You really need to determine which code article applies.
ARTICLE 700?EMERGENCY SYSTEMS
or
ARTICLE 701? LEGALLY REQUIRED STANDBY SYSTEMS
or
ARTICLE 702? OPTIONAL STANDBY SYSTEMS
I highly doubt it is article 700, it might be article 701 but I bet it is more likley an article 702 system. If it is a article 702 system there are no NEC fuel requiments.
On the odd chance it is an article 700 system 700.12(B)(2) and (3) has some fuel requirements and one source of even natural gas does not meet them anyways.
700.12(B)(2) Internal Combustion Engines as Prime Movers.
Where internal combustion engines are used as the prime
mover, an on-site fuel supply shall be provided with an
on-premises fuel supply sufficient for not less than 2 hours?
full-demand operation of the system. Where power is
needed for the operation of the fuel transfer pumps to deliver
fuel to a generator set day tank, this pump shall be
connected to the emergency power system.
(3) Dual Supplies. Prime movers shall not be solely dependent
on a public utility gas system for their fuel supply
or municipal water supply for their cooling systems. Means
shall be provided for automatically transferring from one
fuel supply to another where dual fuel supplies are used.
Exception: Where acceptable to the authority having jurisdiction,
the use of other than on-site fuels shall be permitted
where there is a low probability of a simultaneous
failure of both the off-site fuel delivery system and power
from the outside electrical utility company.