利用三维地震透镜体反射识别C8段河道砂体——鄂尔多斯盆地西南部黄土塬区储层预测实例

Chang 8 channel sand identification with 3D seismic lens reflections in Daijiaping survey,Southwestern Ordos Basin

黄土塬区三维地震资料质量欠佳和砂、泥岩波阻抗相近,导致无论是在常规还是拓频三维地震剖面上都难以识别延长组河道砂岩储层。目标区C8段透镜体在地震剖面上表现为C813底界的波峰同相轴下弯、C812小层对应的波谷加宽或在C81段内部多出一个波峰。与常规三维资料相比,拓频剖面上识别河道砂岩透镜体反射宽度更窄、面积变小,且预测结果与钻井的吻合率并未明显提高,脱离钻井标定更难以确定透镜体反射来自C8段内哪个小层的河道砂体。根据拓频剖面上C8段反射的振幅和波形特征将透镜体反射划分为7类,其中方波有轴、方波谷、圆波谷和圆波谷有轴这四种较发育的透镜体含厚砂层的概率依次减小。延安组煤层产生的多次波是影响C8河道砂岩识别的主要因素之一;某些透镜体反射内并未钻遇砂岩层可能与废弃河道泥质塞或透镜体中心圈定不准等因素有关;部分C8段较厚砂岩未形成透镜体反射可能缘于泥岩中含较多粉砂或钙质从而与砂岩波阻抗十分接近。总之,应用三维地震透镜体反射识别C8河道砂岩储层在目标区水平井开发中发挥了重要作用。

Due to the poor quality of Loess 3D seismic data and few contrast of wave impedance between sands and mud,it is very difficult to identify channel sandstone reservoirs of Chang 8 Mem,Yanchang Formation on both normal sections and broaden-frequency sections in the Ordos Basin.Chang 8 channel sandstones of Yanchang Formation can produce lens reflections both on both normal and broaden-frequency sections when they are thick enough.On seismic sections lens of Chang 8 is shown as falling down event of Chang 8_1~3 bottom,or Chang 8_1~2 as widen trough event or one ad-ditional crest event.According to amplitude and waveform characteristics of Chang 8 on broadenfrequency sections,lens reflection can be divided into seven categories.The first four types are most common,and they are square trough with a peak,square trough,round trough,and round trough with a peak.Sand contains probability in the four types of lenses gets less from the first one to the last one.Multiples caused by coals of Yan'an formation in this area may generate false lens response.This will mainly affect delineating Chang 8 channel sandstone.Drilling does not encounter sandstone this may be due to mud filled plug in abandoned river,or inaccurate lens center delineation on seismic sections.A part of Chang 8 thicker sandstone does not form the lens reflection;this may be because impedance contrast between sand and mud is too small.