Based on core petrographic description

fracture statistics

comprehensive interpretation of seismic and logging data

analogues with other confirmed examples

this paper analyzed the identification marks

development timing

genetic model and distribution of the hydrothermal dolomitization and its alteration on the reservoir quality

so as to analyze the abnormal distribution of dolomite in the Kometan Formation of the Cretaceous oilfield A in Zagros Basin.The most part of Kometan Formation and the lower part of the Shiranish Formation in two wells were altered by hydrothermal dolomitization.The matrix limestone there was replaced by hydrothermal flow with the crystal intergrowth

half automorphic

and coarse grain dolomite.Early extensional fractures were filled with saddle dolomite and anhydrite.Celestine-saddle dolomite-anhydrite is the main hydrothermal mineral assemblage and can be used to identify the hydrothermal origin.The hydrothermal dolomitization preferentially flow upwards along the extensional and transtensional fault systems

where dolomitization mainly occurs on the hanging walls(graben)and extends laterally in the porous and permeable layers.In this case

the geometry of altered rocks was with stratification

the limestone strata near the fractured zone were also altered by hydrothermal dolomitization which display great irregularity. The alteration of hydrothermal dolomitization on reservoir is complicated.The dolostone formed along the main fault has lower matrix porosity than dolomite a little far from main faults

because over-dolomitization usually happened in the position with more active hydrothermal fluid flow.Local reservoir quality is obviously improved with matrix porosity of about 6%.The dolomite is generally more fractured than the limestone

so dolomitization has greatly increased the fracture density and improve the flow capacity of the reservoir.This study is significant for reservoir modeling and production strategies.