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Part II Palaeoenvironmental Indicators |
Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts 01003, USA
Petroleum accumulations in lacustrine environments often possess characteristics different from those in marine settings. In general, lacustrine petroleum fields are smaller, the oil has often not migrated far from the source, and oil shales are common. Some of these differences can be attributed to the relative youthfulness of lacustrine basins and to differences in the type of organic matter present. However, additional influences may arise from the stratigraphic framework and clay composition of the source rocks. The scale of the environment may be a major controlling factor. Gradations between large lakes and marginal marine basins exist, but lake basins will more often be areally small, relatively shallow and subject to frequent lateral migrations in depositional environments. These characteristics make the resulting sedimentary sequences much more heterogeneous, both in stratigraphic relationships and in sediment composition. Stratigraphy will ultimately influence the migration and accumulation of the oil; the sediment composition—particularly clay mineralogy—will affect the generating potential of the source rock. Shifting sediment source areas coupled with lake level fluctuations and early diagenetic reactions may produce thin mudstones containing very heterogeneous clay mineral assemblages, which, accordingly, may not undergo significant transformations during burial diagenesis.
