A New Petrophysical Workflow to Characterize Magnesium Rich Clay Minerals in Pre-Salt Lacustrine Carbonate Reservoirs

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Complex clay types in presalt reservoir rocks make it a challenging task to characterize phyllosilicate and magnesium-rich clays. Reservoir quality and properties such as porosity and permeability are adversely influenced by the presence of magnesium clays that were precipitated within volcanic and lacustrine carbonates deposits under alkaline conditions. Accurate identification and quantification of magnesium clay minerals have a broad business impact on reservoir characterization, basin thermal history assessment, and diagenesis prediction. However, magnesium clays, having high magnesium contents and low potassium and aluminum concentrations in chemical compositions, lack characteristic features in well-log responses, such as GR, which make petrophysical models less effective in correctly interpreting reservoir rock and fluid properties. Although magnesium elemental weight fraction is measured using newer-generation neutron spectroscopy tools, magnesium-rich clay minerals are not properly accounted for in vendor-specific mineral closure models and commercial petrophysical software platforms, and overestimation of dolomite volumes in lacustrine carbonate reservoirs is rather common. A new petrophysical workflow was developed to include a hybrid multimineral solver in which conventional and neutron spectroscopy logs are integrated to solve for mineral volumes. The mineral inversion algorithm, which is simultaneously constrained with log response endpoints and mineral chemistry constraints, allows building new minerals using either core chemistry data, such as X-ray fluorescence (XRF), or regional geological knowledge. The log response endpoints are estimated using nuclear modeling codes such as SNUPAR. The new workflow is applicable to any vendor data and can be easily implemented in any established petrophysical software platform. The new workflow has been successfully applied in a number of wells in the Santos and Campos Basins, offshore Brazil, where magnesium-rich clays were formed commonly as kerolite, stevensite, saponite, and pyroxene. These minerals have distinct features in chemical composition, with magnesium at 18% by weight and aluminum at less than 1% by weight, and their neutron log response is 11 p.u., which is significantly lower than phyllosilicate clays. Results show that the new integrated workflow is capable of resolving presalt reservoir rocks into carbonate, clastic, and volcano-clastic minerals. A comparison of the new mineral model and an existing model is shown in Fig. 1. The existing mineral closure model tends to overestimate dolomite volume, while the new workflow is capable of properly quantifying both conventional and magnesium clays. As shown in Fig. 2, the new clay mineral model agrees with the NMR T2 log mean (T2LM), which is inversely proportional to clay-bound water volume. The log-derived mineralogy is also consistent with petrographic observations of thin-section images. In summary, the new integrated workflow incorporating mineral chemistry has a clear advantage of producing reliable and consistent rock mineral logs for formation evaluation, stratigraphic correlation, rock physics, and petrofacies models across the Campos and Santos Basins.