准噶尔盆地东道海子凹陷下侏罗统浅水三角洲沉积演化及主控因素

Sedimentary evolution and main controlling factors of shallow water delta of the Lower Jurassic in Dongdaohaizi sag,Junggar Basin

准噶尔盆地东道海子凹陷下侏罗统发育浅水三角洲,但其沉积演化与控制因素仍不明确。基于东道海子凹陷及其周缘测录井、岩心、地球化学测试等资料,对该地区下侏罗统沉积相、古地貌、古气候进行分析。结果表明:东道海子凹陷下侏罗统浅水三角洲可识别出三角洲平原、内前缘、外前缘3种亚相,其中平原亚相以分流河道为主,可形成上百米厚的砂砾岩,内前缘水下分流河道发育含泥质夹层的中—细砂岩,外前缘主要为席状砂沉积,形成泥岩与泥质粉砂岩薄互层;古地貌东高西低,根据相对位置与坡度可分为古高地、古斜坡、古洼地3类地貌单元;地球化学指标与沉积特征显示早侏罗世古气候干湿交替,湖平面发生3次湖退—湖进,三角洲平原进退幅度达数十千米。推测古地貌与古气候共同控制了浅水三角洲的沉积展布与演化,其中整体平缓的古地貌为浅水三角洲的形成奠定基础,局部地貌的高低与演化决定沉积体系的展布与变迁,而干湿变化的古气候影响湖平面的升降旋回,进而控制三角洲的进积与退积。综合上述分析,建立了进积型及退积型2种浅水三角洲沉积模式。该认识有利于深化气候和地貌对浅水三角洲控制作用的认识,为准噶尔盆地东道海子凹陷下一步勘探提供借鉴。

A shallow water delta was developed in the Lower Jurassic period in the Dongdaohaizi sag,Junggar Basin,but its sedimentary evolution and main controlling factors were not fully understood.Through the analysis of well logging,core and geochemical data from the Dongdaohaizi sag and its surrounding areas,the sedimentary facies,paleogeomorphology and paleoclimate of this region were analyzed.The study identified three subfacies within the shallow water delta:delta plain,inner front,and outer front.The delta plain subfacies consisted mainly of distributary channels,forming glutenite layers over a hundred meters thick.The inner front featured medium-fine sandstone with argillaceous interlayers,while the outer front was mainly composed of sheet sandstone microfacies interbedded with mudstone and argillaceous siltstone.The paleogeomorphology of the area was higher in the east and lower in the west,categorized into paleo-highland,paleo-slope and paleo-depression based on relative positions and slopes.Analysis of geochemical indicators and sedimentary characteristics suggested fluctuations between humid and arid conditions during the Early Jurassic,with the lake level experiencing three progression-regression cycles.The progradation and regradation of the delta extended over tens of kilometers with the sedimentary distribution and evolution influenced by both paleogeomorphology and paleoclimate.The gentle overall paleogeomorphology provided a foundation for shallow water delta development,while regional paleogeomorphology height and evolution affected sedimentary system distribution and transition.Alternating dry and wet paleoclimate conditions affected lake level fluctuations,thereby controlling delta progradation and regradation.Through a comprehensive analysis,the progradational and regradational depositional models were established for shallow water delta sedimentary patterns,offering valuable insights into the impact of paleogeomorphology and paleoclimate on shallow water delta,and serving as a reference for future exploration in the area.