四川盆地中部地区下寒武统龙王庙组混积特征及其地质意义

Characteristics and geological significance of mixed sediments in the Lower Cambrian Longwangmiao Formation, central Sichuan Basin

宋金民 张亚雄 尹柯惟 刘树根 陈宗于 李智武 田艳红 孙玮 刘颖 金鑫 赵玲丽

Jinmin Song Yaxiong Zhang Kewei Yin Shugen Liu Zongyu Chen Zhiwu Li Yanhong Tian Wei Sun Ying Liu Xin Jin Lingli Zhao

四川盆地中部地区下寒武统龙王庙组混合沉积较发育，但相关的研究和认识尚显薄弱。通过岩心观察描述和岩石学分析，对川中地区下寒武统龙王庙组混合沉积特征和地质意义进行研究。研究结果表明，研究区龙王庙组混合沉积主要为混积型碳酸盐岩，发育碳酸盐岩-混积岩层系与混积岩层系，具有渐变式混积和复合式Ⅱ型混积两种类型。纵向上，混合沉积受海平面变化控制，主要发育在三级海平面旋回的海侵期和海退早期，与风暴事件密切相关，且海侵期混积程度明显高于海退期；横向上，高石梯地区混积强度大于磨溪地区。通过四川盆地中部与北部地区龙王庙组混合沉积特征的对比，发现川北地区与古秦岭洋、摩天岭古陆相接，混积强度与混积颗粒粒度均大于川中地区，指示风暴潮流携带陆源碎屑经川北进入川中地区。四川盆地中部地区龙王庙组混积岩指示高石梯地区处于古地貌斜坡部位，粉砂级石英颗粒卸载形成混积中心。龙王庙组有利储层主要发育在弱混积或未混积颗粒滩微相中，混合沉积不利于优质储层的发育。

The mixed siliciclastic and carbonate sediments that prevail the Lower Cambrian Longwangmiao Formation in the central Sichuan Basin are poorly understood.Their characteristics and geological significances are therefore focused in this study through core description and petrologic analysis.The results show that mixed carbonate-siliciclastic deposition dominates the formation as carbonate-mixed carbonate-siliciclastic sequences and mixed carbonate-siliciclasitic sequences in gradually mixing or type Ⅱ composite mixing styles.Vertically, the sediments are clearly storm-related and controlled by sea level variations in transgression and early regression stages of the third-order eustatic cycle with the mixing range much larger in transgression than that in regression stage.Horizontally, the sediments show higher mixing intensity in the Gaoshiti area than in the Moxi area.A characteristic comparison between the sediments of the formation in the central and northern Sichuan Basin reveals that sediments in the northern part of the basin have higher mixing intensity and greater granular size due to its adjacency to the Qinling Palaeocean and Motianling Palaeoland and a possible influx of terrigenous debris brought by storms through the north part to the center.The mixed sediments in the central Sichuan Basin indicate that Gaoshiti area was lying on a palaeo-slope where silty quartz grains were unloaded to form a mixing deposition center.It is also highly possible that quality reservoirs in the formation occur mostly in the weakly to non-mixing sediments of shoal facies, since highly mixed sediments are not suitable for the formation and development of such reservoirs.

10.11743/ogg20210309