The characteristics of Lower Cretaceous Xiagou Formation pelitic dolostone tight oil reservoir, Qingxi sag
Sun Weifeng1,2, Song Yan1,2,3, Gong Yanjie1,2, Luo Qun3, Wang Chongxiao4
1. Research Institute of Petroleum Exploration and Development, Beijing 100083;
2. State Key Laboratory of Enhanced Oil Recovery, Beijing 100083;
3. College of Unconventional Natural Gas, China University of Petroleum, Beijing 102249;
4. Research Institute of Petroleum Exploration and Development, Yumen Oilfield Company, PetroChina, Jiuquan, Gansu 735000
The Lower Cretaceous Xiagou Formation is the main tight oil reservoir in Qingxi Sag. In this study, the reservoir characteristics including petrologic characteristics, reservoir space types, and pore structures were studied with the methods of thin sections, casting thin sections and high pressure Hg injection. The results show that: 1)The rock types of Xiagou Formation reservoir are mainly dark grey-gray lamellar shaly dolomite and dolomite mudstone with higher content of mud level plagioclase, which can improve rock brittleness and make it easier to form fractures; 2)There are various types of reservoir spaces in the pelitic dolostone reservoir, including primary pores, tectonic micro-fractures and dissolution fractures; 3)The physical properties of pelitic dolostone reservoir are low porosity and low permeability, and there are significant differences among different lithologies; 4)The results of high pressure Hg injection show that the pelitic dolostone reservoir is high in displacement pressure and median capillary pressure, low in pore throat radius. The overall feature is poor pore structure and strong heterogeneity; 5)The pelitic dolostone reservoir in the study area can be divided into three categories based on the parameter of Ep; class Ⅰ, Ep < 1.29, with best pore throat sorting and weakest heterogeneity; class Ⅱ, 1.29 < Ep < 1.79, with better pore throat sorting; class Ⅲ, Ep > 1.79, with poor pore throat sorting.