准噶尔盆地红山嘴地区三叠系克上组两类粗粒三角洲的沉积记录差异及其成因*

准噶尔盆地红山嘴地区三叠系克上组存在扇三角洲与辫状河三角洲这两类粗粒三角洲,其界定在油气沉积学领域一直备受关注,而基于岩相类型及其垂向组合特征的沉积作用研究,辅以砂体宏观展布规律将有助于这两类三角洲的判别。红山嘴地区克上组扇三角洲以发育各种交错层理砾岩相(Gi、Gt、Gp)与块状砾岩相(Gmm、Gcm、Gcs)为特点,而辫状河三角洲则发育各种砂岩相(St、Sp、Sm和Sh等);扇三角洲6种岩相组合显示其碎屑流与密度流沉积相对发育,韵律特征不明显,而辫状河三角洲的7种岩相组合均显示其牵引流沉积作用更为显著,且正韵律居多;坡度陡、物源足造就了扇三角洲“砂体发育而分布局限、切割叠置频繁”的沉积响应特点,辫状河三角洲则因坡度中等且物源供给弱而具有“砂体薄而广布、多为孤立式接触”的响应特点。

Differences in sedimentary records of different coarse-grained deltas and their genesis in the Triassic Upper Karamay Formation,Hongshanzui area,Junggar Basin

The recognization of fan delta and braided-river delta of the Upper Karamay Formation in Hongshanzui area in Junggar Basin has always been concerning the geologists in the field of hydrocarbon sedimentology. Depositional analysis based on lithofacies supplemented by the macroscopic distribution mode of sandbody is helpful for distinguishing the two kinds of deltas. Cross-bedded conglomerate lithofacies(Gi,Gt,Gp)and massive gravel lithofacies(Gmm,Gcm,Gcs)were developed in fan delta,with sandy lithofacies(St,Sp,Sm and Sh, etc.)developed in braided-river delta in Hongshanzui area. Debris flows and density flows were indicated by six lithofacies associations in fan delta system,with unobvious rhythm feature. Seven lithofacies associations in braided-river delta show that the traction currents and positive rhythm feature are more significant. Steep slope and abundant sediments contributed to a thick stacked sandbody with limited distribution in the fan delta system. Gentle slope,insufficient sediments supply caused a thin sandbody with board distribution in the braided-river delta,mainly showing isolated contacts between the sandbodies.