Seepage reflux dolomitization and telogenetic karstification in M ajiagou F ormation in the O rdos B asin, C hina

This study investigates the characteristics of the heterogeneous carbonate reservoir of Majiagou Formation in Jingxi area, Ordos Basin, China, to explain the diagenetic control and distribution of high‐quality reservoir intervals. The carbonates of the study area mainly consist of silt‐sized crystalline dolomite, microcrystalline dolomite, and microcrystalline limey dolomite. Both dolomite content and dolomite size gradually decrease both from top to bottom and from slope to basin. Intercrystalline pores exist in the silt‐sized crystalline dolomite, and vuggy pores and fractures mainly developed in microcrystalline (limey) dolomite. Furthermore, typical geopetal infills and three stages of calcite cements could be differentiated. Based on petrographical and geochemical analysis, the reservoir is dominantly controlled by seepage reflux dolomitization and telogenetic karstification. Four reservoir evolutions could be differentiated. The reservoir in the first model is dominated by intercrystal porosity occurring in silt‐sized crystalline dolostone. It experienced strong dolomitization, telogenetic karstification, and shallow as well as deep burial cementation. The reservoir in the second model consists of only a small amount of high‐angle dissolution fissures and vertical dissolution cavities in microcrystalline (limey) dolostone. It experienced relatively weak dolomitization, strong telogenetic karstification, and two important calcite cementation events. The reservoir in the third model is marked by many vuggy pores and dissolution fractures, occurring in microcrystalline (limey) dolostone. The strata experienced relatively weak dolomitization, strong telogenetic karstification, and two weak calcite cementation episodes. The reservoir in the fourth model is dominated by fractures and their related dissolution pores, which developed in microcrystalline (limey) dolostone. These dolostones experienced relatively weak dolomitization, telogenetic karstification, and calcite cementation. From a reservoir point of view, the reservoirs represented by the first and third models, which respectively are composed of silt‐sized crystalline dolomite occurring in the top of the whole dolostone succession and the dolomite affected by telogenetic karstification, are the most vital exploration targets.