Evolution of diagenetic system and its controls on the reservoir quality of pre-salt dolostone: The case of the Lower Ordovician Majiagou Formation in the central Ordos Basin, China

The evolution of diagenetic system and its impact on the carbonate reservoir quality are of paramount importance. New investigation of the Lower Ordovician Majiagou Formation, central Ordos Basin, China, has revealed that the diagenesis and quality of pre-salt dolostone reservoirs were controlled by the evolution of diagenetic system. Three different reservoir types (grain, crystalline and algal dolostone) and five kinds of reservoir space (intergranular pore, intragranular pore, intercrystalline pore, dissolution pore and vug in karst system, and parallel dissolution pore of laminar algae) are identified in the Majiagou Formation pre-salt dolostone of the central Ordos Basin, based on the petrographic characteristic. The diagenetic process of pre-salt dolostone can be divided into four stages: the penecontemporaneous diagenesis, the early diagenesis, the epidiagenesis and the mesodiagenesis-late diagenesis. Petrographic and geochemical analyses of the studied dolostone reservoir indicated that the diagenetic system was open during the penecontemporaneous diagenesis and became closed during the early diagenesis. Open and closed diagenetic system existed in the west and east of the central Ordos Basin during the epidiagenesis, respectively. After that, the western and eastern pre-salt dolostone reservoirs both evolved into a closed diagenetic system during the mesodiagenesis-late diagenesis. Subaerial meteoric dissolution in an open diagenetic system (secondary pore formation), and subsequent pore preservation in a closed diagenetic system, are the critical factors in the improvement of pre-salt dolostone reservoir quality. Although two cycles from open to closed diagenetic system developed in the western dolostone reservoir, the pore space there was filled with overlying weathering mud in the second open diagenetic system during the epidiagenesis, which reduced the reservoir quality. The eastern dolostone reservoir experienced just one cycle from open to closed diagenetic system, the reservoir quality was improved and then preserved. Thus only a good match among the diagenetic systems during different diagenetic stages will be good for the reservoir formation and preservation. Therefore, the potential high-quality reservoirs in the pre-salt dolostones mainly located in the eastern part of the central Ordos Basin.