The study of carbonate rocks is very important for the understanding and
characterization of hydrocarbonate deposits, since 50% of world oil is associated to
these rocks. Bolivia is a great example, because it is today the largest exporter and
has the second largest natural gas deposit in South America. The Tucavaca Group
occurs in a wide area of eastern Bolivia, denominated Chiquitos-Tucavaca
Aulacogen, which is considered an aborted triple junction rift, also known as
Tucavaca belt. The stratigraphy of the Tucavaca basin is represented by a sequence
made up, from the bottom to the top, of the Boqui Group, followed by the Murcielago
Group and the Tucavaca Group. The Tucavaca Group is composed of
conglomerates of the Pacobillo formation, carbonates of the Pororó formation,
sandstones of the Bocamina formation and shales, mudstones and siltstones of the
Pesenema formation. The group in question is devoid of paleoenvironmental studies
and the age of deposition is still uncertain. So, C, O and Sr isotopes studies were
carried on carbonates of the Pororó formation. For this purpose, carbonate samples
were previously submitted to petrographic and X-ray fluorescence analyses in order
to select the most appropriate samples for isotopic analyses. The carbonates studied
here were individualized in three microfacies: laminated fine dolomite, fine dolomite
and dolomitic breccia. The stable isotope studies were performed on all samples of
the sequence and the samples TUCA – 1, TUCA – 3 and TUCA – 8 were selected for
Sr analysis. Values of 13C and 18O are negative, ranging from -5.24 to -2.56‰ and
-7.16 to -4.59‰, respectively. 87Sr/86Sr ratios are 0.707712 (TUCA – 1), 0.707925
(TUCA – 8) and 0,710183 (TUCA – 3). 18O values are within the limit set as a
parameter of Neoproterozoic carbonates considered isotopically preserved,
determined such as >-10‰. The negative values of 13C indicate low or no biological
activity in the ocean and the possible incorporation of organic matter in carbonate
sediments. The 87Sr/86Sr ratios place the samples at the end of Neoproterozoic
(Ediacaran), between 580 and 560 Ma
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