湖泊浅水三角洲为中国陆相盆地岩性油气勘探的重要目标,为明确其沉积特征及沉积模式,运用岩性、电性、地震资料,对歧口凹陷南大港断层上升盘沙河街组沙二段小型细粒浅水三角洲沉积特征进行了综合研究,建立了沉积模式。通过分析沙二段剥蚀范围、重矿物分布特征,认为沉积物源主要来自东北侧;根据岩性、电性、地震特征识别出水上分流河道、河口坝、水下分流河道、席状砂等沉积微相;最后结合砂地比确定了平面沉积相分布特征。研究表明:湖盆小型细粒浅水三角洲的主要控制因素为构造条件、气候、物源供给、水深、湖平面变化和地形坡度;在构造条件稳定、气候相对干旱、物源供给较小、水深较浅、湖平面下降、地形平缓的背景下,局部隆起提供沉积物来源,发育以三角洲前缘为主、河口坝较发育的小型细粒浅水三角洲。研究成果为全面地认识浅水湖泊三角洲沉积特征与油气勘探前景提供了新的视角。
Lacustrine shallow water delta has become major target for lithologic hydrocarbon reservoir exploration in continental basins in China. To clarify its sedimentary characteristics and sedimentary models, a comprehensive study of the sedimentary characteristics of small fine-grained shallow water deltas in Member 2 of Shahejie Formation in the upthrown block of Southern Dagang Fault, Qikou Sag was carried out on the basis of rock, electrical and seismic data, and a sedimentary model was established. With analysis of the denudation range and distribution characteristics of heavy minerals in Member 2 of Shahejie Formation, the sediment source was mainly from the northeast; the sedimentary microfacies, such as overwater distributary channels, estuarine dams, submerged distributary channels and sheet sands were identified based on rock, electrical and seismic characteristics; finally, the distribution characteristics of planar sedimentary facies were determined in combination with the sand-to-formation ratio. It was found in the study that the main controlling factors of small fine-grained lacustrine shallow water deltas were tectonic conditions, climate, provenance supply, water depth, lake level variation and topographic slope; under the background of stable tectonic conditions, relatively arid climate, low provenance supply, shallow water depth, declining lake level and gentle topography, local uplift provided sediment sources, and small fine-grained shallow water deltas were developed mainly in delta front and more developed at the estuarine dams. The results of the study provide a new perspective for a comprehensive understanding of the sedimentary characteristics of lacustrine shallow deltas and prospects for hydrocarbon exploration.
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