海拉尔盆地牙克石煤田五九矿区大磨拐河组层序-古地理及聚煤作用

Sequence-palaeogeography and coal accumulation of the Damoguaihe Formation in the Wujiu mining area,Yakeshi Coalfield,Hailar Basin

海拉尔盆地牙克石煤田五九矿区煤炭资源丰富且煤层气赋存条件良好,已经成为海拉尔盆地煤及煤层气勘探的地区之一。下白垩统大磨拐河组是其主要含煤地层,为了寻找优势聚煤区,利用岩芯、钻孔资料对其岩相类型、沉积相、层序地层及聚煤作用特征进行研究。大磨拐河组主要由砾岩、砂岩、粉砂岩、泥岩及煤组成,沉积体系有冲积扇、扇三角洲、辫状河、辫状河三角洲和湖泊5种类型。大磨拐河组可根据不整合面和河流下切谷冲刷面等层序界面划分为2个三级层序。层序Ⅰ从大磨拐河组底部到厚层砂砾岩底部,以砾岩、粉砂岩和泥岩为主,煤层总厚度可达13 m,沉积体系主要有冲积扇、扇三角洲和湖泊;层序Ⅱ从厚层砂砾岩底部到大磨拐河组顶部,岩性以粉砂岩和泥岩为主,煤层总厚度可达20 m,沉积体系主要有辫状河、辫状河三角洲、扇三角洲和湖泊。从层序Ⅰ到层序Ⅱ,沉积环境变化有总体变深的趋势,煤层厚度逐渐增大,聚煤作用逐渐增强。依据所获得的沉积模式,运用单因素分析多因素综合作图法,以三级层序为作图单元,恢复了古地理格局。区内古地理单元东部以冲积扇-扇三角洲为主,西部以辫状河-辫状河三角洲为主,中部以湖泊为主,物源主要来源于研究区东部方向。研究区主要成煤环境为滨浅湖,煤层主要形成于高位体系域。聚煤作用受到基底差异性沉降的影响,可容空间增加速率与泥炭堆积速率相平衡,在研究区中东部煤层厚度最大,向西北部和东南部方向逐渐变小。

The Wujiu mining area of the Yakeshi Coalfield in the Hailar Basin is rich in coal resources and has a great potential for coalbed methane exploration.It has become one of the areas for coal and coalbed methane exploration in the Hailar Basin.The Lower Cretaceous Damoguaihe Formation was the main coal-bearing strata,in order to find a favorable coal accumulation zone,the authors studied its lithofacies types,sedimentary facies,sequence stratigraphy and coal accumulation characteristics based on some boreholes data.The Damoguaihe Formation was mainly composed of conglomerate,sandstone,siltstone,and mudstone,with numerous thick coal seams.Five types of sedimentary systems were identified,including alluvial fan,fan delta,braided river,braided river delta and lake.The Damoguaihe Formation could be subdivided into two third-order sequences according to the sequence boundaries represented by regional unconformity,erosional base of the fluvial channel sandstones,and inter-fluvial palaeosols.Sequence I,from the bottom of the Damoguaihe Formation to the bottom of thick sandy conglomerates,was mainly composed of conglomerate,siltstone and mudstone.The total thickness of the coal seams in this sequence could reach 13 m.The main sedimentary systems were those of the alluvial fan,fan delta and lake.Sequence Ⅱ,from the bottom of thick sandy conglomerates to the top of the Damoguaihe Formation,consisted of siltstone and mudstone.The total thickness of the coal seams in this sequence was up to 20 m.The sedimentary systems were those of the fan delta,braided river,braided river delta and lake.Compared with sequence Ⅰ,sequence Ⅱ had a relatively deeper sedimentary environment,and a larger coal seam thickness.According to established sedimentary model,a single factor analysis and multi-factor comprehensive mapping method were applied to recover the palaeogeographic pattern within each third order sequence as the mapping unit.The palaeogeographic units in this area were mainly the alluvial fan and-fan delta in the east,the braided river and-braided river delta in the west,and the lacustrine in the central area.The provenance was mainly from the eastern direction of the study area.The main coal-forming environments in the study area were the littoral-sublittoral lacustrine environments.The coal seams were mainly formed in the high systems tract(HST),where the high peat accumulating rate could be well balanced by the relatively higher rate in accommodation creation.It is predicted that the thickest coal zones are distributed in the central east of the study area,and the coal seams will gradually become thinner towards the northwest and southeast.