An independent petroleum asset valuation is normally commissioned by a target company in defence of a hostile takeover bid. A substantive study of petroleum company defence asset valuations is lacking in the published literature in hydrocarbon economics and other relevant journals. This study evaluates takeover defence valuations in the period ranging from 1988- 2009 based on public domain data. The study focuses on assumptions for the key value metrics in the upstream valuations and how this has changed in the time period. This includes comparative reviews of publicly available data for four petroleum companies. This has been supplemented by published literature on assessment of value.