Igneous rocks identification on well-log data

Abstract

An attempt to discriminate and characterise igneous rocks on well-log data is made in a well drilled on the Cabo Sub-basin (JG-1 well, Sergipe-Alagoas Basin), situated on the northeastern coast of Brazil. Log data from the twelve drilled igneous intervals were analysed on RHOBxPHIN and DTxGRxResistivity cross plots. Similar diagrams were constructed for igneous data from the 1-CPE-1-PE well, drilled on the same basin, and with igneous data from other Brazilian basins. At the RHOBxPHIN cross plot, twelve lithologic fields were discriminated, and their characteristics verified against petrographic data. Two trends are perceptible: the basic rocks set (olivine-basalt, andesite, basaltic pumice and basaltic andesite) distributed along the dolomite line, and the acid rocks set (trachyandesite/quartz-latite, trachyan-desite, trachyte, olivine-trachyandesite and acid pumice) coincident with the calcite line. At the DTxGRxResistivity cross plot, the basic set occurs around low-GR field, and the acid set is distributed around high-GR field. The JG-1 well’s log data were submitted to discriminant analysis, and the results were checked against the CPE-1 well’s curves, for validation. For the first well, the hits ranged from 82 and 100%. For the second well, a perfect lithologic identification was obtained, except for those cases where the lithotypes does not exist in the JG-1 well. One of the main applications for this technique is the stratigraphic correlation for the volcanic successions from sedimentary basins, mainly in the absence of good biostratigraphic markers.