塔里木盆地热液特点及其对碳酸盐岩储层的改造作用

Characteristics of hydrothermal activity in the Tarim Basin and its reworking effect on carbonate reservoirs

塔里木盆地热液活动普遍,为弄清其对该区下古生界碳酸盐岩储层的影响,分析了该盆地碳酸盐岩中热液活动产物的岩石学、地球化学特征,发现该区热液普遍具有贫镁离子、富含CO_2的特点。进一步探讨了该区热液活动对储层的改造作用,发现热液流体成分是这一改造作用的决定性因素。研究结果表明:①热液流体富含CO_2,沿断裂带通道附近溶蚀作用强烈,发育大小不等的溶蚀孔洞,形成规模较小但孔渗性很好的优质储层;②热液流体自身镁离子含量很少,虽然通过溶蚀寒武系白云岩能获得部分镁离子,但热液白云石化作用规模较小;③富含CO_2的热液流体溶蚀围岩形成优质的溶蚀孔洞型储层,其分布明显受到断裂的控制;④热液白云石化作用主要表现为鞍形白云石充填溶蚀孔洞,以及在断裂附近热液活动导致白云石晶体重结晶或过度生长,堵塞已有的孔隙,总体而言是一种破坏性成岩作用。该成果可以为该盆地深部碳酸盐岩储层预测提供帮助。

Hydrothermal activity is common in the Tarim Basin. In this paper, petrologic and geochemical characteristics were analyzed of the products of such activity in carbonate rocks in this basin so as to figure out its effect on the Lower Palaeozoic carbonate reservoirs in this area. It is shown that the hydrothermal fluid in the Tarim Basin is generally high in CO2 content, but very low in Mg^2＋ content. Then, the reworking effect of hydrothermal activity on reservoirs in this area was further discussed. It is indicated that the component of hydro- thermal fluid is the deciding factor for the reworking of reservoirs. The research provided the following findings. Firstly, hydrothermal fluid is rich in CO2. And dissolution is strong near the pathways of fracturing zones and dissolved pores of different sizes are developed.Therefore, small-scale quality reservoirs with good porosity and permeability are formed. Secondly, magnesium content of the hydrother- mal fluid itself is quite low, and some magnesium can be produced from Cambrian dolomites by means of dissolution, but the scale of hydrothermal dolomitization is smaller. Thirdly, quality dissolved pore-cave reservoirs are formed due to the dissolution effect of hydro- thermal fluid with abundant CO2 on the surrounding rocks, and their distribution is dominantly controlled by fracturing. And fourthly, the hydrothermal dolomization is mainly presented in the following two forms. One is dissolved pores/caves filled with saddle dolomites, and the other is a type of destructive diagenesis, based on which, the hydrothermal activities close to the fracturing lead to recrystallization or overgrowth of dolomite crystal and the existing pores are blocked. This paper provides assistance in the prediction of deep carbonate res- ervoirs in the Tarim Basin.