Abstract:

The Ordovician sedimentary and karst environment as well as karst reservoir characteristics in the southern margin are completely different from those in the central-east of Ordos Basin.A shallow well drilled into the southern margin revealed the existence of karst caves filled with clastic rocks.Two sets of strata were recognized as the Benxi and Pingliang Formations, with the former dominated by bauxite mudstone and the latter by grainstone and algal limestone interbedded with multiple sets of mudstone layers.Mineral composition, major element, trace element and rare earth element analyses were carried out to compare the source, weathering alteration degree, redox environment, climate and paleo-tectonic environment of the two sets of mudstones.The results show that the Benxi Formation mudstones are of higher-degree weathering and the Pingliang Formation mudstones were deposited in a more reductive setting.The sedimentary environment of both sets represents a transition from sea to land with frequent climate changes against tectonic backgrounds of active continental edge and continental island-arc in the Late Paleozoic.The Pingliang Formation mudstones of terrigenous clast origin that represents cavities and underground river deposits in the pre-Carboniferous karst period, deposited during wet karst development stage when the southern Ordos Basin were uplifting.Considering the widespread fractures at the upper and lower parts of the formations, we suggest that a karst fracture-cavity system once existed in the southern Ordos Basin.Although the caves revealed by the well are filled with clasts, it can be inferred that there may be residual fractures and cavities at certain paleomorphological locations in the southern Ordos Basin, which are of great value to future oil and gas exploration.

Key words: geochemistry, mudstone, cavity, paleo-karst, Benxi Formation, Pingliang Formation, Upper Ordovician, Ordos Basin