Genesis of Dolomite from Ma5S-Ma510 Sub-members of the Ordovician Majiagou Formation in the Jingxi Area in the Ordos Basin

• Published in: 地质学报（英文版） Vol. 91; no. 4; pp. 1363 - 1379
• Main Authors: LIU Jingdong, JIANG Youlu, LIU Xinshe, YANG Zhiwei, HOU Xiangdong, ZHU Rongwei, WEN Caixia, WANG Feiyan
• Format: Journal Article
• Language: English
• Published: School of Geosciences, China University of Petroleum, Qingdao 266580, China 2017; State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China%School of Geosciences, China University of Petroleum, Qingdao 266580, China%Research Institute of Exploration & Development, Changqing Oilfield Company, PetroChina, Xi'an 710018,China%Marine Oil Production Plant, Sinopec Shengli Oilfield, Dongying 257000, China%Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Science, Guangzhou 510301, China
• Subjects: genesis of dolomite; burial dolomitization; Ordos Basin; microbial dolomitization; Ordovician; seepage reflux dolomitization
• ISSN: 1000-9515

We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma55-Ma510 sub-members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin:(1) Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth-shape dolomite cements.(2) Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle-deep burial stage,forming dolomites mainly consisting of micro crystal and powder crystal structure.(3) Powder dolomite,fine dolomite,and medium-coarse crystalline dolomite formed in pores and fractures in the middle-deep burial stage.The secondary concussive transgression-regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area.The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period.In the sediments,sulfate content was high,which is conducive to the preservation of microbial activity and microbial dolomitization.Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization.Affected by evaporation seawater with increased Mg/Ca ratio,seepage reflux dolomitization was widely developed and formed large-scale dolomite,and underwater uplifts and slopes are favorable areas for dolomite.In the middle-deep burial stage,dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.