Unconformity characteristics of Huaiyuan movement in the northeast of Wushenqi paleo-uplift in Ordos Basin and its implications for karst reservoir generation

Huaiyuan movement during the first and second episodes of the Caledonian was a major tectonic activity in the Early Paleozoic Ordos Basin, featuring long-term sedimentary hiatus and forming a more effective karst reservoir near the paleo-uplift, with the impact second only to the Caledonian (the third episode)-Hercynian movement. To the east side of Wushenqi paleo-uplift, the first member of the Ordovician Majiagou Formation (Ma 1 Member) was in parallel, unconformable contact with the Cambrian Sanshanzi Formation as the result of Huaiyuan Movement, and on either side of the unconformity were carbonate strata with great differences in terms of electrical logging properties, mineral, elemental and isotopic compositions. The Sanshanzi Formation was of fine-crystalline dolomites with irregular dissolved pores and karst reservoirs. The well logging response shows low gamma value and high resistivity at the interval. The reservoirs of the formation were mostly deposited in relatively high-energy shoals near the paleo-uplift with normal seawater, and experienced reflux dolomitization at shallow burial stages. The Ma 1 Member is the first interval of sediments after the Huaiyuan movement, featuring complex lithology and mixed deposition of anhydrite, dolomite, mud and sand due to its proximity to the Wushenqi paleo-land. In terms of the depositional setting, the Ma 1 member was of marine type, that is, mixed tidal flat near the paleo-land in the early transgression period, though with great difference from the Sanshanzi Formation. The climate was relatively arid during its deposition, and the deposits were dominated by marine carbonates with intermittent terrigenous clastic supply. Its dolomite was a result of enecontemporaneous sabkha dolomitization. Controlled by both the primary facies belt with relatively high energy and the dolomitization as well as karstification related to the Huaiyuan movement, the reservoirs in the Sanshanzi Formation were mainly formed by a combination of dolomitization and epigenetic karstification and are widely seen in the vicinity of Wushenqi, Zhenyuan and Lyuliang paleo-uplifts.