四川盆地及周缘前震旦系裂谷特征与油气地质意义

Characteristics and petroleum geological significance of pre-Sinian rifts in the Sichuan Basin and its periphery

四川盆地及周缘已被证实发育较大规模的前震旦系裂谷,但其在盆地内部的展布及发育特征尚不明确,因此在一定程度上制约了对盆地深部构造格局的认识。为此,在前人研究的基础上,利用最新的重磁电资料,通过联合解译和小子域滤波等新技术对新元古界构造形态进行了研究,同时结合区域地震大剖面的精细解释并辅以航磁资料进行验证,分析了新元古代裂谷的特征及其对上覆地层的沉积控制作用。研究结果表明:①盆地内部自西北至东南共发育3条近乎平行的前震旦系裂谷,其中以川中裂谷规模最大;②裂谷内部发育一系列与裂谷走向近平行的次级正断层,将裂谷内部结构复杂化,形成一系列断陷;③裂谷内断层在平面上呈平行/斜列式、相向/相背平行式及同向平行式组合,对应在剖面上呈阶梯式、垒堑式、地堑式及铲式半地堑式等组合特征;④盆地东北部盆缘露头揭示南华系—陡山沱组在裂谷内发育冰期—间冰期交互沉积,主要为三角洲、陆棚与斜坡盆地相碎屑岩和冰碛岩。结论认为,应用重磁联合解译+井震资料修订+露头剖面验证的新方法,证实四川盆地前震旦系裂谷的存在,且南华纪裂谷边界(主干)断裂控制了上覆地层沉积古地理格局和坡折带的发育,部分裂谷控制了寒武系或早古生界沉积中心的分布,并对后期沉降中心、构造圈闭的形成以及含油气系统有着极为重要的影响。

It has been confirmed that large-scale pre-Sinian rifts are developed in the periphery of the Sichuan Basin,but the distribution and development characteristics of the pre-Sinian rifts within the basin have not been clarified,which,to some extent,restricts the understanding on the deep structural pattern of the basin.On the basis of previous researches,combined with the latest gravity,magnetic and electrical data,this paper studies the Neoproterozoic structural pattern by means of combined interpretation,small subdomain filtering and other new technologies.Then,it is verified by virtue of the fine interpretation of regional seismic section as well as the aeromagnetic data.Finally,the characteristics of the Neoproterozoic rifts and the control effect of the rifts on the sedimentation of overlying strata are analyzed.And the following research results are obtained.First,there are three nearly parallel pre-Sinian rifts developed from northwest to southeast within the Sichuan Basin,among which,the one in the central Sichuan is the largest in scale.Second,a series of secondary normal faults which are nearly parallel to the strike of the rifts are developed within the rifts,and they complicate the structures inside the rifts to form a series of fault depression.Third,on the plane,the faults within the rifts are in the patterns of parallel/oblique,synthetic/opposite parallel and concordant parallel combination,which correspond to the combination characteristics of stepwise,horst-graben type,graben type and half graben type on the profile.Fourth,the outcrops along the northeastern margin of the Sichuan Basin reveal the development of the glacial-interglacial alternating sedimentation in the rifts during Nanhua-Doushantuo period,which is mainly composed of clastic rock and tillite of delta,continental shelf and slope basin facies.In conclusion,the new method of gravity and magnetic interpretation+well-seismic data modification+outcrop section verification confirms that there are pre-Sinian rifts in the Sichuan Basin,the boundary(main)faults of Nanhua rift control the sedimentary paleogeographic pattern of the overlying strata and the development of slope break zones,and some rifts control the distribution of sedimentary centers in the Cambrian or Early Paleozoic,which have an extremely important effect on the formation of later subsidence centers and structural traps and the petroleum systems.