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

Abstract

The carbonates in the Middle Ordovician Ma 5 5 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 evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater. The d 18 O 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 d 18 O values (-8.5 to -6.7 %) and Sr 2? concentration and slightly higher Na ? , Fe 2? , and Mn 2? concentrations and 87 Sr/ 86 Sr 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.

Multiphase Diagenetic Processes and Their Impact on Reservoir Character of the Late Triassic (Rhaetian) Kingriali Formation, Upper Indus Basin, Pakistan

“…The early replacive dolomites of Kingriali Formation exhibit high Sr concentration, which suggests that they did not form directly from normal or evaporative seawater, neither display any relics of the Sr purification during recrystallization under burial conditions [80]. Similar to this, the concentrations of Fe 2+ (201.36 and 2201 ppm), Mn 2+ (484.81 and 389.93 ppm), and Na 1+ (293.8 and 253.3 ppm) observed in the early replacement dolomites (RD-I and RD-II) do not exhibit enrichment from any other source and are within the range of the corresponding seawater, as explained by various researchers [84]. RD-I and RD-II have lower salinity values, indicates that the early dolomitization did not proceed with the involvement of complex basinal brines, evaporating seawater, or hydrothermal fluids.…”

“…According to information provided under the dolomite water distribution coefficient, the theoretical equilibrium concentration of Sr in dolomites created by common seawater ranges from 470 to 550 ppm [83]. Lower Sr values are present in finely crystalline penecontemporaneous dolomites formed in an evaporitic environment or by the evaporating dolomitizing fluids [84]. The early replacive dolomites of Kingriali Formation exhibit high Sr concentration, which suggests that they did not form directly from normal or evaporative seawater, neither display any relics of the Sr purification during recrystallization under burial conditions [80].…”

Constraining hydrocarbon migration and accumulation by two-dimensional numerical simulation: Ordovician carbonate reservoirs of the Daniudi Area, Ordos Basin

“…Daniudi area is in the northern of Ordos Basin and the northern of Yishan slope, covering an area of about 2000 km 2 (Figure 1(a) and (b)). It is a gentle uplift in the direction of northeast-southwest (Bai, 2016). It has a low structure in the west and a high structure in the east, and many faults are developed but with short extension lengths (Figure 1(c)).…”

Dolomitization of the Ordovician subsalt Majiagou Formation in the central Ordos Basin, China: fluid origins and dolomites evolution

“…Precipitation of gypsum would increase the Mg/Ca ratios of bottom waters, which also promoted dolomitization. Sea level fluctuations during this stage (Bai et al 2016;Xi et al 2017;Liu et al 2018) controlled the depositional sequence of lagoon-shoal facies and water exchange between the platform interior and the open marine environment. This facilitated continuous dolomitization and alternation of Md1 and Md2 dolomites (Fig.…”