Controls on storage capacity in mudstones. Cementation before sediment compaction and preservation of porosity in lithified rock

Abstract

This contribution tackles the origin of porosity and cements in organic-rich mudstones, both important factors in the production of energy resources from unconventional reservoirs. A thorough chronology of sedimentary processes and early diagenesis depicts how most of the porosity (storage capacity) was preserved by 1) bottom current delivery of sand-size coccolith-rich pelagic fecal pellets to the seafloor, 2) limitation of pervasive early calcite cementation by reverse weathering, 3) cementation of pre-compaction microcrystalline quartz that stiffened the rock and limited the compactional porosity loss.

The presence and chronology of early cements, dissolution fabrics, and migrated oils indicate that significant volumes of fluid flowed through these sediment/rocks, and that fluid transport had a dramatic effect on the evolution of the pore filling histories because much of the cementation occurred prior to compaction. The preservation of porosity due to early cementation enabled subsequent flow of both aqueous and hydrocarbon fluids with consequential impacts on diagenesis and the ability of these mudstones to act as reservoir rather than seal facies.

The results emphasize that the prediction of facies in organic-rich mudstones should be based on pre-compaction diagenetic processes, which can significantly enhance or occlude porosity. In fact, facies in organic-rich mudstones are not satisfactorily predicted by the traditional methods that investigate solely environments of deposition and sequence stratigraphy.