摘要： 滩坝砂体是岩性油气藏勘探的重要储集体,已经成为中国陆相湖盆增储上产的重要现实领域。为了深入评价柴达木盆地西部扎哈泉地区新近系中新统上干柴沟组滨浅湖滩坝砂体的油气勘探前景以及预测有利勘探区带和勘探目标,文中在岩心观察和钻(测)井资料综合分析的基础上,结合岩石薄片、累计粒度概率曲线、暗色泥岩泥地比和砂地比敏感参数分析结果,对该地区上干柴沟组滨浅湖滩坝砂体发育演化特征和古地貌演化特征进行了详细研究。结果表明: (1)扎哈泉地区上干柴沟组沉积时期发育辫状三角洲—滨浅湖沉积体系,滩坝砂体发育,岩性主要为岩屑长石细砂岩—极细砂岩,成分成熟度中等—较差、分选磨圆度中等—好,粒度概率曲线以反映波浪作用的两段跳跃式加悬浮式为主;沉积构造类型多样,主要发育浪成沙纹交错层理、波状层理、丘状交错层理和透镜状层理。(2)上干柴沟组沉积时期,研究区滩坝砂体发育规模先增大后减小;早期到中期,滩坝砂体发育规模不断增大,其中近岸坝主要表现为向南和向东2个方向扩张,呈北偏西方向展布,远岸坝主要表现为南北方向扩张,呈近北西—南东方向展布;中期到晚期,滩坝砂体的发育规模不断减小,其中近岸坝主要表现为向南北2个方向收缩并向东扩展,展布方向变为东偏南转东北方向,远岸坝主要表现为南北方向收缩。(3)上干柴沟组沉积时期,研究区古地貌呈西高东低、南北低中间高的特征;从早期到中期,研究区中部古地貌高点不断向东和东南方向大幅度扩展,范围不断扩大;中期到晚期,古地貌高点范围不断向北和北西2个方向收缩,范围缩小;同时,研究区东南角古地貌高点范围不断向北扩大。综合上述分析结果,认为柴达木盆地扎哈泉地区上干柴沟组滩坝砂体发育演化受控于物源规模大小和微古地貌的发育演化,其中微古地貌发育演化是滩坝砂体发育演化的主要控制因素。研究成果可为相似的地质背景下滩坝砂体成因的岩性圈闭预测提供参考。

关键词: 滩坝砂体, 古地貌, 上干柴沟组, 新近纪, 柴达木盆地

Abstract: The beach bar sand body,which is the significant reservoir for lithologic oil and gas reservoir exploration,has become an important exploration field of increasing reserves and production in China's lacustrine basin. In order to further evaluate the oil and gas exploration prospect of the beach bar sand bodies which developed in the shore-shallow subfacies in the Upper Ganchaigou Formation in Neogene in Zhahaquan area,western Qaidam Basin and predict the favorable exploration zone and target,based on the core observation and comprehensive analysis of drilling and logging data,combined with the analysis results of rock thin sections,cumulative particle size probability curves,dark mudstone mud-to-ground ratio and sand-to-ground ratio sensitive parameters,the development and evolution characteristics of shore-shallow lake beach-bar sand bodies and paleogeomorphology in the Upper Ganchaigou Formation in this area are studied in detail. The results show that: (1)The braided delta-shallow lake sedimentary system was developed in the Upper Ganchaigou Formation in Zhahaquan area. The beach bar sand body was developed,and the lithology was mainly lithic feldspar fine sandstone-ultrafine sandstone. The compositional maturity was medium-poor,and the sorting roundness was medium-good. The particle size probability curve was mainly composed of two-stage jump and suspension reflecting wave action. Sedimentary structure types are diverse,including wave sand ripple cross bedding,wavy bedding,hilly cross bedding and lenticular bedding. (2)During the sedimentary period of the Upper Ganchaigou Formation,the scale of beach bar sand body in the study area first increased and then decreased. From the early to the middle period,the scale of beach bar sand body continued to increase. Among them,the nearshore bar mainly expanded in the south and east directions,and distributed in the north-west direction. The farshore bar mainly expanded in the north-south direction,and distributed in the northwest-southeast direction. From the middle stage to the late stage,the development scale of beach bar sand body decreases continuously. Among them,the nearshore bar mainly shrinks in the north-south direction and extends eastward,and the distribution direction changes from southeast to northeast. The farshore bar mainly shrinks in the north-south direction. (3)During the deposition period of the Upper Ganchaigou Formation,the paleogeographic is characterized as high in the west and low in the east,low in the north and low in the south,and high in the middle part in the study area. From the early to the middle period,the paleogeomorphic high points continued to expand significantly to the east and southeast directions,and the scope was expanding. From the middle to the late period,the range of paleogeomorphology high points continued to shrink in the north and northwest directions,and the range of paleogeomorphology high points in the southeast corner of the study area continued to expand northward. Comprehensive analysis of the above results shows that the development and evolution of beach bar sand body of the Upper Ganchaigou Formation in Zhahaquan area is controlled by the source scale and the development of micropaleogeomorphology. Among them,the evolution of micropaleogeomorphology is the dominated factor for the development of beach bar sand body. The research results can provide reference for lithologic trap prediction of beach bar sand body under similar geological setting.

Key words: beach bar sand body, paleogeomorphology, Upper Ganchaigou Formation, Neogene, Qaidam Basin

中图分类号: 

• P512.2