重庆地区上二叠统沉积层序及幕式聚煤作用

Upper Permian Sedimentary Sequence and Episodic Coal Accumulation in Chongqing Area

重庆地区上二叠统包括龙潭组/吴家坪组和长兴组,其中龙潭组和吴家坪组为同期异相沉积。根据岩性岩相特征及含煤特征分析,上二叠统龙潭组为一套海陆过渡相含煤地层,形成于海湾(潟湖)—潮坪沉积体系,长兴组主要为浅海碳酸盐沉积。通过区内钻井剖面及露头剖面的分析,将研究区划分为2个三级层序,2个层序均反映出海侵自东向西的脉动推进、东深西浅的格局,从而产生各种沉积体系在垂向上和平面展布上的规律分布。层序1煤层分布范围较广,基本覆盖全区。低位体系域一般不含煤层;海侵体系域发育主要煤层C25及C23,其中C25位于含煤地层的底部,分布范围广,其顶、底板均跨越不同沉积相区,具有幕式成煤模式的特征;高位体系域发育主要煤层C18+19,主要分布在重庆天府姚家岩以南至贵州习水,煤层分布稳定。层序2煤层很少,主要为海侵体系域早期发育的C16煤层,煤层分布稳定;高位体系域几乎全部为碳酸盐台地相,不含煤层。海平面的升降打破了原有的沉积环境,而形成新的沉积环境,是控制含煤沉积旋回及聚煤作用的重要因素。

The Upper Permian Series in Chongqing area includes Longtan/Wujiaping, Changxing formations, the Longtan and Wujiaping formations are contemporaneous different facies deposits. According to the analyses of lithologic, lithofacies and coal-bearing character- istics, the Longtan Formation is a suit of continental-marine transitional facies coal-bearing strata formed in embayment （lagoon）--tidal fiat sedimentary system; while the Changxing Formation is mainly neritic carbonate deposit. Through analyses of borehole and outcrop sections in the area, divided deposits in the area into two third order sequences, the two sequences have all reflected the transgression pulsating advancing from east to west and formed a deep east and shallow west pattern, consequently the regular vertical and horizontal distribution of sedimentary systems. Coal seams in the sequence I are widely distributed and basically covered all the area. In the low- stand systems tract has generally no coal seams borne; transgressive systems tract has developed main coal seams C25 and C^3, in which, the C25 is situated in bottom part of coal-bearing strata and widely distributed, its roof and floor all spanned over different sedimentary fa- cies with episodic coal-forming pattern; highstand systems tract developed main coal seam C,8~,9, mainly stably distributed to the south of Yaojiayan, Tianfu coalmine, Chongqing to Xishui, Guizhou. The sequence II has few coal seams, mainly coal seam C16 developed dur- ing early stage of transgressive systems tract, and stably distributed; highstand systems tract nearly all carbonate platform facies, no coal seams borne. The important factor to control coal-bearing sedimentary cycle and coal accumulation is the fluctuation of sea level smashed the original sedimentary environment and formed new one.