辽西八道壕煤盆地层序地层和火山岩层位及找煤远景研究

1、辽西八道壕煤盆地层序地层和火山岩层位及找煤远景研究        普查勘探资料显示阜新盆地外围的彰武-黑山地区分布着18个中生代盆地,其中大部分盆地被第四纪或火山岩覆盖严重。其含煤性和层序地层格架不清。本文选取区内的八道壕煤盆地为例,综合运用了地震地层学、层序地层学、测井、煤岩层对比、火山岩同位素测年等技术手段对其进行了层序划分、沉积相对比以及聚煤规律的研究。同时采得区内大面积分布的火山岩样品20余件,对其中的11件进行了SHRIMP U-Pb同位素分析。研究结果表明:区内八道壕煤盆地的含煤地层八道壕组属早白垩世,

2、可与阜新盆地的沙海组对比。地震剖面、测井和岩芯鉴定成果证实八道壕组形成和保存于西侧有同沉积断裂活动的小型掀斜构造盆地之中。该盆地西深东浅,不对称性明显。八道壕组可分为三个三级层序,下部第1层序( Sq1)为冲积扇注入湖盆形成的水下扇-扇三角洲沉积体系,湖侵域和高位域各含一个煤组,湖侵域退积的水下扇边缘和高位域扇三角洲平原都有厚煤层发育。成煤早期由于盆地西侧医巫闾山的上隆,冲积扇自盆地西南注入湖盆,盆地西北部湖泊深部被浊积扇粗碎屑沉积物充填淤浅首先出现了成煤环境,发育了厚的煤层并自西侧盆缘断裂附近向东超覆。随着冲积扇的继续充填,湖盆覆水较深的部位自西向东迁移,成煤沼泽随之向东移动,因此煤层之上被

3、一套进积的扇三角洲沉积体系所覆盖;Sq1湖侵域的煤层比高位域的煤层分布面积要大。待湖盆逐渐变浅和淤积,碎屑沉积物的颜色由黑、黑灰、灰、灰绿逐渐变为紫灰和紫红,表明晚期陆上冲积扇沉积已经代替了扇三角洲沉积,成煤作用随之结束。所取的11件火山岩样品测得的U-Pb同位素年龄都在122Ma-132Ma之间,且大部分为125Ma-127Ma,时代均属早白垩世中期。其间包括3次火山喷发活动,第1次和第3次为中基性玄武安山岩的裂隙式喷发,为构造活动期的产物;第2次为中性粗安岩的中心式喷发,为构造稳定期的产物。与阜新盆地南部义县附近的义县组火山岩喷发的时代大致相当。因此除非存在构造逆冲的情况,该火山岩下找中生

4、代煤层的希望渺茫。研究表明区内这些盆地的呈“隆、凹”相间的格局,且展布方向有一定的规律性,而且呈NEE方向展布的6个是早侏罗世盆地,其中有的可能存在北票组的含煤地层和可采煤层;而呈NNE方向展布的12个均是早白垩世盆地,其中如八道壕、雷家和谢林台等含煤性较好,可作为找煤的远景区Ther are 18 Mesozoic basins distributed in Zhangwu-Heishan region, west Liaoning province. Most of which are severely covered with the Quaternary strata or volca

5、nic rocks. This paper takes Badaohao Basin in this region for example and combines the seismic stratigraphy, sequence stratigraphy, well logging, correlation of coal seam and strata, and other techniques to research its sequence division, sedimentary facies correlation and the coal accumulation rule

6、. Volcanic rock is widely distributed in this region. Over 20 volcanic rock samples are collected, 10 of which the SHRIMP U-Pb isotope dating analysis is conducted to. The research result shows:Badaohao Formation - coal-bearing strata - in Badaohao Coal Basin is of early Cretaceous Period and can be

7、 correlated to Shahai Formation in Fuxin Basin. The seismic profiles, well logging and core identification show that Badaohao Formation formed and is preserved in small-scaled tilting structure with synsedimentary faulting events in the west. This basin is deep in the west and shallow in the east, a

8、nd shows obvious unsymmetry. Badaohao Formation can be divided into 3 third-order sequences. The first sequence in the lower part is one subaqueous-fanfan-delta depositional system which was caused by alluvial fan intruding lake basin; the lake transgressive system tract and high-stand system tract

9、each contain one coal-seam group; and thick coal seams develop at the edge of subaqueous fan retrograded by lake transgressive system tract and in the region appropriately covered with water in the fan delta of high-stand system tract. At the early stage, due to the uplift of Yiwulv Mountain to the

10、west of the basin, the alluvial fan intruded the lake basin from the southwest of the Basin; and the deep part of the lake in the northwest of the basin is filled by coarse clastic sediments in turbidite fan to form the coal-forming environment, develops thick coal seams and overlaps eastwards from

11、the place near the west basin margin. As the alluvial fan continued filling the deep part of the lake, the part which was covered with deep water in the lake basin migrated from the west to the east and the coal-forming swamp moved eastwards, so the coal seam is covered with a prograding fan-delta s

12、edimentary system and the coal seam in Sq1 lake transgressive system tract is more widely distributed than that in high-stand system tract. As the lake basin shoals and silts up, the color of clastic sediment gradually changes from black, black gray, gray and grayish green to purplish grey and red p

13、urple, meaning that the subaerial alluvial fan deposit of late stage has replaced the fan-delta deposit and that the coal-forming process ends.The U-Pb isotope ages of the 11 volcanic rock samples are 122Ma132Ma and mostly 125Ma127Ma, which belongs to the middle period of early Cretaceous epoch. 3 v

14、olcanic eruptions happened: the 1st and 3rd are fissure eruptions of intermediate-basic basaltic andesite and are resulted from the structures of active period, while the 2nd belongs to central eruption of neutral trachyandesite and is derived from the structure of stabilized period. They are almost

15、 cotemporary with the volcanic eruptions of Yixian Formation near Yi County in the south of Fuxin Basin. Therefore, it is unpromising to find Mesozoic coal seams under volcanic rock unless thrust structure exists.The research shows that these basins alternate between uplift and concave and that their spreading directions follow certain rule. The 6 NEE-trending basins are of early Jurassic epoch, some of which may contain coal-bearing strata and minable coal seams of Beipiao Formation. The 12 NN