La Luna Formation Rocks! Part 1

The Late Cretaceous La Luna Formation is the main source rock of western Venezuela and one of the best known on Earth. With renewed interest in Venezuela’s hydrocarbon sector, the question is changing from what the rock generated to whether it can be produced as an unconventional (shale) resource. Public estimates of that potential differ by more than an order of magnitude: 167 Tcf of gas and 13.4 billion barrels of oil (EIA, 2013) versus 5.7 Tcf and 656 million barrels (USGS, 2017). This article, the first of a series, argues that La Luna should not be evaluated as a single homogeneous unit. Outcrop, core and well studies from Venezuela and Colombia show strong vertical and lateral variability in organic richness, mineralogy, brittleness, pore systems and age. That variability records the changing conditions of the Cretaceous “La Luna Sea”, from oceanic anoxic events to orbital climate cycles. Prospective intervals may be only 20 to 100 ft (6 to 30 m) thick, so landing zones and sweet spots depend on reading the vertical succession in detail. The article reviews the tools available for that reading: biostratigraphy, chemostratigraphy, and organic petrography and palynofacies, including the limitations of maturity measurement in vitrinite-poor marine organic matter. It stresses the value of preserved cores and samples. It also publishes for the first time a regional correlation chart of La Luna and its equivalents from Ecuador and Peru to Venezuela, drawn by hand by the participants of the La Luna Research Conference (Caracas, 2000) and redrawn from the only known photograph.