201 / 2022-02-28 21:41:57

Genesis of Sub-member Ma55 dolostone in the Ordovician Majiagou Formation, NW Ordos Basin: evidence from petrology and major and trace element geochemistry

Genesis of dolostone; Penecontemporaneous dolomitization; Active reflux dolomitization; Ordovician Majiagou Formation; Northwestern Ordos Basin

Dolostone is one of the most significant domains for natural gas exploration in the Lower Paleozoic Majiagou formationm, Ordos Basin. However, owing to its complex texture and diagenesis, the origin of the dolostone and its influence on the reservoirs were not well understood. In this study, the petrology and geochemistry of the dolostone in Sub-member Ma55 of the Ordovician Majiagou Formation in the northwestern Ordos Basin were investigated with the aim of deciphering the genesis of the dolostone. The result shows that :(1)Two types of dolostone were identified in the study area, Type-1 dolostone is mainly composed of micritic dolomite and is characterized by a micritic texture, algae-rich laminae structure, evaporate minerals, and moldic pores. Type-2 dolostone consists of powder-fine crystalline dolomite and is mainly characterized by the apparent residual texture of the primary rock, cloudy center and clear brim textures, and intercrystalline pores. (2) Both types of dolostone have low and concentrated CaO contents and their compositions are close to that of stoichiometric dolomite, with similar ratios as between Fe/Ca and Mn/Ca. In addition, Type-1 dolostone exhibits weak cathodoluminescence for low Mn contents and medium cathodoluminescence for relatively high Mn contents. It has high TiO₂, Al₂O₃, K₂O+Na₂O, Li, and U but low TFe₂O₃ contents. In contrast, except for one sample with weak cathodoluminescence, most of Type-2 dolostone samples exhibit medium-strong cathodoluminescence and have high TFe₂O₃ and low TiO₂, Al₂O₃, K₂O+Na₂O, Li, and U contents. (3) Through comprehensive analysis, it was determined that the genesis of Type-1 dolostone can be described as follow: during a penecontemporaneous dolomitization process taken place in a near-surface environment with low temperatures, the early algae-rich aragonite and calcite were quickly metasomatized by a high-salinity pore fluid in the plaster and calcareous sediments to form the Type-1 dolostone. While Type-2 dolostone may be the result of active reflux dolomitization occurred in shallow burial and high temperature conditions from a reducing environment, a normal seawater environment deposited the underlying permeable grainy limestone, and it was metasomatized by a refluxed Mg²⁺-rich dolomitization fluid coming from an evaporation process of sea water due to density and gravity differences. Making clearing formation mechanism of reservoir is helpful for evaluation and predication of high-quality dolostone reservoir.