Time to depth conversion: Velocity modelling in the Llanos Basin, Onshore Colombia

Velocity modelling for time to depth conversion or depthing, a routine task in oil and gas exploration, only has been discussed in a few case studies in Colombia. A well derived – velocity modelling was performed in the Llanos basin, east Colombia, using a geologically constrained multilayer – cake. The study area, a foreland basin, presents a simple structural geology with short displacement faults mainly located to the base of the Cenozoic sequence; and horizontal and parallel reflectors, typical of a homocline, that pinch out to the east onto Palaeozoic rocks. The Cenozoic rocks are made up of three formations: late Miocene to Pliocene Guayabo formation, early – middle Miocene Leon formation and middle Eocene – early Oligocene Carbonera formation, unconformably resting upon a regional unconformity of Mesozoic (Cretaceous) rocks to the west and Palaeozoic (undefined) rocks to the east known as the Llanos basin unconformity. The velocity modelling has shown that interval velocities in these Formations vary, from top to bottom, in the Guayabo formation from 1830 to 2438 m/s; from 1676 to 2286 m/s in the Leon formation; and, in the Carbonera formation from 2286 to 3353 m/s. Below the Llanos basin unconformity rocks present a strong increment in the interval velocity values that range from 3962 to 5182 m/s. Although well log data is inconclusive in order to offer an explanation for the velocity reversals in both the low – velocity Leon formation and some Carbonera formation intra – layers, these inversions in velocity are a common feature in both formations. The reliability, based in exploration well prognosis, of this conversion method has demonstrated that the differences between predicted and actual drilling data are not greater than 3%. Finally, by incorporating the well – derived velocities in a geologically controlled multilayer – cake a more accurate geometry of the target zone in depth is provided.