Seismic Interpretation of Igneous Intrusions and Their Implications for an Unconventional Petroleum System in Southeastern Parnaiba Basin, Northeastern Brazil.

Abstract:

After almost two decades of little exploratory effort in the Parnaiba Basin, recent gas discoveries have revived the economic interest in this region. A new exploratory play was successfully tested, confirming the efficiency of the igneous sills triggering the thermal effect on source rocks maturation, and also playing an important role as unconventional trap and seal. Parnaiba Basin was affected by two magmatic events: the Mosquito Formation (Average Age: 178 Ma) and the Sardinha Formation (Average Age: 124 Ma), both characterized by basic magmatic rocks. Ten 2D seismic lines, located in the southeastern Parnaiba Basin, were systematically interpreted in order to identify the main diabase sills geometry, some possible emplacement controls and further implications for an unconventional petroleum system. The identified sills were classified in three types: A, B, and C, according to their dominant geometries and the stratigraphic position of their host rocks. From base to top, Type A sills intrude the Pre-Silurian Sequence, that fills in graben structures of the basement. They have short lateral continuity and saucer-shaped geometry. The main graben faults seem to control the steeper sills, which seem to feed upper sills. Type B sills intrude mainly the Silurian Sequence and have a very long lateral continuity when intruding the shales of Tiangua Formation, in which they are mainly horizontal tabular. Type C sills intrude the top of the Devonian Sequence and have two main geometries: saucer-shaped and horizontal tabular. By correlation to the surface geology of the area, Type C sills are interpreted as Sardinha Formation (Lower Cretaceous). Considering an unconventional hydrocarbon play in the area, some potential trap features associated with sill geometry, were identified, using analogous plays successfully tested in recent discoveries. These traps are associated to a specific sill geometry locally known as “inverted saucer-shaped sill”. This shape consists into a horizontal sill placed on the top of a potential reservoir formation, acting also as a seal. On their two tips, this geometry is limited by two steeper sills that may be connected to a lower horizontal sill, which normally intrudes a potential source rock.