Igneability feature: an effective, easy and low-cost way to identify basic igneous rocks using wireline well logs in open hole wells

Abstract

The correct identification of igneous rocks is of fundamental importance during drilling and to the quality of the initial assessment of their role in the petroleum systems. These rocks show characteristics in geophysical well logs that are distinct from those of sedimentary rocks. After calibrating with laboratory data, we propose a method to identify and characterize igneous rocks using basic geophysical logs. This method consists of a crossover between bulk density and photoelectric factor logs to identify basic igneous rocks in sedimentary sections, named "igneability feature". This log feature consists in using the bulk density log on the scale 2.0 to 3.0 g/cm³ in the same track as the photoelectric factor log on the inverted scale, from 12 to 2 b/e. Then, when the density log is to the right of the photoelectric factor, it denotes the presence of basic igneous rocks.Acid igneous rocks were also studied and characterized by a complementary method, which consists in a crossover between the gamma ray log and a factor calculated from the bulk density and photoelectric factor log curves. Thus, this method covers most of the varieties of igneous rock found in the Brazilian basins, such as Santos, Parnaíba and Paraná.