous delta（YSD）and its distal mud（DM）,together with collected borehole data and historical nautical charts,the objectives of this study were to 1)investigate the characteristics and main controlling factors of the distribution of SG,and based on this,estimate methane reserves in gassy area;2)reveal the features of SG migration and its relationship with sedimentary characteristics;and 3)discuss the response of shallow gas-charged deposits to erosion.The main findings of this study were summarized as follows:1.The distribution of SG was identified according to acoustic anomalies in seismic profiles.SG is widely distributed in the mud deposits of highstand systems tract（HST）with thickness greater than 20 m.The relationship between the gas front and seismic units indicates that it tends to be parallel to the adjacent seismic reflector locally,but traverses multiple seismic units regionally.The variation of gas front depth（GFD）is“deep in the north and shallow in the south”and“shallow nearshore and deep offshore”in the YSD and DM,respectively.2.The distribution characteristics of SG and the collected total organic carbon content data indicate that SG in this area is biogenic in origin.Comparison between the distribution of SG and the thickness of HST deposits indicates that a certain thickness of HST deposits is an important prerequisite for the formation of SG.Comprehensive analysis of the sedimentary characteristics revealed by seismic profiles and the distribution characteristics of GFD shows that GFD is mainly controlled by the sulfate-methane transition（SMT）depth determined by the sedimentation rate when the sedimentary environment is relatively stable.While the GFD is also controlled by the sedimentary history in areas with great changes in the sedimentary environment and complex sedimentary history.Based on the distribution characteristics of SG,a conservative estimate of the total methane reserves of 4.35×10¹¹ mol,of which dissolved methane accounts for the majority,was obtained in the gassy area in the YSD.3.The characteristics and distribution of gas chimneys,gas plumes,and pockmarks reflecting the migration of SG in different regions are identified from geophysical data.It shows that the intensity of SG migration decreases seaward,with little active SG migration at the outer margin of the gassy area.Moreover,the migration mode of SG differs greatly between the south and the north of the YSD.Further analysis of the distribution characteristics of SG shows that the cross-shore differences in the intensity of SG migration are mainly attributed to the different rates of methanogenesis due to distinct ages of organic matter in gas-charged sediments between nearshore and offshore areas.Previous studies have revealed that sediment is finer in the south and coarse in the north in the YSD.Comparing it with the distribution of SG migration activities shows that the difference in the permeability of the sedimentary cap bed determined by the grain size of sediment results in different modes of SG migration in the south and north of the YSD.4.Porewater diffusion is an important pathway by which methane is lost from the reservoir.Based on the GFD distribution,the diffusive flux of CH₄ in the gassy area in the YSD was calculated using the free gas depth（FGD）model which is developed based on Fick’s first law.The comparison between the diffusive flux of CH₄ and the thickness of HST deposits revealed an imbalance between methane production and diffusion,which indicates that methane loss from the sediments by bubble ebullition accounts for a considerable portion of methane loss in the nearshore area.This finding agrees well with the contribution of SG to dissolved methane in seawater reflected by the distribution of dissolved methane concentration.Based on the results of the FGD model,it is estimated that the total amount of methane lost by bubble ebullition in the YSD is no less than 1.85×10⁷ mol a^-1,which is of the same order of magnitude as the riverine input,indicating that methane released from sediments is an important source of methane in the water of estuary-delta in this region.5.The bathymetric data and seismic profiles revealed a prominent erosional belt in the nearshore area of the YSD,with greater erosion in the south and slighter erosion in the north.The differences in the characteristics of SG migration between erosion area and the non-erosion area in the south show that the sealing capacity of sedimentary cap bed to SG is undermined by erosion,making it easier for free methane gas in the sediments to migrate through the overlying sediment layer.By analyzing the adjustment process of pore water methane-sulfate profile,it is speculated that erosion of the delta will lead to the increase of methane consumption by sulfate.The results of this dissertation show that the development characteristics of SG in larger river delta system are closely related to the history of sedimentary evolution,and the intensity and mode of SG migration are affected by the sedimentary characteristics.In addition,this study demonstrated SG emission is an important source of methane in the water of the estuary-delta in this region,and revealed that erosion of the delta has increased methane emissions from sediments and will increase methane consumption by sulfate.These findings have implications for other river deltas around the world that are similar to the Yangtze Delta.