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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 49 (1965)

Issue: 3. (March)

First Page: 248

Last Page: 257

Title: Hydrocarbons in Non-Reservoir-Rock Source Beds

Author(s): Ellis E. Bray (2), Ernest D. Evans (2)


Distributions of ratios of odd- to even-carbon-numbered heavy n-paraffins (carbon preference indices) for shales and mudstones in oil provinces have been confirmed by a statistically significant number of examples. Thirty per cent of the shales and mudstones sampled contained petroleum-like mixtures of heavy n-paraffins; sixty per cent contained n-paraffins intermediate between oil and recent sediment n-paraffins; and ten per cent had n-paraffins similar to those in recent sediments, indicating that the freshly deposited sediments of the geologic past contained heavy n-paraffins with strong preferences for odd-carbon numbers.

Carbon preference indices are inversely related to the proportion of hydrocarbon in the organic material of shales and mudstones and, consequently, may be indicative of conversion of organic material to hydrocarbons. Carbon preference indices also provide a unique clue for recognizing petroleum-like mixtures of heavy n-paraffins in non-reservoir rock. Petroleum-like mixtures occur more commonly in organic-rich and hydrocarbon-rich shales and mudstones which, from emprical observation, are inferred to be the source of oil.

The heavy-saturated hydrocarbons of crude oils contain larger percentages of paraffins than do corresponding fractions from extracts of non-reservoir rock. Hypotheses for the primary migration and collection of finely disseminated oil from the source rocks should explain this difference.

The variability of organic material in sediments complicates the search for source rock and presents problem in selection of sites as well as frequency of sampling. Analyses for hydrocarbons in outcrops should be preceded by geological study to determine if the outcropping sediments can be representative of the stratigraphic unit downdip. Outcrop samples must be unweathered; and depositional environment, lithology, and maximum depth of burial should be equivalent to the sediments of interest at depth.

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