Sequence stratigraphy of source rocks applied to the study of the Kim meridgian/Tithonian in the north-west European shelf (Dorset/UK, Yorkshire/UK and Boulonnais/France)

Abstract

In Dorset and Yorkshire (England), the sedimentation of Kimmeridgianfrithonian age (Mutabilis to Pallasioides zones) shows ordered organic sequences. The distribution of the organic matter content (per cent total organic carbon, petroleum potential, hydrogen indices) is not random, but occurs as widespread primary organic-rich cycles nested within each other. These organic-rich cycles, with thicknesses of about 0.5 m, show regular variations in organic carbon contents, reaching a maximum value midway through the cycle. They formed in response to widespread phenomena and can be correlated over long distances. The cycles were deposited in marine environments mainly below the storm wave base; they show little sedimentological evidence of sea-level fluctuation, either transgression or regression. In the Boulonnais (France), a more proximal setting, the comparable strata show a greater lithological contrast and depositional environments corresponding to bathymetric changes are more easily determined. A comparison of the distribution of organic matter through the sequence dated by ammonites, in the three areas studied, shows a good correlation between the distal (Dorset and Yorkshire) and the proximal (Boulonnais) areas. The main result of the biostratigraphic revision in the Boulonnais (France) concerns the Argiles de Châtillon, which can be subdivided into two members. These belong to the Autissiodorensis Zone (Autissiodorensis Subzone) and the Gigas-Elegans Zone, respectively. Thus the Kim meridgian-Tithonian boundary is located within this formation. Transgression does not appear to be the main factor determining the accumulation of autochthonous organic matter (type II). Not every major transgressive facies change seen in the Kimmeridgian/Tithonian of the Boulonnais is characterized by an enrichment in organic matter in the basinal area. Good source rocks with high petroleum potentials result from a conjunction between the physiography, the transgression and the climate. This last causes the variations in primary productivity which produce the organic-rich cycles. Transgressions act mostly to reveal the climatic variations. They are a necessary condition for the accumulation of the autochthonous organic matter, but they are not sufficient by themselves, as demonstrated by the transgressions in the Autissiodorensis and Elegans zones. In the first instance, the strata are depleted in organic matter, in both the distal and proximal areas; in the second instance, type II organic matter accumulated in both distal and proximal environments. The vertical fluctuations in the total organic carbon contents, the types of organic matter and the petroleum potentials of the organic-rich cycles, determine the oil potentials of transgressive sequences in sedimentary basins.