Discovery of syngenetic and eogenetic karsts in the Middle Ordovician gypsum-bearing dolomites of the eastern Ordos Basin (central China) and their heterogeneous impact on reservoir quality

The Middle Ordovician Majiagou Formation in the eastern Ordos Basin (central China) represents a typical case of karst in gypsum-bearing carbonate strata. It is unique in a long karst history of ca. 120 Ma since the deposition to the Late Carboniferous. The lithology is complex with the development of both fine-grained and granular dolomites. Thus, the karstification and its impact on reservoir quality is challenging. To improve the understanding of this critical issue and further provide data for exploration, we collected core samples within the representative first and second sub-members of the fifth Member of the Majiagou Formation (herein referred to as Ma 5(1+2)) and conducted comprehensive analyses of their physical and geochemical properties. Results show that Ma 5(1+2) is generally composed of gypsum-bearing micritic dolomites and grain dolomites. Karst breccias were observed and two periods of karstification were recognized. The first period is syngenetic and shows typical features such as gypsum molds and intragranular pores. It is predominantly developed within gypsum-bearing micritic dolomites due to selective fabric dissolution. Isotopic data indicate the influence of syngenetic meteoric water and seawater, as indicated by slightly depleted delta C-13 values (average 0.03 parts per thousand), relatively low delta O-13 values (average - 8.35 parts per thousand), and slightly elevated Sr-87/Sr-86 values (average 0.70999). The second period is interpreted as inland eogenetic karstification and shows typical features such as non-selective fabric dissolution, including pores and vugs, breccias, and karst fillings in grain rocks. The rocks were subjected to differential dedolomitization. Grain dolomites were strongly altered by meteoric water and record a terrestrial input, as suggested by negative carbon and oxygen isotope excursions (average delta C-13 and delta O-13 values are respectively -3.07 parts per thousand and -8.97 parts per thousand) as well as high Sr-87/Sr-86 values (average 0.71093). The two periods of karstification had a profound and heterogeneous influence on the reservoir quality. Gypsum molds formed due to the alteration of gypsum nodule-bearing micritic dolomite by early syngenetic karstification, and represent the best reservoir space in Ma 5(1+2). This reflects the key role of syngenetic karstification in the formation of high-quality reservoirs. In contrast, the late inland eogenetic karstification is mainly manifested as strong alteration of grain dolomites, with little effect on micritic dolomites due to its relatively poor physical properties. The collapse resulting from strong dissolution destroys the continuity of reservoirs. In addition, relatively high porosity and permeability promote fluid movement, resulting in the infilling of the karst cave system by mud and carbonate sand. Thus, a so-called inversion of physical property occurs, namely the filling of original large pores in the grain dolomites leads to poor reservoir at last, and the preservation of initially small pores in the micritic dolomites leads to relatively good reservoir at last. This indicates that the evolution of reservoir is complex under the influence of karst.