Bush Jr formed Arbusto Oil in mid 80's
Arbusto was failing and was sold to Spectrum 7 (Bush retained some ownership & involvement)
Spectrum 7 began to fail and was being sold to Harken Energy (owned mostly by Soros)
Spectrum 7 and Harken had about reached agreement when Enron announced it began producing from a field 10% owned still by Bush Jr via his interest in Spectrum 7. Ken Lay was in charge of Enron at this time, as I recollect.
The Spectrum/Harken deal fell apart and Spectrum/Enron managed the field together.
It is alleged (unproven to date) the volume of business Spectrum/Enron did was inflated.
Aside from the appearances Bush Sr. Jr, Soros, Ken Lay, etc, these transactions would have gotten little attention. But allegedly, some investors (I don't know in which companies) claim they were mislead. Again this is informed speculation, but I think this is the "smoking gun" such as it is.
Here are 61 "questionable" hits from a google search on George Bush Jr and Arbusto, Harken, Spectrum...
Not much creditability, but it gives you an idea for the basis of the allegations.