川东-渝北地区飞仙关组层序-岩相古地理特征

Sequence-based lithofacies and paleogeography of Lower Triassic Feixianguan Formation in eastern Sichuan and northern Zhongqing area

以沉积相分析和层序界面识别为依据,将川东—渝北地区下三叠统飞仙关组划分为2个三级层序(SQ1和SQ2)和6个体系域,编制各沉积体系域的层序-岩相古地理图。研究表明,SQ1层序时期,研究区受晚二叠世长兴末期的区域构造抬升影响,属于区域性缓慢海进—缓慢海退沉积旋回的产物,一度开阔深水的台盆逐渐变浅而转化为开阔台地沉积环境,台地边缘与台内鲕滩的沉积相分异和沉积作用十分活跃,是形成飞仙关组鲕滩储层的主要时期,尤以海侵期鲕滩最为发育;SQ2层序发育期,受海平面大幅度下降、水循环受限和干旱炎热气候的影响,区域沉积格局由开阔台地转化为局限台地,鲕滩逐渐消失,最终进入频繁暴露的蒸发台地环境,以沉积膏云岩为主,为飞仙关组天然气藏最重要的第一套区域性致密盖层发育层位。

The Lower Triassic Feixianguan Formation in northeast Sichuan can be divided into two third-order sequences and six system tracts in terms of sedimentary facies and sequence interfaces. Characteristics of sequence stratigraphy of the Feixianguan Formation were analyzed and sequence-palaeogeography maps were compiled. It is indicated that, influenced by regional tectonic uplifting of the Late Permian Changxing Stage in the study area during the development of SQ1 sequence, the product was formed in a regional slow transgression -regression depositional environment. The seawater that had been once deep and open became shallow, the depositional evolution changed from basin facies to open-platform facies which almost formed the platform edge or inner oolitic bank. In the second deposition of the third-order sequence cycles （SQ2）, due to arider and hotter climate, sea level falling and water cycle limiting, the sedimentary facies turned into the restricted platform and evaporate platform facies, and the oolitic bank disappeared gradually. Finally, the sedimentary facies was converted to evaporate platform in which gypsum dolomite was developed. The gypsum dolomite served as an important sealant in the Feixianguan Formation, and the brine saved in the same time was helpful to the burial dolomitization in the oolitic bank to form high quality reservoirs in the Feixianguan Formation.