根据低角度前积地震反射特征划分分流砂坝型浅水三角洲相带

Division of distributary sandbar⁃type shallow⁃water delta facies belts based on low⁃angle progradation seismic reflection characteristics

Z6区块钻井分布不均,目标储层砂泥岩构成复杂且砂岩、泥岩波阻抗特征相近,因此对于Z6区块西侧少井地区,分析有利沉积相带展布特征及准确刻画砂体边界存在一定困难。单凭地震反射振幅特征难以区分砂、泥岩,应用常规叠后地震反演等技术无法有效预测储层展布范围。为此,借助三维地震及钻井资料,结合前人的浅水三角洲沉积特征研究成果,建立适合Z6区块的沉积模式和正演模型,通过砂体地震响应特征分析、地震属性优选,预测有利储层展布范围,获得以下认识:①低角度前积或侧积对于下伏地层地震波形的影响是判断砂体边界的关键,适用于划分分流砂坝型浅水三角洲沉积相带,对勘探阶段预测有利储层的延展范围尤为关键;②相较于传统的振幅类属性,瞬时相位和瞬时频率属性更清晰地刻画了Z6区块分流砂坝和分流间湾边界,尤其是瞬时相位属性不受振幅影响,对于岩性尖灭等地质体的相位变化也十分敏感;③靠近物源方向,砂体受分流河道控制较明显,呈条状特征;靠近湖中心位置,砂体受湖浪改造作用更明显,呈坨状、朵叶状。上述研究表明,在少井情况下,在沉积模式的指导下,利用地震属性分析预测的分流砂坝砂体展布范围与钻井资料揭示的砂、泥岩发育情况吻合较好,通过分析有效的地震属性可合理地刻画分流砂坝型浅水三角洲砂体边界。

The distribution of drilling in the Z6 block is non‑uniform,and the target reservoir is composed of complex sandstone and mudstone with similar wave impedance characteristics.Therefore,for the region with few wells on the west side of the Z6 block,it is difficult to analyze the distribution characteristics of favorable sedimentary facies belts and accurately characterize the sand body boundaries.Relying solely on seismic reflection amplitude characteristics makes it challenging to distinguish sandstone from mudstone,and conventional post‑stack seismic inversion techniques are ineffective in predicting reservoir distribution.Thus,a sedimentary model with its forward modeling suitable for the Z6 block is established based on three‑dimensional(3D)seismic and drilling data,coupled with previous studies on the sedimentary characteristics of the shallow‑water delta.Through analysis of the seismic response characteristics of sand bodies and seismic attribute optimization,the favorable reservoir distribution is predicted.The following findings are obtained:①The influence of low‑angle progradation or lateral accretion on the seismic waveform of the underlying strata is the key to determining the boundary of sand bodies.This approach is suitable for delineating the sedimentary facies belts of the distributary sandbar‑type shallow‑water delta,and it is critical for predicting the extension range of the favorable reservoir during the exploration phase.②Instantaneous phase and instantaneous frequency attributes more clearly depict the distributary sandbar and the boundary between the distributary channel bays compared with traditional amplitude‑type attributes,especially the instantaneous phase attribute,which is not affected by amplitude and is sensitive to phase changes in geological bodies such as lithologic pinchout.③Near the source direction,the sand bodies are significantly controlled by distributary channels,exhibiting a linear feature,while near the center of the lake,the sand bodies are obviously affected by the lake wave,exhibiting lump‑shaped and lobeshaped features.This study shows that under the guidance of the sedimentary model,the distribution of the distributary sandbar predicted by seismic attribute analysis and the characteristics of sandstone and mudstone revealed by drilling data match well in the case of few wells.Furthermore,the sand body boundaries of the distributary sandbartype shallow‑water delta can be reasonably depicted through effective seismic attribute analysis.