Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

Pet. Sci. Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China Xiao-Liang Bai 0 1 2 3 4 Shao-Nan Zhang 0 1 2 3 4 Qing-Yu Huang 0 1 2 3 4 Xiao-Qi Ding 0 1 2 3 4 Si-Yang Zhang 0 1 2 3 4 0 Research Institute of Petroleum Exploration & Development, PetroChina , Beijing 100083 , China 1 School of Geoscience and Technology, Southwest Petroleum University , Chengdu 610500, Sichuan , China 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University , Chengdu 610500, Sichuan , China 3 Department of Geology, University of Regina , Regina, SK S4S 0A2 , Canada 4 College of Energy, Chengdu University of Technology , Chengdu 610059, Sichuan , China The carbonates in the Middle Ordovician Ma55 submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized. Two types of replacive dolomite are distinguished: (1) type 1 dolomite, which is primarily characterized by microcrystalline (30 lm), euhedral to subhedral dolomite crystals, and is generally laminated and associated with gypsumbearing microcrystalline dolomite, and (2) type 2 dolomite, which is composed primarily of finely crystalline (30-100 lm), regular crystal plane, euhedral to subhedral dolomite. The type 2 dolomite crystals are truncated by stylolites, indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites. Stratigraphic, petrographic, and geochemical data indicate that the type 1 dolomite formed from near-surface, low-temperature, and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head. The absence of massive depositional Carbonate platform; Dolomite; Middle Ordovician; Dolomitizing fluids; Dolomitization; Ma55 submember - & Shao-Nan Zhang evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater. The d18O values (-7.5 to -6.1 %) of type 1 dolomite are slightly lower than those of seawaterderived dolomite, suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial. The type 2 dolomite has lower d18O values (-8.5 to -6.7 %) and Sr2? concentration and slightly higher Na?, Fe2?, and Mn2? concentrations and 87Sr/86Sr ratios (0.709188–0.709485) than type 1 dolomite, suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial. 1 Introduction The origin of dolomitization has long been a subject of discussion (Warren 2000; Machel 2004; Gregg et al. 2015). Various models have been proposed to explain the origin of dolomite in carbonate platforms (Warren 2000; Machel 2004; Swart 2015), including regional subsurface flow models (sometimes referred to as burial-flow models) at elevated temperatures (Jones and Rostron 2000) and ‘‘early’’ synsedimentary models, such as those involving seepage reflux (Adams and Rhodes 1960). These models have been used to explain the massive dolomite in syndepositional evaporites (Jones and Rostron 2000; Qing et al. 2001) or in seawater with elevated salinity (Rott and Qing 2013; Read et al. 2012; Rivers et al. 2012). However, the development of small-scale, discrete dolomites in platform carbonates in association with elevated salinity seawater and little gypsum is less well documented. This study focuses on the early, pervasive dolomitization of shallow marine platform carbonates by penesaline seawater related to sea-level changes and the subsequent recrystallization of this early-formed dolomite during burial. This study then interprets the discrete, regional-scale distribution of dolomite that may have originated from modified seawater with high-frequency sequence cyclic changes and the precipitation of dolomitic fluids during burial. The carbonates in the Middle Ordovician Ma55 sub member of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized and are an attractive target for hydrocarbon exploration. In particular, the discovery of gas reservoirs in the dolomitic strata in the Jingbian Gas Field was an important finding involving the Ma55 submember, which displays good natural gas exploration prospects among the old dolomite strata of the Ordos Basin. Tests in well Su-203 resulted in a large gas flow rate of over 104 9 104 m3/d from the dolomite reservoirs (Yang and Bao 2011; Zhao et al. 2014), and horizontal well PG3 had a gas flow rate of over 10 9 104 m3/d from the dolomite reservoir of the Ma55 submember. Since then, exploration and development of the dolomite gas pools in the Ma55 submember have been the key goal. The dolomite in the Ma55 submember is characterized by an uneven and discontinuous distribution of dolomitic carbona (...truncated)