Permian Carbonates in the Venezuelan Andes

Abstract

In northern South America, Upper Palaeozoic strata were deposited extensively over peri-cratonic areas associated with restricted seas located between Laurentia and Gondwana, in equatorial Pangea. In many places the successions are rarely exposed, and so are poorly documented; this is largely the result of extensive weathering and dense vegetation in the tropical Andes. However, these strata do contain significant information for palaeogeographic and palaeoclimatic reconstructions, and our understanding of the evolution of northern Gondwana and the finally assembly of Pangea. The main objective of this thesis is the study of Permian carbonates in the Venezuelan Andes, their sedimentology, geochemistry, diagenesis and petroleum potential.
The Palmarito sediments were deposited on a carbonate ramp that dipped basinwards towards the north facing the open ocean. The formation evolved from the underlying fluvial (Sabaneta Formation) through tidal-flat to mid-outer ramp deposits, with all facies recording a well-developed cyclicity. These strata fill an important gap in the regional palaeogeography and hence have revealed important implications for the palaeoclimate and palaeoceanography of the time.
The results of this study of Palmarito strata have significant implications for the palaeogeography of this Permian time. From the facies analysis of the Palmarito Formation, new evidence has been provided for a central Pangean seaway. Furthermore, isotope analyses have improved the time-frame for the succession from 87/86Sr data, and in addition, the long-term stratigraphic trends in the δ13C and δ18O data have permitted interpretations of the climatic and oceanographic controls on Permian carbonate deposition. Moreover, metre-scale cyclicity shows the patterns of short-term controls on sedimentation, where autocyclic and allocyclic processes affected deposition and the vertical stacking of facies. An analysis of the diagenesis of the Palmarito carbonates shows several stages of cementation and alteration, although the strata are mainly fine-grained and coarse cements are rare. Finally the elements of the petroleum system for the Palmarito have been considered for the succession and as a result, it can be proposed that, firstly, fractures in finer-grained facies have the potential to provide reservoir rocks, and secondly, that although high values of Tmax have been obtained from rock-eval analysis from one locality, Palmarito strata do have the potential to form source rocks. Further investigation is required to ascertain the actual hydrocarbon potential of the Palmarito. The high content of finer-grained facies with low permeability provides the Palmarito strata a high potential to perform as a seal, as well as possibility to develop stratigraphic traps.