中上扬子地区晚奥陶世岩相古地理及其油气地质意义

Lithofacies palaeogeography of the Late Ordovician and its petroleum geological significance in Middle-Upper Yangtze Region

中上扬子地区上奥陶统主要发育庙坡组、宝塔组、临湘组(涧草沟组)、五峰组和观音桥组。笔者通过对研究区野外露头及钻井剖面的考察,总结前人研究成果,利用岩石学、古生物学、生态学及室内分析等方法将其划分为潮坪相、浅海陆棚相和深水盆地相3种沉积相类型。潮坪相以沉积灰岩、白云岩、钙质粉砂岩和粉砂岩等为特征。浅海陆棚相主要沉积龟裂纹灰岩、瘤状灰岩、页岩和粉砂质页岩。深水盆地相主要岩性为黑色碳质页岩、粉砂质页岩与硅质页岩,产以营漂浮生活的笔石为主的生物组合。岩相古地理研究表明,晚奥陶世桑比—凯迪早中期,受加里东构造运动影响,华南板内碰撞挤压作用显著,汉南隆起、川中隆起、川西—滇中—黔中—雪峰隆起不断抬升扩大,中上扬子地区表现为海平面相对上升,原本镶边型碳酸盐台地被淹没,沉积了大范围的浅海陆棚相龟裂纹灰岩和瘤状灰岩。凯迪晚期—赫南特期,隆起面积继续扩大,构造围限作用加剧,中上扬子地区发育大面积黑色碳质页岩、粉砂质页岩和硅质页岩。尤其川东南宜宾—泸州,川北旺苍—南江及黔北渝东武隆道真地区五峰组黑色碳质和硅质页岩,具很好的生烃潜力,应视为下一步烃源岩及页岩气勘探开发的重点研究区域。

Miaopo, Baota, Linxiang （Jiancaogou ）, Wufeng and Guanyinqiao Formations were formed during the Late Ordovician in the Middle-Upper Yangtze Region.Based on outcrops and cores ob-servations,combining with the previous research,the lithology,palaeontology,ecology and laboratory a-nalysis were used to divide the facies into tidal flat,shallow marine shelf,and deep water basin facies in study area.Among them,tidal flat facies was characterized by the deposits of limestones,dolomites,cal-careous siltstones and siltstones.Shallow marine shelf facies was mainly composed of “chapping grain”limestones,nodular limestones,shales and silty shales.While black carbonaceous shales,silty shales, and siliceous shales were deposited in the deep water basin,which predominantly produced graptolite as-semblages living by floating camp.The lithofacies palaeogeographic analysis shows that there was a strong inter-plate collision and extrusion in the South China Plate,which was influenced by the Caledonian Tec-tonic Movement in the Late Sandbian-Early Katian Age of the Late Ordovician.During that period,the Central Sichuan and Western Sichuan-Central Yunnan-Central Guizhou-Xuefeng Uplifts gradually swelled up and expanded,meanwhile the relative sea level rose and the rimmed carbonate platform was submerged and deposited a wide range of shallow marine shelf facies“chapping grain”and nodular limestones.In the late Katian and Hirnantian Age of the Late Ordovician,those uplifts continued to expand,and the Middle-Upper Yangtze region mostly began to deposit black carbonaceous shales,silty shales and siliceous shales in the limits of uplifts movements.Especially for the Yibin-Luzhou area in southeastern Sichuan,Wang-cang-Nanjiang area in northern Sichuan and the Wulong-Daozhen area in eastern Chongqing and northern Guizhou,the black siliceous and carbonaceous shales show good hydrocarbon potential,and could be the key study areas for the further exploration and development for source rocks and shale gas.