Water, is it necessary for fluid inclusions forming in calcite?

•Hydrocarbon inclusions were synthesized in calcite with pure crude oil system.•Polar components and organic acid are important for diageneisi in reservoirs.•Diagenesis may be not completely stopped in oil-saturated carbonate rock reservoirs.•Hydrocarbon inclusions can record the whole history of oil accumulation.•Formation fluids can react with calcite in oil-saturated carbonate reservoirs.

It is important to know how did hydrocarbon inclusions form and respond for accumulation of oil and gas in petroleum reservoirs. Also, the process of formation of fluid inclusions in reservoirs can offer the physical and chemical information about diagenesis, which are helpful to evaluate property of reservoirs. In order to understand the formation of fluid inclusions in oil-saturated carbonate reservoirs, we had done experiments on synthetic hydrocarbon inclusions in calcite using pure crude oil system. This article reports that some hydrocarbon fluid inclusions were synthesized in pure crude oil system without adding water in calcite. Reproduced experiment was done to confirm the reliability. The experimental results show that water contained in crude oil could play an important role in transferring materials for calcite growth and formation of fluid inclusions, even though only 0.4 wt% water in experimental crude oil. Some polar components and organic acid in oil can promote dissolution and re-precipitation of calcite. Although we still do not know the details of material transfer and growth of calcite in nearly pure oil fluids, our experimental results provided evidence for that diagenesis may be not completely stopped, and hydrocarbon inclusions can be formed by healing of cracks and record the information of oil accumulation even the carbonate rock reservoirs were saturated with oil. This study indicates that formation fluids can react with calcite or limestone under oil-saturated condition in carbonate reservoirs, which process may be contribute to secondary porosity forming in carbonate reservoirs.