Lower limits of petrophysical parameters allowing tight oil accumulation in the Lucaogou Formation, Jimusaer Depression, Junggar Basin, Western China

摘要

In this study, we investigate the petrophysical parameter values above which oil can accumulate, using a tight oil reservoir in the Jimusaer Depression, Junggar Basin, as an example. Statistical analysis of well-drilling data and effective porosity measurements is integrated with numerical simulations of force balance to illuminate the mechanisms that place a lower limit on the petrophysical parameters of tight hydrocarbon reservoirs. Further, we discuss the factors that influence these limits. The results show that when porosity is observed to be less than 5%, corresponding to a pore-throat radius of more than 0.2 mu m, exploratory wells reach reservoirs comprising only water layers. The lower limit for oil accumulation is reached when irreducible water saturation reaches 100% and porosity is approximately 5%. This fits well with the values arrived at by modeling the force balance. In the reservoirs under consideration, oil charging is driven by excess pressure resulting from hydrocarbon generation and is resisted by capillary force. When the pore-throat radius is larger than 0.2 mu m, the capillary force will be greater than the driving force, and tight reservoirs cannot be charged with oil. This corresponds to a porosity of approximately 5%, matching the results derived from observations. The values of these lower limits depend on many factors, such as source rock quality, depth, temperature, hydrocarbon-generation kinetics, and interfacial tension, and hence vary under different geological conditions. Thus, in lacustrine basins in China with type I kerogen-dominated source rocks, the lower limits are smaller than in those where other types dominate. Differences in the geothermal gradient mean that, at similar hydrocarbon-generation temperatures, the lower limits for reservoirs in Western China are smaller than for those in the east. There is also a smaller lower limitation porosity in tight gas reservoirs than in tight oil reservoirs due to the difference in interfacial tension.