Incised Valleys an Emergent Play in the Llanos Basin Southernmost Border
Disalvo, Alfredo1; Ragazzi, Mariano D.; and Piethé, Ricardo D.
Exploration in the Llanos basin is currently limited due to the high maturity of the traditional plays.
However, of late , reinterpretation of existing data, acquisition of new seismic and wildcat drilling in the southern border of the basin have proved a new stratigraphic play, which in this work we have named Incised Valleys Fill. It consists of a paleogeomorphic trap where fluvial Late Eocene sandstones fill a paleorelief of valleys incised in non permeable Lower Paleozoic rocks which give the lateral seal. Top seal is provided by coastal to shallow marine muds, which indicate the culmination of a Late Eocene to Oligocene transgressive cycle.
Paleovalleys have a SSW-NNE orientation and the sense of sedimentary transport is towards the north. They are controlled by the lithology of the Paleozoic rocks, where the sides of the valleys correspond to ortoquartzites and the bottom to softer rocks, resulting in an irregular topography due to differential erosion.
Sandstones are related to amalgamated fluvial channels, which are confined to the basal fill of the paleovalleys. Upwards, floodplain and coastal influenced fine grained deposits develop. This fining upwards sequence gradually loses its seal towards the east due to a lateral change of facies in the Early Oligocene deposits. The frequency of multistorey channel fills and the sand-shale ratio increase in the whole section, decreasing the capacity of vertical seal. This change of facies has a N-S orientation, with a shale out limit close to Guarrojo-1 well, continuing approximately to well SV-6 in the south, as indicated by net sand to gross thickness maps.
Therefore, Incised Valley fills play distribution is conditioned by two factors: paleovalleys presence and vertical seal distribution.
On the other hand, the lack of vertical seal in the Basal Sandstones could explain the large heavy oil accumulations present to the east at stratigraphically higher units such as C7 in Rubiales-Quifa-Sabanero fields.
This new play has been confirmed by Guairuro and Merlin discoveries and could have the potential for giant reserves (higher than 100 MMbbls recoverable).
By analogy with these recent discoveries, a similar exploration hypothesis where the trap would be given for a stratigraphically younger reservoir/seal pair (C7/C6) could be postulated. This could be located towards the SE border of the basin. The uncertainty about seal continuity towards the southeast would be the main risk factor of this new play.
AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013