Sorry just saw the guide, really comprehensive. Kudos to thread master. However, when given chance to ask question, you can generously ask the human resource exec that the range or ideal figure for the job so you can see things much clear.
Expected salary is has a huge weight into job offers where lower range salary indicates you have poor self confidence, and over max salary range will make you look over confidence. But be assured, when you ask the salary question, don't ask them when they inquire about your expected salary.
Rather than watching fully saturated industry, I think you should look at the job as well.

In my opinion, oil and gas industry still have quite long to go. If I recall accurately, I read early this year that Australia coast has been found for quite massive amount of oil. Onto calculation, assuming are 20 years old by now(which I sure you're not). Even if 2050 where oil depleted, you're already at the age of 50. Which in few years, if no harm, you should be retiring. Of course, in the long run you have to take count of financial saving and how well you manage your extras into investment.

Management trainees can be successful but requires you to stay for more than 3 years. Telcos competition might be tense but rest assured your personal technical skills more important. Example: as a HR manager of xx telco, you can switch to non telco companies. Most telco companies are linked with IT so its still a very broad field