Why are petroleum and natural gas deposits highly sought after

Natural Gas is a gas, it's basically the lightest fraction of petroleum that separates naturally in the reservoir.

You can ship it, it's just hard, you can put it on trucks ala cng as well. We've recently mastered liquefaction, which required cryogenic tech, but this means we can ship it by tanker.

Gas burns much cleaner, but is not as useful as oil for making plastics and other chemicals. It can be easier to move as a gas, however, and engines that run off it can be simpler and far more reliable (turbines love ng, but some can use road gas as well).

Also in the earlier days of oil production no was considered a waste product and simply burned at the pump, but like most things involving oil, they found a way to make the best use of the lightest fraction as well (the heaviest fraction was turned into plastics and highways, it seemed a pity to waste it).

Another note: gasoline was originally a waste product as well, ships ran on diesel and fuel oil, the choice of gasoline for car engines was as much because it was an otherwise wasted fraction (can't really burn it for heat) as anything, though ironically the demand made it one of the most critical fractions.