四川盆地东部上三叠统须家河组层序地层及聚煤特征研究

Sequence stratigraphy and coal accumulation of the Upper Triassic Xujiahe Formation in eastern Sichuan Basin

四川盆地东部上三叠统须家河组共分7段,其中须一段与川西小塘子组为同期异相沉积。须一、须三、须五、须七段为含煤泥岩段,须二、须四、须六段为砂岩段。对区内钻井剖面及露头剖面进行分析,在须家河组中识别出区域性构造不整合面及河流下切冲刷面等层序界面,将须家河组划分为4个三级层序,分别对应于须一段、须二段—须三段、须四段—须五段、须六段—须七段。以地层的岩性、岩相变化特征细化分出低位体系域、湖侵体系域和高位体系域。其中低位体系域以广泛分布的河道砂岩沉积为特征;湖侵体系域与高位体系域以湖滨三角洲相的细砂岩、粉砂岩、泥岩及煤层互层为特征。对层序地层格架下的聚煤特征分析表明,层序Ⅲ聚煤最有利,其次为层序Ⅳ,层序Ⅱ最差;在层序Ⅲ内,高位体系域比湖侵体系域更有利于成煤。煤层厚度变化明显受泥炭堆积速率与可容空间增加速率的控制,高位体系域早期较高的可容空间增加速率与较高的泥炭堆积速率保持平衡,有利于厚泥炭(煤)层的形成。

The Xujiahe Formation of the Upper Triassic in the eastern Sichuan Basin is subdivided into seven members. The first member is correlated to the Xiaotangzi Formation of the western Siehuan Ba- sin, but they have different facies. The first, third, fifth and seventh members are coal-bearing intervals, while the second, fourth and sixth members are sandstone-rich intervals. Four sequence boundaries repre- sented by regional tectonic unconformities and the erosional base of the fluvial channel incised valley filling are recognized in the Xujiahe Formation based on outcrop and borehole data analysis. A total of four thirdorder sequences are subdivided in the Xujiahe Formation, and they are subcorrelated to the intervals of Member 1, Member 2 to Member 3, Member 4 to Member 5, and Member 6 to Member 7, respectively. Each sequence is further subdivided into lowstand systems tract （LST） , lacustrine transgressive systems tract （TST） and highstand systems tract （HST） according to variation in lithology and lithofacies. The EST is characterized by widely distributed channel sandstones, the TST and HST are featured with the lacus- trine-deltaic interbedded fine sandstones, siltstones, mudstones and coals. The further analysis of coal accu- mulation in the sequence stratigraphic framework reveals that, sequence 3 is the most favorable sequence for coal accumulation, followed by sequence 4, sequence 2 is the worst. Within sequence 3, the coal-accumu- lation in HST is better than in the TST. Thickness of coal seams is controlled by the balance between the peat accumulation rate and the accumulation creation rate, and the thick coals tend to be formed in the early HST when the peat accumulation rate is well balanced with the accumulation creation rate.