An integrated approach to characterize diabase sills in Parnaíba Basin, based on geochemical data, cuttings description, and well logs interpretation

Abstract

Igneous rocks play a fundamental role in the formation and evolution of petroleum systems in several basins around the world. In the Parnaíba Basin, northestern Brazil, the diabase sills and dikes, which compound the Mosquito and Sardinha formations, contribute as a thermal source for an unconventional hydrocarbon generation, as well as providing trap and seal for the petroleum system. To understand these intrusions, we propose a methodology to identify the igneous intrusions in well-logs and interpret their compositional variations. The methodology is based on detailed descriptions of cuttings, X-ray fluorescence (XRF), X-ray diffractometry (XRD) and well logs, such as Gamma Ray (GR), Density (RHOZ) and Photoelectric Factor (PEFZ), from six exploratory wells drilled in the main gas-producing area of the Parnaíba Basin. The intrusions present in these wells were identified through the Igneability Feature and classified based on the GR curve as either “Barriguda” or “Caixote” patterns. The “Barriguda” intrusions show variations in the well logs (GR, RHOZ and PEFZ) along their vertical section, as well as variations in aluminum, calcium, potassium, zirconium, iron, titanium, magnesium, nickel and chromium values, evidencing a great textural and mineralogical variation, confirmed by the mineralogical results of XRD. Data interpretation suggests that the “Barriguda” intrusions are characterized in distinct electrofacies- and chemiofacies that represent an internal variation from top to base, related to magmatic differentiation processes.