Título: Chemostratigraphy applied to oil-prone sequences: Mirador Fm. (Middle to Late Eocene - Oligocene), Tachira State, Venezuela

Autor: Reategui, K.; Martínez, A.; Esteves, I.; Martínez, M.; Urbani, F.

Fuente: 8th Latin American Congress on Organic Geochemestry CD-ROM Memorias

Año: 2002

País de publicación: Colombia

Tipo de documento: CD-ROM Memorias

Resumen

Sandstones from Mirador Fm. (Eocene to Early Oligocene) are petroleum reservoir rocks in Rio Zulia oil field, Colombia, and in Tarra area (Colon district, Zulia State), Venezuela. The bottom of the unit can be lithostratigraphically correlated with the lowermost part of Misoa Fm. in the Maracaibo Lake area. Misoa Fm. is the main reservoir unit in the basin. However, the global correlation between both units is a hard task, due to the scatter in the geochemical information available and the poor and non-diagnostic fossil content in Mirador Fm. (Léxico Estratigráfico de Venezuela, 1999). The aim of this study is the generation of chemostratigraphic profiles in Mirador Fm. In the area under study (San Pedro del Rio, Tachira State, Venezuela) searching chemofacies and physicochemical conditions of accumulation. With this information, it will be possible to correlate both units within different oil fields in a regional extent, with a weak biostratigraphic control.

Eighty-nine rock simples were analyzed by AES-ICP for Al, K, Ti, Mg, Ca, Ce, Y, La, Rb, Li, Mn, Zn, Ni, Cr, V, B and S. This study was complemented with a detailed thin section petrography. By multivariate statistical analysis (cluster analysis and multidimensional scaling) was established a statistical correlation between variables and the main changes in the chemical associations for each sample (Figure 1).

The studied section is geochemically characterized in three chemofacies, in agreement with changes in source rock or climatic changes; boundaries between chemofacies are coincident with erosion surfaces. By analysis of sedimentary geochemistry, petrography, lithostratigraphy, statistical and by the following elemental ratios: Cr/V, Y/Ni, Si/Al, Ce/Al, Mg/Al, Mn/Al, Ca/Al, Ni/Al y Zn/Al (Figures 2 and 3), a polimictic source may be inferred, with 2^nd cycle sedimentary rocks, granitoid rocks and granite-derived gneisses too. Volcanoclastic rocks like La Quinta Fm. (Jurassic) were contributing to the sedimentary supply at the bottom of the section. K/Al and Ti/Al ratios (Figure 3) reveal a mainly physical weathering, with dry conditions; at the last interval of the section, a more humid climatic condition was present. Ratios of B/Al and S/Al suggest a more marine influence on the bottom of the section.

This study has confirmed the utility of Chemostratigraphy as an auxiliary tool in the characterization of sedimentary sequences, with emphasis in oil-source rocks and reservoirs.