Mixed signals of the source and thermal maturity for petroleum accumulations from light hydrocarbons: an example of the Beryl field

We have investigated compositional variations of light hydrocarbons in crude oils from the Beryl and adjacent fields of the North Sea in combination with carbon isotopic ratios of light hydrocarbons and biomarker properties of oils. Although there appears to be a strong source influence on various light hydrocarbon maturity parameters, this is a consequence of the mixing of hydrocarbons derived from the Kimmeridge Clay Formation as well as from the Heather Formation and Brent coal at varying thermal maturity levels. Light hydrocarbon parameters based on Mango's reaction scheme involving 3-ring and 5-ring intermediates are supported by carbon isotopic ratios of light hydrocarbons. However, there also is evidence that iso- and cyclo-heptane precursors exist that contribute C7 light hydrocarbons independent of Mango's model. The ratio of 2,4-dimethylpentane to 2,3-dimethylpentane (2,4-DMP/2,3-DMP) and other related parameters appear to be reliable indicators of thermal stress, but must be interpreted within a complete understanding of the petroleum system under study.