Applications of electron microprobe analysis (EPMA) in the study of Venezuelan source rocks: La Luna and Querecual Formations

This work presents the electron probe for microanalysis (EPMA) study of Venezuelan source rocks for stylolites (Querecual Formation), framboids (La Luna Formation), and kerogen (La Luna and Querecual Formations). Distributions of major and trace elements were studied to determine the elemental association with inorganic (minerals) and organic phases (kerogen). Querecual Formation an association of S with Fe, Cu, Zn, and Ni was observed in the stylolites, suggesting the presence of authigenic sulfides, whereas V and Ni are associated with organic matter. In the La Luna Formation, massive and framboidal pyrite was observed, in some cases surrounded by sphalerite; also an association of S with Fe (pyrite) and Ni with Zn (in a lower proportion) was observed. This suggests that Ni and Zn coprecipitate with pyrite, but that Zn also forms a separate phase (sphalerite). In these samples Ni is associated both with the sulfide phase and the organic matter, but V is only associated with the latter. In kerogen from La Luna and Querecual Formation, elemental mapping shows that V is associated with kerogen; it also demonstrates the presence of inorganic phases by elemental associations among C, Ca, and Mg (carbonates); Al, Si, and K (aluminosilicates); and S, Fe, Co, Cu, Zn, and Ni (sulfides); these latter also seem to be associated with kerogen. This means that acidic attack on kerogen does not completely separate mineral phases, possibly because organic matter may be surrounding mineral phases, thus inhibiting this attack. These results demonstrate EPMA potential for studies on the distribution of major and trace elements in source rocks.