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There are few publications on the precise age and palaeogeography of the Palaeozoic rocks of Colombia. In the present study the Pluspetrol Paisa-1 well, located in the central part of the Colombian Llanos Orientales Basin, is investigated palynologically, in order to determine the age, the palaeoenvironment and the palaeogeography of the sediments. The lowermost stratigraphical interval of the well (4939–5040′; ∼1505–1536 m) is composed of alternating sandstone and black shale beds, from which a well-preserved assemblage of Ordovician acritarchs has been identified in five samples (cuttings and sidewall cores). The acritarch assemblage contains some diagnostic taxa, including Arbusculidium, Barakella, Coryphidium, Dactylofusa, Striatotheca, and Veryhachium, among others, indicating an Early to Middle Ordovician age. The presence of a few biostratigraphical index species, such as Dactyolofusa velifera or Coryphidium bohemicum, and the absence of others point to a middle Floian age. As the three diagnostic genera (Arbusculidium, Coryphidium, Striatotheca) are present, the assemblage clearly belongs, in terms of palaeobiogeography, to the peri-Gondwanan acritarch province. Our study thus extends the geographical distribution of this province to the north-western part of South America, extending the palaeobiogeographical distribution map of Early to Middle Ordovician acritarchs. A comparison of the palynoflora with models of acritarch distribution in different palaeoenvironments implies a relatively shallow-water environment. [ABSTRACT FROM AUTHOR]