大型坳陷湖盆浅水三角洲形成条件和沉积模式:以松辽盆地三肇凹陷扶余油层为例

大量地质和地球物理资料分析表明,下白垩统泉头组沉积时期,松辽盆地三肇凹陷的物源主要来自凹陷北部和西南方向,具备了发育大型浅水三角洲的有利地质条件,即盆地基底稳定沉降,盆广坡缓;古气候干热,古温度可达37℃;母源区物理风化作用强烈,形成大量碎屑物质;古水体浅,小于10m,湖平面波动频繁,湖泊水位发生旋回变化等。大型坳陷湖盆浅水三角洲具有以下典型沉积特征:岩性细,成熟度中等,粒度概率曲线具有牵引流沉积特征;砂岩中发育有丰富的强水动力沉积构造;紫红色泥岩大面积展布,生物扰动强烈;垂向上多个间断正韵律相互叠置;具有叠瓦状前积反射地震相;三角洲大面积稳定展布,砂地比值低;发育分流河道、水下分流河道以及席状砂,不发育河口坝。气候是浅水三角洲展布形态的主要控制因素。在干旱气候条件下,湖泊收缩,河流携带碎屑物质长距离搬运,形成枝状三角洲,分流河道在凹陷内的延伸距离一般超过20km,宽度为800～1 800m。在湿润气候条件下,湖泊扩张,受湖泊的阻碍作用,河道频繁分叉改道,形成网状三角洲,分流河道在凹陷内的延伸距离一般不超过15km,宽度为500～1 600m。浅水三角洲分流河道砂体分布广,纵横叠置,利于构成岩性油气藏。

Formation and sedimentary model of shallow delta in large-scale lake.example from Cretaceous Quantou Formation in Sanzhao Sag,Songliao Basin

The research of some core,logging,seismic data from Quantou Formation in Sanzhao Sag has been finished and some Geological ideas have been got,that is,the provenance of Sanzhao Sag was located in the north and southwest of Songliao Basin during the period of Quantou Formation,and the geological conditions were favorable for the formation of the large-scale shallow delta:the basement sank stably and the palaeotopography was gentle and smooth slope;the physical weathering was strong due to the arid palaeoclimate(up to 37℃);the lake level changed cyclically because of two cycles of palaeoclimate change from arid to humid;the lake water was shallow less than 10meters and lake level changed obviously.The shallow delta of Sanzhao Sag has obvious characteristics,that is the finer grain size,plenty of sedimentary structures formed by strong hydropower,wide purple mudstone with lots of bioturbation structure,several non-continuous normal cycles wertically,imbricated progradation seismic facies,large scale sandbody distribution with low percentage of sandstone to mudstone,and very unique distributary channels with few mouth bars.The sedimentation of the shallow delta was controlled by the climate change.When it was dry,the lake shrank,resulting in branched delta and branched distributary channels which were broad and shallow;the length of channels was more than 20km,the width from 800to 1 800m,the average thickness of individual layer was 2.3meters,and the average percentage of sandstone in strata was 35%.When it was wet,the lake expanded,the river branched off diverted frequently due to the lake water obstruction,formed reticular distributary channels that were narrow and deep,the length of the channel was less than 15km,the width from 500to 1 600m,the average thickness of individual layer was 2.8m,and the average percentage of sandstone in strata was 40%.Therefore,the sandbodies of distributary channels are widely distributed,overlapped vertically,and continued laterally,and form favorable lithologic reservoirs.