广西百色盆地古近系始新统沉积相特征及演化

Characteristics and evolution of sedimentary facies of the Eocene of Paleogene in Baise Basin,Guangxi

利用野外露头、地震、测井和分析化验等资料 ,对盆地古近系始新统那读组和百岗组进行层序地层和沉积相研究。那读组和百岗组可划分为 3个Ⅲ级层序 ,9个体系域 ,那读组那三段构成层序Ⅰ ,那读组那二段和那一段构成层序Ⅱ ,百岗组构成层序Ⅲ。层序Ⅰ和层序Ⅱ是盆地断陷期沉积 ,发育冲积扇相、河流相、三角洲相、扇三角洲相、深湖亚相和滨 -浅湖亚相。层序Ⅱ湖泊沉积范围广 ,厚度大 ,厚层的湖相暗色泥岩是盆地主要烃源岩和区域性盖层。层序Ⅲ是盆地坳陷期的沉积 ,发育冲积扇相、扇三角洲相、河流相、三角洲相和滨 -浅湖亚相。盆地中古近系沉积经过了洪积或冲积扇、河流三角洲→湖泊 (扇三角洲 )→河流的发育过程 ,反映了盆地从初期形成→扩张→萎缩的完整的演化序列。在层序Ⅰ和层序Ⅱ的低水位体系域 ,田东凹陷的中央断凹、南部次凹和那笔凸起的北部断鼻等区带发育长轴分布的河流三角洲砂岩 ,这些砂岩处于烃源岩成熟区 ,油气成藏条件优越 ,是河道岩性油藏勘探的有利区。在层序Ⅱ湖侵体系域和高水位体系域 ,北部陡坡发育扇三角洲砂体 ,与烃源岩指状交叉接触 ,有利于油气运聚成藏 ,容易形成大量上倾尖灭岩性油藏。

Based on the identification of sequence boundary through outcrops,seismic data,well logs and analytical data,the Nadu Formation and Baigang Formation of the Eocene of Paleogene in the Baise Basin can be divided into one second-order sequence,and further into three third-order sequences. The Member 3 of the Nadu Formation forms Sequence Ⅰ,the Member 2 and Member 1 of the Nadu Formation form Sequence Ⅱ,and the Baigang Formation forms Sequence Ⅲ. Sequences Ⅰand Ⅱ were deposited during the faulting period of the basin,during the period alluvial fan facies,fluvial facies,delta facies,fan delta facies,shore-shallow lake subfacies,and deep-lake subfacies were developed. Sequence Ⅱ is characterized by a wide range of lake sedimentation with a great thickness,and the thick-bedded mudstones of deep-lake subfacies are the main source rocks and regional cap rocks in the basin. Sequence Ⅲ was deposited during the depression period of the basin,during the period alluvial fan facies,fan delta facies,fluvial facies,delta facies,and shore-shallow lake subfacies were developed. The Paleogene in the Baise Basin experienced a process from the deposition of fluvial facies or alluvial fan facies to fluvial and delta facies,and then to lake facies (fan delta facies),and finally to fluvial facies. This indicates that the Baise Basin experienced a complete cycle of evolution from the early rifting stage to expansion,and then to shrinkage. In the lowstand system tracts of sequences Ⅰ and Ⅱ,sandstones of fluvial and delta facies are well developed in the central faulted sag,the southern sub-sag of the Tiandong Sag and the northern faulted nose of the Nabi Uplift along the long axis. These sandstones are in mature source rock areas with a favorable condition for reservoir formation,and are in the potential areas for the exploration of channel lithologic pools. In the transgressive and highstand system tracts of sequence Ⅱ,fan delta facies were developed along the northern steep slope and their sandstones are interfingered with source rocks which are favorable for the formation of hydrocarbon pools,and are easy to form a great deal of updip pinching lithologic reservoirs.