塔里木盆地塔中西北部多期、多成因岩溶作用地质—地球化学表征——以中1井为例

Geological and Geochemical Implications for Multi-period and Origin of Carbonate Karstification in the Northwestern Tazhong:taking Well Zhong 1 as an example

中1井是一口在塔中西北部的奥陶系碳酸盐岩中取得工业油气流突破的重要发现井。对中1井岩心观察、过井的地震剖面、古生物分析和溶蚀—胶结作用成因的矿物学、岩石学及地球化学研究表明中1井T70(相当于志留系底)、T47(相当于上奥陶统底)存在两至三个构造削截—沉积间断面,与加里东中晚期岩溶有关;奥陶系碳酸盐岩储层方解石充填物的铁、锰、锶和钡含量相对较低、δ18OPDB为-6.4‰^-4.5‰,δ13CPDB为-1.4‰~0.5‰,87Sr/86Sr平均为0.7092;而石炭系巴楚组膏岩、东河砂岩、志留系碎屑岩中的方解石胶结物δ18OPDB为-10.5‰^-16.1‰、平均为-13.9‰,δ13CPDB为-1.5‰^-9.6‰、平均为-4.8‰;87Sr/86Sr为0.7090(志留系)、0.7091(石炭系);研究表明奥陶系碳酸盐岩具有准同生期、浅埋条件下混合水以及较短暴露期下大气淡水下的表生溶蚀的多种复合成因,而志留系和石炭系具有大气淡水溶蚀—充填或埋藏成岩作用特征,海西晚期发生了热液—热水交代有关的溶蚀作用;87Sr/86Sr均低于0.7100,表明中1井区经历的大气淡水条件下暴露期相对较短,陆源物质影响较小,因而不具备沙雅隆起海西早期发生的大规模表生岩溶地质条件。

Well Zhong 1 is an important discovery well of oil and gas in Ordovician carbonate rocks located at the Northwestern Tazhong uplift. This paper presents an updated review of the multi-period and origin of carbonate karsti- fication, which took into account of core observation, palaeo-biological analysis, seismic section and mineralogical and petrological and geochemical of calcite cements and calcites in cleavages and cavities or vuges. The two tectonicrelated truncation and sedimentary gap surfaces have been identified, which hare some extent of relationship with carbonate karstification in Caledonian, the fact that calcites in cleavages and cavities or vuges in Ordovician carbonate rocks with rather low contents of elements of Fe,Mn ,Sr and Ba, and the normal value of δ^18O（ -6.4‰ - -4.5‰） and δ^18C（ -1.4‰- 0.5‰）in comparison with that of the average value of seawater of Ordovicianare believed to occurr as a composite results of syngenesis affected by the mixed fluid at shallow buried condition and later meteoric invasion ; while the calcite cements developed in Bachu, Donghe formations and Silurian sandstones with the ramiform structures of infiltration metasomatism related with magmatic hydrothermalism and the low value of δ^18O（ - 10.5‰ - -16.1‰） and δ^18C（ -1.5‰- -9.6‰） indicated it had undergone an intense hydrothermal buried as well as Hercynian subaerial meteoric diagenetic change; the low data of strontium isotopic compositions （^87 Sr/^86 Sr less than 0. 7100） of filling calcites in cleavages and cavities or vuges in Kalashayi formation and Silurian and Ordovician system of Well Zhong-1 are explained as the results of the little effect of continental clastic sediment due to a rather short period of exposure; moreover, it may be concluded that no similar geological condition or background of a large scale of strong epigenic karstification exists in the Northwestern Tazhong just as occurred in the Tahei oillleld （ Shaya uplift） with Palocave systems widely developed in forms of multiple passage caves in the Middle-Upper Ordovician carbonate.