塔里木盆地顺南地区深层碳酸盐岩热液溶蚀及其油气勘探意义——以顺南蓬1井为例

Hydrothermal dissolution of deep⁃buried carbonate rocks and its significance for hydrocarbon exploration in Shunnan area,the Tarim Basin:taking well Peng⁃1 in Shunnan area as a case

为揭示深层碳酸盐岩储层的成因机理,对塔里木盆地顺南地区蓬1井进行了研究。顺南蓬1井取心段揭示,在埋深超过7500 m的碳酸盐岩中发育有大量的裂缝和溶蚀孔洞,基于详细的岩心观察和描述发现这些溶蚀孔洞主要分布在取心段下部上寒武统白云岩中,而在取心段上部下奥陶统碳酸盐岩中很少。溶蚀孔洞的直径随埋藏深度的增加逐渐增大,表明这些孔洞是由来源于深部的热流体而非大气淡水下渗溶蚀形成的。各成岩矿物(白云岩、方解石)与宿主围岩具有相似的碳同位素值和87 Sr/86 Sr比值,说明成岩流体与原始碳酸盐岩发生了强烈的水—岩反应,主要继承了原始封存的孔隙水的地球化学特征。但是,明显偏负的氧同位素值(平均-13.26‰)表明它们形成于较高温度的成岩流体中,这一推论也得到了流体包裹体显微测温的证实,表明白云岩的重结晶和随后方解石的沉淀都与断裂相关的深部热流体密切相关。此外,全直径CT扫描揭示,裂缝可以明显改善热液白云石化作用形成的白云岩的储集性能,尤其是渗透率,至少提高一个数量级,即从0.02×10^(-3)μm^(2)提高至0.39×10^(-3)μm^(2)。鉴于塔深1井及古城地区具有相似的情况,塔里木盆地深层—超深层碳酸盐岩可能广泛发育热液相关的白云岩储层。因此,在塔里木盆地及其他地区未来的油气勘探中,热液改造型相关的白云岩储层值得重点关注。

In order to reveal the genetic mechanism of deep⁃buried carbonate reservoirs,a study was made based on well Peng⁃1 in Shunnan area of the Tarim Basin.The core section of well Peng⁃1 revealed that a large number of fractures and dissolution pores were developed in the carbonate rocks with buried depth of over 7500 m.Based on detailed core observation and description,it was found that the dissolution pores were mainly distributed in the Upper Cambrian dolomites in the lower part of the core section and few in the Lower Ordovician carbonates in the upper part of the core section.The diameter of pores increases gradually with the increase of burial depth,which indicated that the pores were formed by infiltration of deep⁃seated hydrothermal fluids rather than meteoric water.The similarδ13C values and 87Sr/86Sr ratios of different types of minerals(including dolomite and calcite)with host rocks indicated that the diagenetic fluids inherited the geochemical characteristics of original sequestered pore water through intense water⁃rock action with the original carbonate rocks.However,the significantly negativeδ18O values(with an average of-13.26‰)suggested that they were precipitated from fluids with high temperatures.This inference was verified by the results of fluid inclusion microthermometry,which confirmed that dolomite recrystallization and subsequent calcite precipitation were closely associated with fault⁃related deep⁃seated hydrothermal fluids.In addition,whole diameter CT scanning revealed that fractures can significantly improve the reservoir property and permeability of the dolomite formed by hydrothermal dolomitization.The permeability can be improved by at least one order of magnitude,i.e.from 0.02×10^(-3)μm^(2)to 0.39×10^(-3)μm^(2).Hydrothermal⁃related dolomite reservoirs may be extensively developed in the deep/ultra⁃deep buried carbonates in the Tarim Basin in view of the occurrence of such phenomenon in the well TS1 and Gucheng area.Thus,in the future hydrocarbon exploration in the Tarim Basin and elsewhere,hydrothermal⁃altered dolomite reservoirs deserve more attention.