Organic matter characteristics of Cenomanian-Turonian source rocks: Implications for petroleum and gas exploration onshore Senegal.
Résumé
One hundred and fifteen samples of Cenomanian–Turonian sediments from the Senegal margin basin, believed to be the main source rocks in the area, have been characterised to determine their organic facies and their oil-generating potential. The samples have been subjected to detailed organic petrographic studies (palynofacies, maceral analysis, vitrinite reflectance measurements) combined with Rock-Eval and Leco determinations. Total organic carbon values comprised between 0.3 and 3.5% indicate poor to moderate primary bioproductivity and/or unfavourable organic matter (OM) preservation. Rather high C/N ratio values supported by the presence of well preserved ligno-cellulosic debris and of collotelinite suggest a notable contribution of terrestrial OM. This is supported by hydrogen and oxygen index values that testify to type III OM. S/C ratio values lower than 0.36 together with the presence of abundant reworked organic particles indicate that the studied sediments were deposited in a transitional and mixing zone, between a coastal area with well oxygenated waters and a fully marine environment with more or less pronounced anoxia. The rather low marine productivity and poor OM preservation are here strongly reinforced by a very marked dilution of the OM by mineral inputs. In addition to these combined factors indicating that the OM is mainly of the gas prone-type, a moderate maturity is shown by Tmax and vitrinite reflectance values ranging from 435 to 445 °C and 0.4 to 0.9%, respectively. The reservoir facies consisting of sandstones interbedded with shales, are characteristic of prograding fluvio-deltaic formations developed during Maastrichtian regression. Faults and ‘rollover' structures most probably constitute the main migration pathways and traps for hydrocarbons, respectively. Overall, the characteristics of the studied formations highlight the importance of local factors in OM accumulation, even during the so-called global Cenomanian–Turonian anoxic event.
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