Funny you ask this as I worked on an Oil & Gas value chain alignment project not too long ago! I even created a case losely based off of this project: https://www.preplounge.com/en/management-consulting-cases/interviewer-led-mckinsey-style/advanced/mckinsey-digital-bcg-platinion-oil-gas-upstream-technology-196
Feel free to shoot me a message and we can talk much more in-depth about this industry and the cost/value drivers.
You're got it about right overall. The costs are massive with any of these capex projects. Fundamentally, the largest cost by far is the actual project to setup the sit to dig your resource out of the ground. This is essentially the materials, supplies, contracting/procurement work, etc.
From there, the operational costs are much lower, but consist of labor, IT, maintaining assets/infrastructure, etc.