I'm happy to help as I literally worked on an O&G value-chain intergration project in APAC a couple of years back.
The main problem is that multiple business units (upstream, midstream, and downstream) all individually optimize their own operations to the potential detriment of the business as a whole.
The objective og value chain integration in O&A is align incentives/KPIs etc. so that each part of the body is operating effectively together. I.e., upstream may make a decision that's slightly less beneficial for them (lose 1$ per unit) that is great for the business (make $10 per unit).
Very very tough to accomplish and requires an in-depth understanding of the mechanics of O&G and the dozens of important metrics that O&G firms need to worry about to make profits!