Early dolomitization and subsequent recrystallization of Middle Ordovician carbonates in the western flank of the Ordos Basin, Northern China: Implications for hydrocarbon exploration

The pervasive dolostone from the Middle Ordovician Zhuozishan Formation in the Ordos Basin (Northern China) are considered as potential hydrocarbon reservoirs. Three types of matrix dolomites (Md) are recognized: 1) very fine to fine crystalline, nonplanar-a to planar-e dolomite (Md1); 2) fine to medium crystalline, nonplanar-a to planar-e dolomite (Md2); and 3) medium crystalline, planar-s to nonplanar-a dolomite (Md3). The initial dolomitization of Md1 and Md2 dolomites is inferred to have taken place in the presence of seepage reflux of slightly evaporated seawater in the near-surface to shallow burial settings, and insignificant recrystallization occurred in the process of being buried again, according to petrography, trace elements, and oxygen-carbonstrontium isotopic geochemistry. Coarser crystals with cloudy cores and clear rims consist of the Md3 dolomites, and the texture shows irregular contacts between planar-s and nonplanar-a crystal. Geochemical analysis reveals that the Md3 dolomites have a negative shift in delta O-18 values with increasing Mn contents, and a negative delta O-18 trend with depth in the same well, while the delta C-13 does not tend to correlate with the oxygen isotopes. Additionally, the Sr-87/Sr-86 ratios of Md3 dolomites are relatively higher due to contribution of radiogenic Sr-87. Petrological attribute and geochemical evidence suggest that the massive Md3 dolomites can be due to significant recrystallization of earlier formed dolomites during burial period. The unevenly distributed pores in Md3 dolomites suggest the heterogeneous structure of the precursor dolomites, which may be the Md2 dolomites. The residual particles of Md2 dolomites indicate the precursor limestones were deposited in a relatively high-energy part of the shoal. Hence, the wide distributed Md3 dolomites show salient facies-controlled characteristics, which suggests strategies for hydrocarbon exploration of potential heterogeneous dolomite reservoir.

Automated summary

This study investigated dolostones in the Ordos Basin and identified three types of matrix dolomites. Based on petrography, trace elements, and oxygen-carbonstrontium isotopic geochemistry, it is suggested that the initial dolomitization of Md1 and Md2 dolomites may have occurred in near-surface to shallow burial settings, while Md3 dolomites likely resulted from significant recrystallization during burial.