摘要
近年来海域水合物的实际钻探结果显示,当富生烃凹陷具有适宜的水深、温度、压力等地质条件时,浅部沉积物往往是水合物的潜在富集区域。尽管富生烃凹陷具有优异的生排烃潜力,热成因气较为充足,为水合物的形成提供充足的气体来源,但实际调查表明浅部地层中水合物的气体组分和碳同位素更多地表现为微生物成因气或混合成因气的特征,并没有深部热成因气形成的天然气水合物广泛发育的特征。2007年和2015年在我国南海北部陆坡神狐海域白云凹陷进行了两次水合物钻探,与2007年获取的具有微生物成因特征或热成因气贡献非常小的混合成因水合物相比,在2015年钻探中,有钻孔站位的烃类组分测定揭示了较高含量的乙烷和丙烷,从而直接证实了该站位的水合物气源主要是热成因气。白云凹陷内油气钻井地球化学测试分析显示,虽然深部烃类气体受东沙运动的影响存在大规模的逃逸和散失,但部分热成因气仍然可以通过断层、底辟和气烟囱等垂向通道运移至浅部地层之中形成天然气水合物。通过两个钻探区"深部烃源层系—中部运移通道—浅部水合物矿体"三者的垂向耦合对水合物成藏的控制研究,指出通道类型和运移效能的差异可能是导致同一富生烃凹陷背景下的两个水合物矿藏存在差异的主要原因。对于2007年钻探区而言,在深部热成因气沿着效能较低的通道发生自下而上的长距离运移过程中,可能会发生气体组分分异和碳同位素分馏等作用,导致气体的地球化学更多地表现为"微生物成因气"的特征。而在2015年钻探区,由于断裂、底辟构造发育,深部热成因气可以被更有效地输送至水合物成藏沉积地层中,气体组分和同位素没有发生显著的变化。
The recent drilling results of marine gas hydrate reveal that the shallow sediments in a hydrocarbon rich depression are the potential places for gas hydrate formation when hydrocarbon rich depression is of preferential temperature,pressure and other conditions for hydrate accumulation.However,our investigation found that the gas compositions and carbon isotope of hydrates in shallow strata show biogenic or mixed bio-thermogenic characteristics,but no thermogenic,although hydrocarbon-rich depressions are of excellent potential of hydrocarbon generation and discharge and enough supply of thermogenic gas,both of which can provide sufficient gases for hydrate formation.Two gas hydrate expeditions(GMGS 01&03)were carried out in the Baiyun Depression,South China Sea(SCS),as organized by Guangzhou Marine Geological Survey in 2007 and 2015,respectively.Compared with the results of no or minor contribution of thermogenic gas to hydrate formation at the drilling sites in 2007,higher contents of ethane and propane(up to 5%)in hydrates were detected at the sites drilled in 2015,providing direct evidence that deep thermogenic gas was a significant source for shallow hydrate formation.Geochemical results of industrial boreholes in the Baiyun depression indicate that part of thermogenic gas would migrate from hydrocarbon source rocks through faults,diapirs and gas chimney to shallow strata to form natural gas hydrate,although deep hydrocarbon gas escaped obviously from the reservoir due to the Dongsha movement in the late Miocene.In this paper we discussed the factors associated with hydrate formation,based on the model of vertical coupling relation of "deep hydrocarbon source rockinterconnecting migration channel-shallow gas hydrate formation"in the two Shenhu hydrate drilling areas,and it is found that the type and permeability of gas migrating path might be the reason for the difference in gas source in the two areas.For the 2007 drilling area,long migration of deep thermogenic gas along low energy channels might cause differentiation of gas compositions and fractionation of carbon isotope,resulting in that the gas shows more "biogenic gas"features.2015 drilling result found that through well developed faulting and diapir structures,the deep gases can be transported efficiently to shallow sediment layers in which gas hydrate formed,without oblivious change in chemical and isotopic composition.
引文
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