Marine flooding induced basinal brine mixing and carbonate cementation: An example from Cretaceous ultra-deep clastic reservoirs in the Kuqa Depression, western China

Abstract

Marine flooding can significantly change hydrochemical features of pore water and provide needed ions for reservoir diagenesis. Present basinal brines and contemporaneous cements may record the participation of paleo-marine flooding. The ultra-deep (>6000 m) sandstone reservoirs of the Cretaceous Bashijiqike Formation (K_1bs) in the Kelasu thrust-fold belt, Kuqa Depression of western China, are featured by a CaCl₂-type brine and widespread eodiagenetic carbonate cementation. Through a study integrating petrographic characterization, cathodoluminescence imaging, hydrochemical feature comparison, geochemical indicators, carbon and oxygen isotopes and reactive transport simulations, we have concluded that late Cretaceous marine flooding events provided the essential ion source required for carbonate cementation and controlled major-ion compositions of the K_1bs brine. There appears to be a complementary spatial distribution of calcite and dolomite cements in K_1bs. Calcite cement occurs mainly in the northern paleo-high terrains, which was precipitated from alternating redox conditions due to periodic infiltration of meteoric water. Dolomite cement occurs mainly in the southern paleo-lows, being formed under stable reducing conditions caused by submerged seawater. Calcite precipitation, dolomitization and dissolution of K-feldspar further modified major ionic compositions of the late Cretaceous seawater during burial, leading to the formation of the present CaCl₂-type brines in K_1bs. Reactive transport modeling showed that high-flux meteoric water can promote calcite deposition, while low-flux meteoric water is beneficial to dolomite deposition. The late Cretaceous marine flooding significantly enhanced the major-ion contents, altered the hydrochemistry of the initial pore fluid and induced carbonate cementation within the terrestrial sandstone reservoirs in the seawater-meteoric water mixing zone.