KEROGEN FACIES IN THE MAJOR JURASSIC MUDROCK FORMATIONS OF SOUTHERN ENGLAND AND THE IMPLICATION ON THE DEPOSITIONAL ENVIRONMENTS OF THEIR PRECURSORS

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Abstract
Kerogen types and their hydrocarbon‐generating potentials in three main mudrock units in the Jurassic of Southern England ‐ the Lower Lias, the Oxford Clay and the Kimmeridge Clay ‐ have been investigated using elemental analysis, transmitted‐light microscopy, density fractionation and pyrolysis‐gas chromatography. The results show that, as with the lithology, the three formations display variations in their kerogen composition. The kerogen facies range from the chemical Type II to Type IV in the Lower Lias and the Oxford Clay, whereas only Types II to III are identified in the Kimmeridge Clay. Amorphous organic material constitutes the most abundant fraction of all the kerogen samples. All the identified kerogen types show a potential for generating both oil and gas.The fluctuating oxic‐anoxic depositional bottom conditions during the Jurassic is suggested to have been the major factor controlling the distribution of kerogen facies in the three formations.