河流相砂体识别预测及古气候响应: 以渤中坳陷石南地区明下段为例

河流相砂体的识别预测及其发育演化对古气候变化的响应是当前河流相沉积研究的热点。基于地震沉积学方法,采用三步走的河流相砂体识别预测方法(井震结合建格架→90°相位调整标岩性→切片属性辨砂体),在渤海湾盆地渤中坳陷石臼坨凸起南部(石南)新近系明化镇组下段(简称“明下段”)识别出了顺直和蜿蜒河道砂、河道—堤坝复合体、点砂坝、河口坝、牛轭湖和决口扇6种河流相砂体。依据明下段泥岩颜色,结合藻类种属记录和孢粉记录分析,认为石南地区明下段沉积早期到中期经历了干湿交替→湿润主导的湿润化气候演变; 而明下段沉积中期到晚期经历了湿润主导→干旱主导的干旱化气候演变。润湿化气候演变使得明下段沉积中期河流相砂体宽度和单层厚度减小、弯曲度增大,且带状孤立河道砂体更发育; 而干旱化气候演变使得明下段沉积晚期流相砂体宽度和单层厚度增大、弯曲度减小,且带状河道—溢岸复合带和河口坝砂更发育。研究成果可为其他断陷盆地河流相砂体识别、预测提供参考,对指导构造活动相对较弱的湖盆萎缩期油气勘探实践具有一定的实践价值。

Recognition and prediction of fluvial sand-bodies and their responses to paleoclimatic fluctuations: a case study in the Lower Member of Ming-huazhen Formation in southern Shijiutuo uplift,Bozhong Depression

The recognition and prediction of fluvial sand bodies and their responses to paleoclimatic fluctuations represent a hot topic of concern in fluvial sedimentology. Based on the seismic sedimentology approach,a three-step workflow used for delineation of fluvial sand bodies has been proposed(i.e.,seismic-well tie used for establishment of sequence stratigraphic framework,90° phase rotation to correlate with lithology to seismic events,and stratal slices used to identify sand bodies). This workflow was utilized to delineate fluvial sand bodies in the Lower Member of Neogene Minghuazhen Formation,southern Shijiutuo uplift,Bozhong Depression,Bohai Bay Basin,leading to the recognition of six main types of fluvial sand bodies(e.g.,straight and sinuous channel sandstones,channel-levee complexes,point bars,oxbows,river mouth bars,and crevasse splay). Paleoclimates during the deposition of the Lower Member of the Minghuazhen Formation were reconstructed,based on mudstone colours and records of algae species and sporopollens. Our results suggest that the early,middle,and late evolutionary stages of the Lower Member of the Minghuazhen Formation underwent humid-arid transition,dominantly humid,and predominantly arid conditions,respectively. The paleoclimatic evolution towards a humid condition led to the decrease in sandbody widths and channel sinuosity,and to the better development isolated channel fills characterized by ribbon-shape distribution patterns. The paleoclimatic evolution towards an arid condition led to the increase in sandbody widths and channel sinuosities,and to the better development of channel-levee complexes and river mouth bars characterized by band-shaped distribution patterns. The database and observations of this study provide reference for the recognition and prediction of fluvial sand bodies in other rift basins,and have practical implications for hydrocarbon exploration of fluvial sand bodies in the shrinking evolution period of lacustrine basins characterized by weak tectonic activities.