Characteristics of the Jurassic low-permeable clastic reservoirs in the Qiangtang Basin
Xu Jianhua1, Zhang Shiqi2, Luo Xiaorong1, Wang Zhunbei3
1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029; 2. College of Earth Resources and Information, China University of Petroleum, Dongying, Shandong 257061; 3. Mud Logging Company of Changqing Petroleum Exploration Bureau, China National Petroleum Corp., Xi’an 710021
The Jurassic clastic reservoirs in the Qiangtang Basin consisted of fine sandstones and siltstones with depositional delta,littoral and fluvial facies.Strong compaction and cementation during the burial history resulted in line-concave contact pattern between grains with calcite and siliceous cements.The main pores in the reservoirs were secondary ones including intragranular and intergranular dissolved pores.Most of the dissolved pores were middle to small sized ones with thin or tiny throats.Furthermore,the development of structural fractures played a key role in determining the permeability of the reservoirs.Laboratory experiments classified the reservoirs to be low porous(with a general porosity of lower than 5%)and low permeable(with a permeability of less than 1 mD)ones.According to classifying criteria of low permeable reservoirs,the clastic reservoirs in the studied area could be divided into three categories.Among them,the Xueshan Formation in age of Late Jurassic was the best one,the Quemocuo Formation and Xiali Formation in age of Middle Jurassic took the second and the third positions respectively.The best reservoirs were located in the northern part of the Central Uplift and the better ones lied in the central and northeastern part of the North Depression,while the reservoir qualities in the South Depression were fairly poor.Strong compaction and cementation,and weak dissolution process were main reasons for the development of the low porosity and low permeable reservoirs in this area.