Sedimentary evolution and controlling factors of beach bar sand bodies of the Upper Ganchaigou Formation of Neogene in Zhahaquan area, western Qaidam Basin
YI Dinghong1, SHI Yajun1,2, LI Xiang1,2, LONG Guohui3, NI Xianglong1, LI Jiyong3, WANG Zhaobing3, MA Xinmin1
1 PetroChina Research Institute of Petroleum Exploration and Development-Northwest,Lanzhou 730020, China; 2 Key Laboratory of Reservoir Description,CNPC,Lanzhou 730020, China; 3 PetroChina Qinghai Oilfield Company,Gansu Dunhuang 736202, China
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.
YI Dinghong,SHI Yajun,LI Xiang et al. Sedimentary evolution and controlling factors of beach bar sand bodies of the Upper Ganchaigou Formation of Neogene in Zhahaquan area, western Qaidam Basin[J]. JOPC, 2022, 24(4): 713-727.
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