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Abstract

The Cretaceous passive margin deposits of Colombia form part of northern South America’s most prolific hydrocarbon system, which highlights their source rocks as high-potential, self-sourced to hybrid Unconventional resource plays. This succession is characterized by thick and laterally extensive Type II(S)/III shales and carbonates of the Lower Cretaceous Basal Carbonates (sensu lato) and Upper Cretaceous Villeta/Gachetá/La Luna/Navay formations, the latter group being broadly coeval with the Eagle Ford Formation of North America. Unlike the Eagle Ford, however, the Colombian basins’ world-class source rocks experienced a subsequent complex tectono-stratigraphic evolution, which can either complete and enhance exploration and commercial viability or detract from it.

Basement-involved shortening in Colombia’s supra-subduction zone setting provided the regional framework for these basins to form and has a first-order control on key petroleum system elements and ultimately the location and size of unconventional sweet spots on the play scale. While many of Colombian onshore basins have been Conventionally explored and produced for nearly a century, the understanding of their source rock systems and how they will perform as unconventional plays, is still emerging. In this setting, three key regional evaluation methods that can help to predict resource play behavior are:

  1. Unraveling younger structural activity and its superposition on Cretaceous paleogeography using palinspastic plate reconstructions to constrain original gross depositional environment and identify source rock accumulations of the highest quality and net thickness;

  2. Understanding the areal and temporal interplay between source rock maturation and deformation or uplift; and

  3. Examination of stress regime evolution, natural fracture networks, and the present-day stress field in order to properly plan and execute drilling and hydraulic fracturing.

In conjunction with these geologic factors, which ultimately impact static properties of the unconventional reservoir, source rock maturity and correlative fluid properties exert an additional control on dynamic reservoir performance and in combination dictate well estimated ultimate recovery (EUR) volumes.

Although many of the Colombian basins contain correlative Cretaceous source rock packages, they are unique in terms of their depositional environments, structural history, and degree of thermal maturation. The Cretaceous of Colombia’s various present day basins are situated in different original parts of the broad, northwest South American passive margin, at varying water depth, nutrient supply, and seafloor morphology, and the Tertiary orogenic history has caused different degrees and timing of sedimentary burial and overthrusting of those sections. Regional palinspastic reconstructions illustrate paleogeography at the time of deposition and help to constrain localities of concentrated, high-TOC facies, as shown by an example of Colombia during the Early Turonian (Fig. 1). A transgressive surface marks the transition from Late Cenomanian to Early Turonian, and depositional systems step landward leading to the Cretaceous maximum flooding surface (MFS).

Figure 1.

Palinspastically restored paleogeographic map of the Colombian basins at ~92 ma, Early Turonian (Kennan and Pindell, 2009; Pindell and Kennan, 2009; Tectonic Analysis, Ltd., 2009).

Figure 1.

Palinspastically restored paleogeographic map of the Colombian basins at ~92 ma, Early Turonian (Kennan and Pindell, 2009; Pindell and Kennan, 2009; Tectonic Analysis, Ltd., 2009).

The Early Turonian is a regional condensed section through all the marine sections of Colombia and western Venezuela, characterized in some areas, notably the Upper Magdalena Valley, by a phosphatic deposit about 10 cm thick. This horizon contains phosphate pebbles up to 3 cm diameter and shows various trace and major element abundance peaks. The time interval over which this phosphatic lag was deposited is presently impossible to evaluate.

The Early Turonian condensed section is also expressed as a couple of concretion intervals associated with abundant and diverse fauna, which appear seismically as a pair of high-amplitude reflections within the Cretaceous, and which can be used to locate the underlying Cenomanian lowstand deposits. In the Putumayo basin, this transgression may be represented by Upper Villeta facies that are more argillaceous (deeper water) than the underlying Lower Villeta. Alternatively, the Upper Villeta may record the arrival and eastward migration of a peripheral bulge ahead of converging allochthonous terranes or local enhanced uplift and subsidence events associated with plume-related mafic intrusions.

The Early Turonian in many places (e.g., the Villa de Leiva and Cocuy regions) comprises deeper water facies above shallow-water sandstones or limestones of the Caliza Mermetti. Formation boundaries have been introduced in the literature where this change is obvious. However, although the Caliza Mermetti is regionally extensive, it is laterally discontinuous, and in some regions Early Turonian and Cenomanian shales merge and just one stratigraphic name and age is assigned to both. This has produced nomenclatural correlation problems and misjudgments in sedimentation rates, which in turn have lead to incorrect propositions of large thickness changes in the Upper Cretaceous (growth structures) and unnecessary calls for tectonism at this time (e.g., Sarmiento et al., 2006;, Vásquez and Altenberger, 2005). The Early Turonian facies are very similar to Late Albian facies, and can easily be confused in the field.

