鲁西南地区石炭纪—二叠纪含煤岩系层序地层及聚煤特征

Sequence stratigraphy and coal accumulation of the Carboniferous and Permian coal measures in southwestern Shandong Province

根据钻井、测井和地质等资料,对鲁西南地区石炭纪—二叠纪含煤岩系进行了层序地层学与聚煤作用研究。识别出以区域不整合面、古土壤层底面、海侵方向转换面、下切谷砂体底面为代表的4种类型的层序界面;划分出本溪组、太原组、山西组和下石盒子组中的4个三级层序,从下到上依次为SⅢ1、SⅢ2、SⅢ3及SⅢ4。通过对比研究不同层序的含煤性及煤层的发育特征后认为,形成于分流间湾沼泽环境的SⅢ3煤层发育最好,形成于潟湖与沼泽环境的SⅢ1及SⅢ2煤层发育次之,SⅢ4没有煤层发育。在同一个层序内部,泥炭聚集速率与可容空间增加速率之间的平衡状态决定了煤层的厚度,这种平衡状态保持较长时间是煤层形成的基本条件,而与泥炭聚集有关的可容空间的持续增加需要基准面的不断抬升,因此,发育较好的煤层最可能形成于海侵体系域末期,如SⅢ3的TST末期的3号煤层。

Sequence stratigraphy and coal accumulation of the Carboniferous and Permian coal measures of southwestern Shandong Province were studied using borehole, logging and geological data. Four types of sequence boundary were recognized, including regional unconformity, palaeosols, switching surface of transgression direction, bottom of incised valley fills. Four third-order sequences in the Benxi（C2）, Taiyuan（C2-P1）, Shanxi（P1）and Xiashihezi（P2）Formations were subdivided, named as SⅢ1, SⅢ2, SⅢ3, and SⅢ4 in ascending order. A comparison of the coal-bearing property between different systems tracts and different sequences was made. It was found that coals in SⅢ3, which were formed in the inter-distributary bay swamp, are thicker than coals in SⅢ1 and SⅢ2, which were formed in the lagoonal swamp, with no coals being developed in SⅢ4. Coal thickness is controlled by the balance between peat accumulation rate, and accommodation creation rate, and a longer period of this balance would favor the formation of thicker coals. The increasing rate of the accommodation creation needs to be maintained by a fast-rising sea-level, therefore, the higher quality coals are mainly found in the late transgressive systems tract, as illustrated by the No.3 coal in the late TST of SⅢ3.