川西坳陷中段须四段层序地层格架内沉积特征

为了深入认识川西坳陷中段须四段层序地层格架内沉积特征，综合利用野外露头、地震、岩心、测/录井、分析测试等资料对其进行了系统分析。结果表明须四段为一个三级层序，根据层序界面特征划分出低位体系域、湖侵体系域和高位体系域，并建立了等时层序地层格架。依据沉积相标志识别出冲积扇、辫状河、辫状河三角洲和湖泊4种相类型，明确了层序地层格架内沉积相平面展布及纵向演化特征：平面上，短轴物源自龙门山山前依次发育冲积扇、辫状河、辫状河三角洲和湖泊相，长轴物源主要发育辫状河三角洲前缘沉积，长短轴物源在合兴场-德阳-马井-新繁镇一带地区交汇；纵向上，低位体系域和高位体系域时期以辫状河三角洲前缘沉积为主，水下分支河道砂体纵向叠置、横向连片、分布广泛，湖侵体系域时期以滨浅湖沉积为主。建立了层序-沉积充填模式，指出低位体系域和高位体系域时期的长轴物源和长短轴物源交汇区辫状河三角洲前缘水下分支河道砂体是研究区优质储层发育区。

Sedimentary Characteristics Within Sequence Stratigraphic Framework of the Fourth Member of Xujiahe Formation in Middle Area of Western Sichuan Depression

The sedimentary characteristics within the sequence stratigraphic framework of the Fourth Member of Xujiahe Formation in the middle area of western Sichuan depression were studied by using outcrops, seismic data, cores, logging and analytical testing. The characteristics showed that the Fourth Member of Xujiahe Formation is a three-order sequence,which is divided into lowstand, transgressive,and highstand system tracts according to the sequence boundary characteristics. The isochronous sequence stratigraphic framework was established and its internal sedimentary characteristics was clarified:Four types of sedimentary facies were developed, including the alluvial fan, braided river, braided river delta, and lacustrine. The characteristics of sedimentary facies distribution and vertical evolution in the sequence stratigraphic framework were also clarified. Horizontally, the alluvial fan, braided river, braided river delta, and lacustrine facies are developed in turn in the short axis provenance from the front of Longmen Mountain, while the braided river delta front deposition is mainly developed in the long axis provenance, and the short and long axis provenances are intersected in the area ranging from Hexingchang to Xinfan; Vertically, the lowstand system tract and highstand system tract are dominated by the braided river delta front, and the subaqueous distributary channel sand bodies are vertically superimposed and horizontally contiguous and widely distributed. The transgressive system tract is dominated by the shore-shallow lacustrine. Eventually, a sequence-sedimentary filling model was established. This study suggests that the subaqueous distributary channel sand bodies from the long axis provenance and the convergence areas of short and long axis provenance are quality reservoirs in the lowstand system tract and highstand system tract.