Paleocave architectures and controlling processes of the Ordovician carbonate paleokarst systems in western and central Tarim Basin, northwestern China

The paleokarst systems of the Ordovician carbonate rocks in the Tarim Basin, northwestern China, comprise economically significant oil and gas reservoirs and display complex cave architectures. Based on comprehensive analysis of seismic, well log, core, and outcrop data, the cave architecture and controlling processes of the Ordovician paleokarst systems in the western margin and central uplift belt of the basin are documented. Cave fills of the paleokarst systems are composed mainly of collapse breccias, crackle or mosaic breccias, chaotic breccias, terrestrial sediment fills, and calcareous muddy deposits. Primary architectural elements of the paleokarst systems include surface collapse caves or pits, fractured roofs and walls, sinkholes and associated small-scale caves, cave-level or fault-cave complexes, fractured layers with fractured pores or cavities, and densely spaced small cavity layers. The paleokarst cave structures are characterized by the development of multiple phreatic cave-level or fault-cave complexes and were constrained mainly by the interplay of changing phreatic zones due to multiple stage uplifts or relative sea-level falls and existing high-angle fault belts. They were also transformed by karstification with composite unconformities and hypogenic process. The paleokarst system in the central uplift belt formed in a carbonate island environment during the late Middle Ordovician, whereas the system in the western basin margin developed in an attached carbonate platform setting at the end of the Late Ordovician. Partially filled cave-level or fault-cave complexes, fractured cave roofs, or fractured intervals with fractured pores or cavities comprise the most significant reservoirs in the paleokarst systems.