SEQUENCE STRATIGRAPHY OF SOURCE ROCKS APPLIED TO THE STUDY OF THE KIMMERIDGIAN TITHONIAN IN THE NORTH-WEST EUROPEAN SHELF (DORSET UK, YORKSHIRE/UK AND BOULONNAIS/FRANCE)/

Dorset and Yorkshire (England), the sedimentation of Kimmeridgian/Tith onian age (Mutabilis to Pallasioides zones) shows ordered organic sequ ences. 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 eac h other. These organic-rich cycles, with thicknesses of about 0.5 m, s how 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 d eposited in marine environments mainly below the storm wave base; they show little sedimentological evidence of sea-level fluctuation, eithe r transgression or regression. In the Boulonnais (France), a more prox imal setting, the comparable strata show a greater lithological contra st and depositional environments corresponding to bathymetric changes are more easily determined. A comparison of the distribution of organi c matter through the sequence dated by ammonites, in the three areas s tudied, shows a good correlation between the distal (Dorset and Yorksh ire) and the proximal (Boulonnais) areas. The main result of the biost ratigraphic revision in the Boulonnais (France) concerns the Argiles d e Chatillon, which can be subdivided into two members. These belong to the Autissiodorensis Zone (Autissiodorensis Subzone) and the Gigas-El egans Zone, respectively. Thus the Kimmeridgian-Tithonian boundary is located within this formation. Transgression does not appear to be the main factor determining the accumulation of autochthonous organic mat ter (type II). Not every major transgressive facies change seen in the Kimmeridgian/Tithonian of the Boulonnais is characterized by an enric hment in organic matter in the basinal area. Good source rocks with hi gh petroleum potentials result from a conjunction between the physiogr aphy, the transgression and the climate. This last causes the variatio ns in primary productivity which produce the organic-rich cycles. Tran sgressions act mostly to reveal the climatic variations. They are a ne cessary condition for the accumulation of the autochthonous organic ma tter, but they are not sufficient by themselves, as demonstrated by th e transgressions in the Autissiodorensis and Elegans zones. In the fir st instance, the strata are depleted in organic matter, in both the di stal and proximal areas; in the second instance, type II organic matte r accumulated in both distal and proximal environments. The vertical f luctuations in the total organic carbon contents, the types of organic matter and the petroleum potentials of the organic-rich cycles, deter mine the oil potentials of transgressive sequences in sedimentary basi ns.