Paired with paleogeographic maps of the same interval, stratigraphic cross sections can highlight the diachronous nature of the Cretaceous deposits (Fig. 2), which are controlled by the regional tectonic configuration. Stratigraphic cross sections do not show the areal and regional distribution of facies but rather a temporal framework for deposition. The section begins with basement and initial rift sediments that then get transgressed by the Cretaceous systems. The base of the section shows the Caqueza Group and equivalent units containing abundant turbidites (basin floor fans, channel-levee systems, and classical turbidites). This unit is prograded across by a shallow-water sandstone, the Caqueza sand, and then transgressed by a regionally extensive surface, the base of the Villeta Group in the Eastern Cordillera.

Figure 2.

West to East sequence stratigraphy slug diagram of Jurassic-Cretaceous Formations of the Middle Magdalena Valley, Eastern Cordillera, and Llanos Basins (from Villamil and Pindell, 1998).

Figure 2.

West to East sequence stratigraphy slug diagram of Jurassic-Cretaceous Formations of the Middle Magdalena Valley, Eastern Cordillera, and Llanos Basins (from Villamil and Pindell, 1998).

The Hauterivian-Barremian boundary at the base of the Villeta cannot be traced to the Upper Magdalena Valley but can be recognized in the Villa de Leiva region as the base of the Paja Formation. The Late Barremian contains abundant calcareous concretions rich in ammonite assemblages and represents a relatively condensed interval. The Aptian stage includes turbidites of the Socotá Formation in the central portions of the Eastern Cordillera and shallow-water facies to the east, near the Llanos. The Albian is represented by the Hilo Formation and its coeval Une Formation to the east. The Middle Albian is a major condensed section that can be recognized regionally in Colombia. The Cenomanian stage is a general regression and a sequence boundary during the Late Cenomanian. Facies of this age are called Une in the eastern portions of the Eastern Cordillera and they are unnamed in the central and western parts.

Above the Late Cenomanian lowstand, there is a transgressive surface followed by deposition of the La Frontera Formation condensed section (see Villamil and Arango, 1994). The early Turonian MFS (see also Fig. 1) is represented in the Llanos by relatively shallow-water facies rather than distal offshore shales, hemipelagic limestones, and calcareous concretions. This unit is called Chipaque and by many other names in different regions of Colombia. This Cretaceous MFS is prograded over by Cretaceous depositional systems (low-order highstand systems tracts). The first units to prograde over it are some unnamed shales of the Villeta (La Frontera and others), then siliceous shales at the top of the Villeta that are correlative to the Olini, Conejo and other units, and finally by the shallow-marine sandstones of the Guadalupe and coastal plain deposits of the Guaduas Formation.

Plate interaction and tectonic history from Upper Cretaceous to Present day exerted the primary control on source rock maturity through combinations of regional burial, exhumation, and structural activity (Fig. 3). Contemporary foredeep maturation developed farther from the original thrust fronts, as a function of burial and outward propagation of younger thrust faults. In the Lower Magdalena Valley, the San Jorge and Platorifts were deep enough to host maturation within them, but the Plato was uplifted in the Pliocene and maturation was arrested.

Cauca Valley sedimentation is now sufficiently thick to have resulted in at least local maturation of Cretaceous and early Tertiary source rocks, if they are present. The Sinú Belt has been thickened via ongoing sediment accretion to the point of driving maturation within it. Although some are by now passed their prime in terms of maturation potential, the greatest number of areas within Colombia presently have active, ongoing maturation. Turonian source-rocks are under mature in the Upper Magdalena Valley, while they are mature in the Eastern Cordillera. The reason for this difference may be the thicker initial overburden of the Eastern Cordillera compared to that of the Upper Magdalena Valley. Detailed 3D basin modeling on the Middle Magdalena Valley (Figs. 4a, b) suggests that present day burial depth is a proxy for maturity, but it is the regional-scale tilting and differential uplift and erosion of the basin between the Central and Eastern Cordillera, as well as smaller-scale structural activity, that ultimately dictate source rock maturity through time.

Figure 4A, B.

Oblique view of 3D basin modeling results for the (A) La Luna Formation and (B) Tablazo Formation in the greater Middle Magdalena Valley. Colors show predicted presentday maturity, and contours are derived from regional depth structure maps.

Figure 4A, B.

Oblique view of 3D basin modeling results for the (A) La Luna Formation and (B) Tablazo Formation in the greater Middle Magdalena Valley. Colors show predicted presentday maturity, and contours are derived from regional depth structure maps.

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