2013 Volume 122 Issue 1 Pages 69-89
Silica mineral phase transformations in addition to mechanical compaction and cementation, affect burial diagenetic processes in diatomaceous sediments. Thus, the porosity and/or permeability of siliceous sediments abruptly change with the transformations. Among these diagenetic changes, the transformation from opal-CT to quartz is the most important process in petroleum geology. In this transformation process, permeability improves due to the improvement of pore connectivity induced by the net-like development of new pores. If enough silica is supplied during the process, the silica attaches selectively to portions with high permeability. In this process, the formation of chert just below the transformation boundary is possible. Chert is the most brittle of siliceous rocks and is capable of forming fractured reservoirs. If insufficient silica is supplied, quartz porcelanite with a low clay content shows high reservoir quality within the one-thousand meter interval below the transformation depth. If a clay-rich layer is interbedded in that interval, mechanical compaction progresses selectively in the layer. As a result, opal-CT porcelanite directly above the transformation boundary and clay-rich quartz porcelanite layer below the boundary seal hydrocarbons that have migrated from deeper zones. Meanwhile, if the basin uplifts and the transformation depth becomes shallower than about 500 meters, the permeability of opal-CT porcelanite above the transformation boundary could be as high as the permeability of the quartz porcelanite below it. In that case, opal-CT porcelanite directly above the transformation boundary does not provide an effective seal.
