Punitive tax -> higher oil price -> shipping oil from overseas is more competitive -> more fuel used and environmental impact to get the oil from overseas
You should probably just be advocating a carbon tax since that addresses this externality in a more general way.
A carbon tax makes sense, but probably wouldn't impact this behavior explicitly, since the natural gas is being burned before it even reaches the market.
A carbon tax can apply here as well. In fact, it's probably best to make producers pay carbon tax where possible, since they will be simpler to tax (and the tax will be harder to evade for them).
You should probably just be advocating a carbon tax since that addresses this externality in a more general way.