Sabai Sabai,
I read the advertisement several times and make these observations.
1. You would be working 17.5 hours contact time for English each week or 70 hours per month, that's just over 71 USD per hour with accom plus, plus. Or looking at it another way 60,000 USD per year.
2. This guy insists that he and his family is not very good at speaking English but his letter is articulate, yes, he could have got someone else to write it.
3. On top of your salary, you would receive a place to live, mod cons, vehicle etc. at no expense to you.
Government minister do not get paid this much or have these perks, so my suspicion is raised. Looks nice... very attractive. However, Nigeria is poverty stricken even though it has great mineral wealth, most if not all expatriates live in compounds, often onsite and to be honest not that many have their families with them.
IMHO I believe this to be a variation of the 419 scam (419 refers to the Nigerian legal code to do with misrepresentation and fraud), it generally works by offering something to good to be true and for some reason or another (problems with your visa, ticket, validation of educational certificates etc. etc.), you will be asked to pay money upfront which will be refunded when you get to Nigeria (you of course will not see this money again), and after numerous attempts to contact your new employer fails you are left to ponder what happened. However, DO NOT travel to Nigeria under your own steam thinking you will get your money back or that it might be just one big mistake or miscommunication.
Now, I might be wrong and therefore I would contact Nova Gas Incorporated independently to confirm that your prospective new employer works for them, you might even say why you are enquiring to support your request.
I would also ask for the ID page of the passport of each member of the family (are they really French or are they just saying they are from France), pictures of the residence, your accom, proof of funding resources for your salary, names and addresses of the families referees etc. etc. or until you were thoroughly satisfied that everything was aboveboard and you were 110% certain that this was real.
Nigeria is a risky and notorious place known for scams and is a dangerous place (check with your consultate or nearest embassy), so please be careful.