Secondary Migration of Petroleum as a Self-Adjusting Colloidal Flow Explains Many of the Enigmas of Conventional and Unconventional Plays
Molecular and colloidal transport processes play an essential role in petroleum systems, yet they remain understudied. In this talk, I enlarge on a hypothesis I have developed in which secondary migration of petroleum occurs as a self‐adjusting colloidal flow of nano‐ and microdroplets (Stainforth, 2012, 2016). This mechanism avoids the difficulties of the popular mechanism of migration as a continuous phase Darcy flow. In the new hypothesis, the controlling viscosity is that of the pore water rather than the petroleum, so the mechanism works equally well for all petroleum types regardless of their viscosity. In addition, capillarity does not enter the problem as long as the nano‐ and microdroplets are smaller than the pore throats of the carrier beds. Thus, the permeability of the rock has little influence on the migration down to a critical pore throat size. The mechanism is also much faster and more efficient than Darcy flow of continuous phases of petroleum. It leaves little evidence of the passage of petroleum except for colloidal concentrations in pore waters and fluid inclusions (which are samples of the petroleum colloid) where cementation occurred during migration. This colloidal migration breaks down under three conditions in which the effluxes of petroleum from an elemental volume of the carrier beds cannot keep up with the influxes: 1) When the dip of the carrier beds tends to zero: this leads to the accumulation of conventional petroleum pools. 2) The density of the petroleum is heavy and tends towards that of the surrounding pore water. This leads to the accumulation of unconventional heavy oils and tars. This explains why the tar deposits are not significantly denser than the surrounding pore water and raises the possibility that biodegradation of the major tar deposits of the world has been greatly exaggerated. 3) The pore throats of the carrier become so small, by advancing diagenesis, that the colloidal flux balances cannot be maintained. This leads to the accumulation of continuous phases of gas of the “basin‐centered” type in carrier beds and of the “shale gas” type in adjacent source rocks. The point at which this happens is sensitive to the temperature history of the rocks because of the very different kinetics of carrier bed diagenesis and petroleum cracking reactions. Regarding the “basin centered” gas plays, colloidal secondary migration explains both the high gas saturations (in spite of prohibitive capillary entry pressures) and the under‐pressures in the continuous gas phase.
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019