西山煤田山西组震积岩特征及其煤系气勘探开发意义

Seismites characteristics and significance of exploration and development from coal measure gas of Shanxi Formation in Xishan Coalfield

太原市西山煤田山西组地质勘探历程较早,但煤层气勘探开发起步较晚,虽然近年来投产的煤层气井不断增加,但整体产气量并不理想。西山煤田山西组2煤顶板、3煤顶板和1煤底板分别发育有3层震积岩,为查明震积岩特征及其煤系气勘探开发意义,开展了露头系统观测和室内测试分析。结果表明:①山西组共识别出同沉积断裂层、震积角砾岩、阶梯状小断层、震褶岩、液化均一层、球枕状构造、泄水构造、生物逃逸构造等8种震积构造,3层震积岩具有不同的垂向序列;②早二叠世晚期频发的地震事件对泥炭沼泽演化具有抑制作用,主要表现为破坏了地壳沉降速率和泥炭堆积速率的平衡,使得泥炭沼泽的可容纳空间减少,由此造成了泥炭沼泽的消亡,地震事件是造成山西组煤层相对较薄且分布极不稳定的主因;③2煤顶板震积岩形成于泥炭沼泽演化末期,总有机碳含量为1.22%~13.73%,平均为4.51%,是较好的烃源岩;孔隙度为2.53%~7.81%,平均为4.22%,地震作用形成的阶梯状小断层、震积角砾岩等构造增大了泥页岩的孔隙度,为煤系气的赋存提供了空间,为其运移提供了通道;脆性矿物含量为32.1%~75.9%,平均50.6%,震积岩可改造性较强;④在煤系气开发中,2煤可与顶板震积岩联合改造,这种联合改造可充分利用煤层的气资源优势和震积岩的可改造优势,突破以往在煤层中建立流态运移产出通道的技术瓶颈。

The geological exploration history of Shanxi Formation in Xishan Coalfield of Taiyuan City is earlier,while the exploration and development of coalbed methane( CBM) started late. Although the number of CBM wells put into production is increasing in recent years,the overall gas production did not achieve the desired effect. There are three layers of seismites developed in roof of No.2 coal,No. 3 coal and bottom of No. 1 coal in Xishan Coalfield,respectively. In order to study the characteristics and its significance of exploration and development in coal measure gas of the seismites of Shanxi Formation in Xishan Coalfield in Taiyuan City,outcrop observation,sampling and laboratory testing were carried out. The results show that:(1) there are 8 kinds of deformation structures in the Shanxi Formation,including synsedimentary faults,seismogenic breccia,stepped small faults,seismic fold rocks,liquefy homogeneous layer,ball-and-pillow structures,water escape structures and biological escape into caves. The three layers of seismites have different vertical sequence.(2) The frequent seismic events at the late early Permian in the study area inhibited the evolution of peat bogs. The balance between crustal subsidence rate and peat accumulation rate is destroyed,which reduces the accommodating space of peat bogs,thus leading to the extinction of peat bogs. Seismic events are the main reason for the relatively thin and unstable coal seam distribution in Shanxi Formation in Xishan Coalfield.(3) Seismites on the roof of No.2 coal were formed at the late of peat swamp evolution,and its total organic carbon contents are ranges from 1.22% to 13.73%,with an average of 4.51%,the seismites are a better source rock. The porosity of seismites is ranges from2.53% to 7.81%,with an average of 4.22%,the small stepped faults and seismic breccia formed by earthquakes increased the porosity of mud shale,and it’s also provided space for occurrence and migration of coal measure gas. The brittle-mineral contents of seismites are ranges from 32.1% to 75.9 %,with an average of 50.6%,showing strong reformability.(4) In the development of coal measure gas,the No.2 coal can be combined stimulation with roof seismites,which can make full use of the advantage of coal seam gas resources and seismites fracability,and also break through the technical bottleneck of establishing flow migration and output channel.