"I would remind you that plenty of countries, including Australia, operate on tax laws and not PSAs."
While plenty of countries operate on tax laws and not PSA's, those countries that operate on tax have specific miniming taxes.
The starting point is that petroleum resources are the property of the country as representated by the Government (USA is one expection to this) and the company enters into agreements which give them to get the rights to extract petroleum and compensate the country by abiding by the specific petroleum fiscal regime set by the country. Australia has a set of petroleum specific royalties to fo this, Zimbabwe has stated it will take the PSA route to get its share of the value of the petroleum produced.
So the signing of a PSA prior to producing oil and gas in Zimbabwe is a concern for investors.
Daniel Johnsons write a great book on this subject titled "International Petroleum Fiscal Systems and Production Sharing Contracts"
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