定边地区中侏罗统延安组辫状河沉积模式

Braided river sedimentary model of Middle Jurassic Yan’an Formation in Dingbian area

辫状河具河道宽而浅、弯曲指数<1.5、坡降大、易迁移、心滩发育的特点。为明确定边地区延安组辫状河沉积特征及沉积模式,利用野外露头、岩芯和测井相分析辫状河沉积微相、砂体连通模式和心滩分类。结果表明定边地区延安组辫状河识别出心滩、辫流水道、串沟水道、泛滥平原等沉积微相,发育槽状交错层理、板状交错层理、平行层理、块状层理等;辫状河砂体连通模式特征主要与水流的水动力强度和河道消亡时间有关,在顺水流方向和垂直水流方向上各有4种表现形式;心滩是辫状河沉积的主要构成单元与成藏单元,发育顺流加积、垂向加积、侧向加积等叠加方式,按照水动力强度的变化分为纵向沙坝、斜列沙坝、横向沙坝等。辫状河沉积模式研究对定边地区延安组辫状河砂体特征与构型特点具有一定的指导意义,同时为同一盆地和其他盆地其他层位辫状河沉积模式提供借鉴。

Braided river is characterized by wide and shallow channel,bend index less than 1.5,large slope,easy migration and development of diara.In order to clarify the sedimentary characteristics and sedimentary model of the braided river of Yan’an Formation in Dingbian area,the sedimentary microfacies,sand body connection model and diara classification of the braided river were analyzed by using field outcrops,core and logging facies.The results show that the sedimentary microfacies of Yan’an Formation braided river in Dingbian area can be identified as diara,braided channel,cascade channel and flood plain;trough cross bedding,plate cross bedding,parallel bedding and massive bedding are developed.The characteristics of braided river sand body connection mode are mainly related to the hydrodynamic intensity of flow and the extinction time of channel,and there are four forms in the direction of flow and vertical flow respectively.The diara is the main component unit and reservoir forming unit of braided river deposition.It develops superposition modes of upstream aggradation,downstream aggradation,vertical aggradation and lateral aggradation.According to the change of hydrodynamic intensity,it can be divided into longitudinal bar,inclined bar and transverse bar.The study of braided river sedimentary model has a certain guiding significance for the characteristics and configuration of the braided river sandbodies of Yan’an Formation in Dingbian area,which provides a reference for the braided river sedimentary model of the same basin and other basins.