鄂尔多斯盆地东南部本溪组地层水化学特征与天然气成藏意义

Chemical characteristics of Benxi Formation water and its significance for natural gas accumulation in Southeast Ordos Basin

通过对本溪组地层水性质与地球化学特征的详细研究表明:研究区地层水呈弱酸性,总矿化度介于50.68~391.74 g/L,阴、阳离子分别以Cl^-,Ca^(2+),K^++Na^+含量占优势为特征,水型为单一CaCl_2型;钠氯系数(rNa/rCl)介于0.18~0.49,平均为0.35;脱硫酸系数(100×rSO_4/rCl)在0~0.53,平均为0.1,指示其经历了强烈的浓缩变质与脱硫酸作用的改造。研究后认为,纵向上,随着深度的增加,地层水各特征参数的化学剖面在宏观上呈现出3种变化类型:先增后减型、增减交替型、波状递增型;整体上TDS(本2)<TDS(本1),rNa/rCl(本2)>rNa/rCl(本1),rSO_4×100/rCl(本2)>rSO_4×100/rCl(本1),这表明在流体(气、水)运移过程中,作为优势通道的不整合面使得与其相邻的本2段较本1段具有更强的开放性和流动性,且储层砂岩次生孔隙更为发育{rCa/rMg(本2)>rCa/rMg(本1)}。平面上天,然气在整体自西南向北、东向运移的路径上,其组分中CH_4含量逐渐增高,而N_2含量逐渐降低,呈现出较强的水溶气运聚特征。在运移过程中,天然气遇到合适的圈闭,则逐渐聚集起来形成气藏,且地层趋于封闭的还原环境,有利于天然气的保存。

Statistical analyses of chemical characteristics of the Benxi Formation water indicate that the formation water containing mainly cation Cl-,Ca（2＋）and K＋＋ Na＋is of CaCl_2 type with high TDS（50. 68 g/L ～391. 74 g/L） and slight acidity. The rNa/rClis in the range of 0. 18 ～ 0. 49（0. 35 on average） and the 100 ×SO_4）/rClranges 0 ～ 0. 53（0. 1 on average）,which suggest the formation water experienced serious concentration effect of evaporation and sulphate reduction reaction. Vertically,the chemical composition and ionic ra-tion of the formation water with depth increase presents three typical patterns：increase-decrease mode,increase-decrease alternation mode and undulance increase mode. Also,TDS（b2） TDS（b1）,rNa/r_（Cl）（b2） rNa/rCl（b1）,r_（SO_4）× 100/r_（Cl）（b2） 〉r_（SO_4）× 100/r_（Cl）（b1）,rCa/r_（Mg）（b2） 〉rCa/rMg（b1）,which suggest the unconformity surface at the bottom of the Benxi Formation acted as a favorable migration pathway,resulting in more openness,mobility and secondary porosity in the 2ndmember of Benxi Formation（b2） than the 1stmember（b1）. As natural gas migrates from west to east and south to north,the content of CH_4 increases showing a tendency of methanation,while N_2 decreases,which displays characteristics of water-solved natural gas. The gas pools when there is a right place,and the circumstance evolves to a closed-stagnation-closed state with the time passed,which is excellent for natural gas accumulation and preservation.