Characteristics and exploration strategy of the Middle Permian hydrothermal dolomite in southwestern Sichuan Basin

Hydrothermal dolomitation can form high-quality dolomite reservoirs. Through mineralogic observations, geochemical analyses and comprehensive interpretations of seismic profiles, the Middle Permian saccharoidal dolomite in Qixia and Maokou formations which are related with hydrothermal dolomitization have following characteristics: (1) higher occurrence frequency, greater thickness in a single layer (5~40 m), host limestone dominated by bioclasitc limestone or bioclasitc micrite; (2) residual bioclasitc textures developed in mesocrystalline-macrocrystallinpore dolomites and bioclast-dissolved pores partly filled with authigenic quartz and saddle dolostone mostly with intergranular and corroded pores, which porosity averages between 6%and 10%; (3) an obviously higher homogenization temperature of dolomite matrix (125~178°C) than the stratum temperature during dolomitization, which is indicated by more negative oxygen isotopes (-10.1‰~-11.42‰), higher 87Sr/86Sr(0.709194~0.710174)and positive Eu anomaly; (4) well-developed basement faults adjacent to dolomite-producing wells and a sunken reflection in corresponding seismic profiles. Generally speaking, close to basement fault zones, hydrothermal fluid dolomitization is intensive and dolomite reservoirs are thick and in good lateral continuity, where structure traps are exploration target. On the contrary, far from basement fault zones, hydrothermal fluid dolomitization is weak and dolomite reservoirs are thin and in poor lateral continuity, where hydrocarbon exploration should focus on litho-structure traps.