Application of borehole images in the characterization of volcanic paleoenvironments with implications for the exploration of hydrocarbons in Brazilian basins

Conventional borehole image log interpretation of linear fractures on volcanic rocks, represented as sinusoids on unwrapped cylinder projections, is relatively straight-forward, however, interpreting non-linear rock structures and complex facies geometries can be more challenging. To characterize diverse volcanic paleoenvironments related to the formation of the South American continent, this study presents a new methodology based on image logs, petrography, seismic data, and outcrop analogues. The presented methodology used pseudo-boreholes images generated from outcrop photographs with typical igneous rock features worldwide simulating 2D unwrapped cylinder projections of a 31 cm (12.25 in) diameter well. These synthetic images and standard outcrop photographs were used to define morphological patterns of igneous structures and facies for comparison with wireline borehole image logs from subsurface volcanic and subvolcanic units, providing a “visual scale” for geological evaluation of volcanic facies, significantly enhancing the identification efficiency and reliability of complex geological structures. Our analysis focused on various scales of columnar jointing and pillow lava lobes with additional examples including pahoehoe lava, ignimbrite, hyaloclastite, and various intrusive features in Campos, Santos, and Parnaíba basins in Brazil. This approach increases confidence in the interpretation of subvolcanic, subaerial, and subaqueous deposits. The image log interpretation combined with regional geological knowledge has enabled paleoenvironmental insights into the rift magmatism system related to the breakup of Gondwana with associated implications for hydrocarbon exploration.