川南威荣页岩气田五峰组—龙马溪组页岩沉积相特征及其意义

Characteristics and significance of sedimentary facies of Wufeng-Longmaxi formation shale in Weirong Shale Gas Field, southern Sichuan Basin

以川南威荣页岩气田五峰组—龙马溪组典型取心井为例,基于岩心观察,有机碳测试、生物鉴定、现场含气量解析和测录井等资料开展了富有机质页岩微相划分。五峰组—龙马溪组一段沉积相主要为深水陆棚亚相,根据矿物组成不同可划分为6种微相:黏土质硅质、富硅生物、硅质黏土质、钙质黏土质、含钙富黏土和富黏土深水陆棚微相。纵向上,前4种页岩相带具有高有机质丰度(≥3%)、高孔隙度(≥5%)和高含气量特征(≥2 m ^3 /t),其中高有机质丰度为页岩气生成提供物质基础,同时生烃过程中产生大量的有机孔为页岩气赋存提供空间,尤其是富硅生物页岩脆性矿物含量高,具有高孔、高渗特征,工程上易于压裂,是页岩气富集最有利相带和页岩气勘探开发的甜点层段;横向上,这4种有利相带在威荣页岩气田的西区较厚,其勘探潜力优于东区。

The sedimentary microfacies of organic-rich shale in the Wufeng-Longmaxi formations in the Weirong Shale Gas Field of the southern Sichuan Basin were studied based on core observation, organic carbon content, biological identification, in situ gas content and well logging data. The shale from the Wufeng Formation to the first member of Longmaxi Formation mainly developed in a deep-water shelf environment. They were divided into six microfacies mostly depending on mineral composition, including clayey siliceous, siliceous-rich biological, siliceous clay, calcareous clay, calcium-rich clay and clay-rich deep-water shelf micro-focies. Vertically, the first four facies are characterized by high TOC content (≥3%), high porosity (≥5%) and high gas content (≥2 m^3 /t). The high abundance of organic matter provided a material basis for shale gas generation, and a large number of organic pores generated during hydrocarbon generation provided space for shale gas occurrence. Moreover, siliceous- rich biological shale is the most favorable facies zone and dessert zone for shale gas exploration and development , due to its high content of brittle minerals, high porosity and permeability, and easy fracturing . From the view of horizontal distribution, the west block of Weirong Shale Gas Field has a greater gas exploration potential than the east block because of greater thickness of the four favorable facies.