La Luna Formation is known as one of the most prolific source rocks in the northern Andes and worldwide. However, despite this proven potential and the increasing interest in La Luna Fm. as an unconventional play the regional sequence-stratigraphic framework and its depositional system remain poorly understood. Then to understand this variability for Luna Fm. in the Magdalena Valley Basin we use the results from a complete set of wireline logs, and description and analysis of multiple cores to propose a new stratigraphic model that relates facies and depositional environments distribution with rock properties.
Our method relies on describing and identifying key lithofacies, stacking patterns, erosional and flooding surfaces, unconformities, as well as regional markers such as concretions levels, volcanic ash and phosphate-rich layers that we tie with detailed age determinations. Then by correlating these stratigraphic and sedimentological models for each different well with the geochemical data and our log-derived petrophysical analyses we are able to extrapolate and correlate time-lines in order to establish a regional sequence stratigraphy model and simultaneously to relate rock properties such as organic richness, porosity, permeability, and brittleness to regional sequences.
We identify three low-frequency depositional stages bounded by key regional surfaces that are differentiable within La Luna Formation. The lowest interval is an organic-rich level, consisting of grey to black, calcareous mudstones and limestones with nodules, interbedded with bioclastic and recrystallized wackestones and packstones; the middle level is an organic-poor interval consisting of grey laminated siliceous, argillaceous and non-calcareous mudstones and siltstones, bioturbated, and the upper level is an organic-rich interval consisting of grey to black, silica-rich mudstones, interbedded with foraminiferal wackestones, phosphatic and chert beds. Our correlations for this part of the basin show the regional development of three these sequences with similar lithofacies and related rock properties distribution and with small changes in depositional environments and thicknesses.
Then by correlating stratigraphic interpretations with rock properties between key surfaces we are able to start developing a model that explains the regional distribution and continuity of both depositional systems and rock properties in one of the newest and most promising unconventional targets.
