Are Gas Shales Suitable Analogs for Oil Shale Exploration?
Gas shales are not suitable direct exploration analogs for oil resource plays. First, reservoirs with properties sufficient for gas production may be insufficient for oil production. Second, oil in resource plays may have migrated from their source beds, whereas gases in mudrock reservoirs are almost always self sourced. Finally, gas sorbed in reservoirs can contribute to production whereas oil sorbed in the reservoir cannot be desorbed.
Subsurface oil viscosity is about 100 times higher than gas viscosity. Oil reservoirs therefore need about 100 times higher permeability to produce at equivalent subsurface volumetric rates. Oil reservoirs have less drive. The oil reservoir must have higher permeability to compensate for the lower drive. Oil drive increases with higher initial pressure and dissolved gas content. Fractures may compensate for lower matrix permeability, but they will not compensate for lower drive and the displacement nature of oil production.
Gas migration into nanoDarcy reservoirs is difficult or impossible. Gas-charged nanoDarcy reservoirs must be self sourced. The gas-productive fairway can be predicted from the thermal maturity and source rock quality. In contrast, oil can migrate into reservoirs with microDarcy permeability; self sourcing is not needed. Oil can accumulate in thermally immature, low permeability reservoirs, depending in part on migration and sealing patterns.
Oil sorbs so tightly to organic matter that sorbed oil cannot be produced by any primary production mechanism. Organic matter increases reservoir sorption, so high organic content may reduce the oil recovery efficiency from a tight oil reservoir.
Characteristics favorable for oil resource plays are the following.
- Permeability should exceed a microDarcy or so, and oil saturation should be high to give a high oil relative permeability. Oil saturation needs to be corrected for the oil sorbed on kerogen so that calculated saturation is that of the volumetrically stored oil.
- In situ GOR and pressure need to be relatively high. This requires suitable oil thermal maturity and relatively deep reservoirs.
- Reservoirs need to be in a position favorable for charging from a thermally mature oil source rock. The reservoir itself can be thermally immature.
AAPG Datapages/Search and Discovery Article #90214 © 2015 Southwest Section AAPG Annual Convention, Wichita Falls, Texas, April 11-14, 2015