大巴山地区早古生代黑色岩系岩相古地理及页岩气地质意义*

大巴山地区早古生代发育下寒武统水井沱组(巴山组—鲁家坪组)和上奥陶统五峰组—下志留统龙马溪组2套黑色岩系。沉积学研究表明,2套黑色岩系均发育于扬子北缘被动大陆边缘盆地之上,早期快速海侵,沉积环境以深水陆棚为主,晚期随着海平面下降逐渐变为浅水陆棚沉积,局部发育滩礁沉积。早寒武世和晚奥陶世—早志留世,研究区均处于被动大陆边缘盆地,基底不平整,陆架边缘呈现多个小岛阻隔的古地理格局,但两者的形成机制却完全不同:早寒武世的构造格局沿袭了陡山沱期的地堑和地垒分布格局,地垒处表现为局部隆起或水下潜隆;晚奥陶世—早志留世的构造格局则与扬子板块向华北板块俯冲有关,是在扬子北缘被动大陆边缘基础上发育起来的前陆隆起。这种受限的滞留海域有利于形成厚度大、有机碳含量高(2%~6%)、脆性矿物含量高(40%~65%)的富有机质页岩,虽然其是页岩气勘探的有利目标层系,但仍需加强构造保存条件的研究。

Lithofacies palaeogeography of the Early Paleozoic black rock series in Dabashan region and their shale-gas geological significance

Two suites of black rock series of the Lower Cambrian Shuijingtuo Formation(the Bashan Formation to the Lujiaping Formation)and the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation were developed in the Dabashan region in the Early Paleozoic. Sedimentological research reveals that the black rock series were deposited in a passive continental margin basin in the northern margin of the Yangtze Plate. Their sedimentary environments were mainly a deep shelf due to rapid transgression during the earlier period and gradually changed into the shallow shelf with sea-level decline during the later period with some local reefs and shoals. The Early Cambrian and Late Ordovician to Early Silurian basements of the passive continental margin basin in the study area were not flat,which would result in the multi-island obstructively palaeogeographical pattern in continental margin. But their formation mechanisms are totally different. The Early Cambrian palaeogeographic pattern was inherited from graben-horst pattern of the Doushantuo Stage,and local uplift or submerged uplift remained in garben;but foreland uplift during the Late Ordovician to Early Silurian developed in passive continental margin basin in the northern margin of Yangtze Plate was related to Yangtze Plate subduction to North China Plate. The restricted stagnant ocean is more conducive to high-quality shale formation with heavy thickness,high TOC(2%~6%)and high content of brittle mineral(40%~65%),which are usually sweet target layers for shale-gas exploration,but it is necessary to enforce structural research of preservation conditions.