Effects of diagenesis on shale nano-pore structure and implications for sealing capacity

Abstract

A B S T R A C T:The effect of diagenesis on shale petrophysical characteristics is being investigated as part of a study on shale sealing capacity because of its significance for modelling hydrocarbon charge histories of sedimentary basins. Results to date indicate that diagenesis (degree of cementation and dissolution) significantly affects porosity and inter-connectivity of the nano-pores (0.3~0 nm), the pores constituting the main pore-throats for tight shales. Diagenesis causes tight shale permeabilities to vary over a range exceeding an order of magnitude (113 21 -6 x 10 2o m 2) and porosities to vary between 1 and 12%. In addition, diagenesis significantly influences shale nano-pore resistance to collapse during compaction and burial, mainly at depth > 2-3 km, affecting hydrocarbon trapping, overpressure and sealing capacities. Dissolution tends to delay the timing of excess pressure pulses, while cementation has a reverse effect. The significance of diagenesis is reduced at shallower depths.

Preferred Orientation of Phyllosilicates in Gulf Coast Mudstones and Relation to the Smectite-Illite Transition

“…With further increase in depth, porosity is constant or slightly larger. Similar results showing continuous increases in porosity as a function of depth, with no discontinuities associated with the S-I transition, have been reported by many investigators (e.g., Katsube and Williamson, 1994;Vernik and Liu, 1997). Hence, although there are minor changes in porosity with increasing depth correlating approximately with the S-I transition, such changes are subtle compared with the marked change in preferred orientation.…”

Modeling free gas content of the Lower Paleozoic shales in the Weiyuan area of the Sichuan Basin, China

“…The main factors that influence the porosity of a shale include its inorganic and organic compositions, and its diagenesis or thermal maturation experienced (Jarvie et al, 2007;Katsube et al, 1992Katsube et al, , 1994Katsube et al, , 2000Kuila and Prasad, 2010). While the original intergrain pores in shale will decrease with increasing diagenesis, the secondary pores, especially the intragrain organic nanopores could be formed (Katsube et al, 1992(Katsube et al, , 1994(Katsube et al, , 2000Kuila and Prasad, 2010).…”

“…While the original intergrain pores in shale will decrease with increasing diagenesis, the secondary pores, especially the intragrain organic nanopores could be formed (Katsube et al, 1992(Katsube et al, , 1994(Katsube et al, , 2000Kuila and Prasad, 2010). As illustrated by Jarvie et al (2007), the organic nanopores will be created when a shale is matured to the stage of oil window as a result of the hydrocarbon generation and expulsion, and the pore sizes and volumes will increase with further increasing the thermal maturity.…”

Multiscale porosity estimates along the pro-and retrograde deformation path: an example from Alpine slates

“…Diagenetic cementation processes play an important role as porosity reducing mechanism in these slates accompanying dissolution by pressure solution (e.g. Katsube and Williamson, 1994).…”