The behavior of REEs indicates the dolomite petrogenesis and a unique dolomitization model of the Middle Permian Qixia formation in Northwest Sichuan Basin, China

The study on dolomite characteristics and its forming mechanism is of great significance for dolomitization theory and dolomite reservoir exploration. In recent years, the exploration of dolomite reservoirs in the Sichuan Basin has reached a bottleneck state. This study takes the Qixia Formation, Middle Permian in Northwest Sichuan basin as an example, the characteristics, distribution, diagenetic fluid, diagenetic environment of dolomite, and the dolomite formation mechanism were analyzed by means of observation and sampling of outcrops and cores, electronic scanning microscopy (SEM), and measurements of rare-earth elements (REEs), carbon isotopes, oxygen isotopes, and strontium isotopes. The results show that the dolomite exists in two forms, P-dol (dolomite in the dolomitic part of the mottled dolomitic limestone), and C-dol (crystalline dolomite). Core photos show that C-dol can be divided into two types: dolomite-A (dol-A) and dolomite-B (dol-B). The samples are low ∑REEs + Y, 13 C (1.32‰–4.32‰), negative δ 18 O (− 5.87‰ to − 1.99‰) and δCe, and high in δEu and 87 Sr/ 86 Sr ratio (0.70724–0.70923). The partition pattern of REEs in the samples shows seawater characteristics of LREE loss, La enrichment, Ce weak loss, and Gd enrichment. These characteristics indicate that the diagenetic fluid of dolomite in the study area was a hybrid of seawater and freshwater and the diagenetic environment was at relatively high temperature. Moreover, after extending the “Mg cage effect” theory proposed by Folk, it is found that Mg 2+ removed from sediments that were flushed by fresh water during the syndiagenetic stage facilitate the formation of dolomites. In accordance with the geological setting, a unique dolomitization model of the Middle Permian Qixia formation in the northwestern Sichuan Basin was built. The stratum in this model can be divided into four zones from top to bottom: freshwater vadose zone, mixed vadose zone, mixed phreatic zone, and seawater phreatic zone. The mottled dolomitic limestone is developed in the mixed vadose zone and the crystalline dolomite in the mixed phreatic zone. This study provides important evidences for the formation mechanism of dolomite and a reference to the study of ancient dolomite genesis and reservoirs of the Middle Permian Qixia formation in the northwest Sichuan Basin.