longtimer, CPC has 100% of the block and has/had to give up 50%, SHT (chad) keeps 30% of all oil but does not participate in development costa. So the way I have interpited it CPC had 100% ownership which gets to keep 70% of the oil, they then pay 12.5% royalty and income tax of 50%.
JMHO, I may be missing something, but this apears to be the structure on the PSC. The 50% relinquishment is also open to interpitation, I did not think they were giving up 50% of their 100%, but thought it would be 50% of the acreage.