APPLICATION OF LIGHT-HYDROCARBONS (C-4-C-13) TO OIL SOURCE ROCK CORRELATIONS - A STUDY OF THE LIGHT-HYDROCARBON COMPOSITIONS OF SOURCE ROCKS AND TEST FLUIDS FROM OFFSHORE MID-NORWAY/

Relatively little work has been published on the correlation between t he light hydrocarbon distributions in reservoir fluids and their propo sed source rocks [Philippi, G. T. (1981)]. The aim of our work was to study this relationship in detail for samples from Mid-Norway. The mai n source rocks off shore Mid-Norway are the marine shales of the Late Jurassic Spekk Formation and the coals and paralic shales of the Early Jurassic Are Formation. Reliable light hydrocarbon (C-4-C-13) data fo r source rock samples were acquired by thermal extraction-GC of the so urce rocks. Of these, notably the hydrocarbons in the C-6-C-8 range (r outinely measured in test fluids) were used to discriminate between th e Spekk and Are Formation samples. A total of twenty-six samples from the Spekk Formation and twenty-four samples from the Are Formation at different maturity levels and facies were analyzed. In general, the tw o source rock types differ in their light hydrocarbon composition by t he presence of relatively more aromatics and cyclohexanes in the Are s amples, while the Spekk samples are richer in cyclopentanes and acycli c hydrocarbons. We show that source rock facies is a more important in dicator of light hydrocarbon composition than maturity variation. Diff erences in the chemical composition, which can be used to discriminate between the two source rocks, were supported by differences in the ca rbon isotope composition of individual components of the same fraction , as determined by GHM-IR-MS analysis of eleven samples. Further, the light hydrocarbon compositions of reservoir fluids (oils and condensat es) were compared with those for the source rock(s). Sixty-six gas chr omatograms of oils and condensates, representing most of the known pet roleum accumulations in Mid-Norway, were collected. Of these, five oil samples were selected for detailed isotopic analysis of individual co mponents (GC-IR-MS). When using a classification scheme based on data from sediment samples, data for the light hydrocarbon fraction of oils and condensates indicate that the Spekk Formation is the dominant sou rce for most of the fields from Mid-Norway, with a significant contrib ution from the Are Formation being detected principally in one field. Differences in the chemical composition of the C-6-C-8 fractions were supported by differences in the carbon isotope composition of individu al components, which also discriminate between the oils. Although the classification diagrams used in this study are based on source rock da ta from Mid-Norway, the method can be applied to other areas, providin g that the diagrams are calibrated with source rock data from the area of interest. (C) 1998 Elsevier Science Ltd. All rights reserved.