About Tax And Type Of Labour

Please i'd really love for tax, labour and other economic gurus, scholars and practitioners to throw some light on this matter - it is a true scenario maybe a bit sketchy, i hope you can make something out of it and advise, i'll be glad to offer more details where i can:

A group of workers are contracted to a major O&G company XLT via a contracting agency ABC. Now, agency ABC has explicitly declared that these fellas are not their employees and XLT only regards them as contractors. These guys reside in various states of the federation but work in the niger-delta area. Work is rotary - 2weeks on and 2weeks off - wages are worked on a day-rate basis and it is a pay-as-you-work arrangement.
According to terms of service agreed upon by the workers and Agency ABC - just about the only thing that ABC does is to pay the workers their wages plus any other extras as approved by XLT mgt.

My 1^st question is, according to Nigerian Labor laws, under what category of workers will these guys fall ?
What kind of taxes are they eligible to pay and do they pay their taxes individually or does the agency ABC deduct it from what they earn? Who do these guys pay their taxes to - state of residence or work location state?