Fluid Inclusion, Isotopic, and Elemental Geochemistry Studies of Cave-Filling Calcite in the Lower-Middle Ordovician Yingshan Formation of Tahe Oilfield, NW China: Implication for Karstification in Non-exposed Limestone

Improving the recognition of paleo-fluid circulation history is of great significance to reconstruct pore evolution during carbonate diagenesis. Integrated petrography, fluid inclusion, isotopic and elemental geochemistry (laser ablation inductively coupled plasma mass spectrometry) studies, calcites generated in paleocaves, and fractures were investigated. This study aims to reveal the paleo-fluid origin and karstification event within the nonexposed limestone of the Lower-Middle Ordovician Yingshan Formation in the Tarim Basin. The only generation of blocky calcite growing along the karst paleocave and fracture walls [cave-filling calcite (CFC)] crosscuts burial stylolites. The secondary fluid inclusions obtained from CFC are characterized by the coexistence of liquid-only and liquid-dominated aqueous inclusions with low salinities values (0-2.4 wt%), suggesting that the CFC has experienced a low-temperature environment (<50 degrees C). The depleted delta O-18 values (-15.32 parts per thousand to 12.45 parts per thousand), seawater-like yttrium and rare earth element patterns, and low sigma REE (<0.65 ppm) have recorded the major episode of meteoric water leaching the Yingshan limestone. This view was further confirmed by the calculated delta O-18(water) values of parent fluids (-14.3 parts per thousand to 2.2 parts per thousand). The relatively higher Sr-87/Sr-86 ratios (0.70942-0.70994) are interpreted as the result of meteoric water interacting with the overlying Silurian detrital rock when percolated downward. The geochemical evidence recorded by CFC indicates that the karstification event for Yingshan nonexposed limestone possibly took place in the Early Hercynian period during the late Devonian. Therefore, meteoric water percolating downward along the fractures penetrating insoluble strata and/or migrating laterally along the permeable strata is deemed responsible for the karstification event, although the overlying thick insoluble strata in the coverage area.