Hypogenic karstic cavities formed by tectonic-driven fluid mixing in the Ordovician carbonates from the Tarim Basin, northwestern China

Abstract

Enhanced hydrogeologic circulations promoted by tectonics are commonly linked to karstic cavity formation in carbonate rocks, providing superb reservoirs for hosting energy resources (i.e., hydrocarbon and geothermal) in sedimentary basins. Predicting such cavern reservoirs in the deep subsurface is difficult mainly due to uncertainties in timing the tectonics and characterizing their associated fluids, which hamper the related hydrocarbon exploration. By combining carbonate U-Pb chronology, geochemistry, and seismic data analyses of fracture and cave-filling carbonates in cavern reservoirs from the Ordovician units of the Tarim Basin, northwestern China, the current study sought new evidence for fluid activities related to tectonics. Crucially, carbonate U-Pb ages confirm that these karstification events were closely related to syn- and/or postmineralization faulting by local tectonics. Geochemistry signatures in the authigenic minerals of fractures further suggest that the episodically developed meteoric water mixed with deep basinal brine. The carbonate dissolution rate might have been markedly enhanced by active hydrologic circulation and fluids mixing or even the formation of sulfuric acid, thus promoting the formation of karstic cavities that was closely related to the deep-rooted fractures and faults. This study highlights the indispensable role of hypogenic karstification in the formation of cavern carbonate reservoirs in the Ordovician units of the Tarim Basin and the outcome from this new contribution may provide useful guidelines for hydrocarbon exploration in the basin and other global analogues.