四川盆地及周缘上奥陶统五峰组岩相特征

上奥陶统五峰组分为页岩段和观音桥段。对比分析四川盆地及周缘五峰组页岩气钻井岩心以及测试数据,根据岩心和全岩矿物组分特征,以硅质矿物(石英)、黏土矿物和碳酸盐矿物为三端元,进行岩相划分。页岩段可识别出4种岩相类型:硅质页岩、含黏土硅质页岩、含灰硅质页岩以及含灰黏土质页岩;观音桥段发育含介壳泥质灰岩。页岩气钻井实测数据表明,页岩段4类页岩岩相品质(有机碳含量、脆性矿物含量以及含气量)差异较大,硅质页岩品质最优,含灰黏土质页岩品质最差。页岩段岩相类型以及观音桥段分布特征受控于沉积环境。川南永川地区等相对浅水的区域页岩厚度较大,但是硅质页岩厚度较小,页岩段整体品质较差,不建议作为水平井靶窗;川南威远地区受古陆和古地形影响,硅质页岩厚度较小,也不建议作为水平井靶窗;川东南焦石坝、武隆、丁山以及南川地区硅质页岩厚度超过4 m,可作为水平井靶窗。

Lithofacies of Upper Ordovician Wufeng Formation in Sichuan Basin and its periphery

The Upper Ordovician Wufeng Formation was divided into the shale and Guanyinqiao sections. Based on the comparative analysis of shale gas drilling cores and the whole rock mineral composition of the Wufeng Formation in the Sichuan Basin and its periphery, three end members including siliceous minerals (quartz), clay minerals and carbonate minerals were chosen for lithofacies classification. Four lithofacies types were identified in the shale section including siliceous shale, clayey siliceous shale, gray siliceous shale and gray clayey shale, while in the Guanyinqiao section, shell marl developed. The shale gas drilling showed that the four lithofacies types vary in TOC, brittle mineral and gas contents. The quality of siliceous shale is the best, and the quality of gray clayey shale is the worst. The lithofacies types of the shale section and the distribution characteristics of the Guanyinqiao section were controlled by the sedimentary environment. The shale thickness in Yongchuan area and other relatively shallow water areas in the southern Sichuan Basin is relatively large, but the thickness of siliceous shale is small, and the overall quality of the shale section is poor, so it is not recommended as the target window of horizontal wells. The thickness of siliceous shale in Weiyuan area in the southern Sichuan is small due to the influence of ancient land and topography, and it also is not recommended as the target window of horizontal wells. The thickness of siliceous shale in Jiaoshiba, Wulong, Dingshan and Nanchuan areas in the southeastern Sichuan Basin is more than 4 m, which can be regarded as a target window of horizontal wells.