BURIAL AND THERMAL CONDITIONS OF DIAGENESIS IN THE LOWER CRETACEOUS BARRA-DE-ITIUBA SHALE FORMATION, SERGIPE-ALAGOAS BASIN, BRAZIL

Vitrinite reflectance and clay mineral composition of shales from five wells in the Lower Cretaceous Barra de Itihba Formation, Sergipe-Atag oas basin, Brazil, have been examined in order to estimate the thermal conditions of diagenesis and the thickness of the eroded piles using the Sweeney and Burnham [1990] and Velde and Vasseur [1992] kinetic mo dels for organic matter maturation and smectite-to-illite reactions re spectively. In spite of large uncertainties, the thermal gradient and the surface temperature at time were estimated at 32 degrees C.km(-1) and 25 degrees C respectively. The evolution of the vitrinite reflecta nce with depth indicates that erosion varies from 1 500 m to 0 m accor ding to the location of the wells in a northwest to southeast transect across the sedimentary basin. These data are in good agreement with b urial calculated from the position of analcime in each well. Indeed, t his mineral has probably precipitated in surface conditions by reactio n of clay minerals with alkaline solutions in shallow sedimentary envi ronments. Thus, burial can be estimated using the maximum depth at whi ch analcime is identified. A mathematical decomposition of XRD pattern s using the DECOMPXR program shows that the illite-smectite mixed laye r particle population present three different states: ordered (R=1) il lite-smectite mixed layer minerals (I/S); poorly crystallized illite ( PCI) and well crystallized illite (WCI). The amount of expandable laye rs in the ordered interstratified I/S steadily decreases from 20 % to 10 % and the smectite weight % of the I/S + PCI + WCI fraction of clay samples decreases from 15 to 6%. The kinetic model from Velde and Vas seur [1992] does not depict correctly the VS illitization process in t he 20-0 % smectite composition range. This is due to the fact that ill ite precipitates either on illitic particles as well as on large US cr ystals. The remaining smectite layers in the I/S crystals are not diss olved.