塔里木盆地塔河地区中下奥陶统鹰山组碳酸盐岩层系内幕不整合识别特征*

为论证T⁶₇界面性质并探索碳酸盐岩层系不整合的识别方法,文章综合利用柯坪—巴楚地区露头和塔河地区钻井、测井和地震资料,研究了塔里木盆地塔河地区中下奥陶统鹰山组内幕不整合面T⁶₇。在露头上, T⁶₇界面表现为岩相转换面,但在不同剖面的暴露强度存在差异。在地化数据上, T⁶₇不整合面之下碳酸盐岩地层碳氧同位素值出现明显偏负;Sc,Co,Ni,Ba,U,Sr等微量元素表现出同样的特点,而Li和V在T⁶₇界面之下富集。在自然伽马测井与自然伽马能谱测井曲线上, T⁶₇不整合面之下的鹰山组下段自然伽马测井曲线明显较界面之上的鹰山组上段齿化剧烈,经过滤波处理的Th/U曲线在T⁶₇界面处呈现出明显的低值。在地震剖面上,难以从地震同相轴反射终止现象中识别出T⁶₇界面,但“串珠”状反射沿T⁶₇不整合面发育,这些串珠状反射被解释为与喀斯特作用相关的岩溶洞穴。T⁶₇界面属于海平面相对下降形成的短暂暴露面(<1,Ma),其主要受控于海平面升降和古地貌,在塔河不同区域的暴露、剥蚀强度不同,强暴露区主要位于中东部,弱暴露区域主要位于西北部。T⁶₇不整合面对塔河地区鹰山组内幕碳酸盐岩储集层发育具有重要意义,既可为层间岩溶的发育提供条件,亦可与断裂构成连通体系为大气淡水提供通道,从而对塔河地区深部碳酸盐岩储集层起到积极的改造作用。

Identification methods and features of unconformity within carbonate successions of the Lower-Middle Ordovician Yingshan Formation in Tahe area,Tarim Basin

This paper integrates outcrops,well log and seismic data to describe the features of unconformity T⁶₇ within the Yingshan Formation in Tahe area, Tarim Basin. The unconformity T⁶₇ is characterized by lithofacies transitional surface at outcrops. δ¹³C and δ¹⁸O values below the unconformity T⁶₇ are significantly more negative,the same as the trace elements of Sc,Co,Ni,Ba,U,and Sr. The contents of Li and V below the unconformity T⁶₇ are relatively high. The GR logs in the lower part of the Yingshan Formation are more variable and have higher values. The Th/U values amended by filtering method are low at the section of unconformity T⁶₇ . It is difficult to identify the unconformity T⁶₇ based on the phenomena of reflection termination such as truncations,onlap and toplap in seismic profile,but some “beads-like” reflection anomalies are found along the unconformity T⁶₇ surface. These “beads-like” reflections are interpreted to indicate karst-relate palaeo-caverns. The unconformity T⁶₇ results from a short-term exposure(<1,Ma)and is controlled by sea-level changes and the palaeogeomorphology. The exposure degree of the unconformity T⁶₇ is variable in Tahe area,with the largest in the middle-east part,and the lowest in the northwest part. The unconformity T⁶₇ has great importance for the formation of carbonate reservoirs in the Yingshan Formation. It not only can provide the geological background for the interlayer karst development,but also can connect the fracture system to provide channels for the meteoric water flowing.