川东北五宝场地区侏罗系沙溪庙组沉积特征

Sedimentary characteristics of Jurassic Shaximiao Formation in Wubaochang area,northeastern Sichuan Basin

根据基准面旋回变化原理,利用野外露头、钻井、测井和地震地层切片等资料,对川东北五宝场地区侏罗系沙溪庙组二段开展了层序结构分析和不同可容空间下河道构成特征及河型转换模式等方面的研究。研究结果表明:(1)川东北五宝场地区侏罗系沙溪庙组二段主要发育1个三级及3个四级基准面旋回,各四级层序在低可容空间下为厚层、叠置箱形砂体,随着基准面上升,可容空间增大,到层序顶部过渡为相对孤立的砂体沉积;(2)沙二段沉积时期,研究区气候干燥,砂岩粒度大、成熟度低,发育强水动力构造,地震剖面上为平行反射,河流相沉积特征明显,发育多个低可容空间下的间断正韵律相互叠置的辫状河沉积体系及高可容空间下具“二元”结构的曲流河沉积体系;(3)沙二段河流相地层自下而上具有“辫—曲”转换特征,低可容空间下,砂体大面积展布,地震地貌上表现为相互叠置的辫状河河道,当基准面迅速上升时,可容空间增大,地震地貌上表现为相对孤立的曲流河河道;(4)研究区可容空间和沉积物供给比值(A/S)主要受大巴山物源区强烈的构造活动及温暖潮湿向炎热干旱的气候变化影响。

Based on the principle of base level cycle change,the data of field outcrop,drilling,logging and seismic stratigraphic slice were used to analyze the sequence structure,river channel composition characteristics under different accommodation spaces and river type conversion model of the second member of Jurassic Shaximiao Formation in Wubaochang area of northeastern Sichuan Basin.The results show that:(1)There are mainly one third-order and three fourth-order base level cycles developed in the second member of Jurassic Shaximiao Formation in Wubaochang area of northeastern Sichuan Basin.Each fourth-order sequence is a thick layered stacked box type sand body under the low accommodation space.With the base level rising,the accommodation space increases,and it transits to a relatively isolated sand body deposition at the top of the sequence.(2)The paleoclimate of the second member of Shaximiao Formation was dry.Due to the near provenance deposition,the sandstone was coarse and the maturity was low,the strong hydrodynamic structure was developed,it was parallel reflection on seismic profile,and the fluvial facies characteristics were obvious.There are many braided river sedimentary systems with discontinuous positive rhythms superimposed on each other in low accommodation space and meandering river sedimentary systems with binary structure in high accommodation space.(3)The fluvial facies strata of the second member of Shaximiao Formation have a“braid curve”transformation law from bottom to top,which is specifically shown as follows:under the low accommodation space,sand bodies are distributed in a large area,and the seismic geomorphology is shown as mutually overlapping braided river channels.When the base level rises rapidly,the accommodation space increases,and the seismic geomorphology shows a relatively isolated meandering river channel.(4)The accommodation space and sediment supply ratio(A/S)is mainly affected by strong tectonic activity and change from warm and humid to hot and dry climate in Daba Mountain provenance area.