北部湾盆地乌石凹陷东区古近系流沙港组二段砂泥薄互层沉积相特征及演化*

北部湾盆地乌石凹陷东区古近系流沙港组二段构造复杂,沉积差异大,横向变化快,对物源和沉积相的认识存在争议。为了查明研究区沉积相分布与演化规律,文中以岩心、测井、地震和分析化验资料为基础,结合区域构造背景,对研究区流沙港组二段沉积物源、沉积相特征及演化规律展开研究。分析认为,流沙港组二段物源来自于北部高地势地区,发育三角洲前缘—深水湖泊沉积序列,主要发育相对远物源的正常三角洲前缘沉积,砂体单层厚度小、多层交互式分布。井震结合由下到上将流沙港组二段划分为4个四级层序,其中MSC2和MSC3层序为短期快速水退到较长期水进过程,物源供给较为充足,砂体发育程度高,是主力砂体展布层段;流沙港组二段储集层物性主要受埋深及沉积相带约束,B油田物性好于A油田,三角洲前缘近源端发育的水下分流河道、河口坝为有利储集层分布区。

Sedimentary characteristics and evolution of thin interbeds of the Member 2 of Paleogene Liushagang Formation in eastern Wushi sag,Beibu Wan Basin

The structure and sedimentary characteristics of the Member 2 of Liushagang Formation in the eastern Wushi sag,Beibu Wan Basin are very complex. The sedimentation is of great variation with sharp lateral changes. The understanding of sediment provenance and sedimentary facies is controversial. Using core,logging,seismic,and laboratory data,the present study attempts to: (1)investigate sediment provenance,sedimentary facies and evolution of the Member 2 of the Liushagang Formation in the study area;(2)determine the distribution and evolution of sedimentary facies in the study area;(3)predict favorable reservoir distribution characteristics;and(4)to better serve the oil and gas exploration and development. Results from this study suggest that the northern high-lying area represents the sediment provenance of the Member 2 of the Liushagang Formation,which is characterized by the delta front-deep lake sedimentary sequence. The normal deltaic sediments were well developed,and the thickness of individual sand bodies is small,which manifests as multilayer interactive distribution. The Member 2 of the Liushagang Formation is divided into four 4th￣order sequences from bottom to top. MSC2 and MSC3 sequences formed during the short-term rapid water regression were involved into the process of longer-term water transgression. The sediment supply was sufficient,sand bodies were,thus,well developed. Observations from this work suggest that the buried depth and sedimentary facies play a pivotal role in the modulation of the reservoir properties of the Member 2 of the Liushagang Formation. Reservoir properties of B oil field are better than those of A oil filed. The subaqueous distributary channel and estuary dam developed near the source of the delta front are favorable reservoir distribution areas.