Characteristics and genetic mechanism of weathering crust on carbonate rocks on the top of the Ordovician in southern margin of Ordos Basin
XIONG Jiabei1,2,3, HE Dengfa2,3, CHENG Xiang2,3, LUO Yufeng1
1 PetroChina Zhejiang Oilfield Company,Hangzhou 311100,China; 2 School of Energy Resources,China University of Geosciences(Beijing),Beijing 100083,China; 3 Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Enrichment Mechanism(Ministry of Education), China University of Geosciences(Beijing),Beijing 100083,China
Abstract：During the Caledonian tectonic movement period,the Ordos Basin was uplifted as a whole in the Middle and Late Ordovician,and underwent deposition again in the Late Carboniferous. It has experienced nearly 150 million years of weathering and erosion,resulting in the development of an Ordovician carbonate weathering crust,which is highly important for natural gas accumulation. In this paper,a typical well was selected from the southern margin of the Ordos Basin. Based on the mineral composition,major and trace elements,and logging response characteristics,the vertical structure,degree of weathering and alteration,and paleoclimate environment of the weathering crust were analyzed,and the formation process and preservation mechanism were reconstructed. The results indicate that: (1)The weathered crust can be divided into a weathered residual layer,a strongly weathered layer,a weakly weathered layer,and a bedrock layer from top to bottom. The overall weathering degree is strong,and the logging response characteristics in each layer segment are very significant. (2)In this weathering crust profile,the main elements TiO2,Al2O3,and Fe2O3 are enriched,with slight depletion of SiO2 and strong depletion of Ca,Mg,and Na. (3)Based on paleolatitude data and relevant geochemical indicators,it is believed that the southern part of the Ordos Basin developed transitional sedimentary facies between sea and land during the late Paleozoic. The average annual temperature during the formation of weathered crust was 28.15 ℃,and it was in a humid-semi-arid environment,belonging to a tropical-subtropical climate. (4)The widespread development of cracks in each layer of the weathered crust indicates the existence of a karst fracture-cave system in the southern region of the Ordos Basin. Although the thickness of each structural layer of the weathered crust varies due to the influence of terrain elevation differences in the later stage,the probability of preserving the weathered crust structural layer is greater in areas with relatively flat terrain,suggesting that this location is favorable for later exploration of weathered crust gas reservoirs. These results can provide a reference for the exploration of oil and gas reservoirs in karst weathering crusts in the Majiagou Formation and aluminum-bearing rock series in the Benxi Formation in the Ordos Basin in the future.
XIONG Jiabei,HE Dengfa,CHENG Xiang et al. Characteristics and genetic mechanism of weathering crust on carbonate rocks on the top of the Ordovician in southern margin of Ordos Basin[J]. JOPC, 2024, 26(1): 100-118.
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