小型断陷湖盆扇三角洲沉积特征与发育模式——以徐家围子断陷北部沙河子组为例

Sedimentary characteristics and development model of fan delta in small faulted basin:a case of Shahezi Formation in northern Xujiaweizi Fault Depression,NE China

为了深化小型断陷湖盆扇三角洲沉积地质认识,促进该类盆地油气勘探开发,以松辽盆地徐家围子北部沙河子组为研究对象,综合运用岩心、录井、测井以及地震资料对小型断陷湖盆扇三角洲沉积特征与发育模式进行了深入分析。结果表明:徐家围子北部沙河子组扇三角洲沉积发育,岩性以低成熟度的褐色、灰色砂砾岩和褐色、黑色泥岩为主,局部可见碳化泥岩;砂砾岩在测井曲线上多呈中幅齿状或齿化箱状,成像测井上常为暗色背景下大小不一的亮斑响应;在地震剖面上,扇三角洲沉积多表现为中低频楔状前积反射、丘状或楔状杂乱反射特征。平面分布特征显示,扇三角洲的发育与盆地范围以及构造特征均密切相关:沙河子组沉积初期,由于盆地范围小、沉积物供给充足且搬运距离短,扇三角洲沉积不仅发育于西部陡坡带,东部相对缓坡区同样发育;随着控盆断裂持续活动和盆地范围的扩大,东部缓坡带扇三角洲沉积逐渐向辫状河三角洲沉积转变,不同构造带上的沉积相开始出现分异,据此建立了强烈断陷期小型断陷湖盆扇三角洲的发育模式,可为类似小型断陷湖盆扇三角洲的油气勘探提供地质依据。

The geological knowledge of fan delta in small-scale faulted basins will be of important significancefor oil ＆ gas exploration. Taking Shahezi Formation in the northern Xujiaweizi Fault Depression as an example, the data of core, logging and seismic were used to study the sedimentary characteristics and development model of fan delta in a small faulted basin formed during the intensive rifting stage. The results show that fan delta develops in Shahezi Formation within the study area, and the sediments of fan delta are mainly composed of brown to grey sandy conglomerate with low maturity, brown and black mudstone, and locally distributed carbonized mud- stone. On the logging curves, the sandy conglomerate appears as zigzag shape with medium amplitude and zig- zag box; on the imaging logging profiles, the conglomerates with high content of argillaceous sediments appear as bright spots with different size on black background. Furthermore, according to well-seismic calibrating, the sediments of fan delta typically show sphenoid progradation reflections, moundy and sphenoid disorder reflec- tions on seismic profiles. By the analysis of fan delta distribution in Shahezi Formation, it indicates that the develop- ment of fan delta is closely related to basin scope and tectonic features. During the early sedimentary stage of the Shahezi Formation, not only the west steep slope but also the east gentle slope was dominated by fan deltas due to the small scope of the basin and sufficient sediments with short transportation distance. However, with the con- tinuous activity of basin-controlling fault and the extension of basin, the fan delta on east gentle slope was gradually replaced with braided delta, and the differentiation characteristics of sedimentary facies on different tectonic belts become obvious. According to these features above, the development model of fan delta in a small fault basin formed during the intensive rifting stage was proposed, which will be very important for the oil and gas explora- tion in similar basins.