重庆綦江观音桥剖面五峰组—龙马溪组地层特征及其对页岩气勘探开发的启示

STRATIGRAPHIC CHARACTERISTICS OF WUFENG FORMATION—LUNGMACHI FORMATION IN GUANYINQIAO SECTION OF QIJIANG, CHONGQING, CHINA AND THEIR IMPLICATIONS FOR SHALE GAS EXPLORATION AND DEVELOPMENT

重庆市綦江区安稳镇观音桥剖面是我国南方扬子地区五峰组—龙马溪组页岩代表性剖面之一,也是綦江丁山和东溪地区页岩气勘探开发的重要参照剖面。该剖面岩性主要有碳质页岩、泥质粉砂岩、泥岩和含泥泥晶灰岩等。生物地层研究表明,该剖面五峰组至龙马溪组下部页岩地层完整连续,笔石带最高发育至Lituigraptus convoltus带,其中Dicellograptus complexus带至Cystograptus vesiculosus带地层厚4.69 m。受黔中隆起向四川盆地方向延伸形成的丁山水下高地的影响,该地区富有机质页岩的厚度较薄,而向四川盆地内部富有机质页岩的厚度有增大趋势。后期构造运动中,研究区受齐岳山断裂影响较大,靠近断裂地区表现为常压页岩气藏,距离断裂较远的四川盆地内部地区, WF2—LM4笔石带页岩的厚度较大,页岩气保存条件变好,为超压页岩气藏。

The Guanyinqiao section, located in Anwen Township, Qijiang District, Chongqing Municipality, is one of the classic sections of Wufeng Formation-Lungmachi Formation shale in the Yangtze region of South China. It is also an important reference section for shale gas exploration and development in Dingshan and Dongxi areas of Qijiang District. Herein we conduct a study of the stratigraphic characteristics of the section, aiming to reveal its implications for shale gas exploration and development in Yangtze Region, South China. Our study shows the lithological components of the strata in the section includes mainly carbonaceous shale, argillaceous siltstone, mudstone and micritic marlstone. The biostratigraphic study shows that the shale of the Wufeng Formation to the lower part of the Lungmachi Formation in this section are continuous with the topmost part corresponding to graptolite Lituigraptus convoltus Biozone. In the gas shale, the total stratigraphic thickness of Dicellograptus complexus Biozone(late Katian, Ordovician)to Cystograptus vesiculosus Biozone(Rhuddanian, Silurian) is 4.69 m, which is relatively thin and is mainly controlled by Dingshan bathymetric high as an extension of Central Guizhou Uplift towards the Sichuan Basin, whereas the thickness of this organic-rich shale increases progressively into the interior of the basin. The gas shale was additionally affected by later tectonic movements, among the most significant results is the occurrence of Qiyueshan Fault, which serves to define the southeast boundary of the Sichuan Basin. Specifically, in the areas close to the fault the shale gas reservoir is characterized by normal-pressure, while in the interior of the basin far away from the fault the shale gas reservoir shows overpressures and the preservation conditions become more favorable.