The moldic pore evolution of the Middle Ordovician sabkha dolostone in Ordos Basin, China: A study based on the petrographic and geochemical characteristics of pore fillings

In the Ordos Basin, the sabkha dolostone reservoir with moldic pores of the fifth Member in the Middle Ordovician Majiagou Formation (M5) dominated the gas production of the Lower Palaeozoic. Recent research studies revealed the complex history of the moldic pore evolution, which intensively affected the reservoir potential evaluation. The moldic pores were formed by dissolution of the gypsum concretion and then experienced a different extent of multi-phase filling. In order to understand the evolution of the moldic pores and explain its affection on the development of reservoirs, comprehensive petrographic and geochemical analysis including the cathodoluminescent, scanning electron microscope, trace and rare earth elements in situ, and strontium isotopes have been utilized to study the moldic pore fillings. The results reveal the moldic pore evolution: (a) the gypsum concretions were dissolved and the moldic pores were formed in the early diagenetic stage; (b) the calcite fillings formed in the vadose zone and phreatic zone during the early diagenetic stage in the sabkha environment; (c) the ferroan dolomites formed during the process of conversion of montmorillonite to illite at the burial diagenesis stage, at the burial depth of 3,500 m-4,000 m in the middle-Late Jurassic. Significantly, filling in the late burial period is more conducive to the pore preservation compared with the filling in the early diagenetic stage, and the formation of the sabkha dolostone reservoir lies in the preservation of pores in the early diagenetic stage. Consequently, the results of this study provide a framework to differentiate the petrographic and geochemical characteristics of kinds of pore fillings and to understand the moldic pore evolution and its affection on the development of reservoirs in similar sedimentary environments.

Automated summary

In the Ordos Basin, the sabkha dolostone reservoir with moldic pores of the fifth Member in the Middle Ordovician Majiagou Formation (M5) dominated the gas production of the Lower Palaeozoic. Recent research studies revealed the complex history of the moldic pore evolution, which intensively affected the reservoir potential evaluation.