Control on paleokarst reservoir heterogeneity: integrating surface-subsurface water systems and paleogeomorphology in the Lungu area, Tarim Basin

Deeply buried Ordovician carbonate reservoirs in the Tarim Basin face significant exploration risks due to extreme heterogeneity. Focusing on the Lungu area, this study integrates high-resolution 3D seismic, well-log, and core data to unravel the multiscale genesis of epikarst fracture-vug reservoirs. We reconstruct the pre-Carboniferous paleogeomorphology and delineate karst water systems to quantify their control on reservoir development. Results indicate that the Yingshan Formation is dominated by sparry grainstones, with reservoir space governed by secondary dissolution. Seven secondary geomorphic units were identified, revealing a stepwise decrease in epikarst zone thickness from ∼45 m (highlands) to ∼10 m (basins). Crucially, we identify seven secondary karst water systems, demonstrating that their boundaries (watersheds, faults, dense fillings) act as primary controls on reservoir. A “Tectonics-Paleogeomorphology-Hydrodynamics” ternary model is established: highlands favor fracture-type reservoirs via rapid vertical percolation, while gentle slopes develop superior fracture-vuggy reservoirs through diffuse flow and prolonged water-rock interaction. The coupling of NEE-trending master faults and surface drainages creates optimal “sweet spots” with large cave systems. This research shifts the paradigm from static geomorphic mapping to dynamic water-system-constrained evaluation, providing a robust framework for predicting fracture-vug units in deep karst plays.