神府南区直罗组下段古河道冲刷带水动力条件的定量表征

Quantitative characterization of hydrodynamic conditions in the scour zones of the lower Zhiluo Formation in Shenfu south area

古河道冲刷带是造成煤炭开采过程中水害的关键地质因素,然而,目前国内外对古河道带冲刷带的古水动力条件量化表征方面的研究很少,限制了对含煤岩系中河道冲刷带形成过程的深入理解和精准预测。以鄂尔多斯盆地东北部神府南区直罗组下段古河道冲刷带为例,通过粒度分析方法量化表征了古河道冲刷带形成的古水动力条件。研究发现,直罗组下段从下往上可识别出a,b,c,d四个沉积旋回,从a旋回到d旋回,直方图中粒径逐渐分布集中;概率累积曲线以两段式为主,悬浮总体含量逐渐增多;颗粒粒径(M_(z))表现为以细砂为主,含少量极细砂、粗砂;标准偏差(σ)集中在0.50~1.16,指示分选性呈中等~较好;偏度(S_(k))以对称和正偏为主;峰度(K_(g))主要表现为中等到尖锐;在C-M图上对应为PQ和QR段。a旋回到c旋回为辨状河三角洲平原分流河道沉积,而d旋回为曲流河三角洲平原分流河道沉积。以粒度参数及沉积环境为约束计算古水动力条件显示,a旋回底部砾石沉积的平均起动流速为1.78 m/s,弗劳德数为1.05,属于高流态河道搬运,而a旋回到d旋回中砂岩沉积的起动流速分别为0.27,0.26,0.25,0.25 m/s,弗劳德数为0.16,0.15,0.14,0.14,属于低流态河道搬运。直罗组下段底部砾岩沉积时期是形成河道冲刷带形态的关键时期,而其中砂岩沉积时期为对早期形成的古河谷的充填作用。

Palaeochannel scouring zone is the key geological factor causing water damage in coal mining.However,there are few studies on the quantitative characterization of palaeohydrodynamic conditions of palaeochannel scour zone worldwide,which limits the in-depth understanding and accurate prediction of the formation process of palaeochannel scour zone in the coal-bearing rock series.Taking zhiluo Formation in Shenfu South area of northeastern Ordos Basin as an example,this paper quantifies the paleo-hydrodynamic conditions of the formation of the palaeo-channel scour belt by means of grain size analysis.It is found that four sedimentary cycles a,b,c and d can be identified in the lower member of Zhiluo Formation from bottom to top,from a cycle to d cycle.The average particle size of histogram is gradually distributed and concentrated.The probability accumulation curve is dominated by two sections,and the suspended total content increases gradually.The particle size(M_(z))is mainly fine sand,with a small amount of very fine sand and coarse sand.The standard deviation(σ)ranges from 0.50 to 1.16,indicating that the sorting is medium to good.The skewness(S_(k))is mainly symmetrical and positive skewness.Kurtosis(K_(g))is mainly moderate to sharp.On C-M graph it corresponds to PQ and QR segment.The a cycle back to c cycle is the distributary channel deposits in the plain of the braided river delta,while d cycle is the distributary channel deposits in the plain of the meandering river delta.The calculation of paleo-hydrodynamic conditions with grain size parameters and sedimentary environment as constraints shows that the average starting velocity of gravel deposition at the bottom of a cycle is 1.78 m/s,and the Froude number is 1.05,which belongs to the high-flow fluvial transport.The starting velocity of sandstone deposition in a cycle back to d cycle is 0.27,0.26,0.25 and 0.25 m/s respectively,the Froude number is 0.16,0.15,0.14 and 0.14,belonging to low flow state river transport.The conglomerate deposition period of the lower member of Zhiluo Formation is the key period for the formation of channel scour zone,and the sandstone deposition period is the filling of the ancient valley formed in the early stage.