The Role of Clay Minerals in the Petrophysical Properties of Carbonate Rocks from the Cotinguiba Formation (Sergipe-Alagoas Basin, Brazil)

The Cotinguiba Formation, of Cretaceous age, located in the Sergipe-Alagoas Basin, Brazilian Northeastern coast, comprises carbonate breccias and shales interlayered with fine to coarse clastic sediments (mudstones, marls), deposited maximum sea level episode during Cenomanian-Coniacian transgressive event. Core sections were selected from formation materials, which 20 plugs obtained, representing main horizons present Formation. samples analyzed petrophysical physical methods (air porosimetry, mercury injection, acoustic wave velocity) mineralogical (optical microscopic analysis, x-ray diffraction (XRD) scanning electron microscopy (SEM)). Results showed that clay minerals’ content carbonates varies 2% 20%, illites/micas interstratified illite/smectite as most abundant minerals, containing also traces chlorite kaolinite, presence palygorskite varying trace 8%. According percentages calcite, dolomite, siliciclastic + minerals obtained XRD lithologically identified limestone, impure dolomitic calcitic dolostone, dolostone. rocks have a good porosity, 6% 20% carbonates, predominantly related microporosity. Clay influenced grain density properties, albeit not having same role porosity permeability. SEM images, though, show they an important Grain variation phyllosilicate was observed more significantly limestones higher than 8%, pronounced decrease palygorskite. Microporosity is factor for reducing Vp Vs velocities those values exceeding 5341 m/s 3026 but, when 4%, do exceed 4100 2480 m/s, respectively. Therefore, research allowed evaluating influence textural properties