上扬子地区五峰组—龙马溪组黑色页岩高频层序划分

Division of high-frequency sequences in black shales of the Wufeng-Longmaxi formations,Upper Yangtze Region

上扬子地区上奥陶统五峰组—下志留统龙马溪组是中国页岩气勘探开发最成功的层位。开展该套地层黑色页岩高频层序划分,有助于理解有机碳在页岩中的分布特征。以上扬子地区N211井的五峰组—龙马溪组黑色页岩为研究对象,分析了自然伽马(GR)与总有机碳含量(TOC)特征,基于旋回地层学理论划分了黑色页岩高频层序。结果表明:上扬子地区五峰组和龙马溪组下部黑色页岩GR值与TOC值整体上由五峰组底部到五峰组顶部的观音桥段逐渐变大,观音桥段出现极大值,再由观音桥段向龙马溪组逐渐变小。利用GR频谱分析及其滤波曲线,在五峰组和龙马溪组下部分别识别出约6个和7个长偏心率周期,据此,将五峰组和龙马溪组下部划分为12个完整的四级层序和1个不完整的四级层序。该研究可为黑色页岩高频层序划分提供典型案例,并为上扬子地区五峰组—龙马溪组页岩气甜点段预测提供参考,即:在五峰组—龙马溪组页岩高频层序结构中,层序边界附近的黑色页岩有机质富集程度相对较高。

The Wufeng-Longmaxi formations in the Upper Yangtze Region are the most successful horizon of shale gas exploration and development in China. The high-frequency sequences division in the Wufeng-Longmaxi formations black shales is helpful to clarify the distribution of organic carbon. The Well N211 black shales were studied to determine Natural Gamma(GR) and total organic carbon content(TOC), and high-frequency sequences were divided based on cyclostratigraphy. The GR value and TOC gradually increase from the bottom of Wufeng Formation to the Guanyinqiao Member(maximum value appears in Guanyinqiao Member), gradually decrease from Guanyinqiao Member to Longmaxi Formation. Using GR spectrum analysis and its filter curve,about six and seven long eccentricity cycles were identified in the Wufeng and lower Longmaxi formations, respectively. The Wufeng and lower Longmaxi formations can be divided into 12 fourth order sequences and one incomplete fourth order sequence. This study can provide a typical case for high-frequency sequences division of black shales, and a reference for shale gas sweet spot prediction in the Wufeng-Longmaxi formations. In the shale high-frequency sequence structure of the Wufeng-Longmaxi formations, the black shales near the sequence boundary are relatively rich in organic matter.