--> Basement Control in the Deposition of the Basal Sands (Mirador-Carbonera Fms) of the Southeastern Llanos Basin of Colombia and its Implication in the Distribution of Stratigraphic Traps

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Basement Control in the Deposition of the Basal Sands (Mirador-Carbonera Fms) of the Southeastern Llanos Basin of Colombia and its Implication in the Distribution of Stratigraphic Traps

Abstract

Abstract

The Pendare-1 well, drilled in 2012, discovered 140 API oil at the top of the Oligocene Basal Sands (top of C7 member of the Carbonera Fm) in the CPO-13 Block of the Llanos Basin, Colombia. Since then 108 Km2 of high quality 3D seismic was acquired and 3 appraisals wells were drilled, finding a second reservoir at a deeper level. The results of the wells and the interpretation of the 3D seismic data have changed the play concept from structural to stratigraphic, but understanding the lateral limits of the accumulations is still a challenge.

The basement of the southeastern Llanos Basin is composed of Ordovician, highly compacted siliciclastic rocks, structured in a low angle monocline dipping toward the west, and exposed to erosion until the Tertiary. Quartzite sections, more resistant to erosion, developed cliffs and low altitude ranges oriented north-south while Ordovician shales, less resistant to erosion, formed valleys. This paleo-topography of cuestas controlled the upper Eocene to Oligocene sedimentation with thicker deposits in the lows and thinner deposits in the highs.

The 3D seismic data shows that Sandstones of the Mirador Fm (upper Eocene-lower Oligocene) filled the north-south oriented valleys. The overlaying sediments of the lower Carbonera Fm (C7 member) were deposited in a fluvial meandering environment where channels flowed towards the northwest, ignoring the north-south paleo-barriers. Oil accumulations in the Pendare Field are present in the paleo-highs of this pre-existing topography. Today we are interpreting that in areas above paleo-valleys, more accommodation space was created due to differential compaction of sediments, thus affecting the sedimentation pattern during the deposition of the entire C7 member. The differentially compacted lows could have originated poorly drained areas which may have resulted in the deposition of finer grained sediments that eventually developed into lateral seals. This lateral change in facies could be controlling the up-dip seal of the accumulations where no structural limits have been observed. The analysis of changes in elastic properties of high quality PreStack inverted seismic data shows a lateral distribution of facies that tends to confirm this model. In addition, other factors such as hydrodynamics may play an additional role in the trapping configuration contributing to the complexity of defining stratigraphic traps in the southeastern Llanos Basin.