陆相断陷湖盆基准面旋回对沉积砂体的控制——以海拉尔盆地贝尔凹陷贝西地区为例

Effect of Base-level Cycles on Sedimentary Sandbodies in Continental Faulted Basins: Case study of Beixi area,Beier Depression,Hailar Basin

依据岩芯、录井及测井等方面的资料,对海拉尔盆地贝尔凹陷贝西地区南屯组内各级次基准面旋回的演化规律及高分辨率层序发育模式进行研究。研究结果表明:研究区南屯组发育有5类层序界面,共可划分为3个长期基准面旋回和12个中期基准面旋回,地层厚度受古地貌和基准面升降的双重控制,其中古地貌对于南屯组的厚度变化影响较大,形成了由"南厚-北薄"逐渐过渡为"北厚-南薄"的地层发育模式;以南一段下部LSC1为例分析了基准面升降对相序、相域及源储分布的控制作用,认为在LSC1沉积时期,基准面的变化共经历了早期的缓慢上升阶段和快速上升阶段、中期的缓慢上升至缓慢下降阶段以及末期的快速下降阶段,主要发育有扇三角洲、湖底扇和湖泊沉积体系。其中储集体主要发育在MSC1、MSC2和MSC5沉积时期内,类型为向上变深型非对称中期基准面旋回在上升早期所形成的厚层分流河道砂体,砂体物性较好、几何形态以多层式为主。MSC3沉积时期为最大湖泛期,深湖相泥岩广泛分布、为优质烃源岩层,源储上下紧邻、成藏条件优越,是有利的勘探层系。

On the basis of core logging,well logging and other data sources,this study examined the mode of highresolution sequence development and how the base level cycles evolved at all levels in the Beixi area of the Beier Depression in the Hailar Basin. The results showed that five types of sequence boundary developed in the Nantun Formation within the study area. These are divided into three long-term base-level cycles and 12 medium-term base-level cycles. Stratigraphic thickness is controlled by changes in paleogeomorphology and datum surface. The ancient landscape has had a great influence on thickness variation,forming the‘south-thick gradual transition to north-thick’stratum model. As an example,the effect of datum elevation on phase sequence,phase domain and source reservoir distribution was analyzed in LSC1. It is considered that during LSC1 deposition,the change of base level comprised an early slow-rising stage,a fast-rising stage,a further slow-rising stage,a slow decline stage,and finally a rapid decline stage. A fan delta,a lake bed fan and a lake depositional system were developed in the study area. The sandbodies in MSC1,MSC2 and MSC5 are well developed and have beneficial physical properties. The geometrical shape is predominantly multi-layered. Thick-layer distributary channel deposits constitute the main reservoir sandbodies.MSC3 represents the longest time period of the largest lake,forming high-quality hydrocarbon source rocks. The source reservoir is close to the bottom,and a superior reservoir formation has resulted.