四川盆地西北部二叠系栖霞阶层序地层特征及地质意义

基于露头、钻井岩心和测录井资料,采用层序地层学理论与方法,将四川盆地西北部二叠系梁山组+栖霞组作为一个整体予以解剖,识别出梁山组底界面(Ⅰ型)、栖一段与栖二段界面(Ⅱ型)和栖霞组与茅口组界面(Ⅰ型)3个三级层序界面,将中二叠统栖霞阶划分为2个三级层序: 下部的SQ1层序对应梁山组+栖一段,上部的SQ2层序对应栖二段。通过栖霞阶层序地层格架分析,发现栖霞阶地层存在“底超顶削”的充填规律,统计并绘制SQ1与SQ2层序地层厚度等值线图,对研究区栖霞阶SQ1初期和SQ2期构造—古地理格局进行了恢复,在明确SQ2期为栖霞阶主要成滩期的基础上,结合岩溶发育单元及白云岩展布特征,讨论栖霞阶油气储集意义。结果表明:区内栖霞阶存在北西、北东向隆坳分异,汉南隆起、川北隆起、北缘隆起与广元—旺苍凹陷始终控制了区内沉积格局,指出广元—旺苍海槽雏形始于二叠系栖霞阶。研究区岩溶白云岩孔洞型储集层区与环广元—旺苍凹陷周缘的灰岩岩溶型储集层区为有利的勘探区。

Sequence stratigraphic characteristics and geological significance of the Permian Qixia Stage in northwestern Sichuan Basin

Based on outcrops, drilling, cores, and logging data, the Liangshan Formation and Qixia Formation in the northwestern Sichuan region are analyzed using the theory and method of sequence stratigraphy. Three third-order sequence boundaries are identified, including the basal boundary of Liangshan Formation (type Ⅰ), the boundary between Qi 1 Member and Qi 2 Member (type Ⅱ), and the boundary between the Qixia Formation and the Maokou Formation (type Ⅰ). The Middle Permian Qixia Stage can be subdivided into two third-order sequences (from base to top: SQ1 and SQ2). The SQ1 sequence corresponds to the Liangshan Formation and the Qi 1 Member, and the SQ2 sequence corresponds to the Qi 2 Member. Based on the sequence stratigraphic framework reconstruction of the Qixia Stage, the sedimentary filling pattern of the “onlapping at the base and truncation at the top” is discovered. The contour maps of SQ1 and SQ2 sequence stratigraphic thickness are calculated and plotted. Combined with the stratigraphic filling pattern, the tectonic-palaeogeographical framework of the early SQ1 and SQ2 phases of the Qixia Stage, their geological significance is discussed. At the Qixia Stage, the differentiation between uplift and depression occurs in the northwest and northeast trends. The Hannan uplift, the northern Sichuan uplift, the northern margin uplift and the Guangyuan-Wangcang sag have controlled the sedimentary pattern in the study area. It is indicated that prototype of the Guangyuan-Wangcang Trough began in the Middle Permian Qixia Stage. In the study area, the karst cave-type dolostone reservoir and the karst-type limestone reservoir around the Guangyuan-Wangcang sag rim are favorable exploration areas.