Abstract
The 5th sub-member in 5 Member Ordovician Majiagou Formation in Daniudi Gas Field, Ordos Basin, is deposited in an environment consisting of an ancient epicontinental sea, where very fine crystalline dolostone reservoir has developed. In this study, based on the petrological and geochemical characteristics, the genesis of the dolomite developed in M55 were studied by analyzing the properties and sources of the dolomitization fluids, and the influence of the paleogeomorphology differences on the distribution of dolostone was also discussed in order to clarify the distribution of the dolostone developed in the lime flat of the epicontinental sea. The dolostone of the M55 had a crystal structure, mainly including microcrystalline and very fine crystalline. The content of MgO and CaO in dolomite was negatively correlated, indicating that it was the result of replacement. The dolomite was dark red under cathode luminescence, and the distribution mode of rare earth elements showed the negative anomaly of Ce and Eu, indicating that the dolomitization fluid was sea-sourced fluid. The δ13C, δ18O, and 87Sr/86Sr isotope range of limestone was similar to that of Ordovician seawater in the study area, whereas the δ13C, δ18O, and 87Sr/86Sr of dolostone were obviously more positive than that of limestone. The substitute index of the salinity (Z) of the dolomitization fluid was higher than 122, which is higher than limestone (Z = 120.5), indicating that the dolomitization fluid was slightly evaporated seawater. The wormholes observed on the core and the gypsum in the penecontemporaneous period observed in the thin sections indicated that the dolostone was formed in a period when the sea level was relatively low, and it was the result of seepage–reflux dolomitization. By analyzing the correlation between the thickness of dolostone and the paleogeomorphology of the M55 of the sedimentary period, it was found that the thickness of dolostone at relatively high altitude was significantly larger than that of other areas. The development of dolostone was controlled by sea level, and the local paleogeomorphology controls the distribution of dolostone during the period of low sea level. There were many more limestone–dolostone cycles and larger cumulative thicknesses of dolostone at relatively higher topography. This study provides a theoretical basis for the prediction of the distribution of dolostone reservoirs in the carbonate tidal flat environment dominated by lime flats under the background of the ancient epicontinental sea.
Highlights
Dolostone, as an important oil and gas reservoir rock, accounts for more than half of carbonate reservoirs [1,2,3]
In order to better predict the distribution of dolostone reservoirs, geologists have long been focusing on the study of the dolomitization mechanism and many achievements have been accomplished such as the penecontemporaneous sabkha model [4,5,6] and the seepage–reflux model under an evaporation environment [7,8,9], which are used to explain the genesis of island dolostone [10,11,12], the hydrothermal fluid model related to hydrothermal solution [13,14,15], the fault-controlled model of the combination of hydrothermal or formation fluid [16,17,18], and the intermediate–deep burial dolomitization with large burial depth, which is often accompanied by hydrothermal development [19,20]
These dolomitization models explain the origin of dolomite from different sources and the dynamics of magnesium-enriched fluids, and they have played an important role in the prediction of dolostone reservoirs
Summary
As an important oil and gas reservoir rock, accounts for more than half of carbonate reservoirs [1,2,3]. In order to better predict the distribution of dolostone reservoirs, geologists have long been focusing on the study of the dolomitization mechanism and many achievements have been accomplished such as the penecontemporaneous sabkha model [4,5,6] and the seepage–reflux model under an evaporation environment [7,8,9], which are used to explain the genesis of island dolostone [10,11,12], the hydrothermal fluid model related to hydrothermal solution [13,14,15], the fault-controlled model of the combination of hydrothermal or formation fluid [16,17,18], and the intermediate–deep burial dolomitization with large burial depth, which is often accompanied by hydrothermal development [19,20] These dolomitization models explain the origin of dolomite from different sources and the dynamics of magnesium-enriched fluids, and they have played an important role in the prediction of dolostone reservoirs. The sedimentary of the 5th sub-member in 5 Member of the Ordovician
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