Development regularity and genetic mechanism of weathering crust reservoirs in the western Ordos Basin: take the sub-members 1-4 of Member 5 of Majiagou Formation in Tao 2 block as an example
Feng Qiang-Han1, Xu Shu-Mei2,3,4, Chi Xin-Qi2,3, Shu Peng-Cheng2,3, Kong Jia-Hao2,3, Cui Hui-Qi2,3, Ma Hui-Lei2,3
1 Changqing Oilfield Company,PetroChina,Xi’an 710021,China; 2 Key Lab of Submarine Geosciences and Prospecting Techniques,Ministry of Education,Ocean University of China,Shandong Qingdao 266100,China; 3 College of Marine Geosciences,Ocean University of China,Shandong Qingdao 266100,China; 4 Frontiers Science Center for Deep Ocean Multispheres and Earth System,Shandong Qingdao 266100,China
Abstract The top of the Majiagou Formation of the Lower Paleozoic in the Ordos Basin suffered from long-term weathering and denudation and formed a weathered crust karst reservoir dominated by gypsodolomite. The sub-members 1-4 of Member 5 of Majiagou Formation of the Tao 2 block in the western part of Ordos Basin are mainly made up of gypsodolomite interbedded with dolomite and calcite dolomite. Based on core observation,polarized microscopic analysis,and scanning electron microscopy analysis,the multi-layered gypsodolomite are identified. Through comparing the physical and chemical properties among anhydrite,dolomite and calcite,it is concluded that the main pore space of the sub-members 1-4 of Member 5 is dissolution pores of anhydrite nodules and crystals and the accompanying swelling microfracture. The pore size is self-limited,which is restricted by the size of anhydrite nodule and crystal. Gypsodolomite and anhydrite bearing tidal flat of the supratidal zone is the prerequisite material basis and environmental condition for the formation of reservoirs,and thus lead to the strata-controlled distribution of gypsodolomite gas reservoirs. Four stages of pores dissolution are identified,with fabric selective dissolution and phased filling dissolution pores of anhydrite nodule and crystal are the key processes of reservoir formation. The reservoir is a lithologic gas reservoir controlled by the karstification of anhydrite bearing dolomite. Karstification palaeogeomorphology no longer has a critical control on the reservoir distribution. The diffusive percolation corrosion of the dispersive fresh water on the gypsodolomite strata is the main reason for the lack of the large cavities. The viewpoint of “stratified and karstified sedimentary facies controlled lithologic gas re servoirs” will promote the deep transformation of reservoir forming mechanism in the Lower Paleozoic of the study area from “karstification palaeogeomorphic-controlled gas reservoirs” to “sedimentary facies controlled lithologic gas reservoirs”.
Fund:Financially supported by the Research Project of PetroChina Changqing Oilfield Company Limited(No. DSCQC-2019-370)
Corresponding Authors:
Xu Shu-Mei,born in 1970,is a professor.She is engaged in studies of sedimentology and basin analysis. E-mail: xsm@ouc.edu.cn.
About author: Feng Qiang-Han,born in 1968,is a senior engineer.He is engaged in studies and management of gas exploration. E-mail: fqh_cq@petroChina.com.cn.
Cite this article:
Feng Qiang-Han,Xu Shu-Mei,Chi Xin-Qi et al. Development regularity and genetic mechanism of weathering crust reservoirs in the western Ordos Basin: take the sub-members 1-4 of Member 5 of Majiagou Formation in Tao 2 block as an example[J]. JOPC, 2021, 23(4): 837-854.
Feng Qiang-Han,Xu Shu-Mei,Chi Xin-Qi et al. Development regularity and genetic mechanism of weathering crust reservoirs in the western Ordos Basin: take the sub-members 1-4 of Member 5 of Majiagou Formation in Tao 2 block as an example[J]. JOPC, 2021, 23(4): 837-854.
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