川西上侏罗统遂宁组沉积相特征

Characteristics of sedimentary facies of the Suining Formation of Upper Jurassic in western Sichuan Province

川西地区在晚侏罗世遂宁期处于稳定坳陷阶段,构造活动相对较弱,主要沉积1套稳定的鲜紫红色泥(页)岩夹长石岩屑细—粉砂岩地层,构成了1个相对独立和完整的沉积旋回。根据不同级次基准面旋回分析,川西遂宁组可划分出1个长期层序、3个中期层序(对应于遂宁组的3个岩性段)和十余个短期层序。以各中期层序的上升和下降相域为编图单元编制的沉积相图表明遂宁期川西地区以广泛发育被湖湾分隔的曲流河三角洲沉积体系为特征,并形成各自相对独立的储、盖组合和油气聚集成藏条件。遂宁期沉积演化具明显受基准面旋回控制的继承性,遂宁期早期(遂宁组一段沉积时期),基准面不断上升,沉积作用以进积→加积为主,沿龙门山前缘发育有小型冲积扇或带状剥蚀区,向前分别在丰谷、新都、回龙镇各发育1个大型且稳定的三角洲;遂宁期中期(遂宁组二段沉积时期),随着基准面由缓慢上升转为下降,沉积作用由加积→加积演化,遂宁期早期形成的3个三角洲分裂成4～5个发育较均衡的三角洲;遂宁期晚期(遂宁组三段沉积时期),随着基准面不断下降,沉积作用由加积→进积演化,三角洲不断向湖推进,同时,由于受陆源物质供给不均衡的影响,该时期川西地区三角洲发育有较大变化,除在德阳发育1个较稳定的三角洲外,在新都发育的三角洲逐渐消亡,而在新津则由浅湖沉积逐渐演变为大型三角洲沉积。

During the Suining Age of Late Jurassic, western Sichuan Province was in stable subsidence, where the tectonic movement was relatively inactive. The Suining Formation was mainly a set of fresh purple-red mudstone or shale interbedded with feldspathic fine-grained sandstone to siltites, which constituted a complete sedimentary cycle.According to the analysis of base-level cycles with different scales, the Suining Formation can be divided into one long sequence cycle, three middle sequence cycles （corresponding to the three lithologic members of the formation ）, and some short sequence cycles. On the basis of the rising and falling base-level phases of each middle sequence, the sedimentary facies maps of the Suining Formation have been compiled in western Sichuan Province. The maps indicated that at the Suining Age, the study area was dominated by the deposition of meandering fluvial delta system which was detached by gulfs. The relatively independent reservoir-cap associations were formed and the accumulation and hydrocarbon forming conditions were also formed. During the Suining Age, the sedimentary evolution was obviously controlled by base-level cycles, and can be inherited.During the Early Suining Age （corresponding to the deposition of Member 1 of Suining For- mation ）, with base-level continuously rising, the sedimentation evolved from progradation to aggredation. There were some small-scale alluvial fans or zonal eroded area in front of the Longmen Mountains. At the same time, three big-scale and stable deltas were respectively developed in Fenggu, Xindu and Huilongzhen, which are located in western Sichuan.During the Middle Suining Age （corresponding to the deposition of Member 2 of Suining Formation ）, with base-level changing from slowly rising to falling, the sedimentation evolved from aggredation to retrogradation and then to aggredation.The three deltas previously developed divided into 4-5 balancing deltas. During the Late Suining Age （corresponding to the deposition of Member 3 of Suining Formation ）, with base-level continuously falling, the sedimentation evolved from aggredation to progradation, and the deltas were pushed to lake continuously.At the same time, the terrigenous materials supply was unbalanced, thus the development of deltas in western Sichuan changed greatly. Except for a stable delta developed in Deyang, the delta in Xindu was gradually shrinking, while in Xinjin, the sedimentary evolution was from shallow lake to a large-scale delta.