四川盆地高石梯—磨溪地区震旦系—寒武系天然气TSR效应及气源启示

Thermochemical sulphate reduction of Sinian and Cambrian natural gases in the Gaoshiti-Moxi area,Sichuan basin,and its enlightment for gas sources

综合分析四川盆地高石梯-磨溪地区(高-磨地区)震旦系-寒武系天然气、储层沥青及膏盐分布等,发现高-磨地区天然气发生过不同程度硫酸盐热化学还原作用(TSR)反应。主要基于:①天然气中含一定丰度H2S,震旦系灯影组H2S含量为0.6%~3%,寒武系龙王庙组为0.2%~0.8%;其δ^34S值普遍较重(21‰~23‰),为TSR反应产物;②储层沥青S/C原子比介于0.06~0.4之间,远远超过有机质裂解生成沥青中S/C比的最高上限(0.034),峰值甚至超过了TSR反应强烈的川东北普光气田飞仙关组储层沥青的比值(0.06~0.12),为TSR过程无机S加入所致;③四川盆地寒武系底部发育膏盐类沉积,为TSR反应提供了SO4^2-和Mg^2+等物质,灯影组发育富Ca^2+/Mg^2+、贫Na^+/K^+型地层水,证明盐、膏类溶解的普遍性。地层水中相对缺乏SO4^2-,应为TSR反应消耗所致。TSR反应明显氧化乙烷,导致天然气干燥系数增加、δ^13C2变重;TSR反应程度不同造成了龙王庙组和灯影组天然气特征的差异,龙王庙组TSR反应程度相对较弱,天然气甲乙烷碳同位素明显倒转;而灯影组TSR反应程度相对要强,甲乙烷同位素正序分布。考虑TSR效应,恢复原始组成,高-磨地区寒武系-震旦系天然气应有明显的甲烷、乙烷碳同位素倒转现象,这种倒转跟该盆地及世界高-过成熟页岩气特征高度一致,暗示高-磨地区主力气源可能为源岩晚期所成天然气。这一认识可以很好诠释甲烷δ^13C1值较重、普遍低于储层沥青这一为现在主流认识(高-磨地区主体为原油裂解气)所不好解释的现象。对于重新认识天然气成藏聚集规律具有重要意义。

Integrated analysis for natural gases, asphalt and gypsum deposits in the Gaoshiti-Moxi(Gao-Mo) area indicates that natural gases in the three reservoirs of the area underwent various degrees of Sulphate Reduction(TSR). The main evidence includes: 1) natural gases contain abundant H2S with a content of 0.6%~3% at the Sinian Dengying Formation reservoir and 0.2%~0.8% at the Cambrian Longwangmiao Formation reservoir, and heavey δ^34S values of H2S(from 21‰ to 23‰), suggesting that S may source from inorganic gypsum through TSR reaction. 2) The S/C atomic ratios of the reservoir asphalt was about 0.06~0.4, much higher than the upper limit value(0.034) of the asphalt formed through organic matter thermal cracking, and even a little higher than the S/C ratios(0.06~0.12) of the asphalt from the Puguang gas field where TSR had been proven strongly. 3) Gypsum deposited widely at the lower Cambrian in the Sichuan basin provided the materials as SO4^2- and Mg^2+ for TSR. Formation waters in the Dengying Formation reservoirs contain high Ca^2+/Mg^2+ and low Na^+/K^+, indicating that dissolution of the salt and gypsum are quite common. Relative lack of SO4^2- in the formation water may result from the consumption by TSR reaction. Ethane should have been oxidized during TSR reaction, resulting in the increasing dry coefficient and much heavier δ^13C2 values. Therefore, TSR reaction extents can result in differences of the natural gases in both Longwangmiao Formation and Dengying Formation. Relatively weak TSR reaction of the Longwangmiao Formation leads to reverse trend(δ^13C1>δ^13C2) of C isotopic composition in methane and ethane of natural gas, while relatively strong TSR reaction makes natural gases have normal distribution(δ^13C1<δ^13C2) of C isotopic composition in methane and ethane. If without oxidation of TSR, the reverse trend of C isotopic compositions in methane and ethane for Sinian and Cambrain natural gases in the Gao-Mo area should be commonly. The inverse trend(δ^13C1<δ^13C2) is consistent to that of highly mature shale gases in the basin and other areas in the world, indicating that primary origin of the natural gases in the Gao-Mo area should be correlated with the late-generated gases by shales. That can explain reasonably why the δ^13C of methane is generally heavier than that of reservoir bitumen. This study is important to revisiting the accumulation mechanisms of the natural gases in this area.