Characteristics and evolution of microbial carbonate reservoirs in the Pingliang Formation on the southern margin of Ordos Basin

Abstract

As an important type of hydrocarbon reservoir, microbial carbonate contains great amount of hydrocarbon resources. The recent discovery of industrial gas in the Majiagou Formation in the east-central part of the Ordos Basin indicates that this type of reservoir also has very extensive exploration prospects in the Ordos Basin. Similar reservoirs were also found on the southern margin of the basin, but no systematic research has been achieved. In this paper, petrological and geochemical anatomy of microbial carbonate reservoirs of the Pingliang Formation on the southern margin of the Ordos Basin were carried out. Results show that the study area developed a larger-scale microbial mound and beach complex than that in the basin, and the thickness can be more than 100 m. The microbial rock is mainly agglomerate limestone with dissolution pores. The microbial rock reservoir of the Pingliang Formation has experienced two stages of karsting, quasi-syngenesis and epigenesis, which are the key for the formation of reservoir, and there is oil and gas charging during the burial period. Among them, the quasi-syngenetic stage experienced a brief exposure, forming a large amount of irregular-shaped solution pores, and radial fibrous calcite pores filled in the early pores, with similar oxygen/carbon isotope and trace element contents such as Sr and Mg as the matrix calcite, it is then indicated that they were originated from seawater in the same period. During the epigenetic stage, the early solution pores expanded, forming a large number of fractures and cavities, accompanied by the filling of coarse-crystal calcite and mud, and some of the coarse crystalline calcite shows the characteristics of atmospheric precipitation. There is oil and gas charging during the burial period. There are bituminous residues in the present-day solution holes and fractures, and a large amount of liquid hydrocarbon-bearing inclusions can be observed in the calcite filling during the burial period. In summary, the microbial carbonate of the Pingliang Formation at the southern edge of the Ordos Basin, where both reservoir formation and hydrocarbon filling processes exist, is a potential exploration field.

Keywords

• radial fibrous calcite
• dissolution
• microbial carbonate
• pingliang formation
• ordovician
• southern margin of ordos basin