Abstract:

In recent years， several exploration wells have revealed low-yield natural gas in the concealed structures of the 8^th member of the Upper Paleozoic Shihezi Formation and the 1^st member of the Upper Paleozoic Shanxi Formation in the thrust zone along the western margin of the Ordos Basin. Hence， investigating the concealed structures in the footwall of nappes is significant for natural gas exploration in the study area. Using the latest drilling results， seismic data， observations of petrographic thin sections， and organic geochemical data， we explore the characteristics of the Paleozoic concealed structures in the footwall of thrust nappes within the central section of the western margin of the Ordos Basin. Accordingly， the geological conditions for hydrocarbon accumulation and hydrocarbon exploration potential of the Ordovician Kelimoli Formation is analyzed and favorable exploration targets are proposed in the concealed structural zone. The results indicate that， under the influence of intense W-E-oriented stress of nappes during the Yanshanian movement （Jurassic to Cretaceous）， strata in the footwall of nappes in the study area deformed under passive compression and folding， resulting in the extensive development of SN-oriented concealed structures distributed in rows. These concealed structures exhibit large-scale traps and underdeveloped faults， suggesting favorable sealing performance. The Ordovician Kelimoli Formation contains high-energy ramp shoal reservoirs consisting primarily of medium-to-coarse-crystalline dolomites. These reservoirs exhibit storage spaces dominated by intercrystalline pores， intercrystalline dissolution pores， and fractures， with porosity ranging from 3 % to 6 % and permeability from 1 × 10^-3 to 8 × 10^-3 μm². Two suites of high-quality marine source rocks occur in the Wulalike and Kelimoli formations， with kerogen dominated by Types I and II1 and vitrinite reflectance （R_o） ranging from 1.50 % to 1.90 %. The study area exhibits both vertical and lateral hydrocarbon supply， a favorable source rock-reservoir configuration， and the formation of the concealed structure traps is well matched with the main hydrocarbon generation and expulsion period of source rocks. The superimposed areas of low-amplitude concealed structural zones and high-energy shoals serve as natural gas enrichment areas， with the favorable superimposed areas of 300 km² from preliminary prediction. Additionally， the Yandunshan， Huianbu， and Majiatan concealed structural zones in the Majiatan section along the western margin of the Ordos Basin are identified as favorable areas for future natural gas exploration in the Kelimoli Formation.

Key words: high-energy shoal zone, concealed structure, hydrocarbon accumulation potential, Wulalike Formation, Kelimoli Formation, Ordovician, western margin of the Ordos Basin