PRIMARY PETROLEUM MIGRATION FROM SHALES WITH OXYGEN-RICH ORGANIC MATTER

The mechanism of primary petroleum migration (PPM) is no better understood today than 20 years ago. Serious unanswered problems exist with the popular model of monophasic or “bulkphase” PPM from shales with oxygen‐rich organic matter (OM). Among these problems are: (1) How are molecularly dispersed hydrocarbons (HCs) gathered and concentrated into microdroplets and/or filaments for migration? (2) How are astronomical capillary‐displacement pressures overcome at the low burial temperatures (50d̀ to 60d̀C) over which bulk‐phase migration mechanisms have been proposed to operate? (3) What accounts for the significant compositional differences between crude oils and shale bitumen?The hypotheses of structured water and chromatographic adsorption were proposed to solve these problems. Both hypotheses are theoretically improbable. The two hypotheses contradict eachother and no evidence has been advanced of their existence in Nature. Substantial evidence from both Nature and the laboratory suggest that chromatographic adsorption does not exist. The hypothesis of structured water compounds the problem it was proposed to solve.PPM by gaseous solution at minimal burial temperatures of 150d̀C removes the problems associated with monophasic PPM. These burial temperatures, either alone or combined with microfracturing, remove the barriers created by high capillary‐displacement pressures. Microfracturing of shales must occur prior to and during PPM, no matter what the migration mechanism. Evidence of microfracturing exists from both the laboratory and Nature. The HC‐ gathering problem is solved by a difusive partitioning (“scavenging”) of HCs by aqueous solution through shale‐pore water to dispersed HC gas bubbles. Compositional differences between shale bitumen and crude oil are not only explained but are predicted by PPM by gaseous solution, and thus serve as evidence for PPM by gaseous solution. Laboratory data demonstrate that, even at mild burial temperatures, an HC‐gas phase has sufficient carrying capacity to account for oil deposits. Evidence of PPM by gaseous solution exists, from studies carried out in different sedimentary basins.The contention is invalid that if PPM by gaseous solution occurs at all, it only occurs at high maturation ranks, the only time when sufficient quantities of the C1 to C4 HC gases are thought to be generated. Significant generation of C1 to C7 (but especially C1 to C4) HCs commences in shales with oxygen‐rich OM at burial temperatures of 66d̀C (Ro = 0.40), as shown by data from both the laboratory and Nature. This generation at low maturation ranks has previously gone undetected because of inadequate analytical procedures. The contention is also invalid that the low gas‐oil‐ratios (GORs) of many oil pools invalidate the possibility of PPM by gaseous solution. Due to the high generation capacity of oxygen‐rich OM for the C1 to C4 gases, compared to C6+ HCs, GORs are more than adequate at the time of PPM to account for PPM. Post‐PPM processes significantly lower the GORs of oil deposits. In the Author's opinion, most HCs move from shales with oxygen‐rich OM by gaseous solution.