Discovery and implications for hydrocarbon exploration of the Shenmu-Zhidan low paleo-uplift in the 4^th member of the Ordovician Majiagou Formation, eastern Ordos Basin

Breakthroughs in hydrocarbon exploration in well Mitan 1 demonstrate that the 4^th member of the Ordovician Majiagou Formation (also referred to as the Ma 4 Member) in the eastern Ordos Basin has great potential for hydrocarbon exploration. However, the limited understanding of the paleotectonic framework of the Ma 4 Member in this region has restricted natural gas exploration in this member. Using aeromagnetic data, seismic sections, and drilling data, outcrop observation, and geochemical analysis, we identify the Shenmu-Zhidan low paleo-uplift within the Ma 4 Member in the eastern Ordos Basin, as well as the formation mechanisms of the paleo-uplift. The controlling effects of the paleo-uplift on the sedimentary reservoirs of the Ma 4 Member are thereby discussed. The results indicate that the Shenmu-Zhidan low paleo-uplift extends in the NE direction, with a width of approximately 140 km, a length of around 250 km, and an area of 3.8×10⁴ km². This low paleo-uplift was formed during the deposition of the Ma 4 Member under the combined influence of the compositional differences of the paleobasement and syndepositional normal faults. Specifically, under the effects of syndepositional normal faulting, the paleobasement in the footwall of the normal faults evolves into relatively high-lying low paleo-uplifts, while that in their hanging wall underwent subsidence and evolves into relatively low-lying depressions or sags. The Shenmu-Zhidan low paleo-uplift is relatively high-lying, containing microbial mounds, mud mounds, and psammitic shoals, which can develop into high-quality dolomite reservoirs in the presence of penecontemporaneous dolomitization. Additionally, this paleo-uplift exhibits a sequence of widely developed anhydrite layers with thicknesses varying from 1 to 12 m. The dolomite reservoirs in the Ma 4 Member within the low paleo-uplift zone are sealed by tight limestones laterally and anhydrite layers vertically, allowing them to form lithologic traps with excellent sealing performance. The lithologic traps of dolomites in the Ma 4 Member within the Shenmu-Zhidan low paleo-uplift, located in present-day structurally high parts, exhibit well-developed faults and effective sealing ability, which create favorable conditions for natural gas migration and enrichment.