沾化凹陷渤南—罗家地区沙四上亚段层序地层及沉积演化研究

SEDIMENTARY FACIES OF THE UPPER PART OF THE FOURTH MEMBER OF THE SHAHEJIE FORMATION IN THE BONANLUOJIA AREA OF THE ZHANHUA SAG

陆相断陷湖盆特殊的凸洼相间分布的地貌条件和多断裂发育的构造格局决定其物源的多方向性、沉积相的多类型性和平面分布的复杂性。综合地震、钻井岩芯、录井和粒度分析等各项测试资料,以及通过识别层序界面和总结沉积相变规律建立渤南—罗家地区沙四上段层序地层格架,并在此基础上通过统计砂体连通性和岩屑组分类型确定物源方向,通过岩芯观察、单井-连井沉积相分析、地层等厚图等单因素图件分析,阐释渤南—罗家地区沙四上段平面沉积相展布特征和演化过程。研究表明,渤南—罗家沙四上亚段系由低位域、湖侵域和高位域组成的三级层序单元。在沙四上段沉积初期,湖盆中所充填地层终止于渤南断层,范围局限;至沙四上段沉积末期,湖水持续上涨,湖盆范围扩展到南部陈家庄凸起。低位域沉积地层主要分布于渤南断裂带以北部位并向南超覆于下伏地层,主要为来自于北部和东北部物源的厚层陡坡扇三角洲前缘沉积。湖侵域和高位域沉积地层则在全区均有分布,其中前者包括北部和东北部陡坡扇三角洲、南部和东南部缓坡扇三角洲以及中西部滩坝沉积;高位域沉积地层在盆地中心以深湖—半深湖沉积的厚层泥膏岩为特征,而在南部由于较长的搬运距离和相对低缓的地形条件而以缓坡扇三角洲沉积、浅水低隆起区碳酸盐岩沉积及其二者的混合沉积为主。

Fault-bounded continental basins are characterized by complex sources of sediment supplies and complex sedimentary facies distribution,due to local topographic variations.In this study,we analyzed the sedimentary facies distribution and evolution as recorded in the upper part of the fourth member of the Shahejie Formation in the Bonan-Luojia area in the Zhanhua sag,on the basis of integrated seismic,well-log,drill core,and particle size data.Particle size data,composition of lithic fragments,and heavy minerals were used to determine sediment sources.Sedimentary microfacies analysis was based on petrographic analysis of drill cores and analysis of well logs.A sedimentary model was established by analyzing east-west and north-south transects of drill core and well log data.Isopachous maps of stratigraphic thicknesses and isopleth maps of sandstone content were constructed to analyze the spatial distribution of depositional elements and stratigraphic units.Fluctuations of lake-level and evolution of sedimentary sequences were inferred on the basis of integrated sedimentary analysis.The upper part of the fourth member of the Shahejie Formation is interpreted as a third-order depositional sequence which includes a lowstand system tract,a transgressive system tract,and a highstand system tract.The lake shore was initially near the Bonan fault,but subsequently migrated southward toward the Chenjiazhuang uplift as lake-level rose.Consequently,the distribution of the lowstand system tract is restricted to the north of the Bonan fault,and it onlaps southward over older strata as fan delta front deposits were formed with sediments sourced from the north and northeast.The transgressive and highstand system tracts are more widely distributed in the study area.The transgressive system tract is represented by steep fan delta front deposits to the north and northeast,gentle fan delta front deposits to the south and southeast,and beach bar deposits to the midwest.The highstand system tract is represented by thick lacustrine evaporates in the central part of the basin,and gentle fan delta front deposits in the southern part of the basin due to long sediment transportation distance and relatively gentle topography.