Middle Ordovician multi-stage penecontemporaneous karstification in North China: Implications for reservoir genesis and sea level fluctuations

Palaeokarsts contain important information about subaerial exposures due to sea-level regression, which has been considered as a dominant genesis of reservoirs. Herein we describe a typical sedimentary core section of the Middle Ordovician Majiagou Formation that records multi-stage penecontemporaneous karstification in North China. Petrographic observations and stable isotopic data reveal high-frequency penecontemporaneous subaerial exposure and resulting karstification. Evidence for this includes erosional surfaces, gypsum breccias, facies-controlled dissolution vugs and vadose infillings, fabric-selective dissolution, and negative carbon and oxygen isotopic excursions. Karstification generally occurred in the middle and upper parts of an upward-shallowing sequence within a single depositional cycle, leading to meteoric water dissolution of shoals and mounds. This significantly enlarged the primary pores and generated new reservoir space. As such, high-quality hydrocarbon reservoirs became well developed, which are regularly and cyclically stacked at locations a few meters below the multiple exposure surfaces. Reservoir porosity also varies systematically with the high-frequency sedimentary cycles, and is generally at a maximum in the middle and upper parts of a cycle. The multi-stage penecontemporaneous subaerial exposure and resulting karstification reflect high-frequency changes in relative sea level in North China at this time, thereby providing a framework for identifying thin carbonate reservoirs in and around the Ordos Basin.