Hydrothermal activity and its influence on hydrocarbon accumulation in deep dolomite reservoirs of the Ediacaran Dengying Formation, Sichuan Basin

There are abundant hydrothermal events within the Dengying Formation dolomite of the Precambrian system in southwest China. Methods including petrography identification, fluid-inclusion observation, in-situ U-Pb dating, and in-situ measurement of rare earth element (REE), etc. are integrated to characterize hydrothermal activity process within the Dengying Formation dolomite. The hydrothermal activity therein can be divided into four stages on the basis of in-situ U-Pb dating results of saddle dolomite cements. The 1st-stage (415.0–400.0 Ma) and 2nd-stage (259.4–248.0 Ma) hydrothermal events are characterized by saddle dolomite filling along the margin of fractures, or filling within dilational breccia and zebra textures. Compared with matrix dolomite and seawater-derived fibrous dolomite, saddle dolomite exhibits obvious negative anomalies of Ce elements. The 3rd-stage (225.6–199.0 Ma) hydrothermal event is represented by galena, sphalerite and other Mississipppi Valley-type (MVT) mineral cements in residual space. The formation of lead-zinc ore is due to the precipitation of metal sulfide caused by the thermo-chemical sulfate reduction (TSR) reaction between hydrothermal fluids and hydrocarbons during the large-scale hydrocarbon charging period. The 4th-stage (130.0–41.0 Ma) hydrothermal event is characterized by quartz and a small amount of fluorite filling the residual pores with dolomites. Quartz and fluorite record the migration of deep high-temperature hydrothermal fluid along early fractures and residual pores. During this period, the hydrothermal fluids result in the heterogeneous structure of bitumen, which is a clear response to high-temperature hydrothermal activity.