Neogene siliceous rocks are known to act as both reservoirs and seals. However, the role of diagenetic alteration in the development of hydrocarbon reservoirs, seals, and traps in such formations is poorly understood. This study proposes two new models for hydrocarbon traps involving siliceous rock reservoirs with well-developed matrix porosity. The models are based on observations from the Yurihara and Toyotomi fields in Japan to evaluate changes in petrophysical properties associated with confining pressure and diagenetic transformations from opal-CT to quartz.
When this diagenetic transformation boundary is located at depths greater than approximately 1000 m (∼3300 ft), the overlying opal-CT porcelanite layer forms a seal and the underlying clay-poor quartzose porcelanite forms a reservoir, facilitating the development of a hydrocarbon trap. The quartzose porcelanite containing less than 6% Al2O3 (an indication of clay content) can act as a reservoir even 1000 m (3300 ft) below the boundary.
When the boundary is located at depths shallower than approximately 500 m (∼1650 ft), the overlying opal-CT porcelanite is unable to form a seal because of lower confining pressures—although clay-poor quartzose porcelanite continues to be a good reservoir. However, clay-rich quartzose porcelanite intercalated with the reservoir can act as a seal. When the quartzose porcelanite contains greater than 15% Al2O3, it has the potential to seal at the depth of the boundary. The requirement for a seal drops to greater than 8% Al2O3 at 1000 m (3300 ft) below the boundary.
Such traps have not been reported outside Japan; therefore, these models are likely to be useful for hydrocarbon exploration in siliceous rock formations elsewhere, such as in the Monterey Formation of California.