Phanerozoic magmatism in the Parnaíba Basin: characterization of igneous bodies (well logs and 2D seismic sections), geometry, distribution and sill emplacement patterns

The Parnaíba Basin, like other Brazilian Paleozoic intracratonic basins, contains intrusive igneous bodies in its sedimentary section in the form of dykes and, mainly, thick sills. The identification of the geometry and the emplacement mechanism are important for the determination of the traps controlling gas and/or oil accumulations. A variety of models available in the literature suggests a complex relationship between the magma and the host rock. Several factors, such as the density and pressure of the magma injection, steam generation, regional and local stress fields, and the rheology of the host rock, work together controlling emplacement, expansion and the final geometry of intrusions in the sedimentary column. Well logs show a conspicuous signature (e.g. gamma-ray curve with paunchy feature) for the igneous bodies (sills). Many sills occur parallel to the bedding, generally intruding thick sections of shales along the contact of the lithostratigraphic units of the Longá-Poti, Pimenteiras-Cabeças and Tianguá-Jaicós formations. Seismic sections reveal a variety of geometries, including layer parallel, saucer-shaped, planar transgressive and fault block. By far the most common sill geometry is the layer parallel, which is able to reach up to hundreds of kilometres in extent, and has minimal thickness ranging between 2 and 5 m, and with a maximum of around 250 m.