川南威远—泸州页岩气井区奥陶系顶部观音桥组岩相和沉积环境

Lithofacies and sedimentary environments of the uppermost Ordovician Kuanyinchiao Formation from the wells in Weiyuan-Luzhou, southern Sichuan

上扬子地区四川盆地南部威远—泸州地区上奥陶统凯迪阶五峰组和志留系底部兰多维列统龙马溪组黑色笔石页岩沉积于滞留缺氧海底且富含有机质,是页岩气的主产层位,介于五峰组和龙马溪组之间的奥陶系顶部赫南特阶观音桥组通常是含腕足类以及三叶虫的灰岩沉积,其化石组成是冈瓦纳冰川事件最盛期在低纬度陆表海区的产物,岩石学微相特征研究能进一步诠释当时海底沉积环境。本文根据威远地区的W2、W4H10井,泸州地区的L2井、L4井、 L5井、 L6井、 L7井、 L8井、 L9井观音桥组微相鉴定识别生物多样性,除常见的Hirnantia腕足动物群、三叶虫Dalmanitina等壳相化石外,还出现海百合、腹足类Homotoma、双壳类、海绵动物碎片和虫管遗迹化石Chondrites(丛藻迹),其化石组成记录了该层位的生物多样性。观音桥组沉积时该区海底富氧环境有利于底栖动物生存,远岸区海底水动力弱,化石粒度偏细且部分化石破碎程度高,未见大量复体珊瑚和钙藻等典型暖水区化石,反映出远岸相观音桥组海底水温可能偏低。

Graptolitic black shales throughout the Katian(Upper Ordovician)Wufeng Formation and Lower Silurian Lungmachi Formation are deposited in stagnated and anoxic marine-floor rich in organic matter, and considered as the key idealized source rocks and targets on exploration of shale gas in Weiyuan-Luzhou well blocks of southern Sichuan, outer-shelf of the Upper Yangtze region. The Hirnantian(uppermost Ordovician)Kuanyinchiao Formation between the Wufeng and Lungmachi formations is generally limestone yielding brachiopods and trilobites due to the impact of the Gondwana major glacial event in the epicontinental sea of the lower latitude.Nevertheless, their lithological features were significant proxies for insight palaeoenvironment of marine-floor. Microfacies of the Kuanyinchiao Formation from the boreholes of the wells W2 and W4 H10 in Weiyuan, and L2, L4, L5, L6, L7, L8 and L9 are identified. Bioclastic components of the crinoids, gastropods, bivalves, spongia and ichnofossil of Chondrites additionally imply a higher biodiversity except brachiopods and trilobite, which of the latter were frequently recorded. Bioclastics were deposited in weak turbulent conditions;oxygenated marine-floor was favorable for inhibitations of benthic fauna. Bioclasts are generally fine in sizes and high in breakage. Nevertheless, typical warm-water biotic elements such as coral and calcareous algae are few;therefore, Kuanyinchiao Formation in outer-shelf possibly maintained a cool environment.