June 22, 2017

Controls on petroleum storage in shales: “Porosity makes the difference”

GEOS4 staff has co-authored a research paper showiing that while the amount of petroleum retained in shales is a function of total organic carbon (TOC) content as well as chemical structure of the macromolecular organic matter, porosity is the fundamental control of how much recoverable oil can be stored. Thus, predicting the occurrence of porosity is a key element in the exploitation strategy not only of conventional but also of unconventional reservoirs.

Porosity evolution in the Niobrara, Barnett and Posidonia Shales is largely controlled by changes in kerogen density brought about by swelling and shrinkage as a function of thermal maturation.

While porosity development in organic matter can sometimes be linked with secondary cracking, secondary organic pores mainly form after the maximum kerogen retention (swelling) ability is exceeded and kerogen starts to shrink. Kerogen shrinkage begins at ~445°C Tmax (~0.8% Ro). Shirkage is accompanied by a decreasing oil retention capacity (reflected by decreasing OSI = S1*100/TOC values).

Han, Y., Horsfield, B., Wirth, R., Mahlstedt, N., Bernard, S. (2017): Oil retention and porosity evolution in organic-rich shales. - AAPG Bulletin, 101, 6, p. 807-827.