Generation of low molecular weight hydrocarbons from organic matter in source beds as a function of temperature and facies

Abstract

About 250 canned rock samples from three exploration wells have been analysed by a new technique for content and composition of light hydrocarbons (C 2/1bC 7). From depth plots of these data the process of hydrocarbon generation in source beds can be monitored as a function of the temperature and facies of their organic matter. Over a more than 1000 m thick source-bed-type shale unit of Jurassic age, progressive generation of low molecular weight alkanes is evidenced, on the basis of yield increases, by the following approximate orders of magnitude: 2.5 for ethane, 2 for propane, 1 for butane. Non-generation of pentanes to heptanes is indicated by yields which remain essentially constant. This amounts to an overall tenfold increase of the total (C 2/1bC 7) alkane content (the concentration units are all carbon-normalized). For selected samples from the same well, evidence is presented for preferential loss of the light hydrocarbons due to improper sample storage. At equal rank, a coal sample typifying organic matter derived from terrestrial vegetation is about three orders of magnitude lower in light hydrocarbon content than a marine shale from the same stratigraphic sequence. The differences in the distribution of C 6-hydrocarbons are most pronounced in the relative proportion of benzene which constitutes 80% for the coal versus 20% for the marine shale. Finally, relative differences in hydrocarbon generation capability are established for the source rock units studied by comparing their light hydrocarbon yields on the basis of equal maturation levels determined from vitrinite reflectance data.