Let's assume the gas flow rates and the extent/volume is known....

Let's assume the gas flow rates and the extent/volume is known. TPD and Petrochem have entirely different NPV valuations.
If TPD attempt to bootstrap the development it will take at least ten years to get scale and volume. At that time the earlier wells' production will decline. Assume NPV $100m.

Petrochem can immediately throw free CCP money to get gas volume up to max in two years. Their NPV is, say, $1000m.

It is classic valuation gap negotiation. The sale price will be somewhere between the two numbers. With a single buyer it will be close to the bottom. With multiple bidders it will be nearer the top. Mongolia is a non-mature market so not many bidders. Furthermore, anyone bidding for gas will eventually have sell to the monopoly gas buyer in China (Petrochem).
TPD could maintain a free carried 10-15% based on revenue or profit, but it is easy to manipulate intercompany sales producing zero book profit.

In the end, we will get a good price to sell but it will be a fraction of the ultimate realised value. But we will then have cash in the kitty.