Regional Sequence Stratigraphy of the Upper ...

URTeC ID 1934959

Regional Sequence Stratigraphy of the Upper Cretaceous La Luna Formation in the Magdalena Valley Basin, Colombia.

H.A. Galvis-Portilla*1, N. Marfisi1, I. Higuera-Diaz1, S. Cespedes1, C. Ballesteros1, S. Forero1, M. Cantisano1, E. Pineda2, Z. Pachon2, R.M. Slatt3, R. Ramirez4, G. Guzman4, A. Torres4 1 Unconventional Resources Program, Ecopetrol, Bogota, Colombia. 2 Colombian Petroleum Institute, Ecopetrol, Santander, Colombia. 3 The University of Oklahoma, Norman, OK, USA. 4 Genesis Consulting, Bogota, Colombia. Copyright 2014, Unconventional Resources Technology Conference (URTeC) This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, 25-27 August 2014. The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper without the written consent of URTeC is prohibited.

Abstract 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. Key words: Lithofacies, Sequence Stratigraphy, La Luna Formation, Magdalena Valley Basin, Colombia.

Introduction The Middle Magdalena Valley Basin (MMVB) is a mature basin in terms of oil exploration and production; its activity has been largely focused on Tertiary conventional hydrocarbon reservoirs. Since the first giant oil field discovery in 1918 (Cira-Infantas Oil Field), to the present in the MMVB had been discovered of about 1900 MMBOE and 2.5 TCF over 40 oil fields (ANH, 2007). Then, given these proven conventional oil accumulations confirming an active and effective petroleum system, it is important to address the characteristics and potential of the source rock in this basin. In theory, in source rocks 40% of its generated hydrocarbons are retained and the rest 60% are expelled-out (Jarvie et al., 2007). If that were the case in the MMVB there is a great potential for hydrocarbon accumulations in unconventional organic shales in this region. Moreover, the MMVB in Colombia has begun to attract efforts in looking for these retained hydrocarbons into the main source rock, the Upper Cretaceous La Luna Formation. Recent assessments of technically recoverable shale oil and shale gas resources estimates 4,8 Billion bbl of oil in-place and 18 Tcf of gas in-place within cretaceous units in the MMVB (EIA, 2013). As today the regional studies of the stratigraphy and extension of La Luna Fm. in the MMVB (Figure 1), and the UMVB, are based either on surface outcrops along the eastern limit of the basin or on indirectly and segmented information from well logs and/or seismic data (Bernal, 2009; Burgl, 1962; Macellari, 1988; Morales et al., 1958; Rangel et al., 2000; Villamil, 1998 and Wheeler, 1929). Recent stratigraphic wells in the MMVB have for the first time drilled the entire Cretaceous succession and made the acquisition of more than 5000 feet of cores then, marking a milestone in the exploration and understanding of these rocks in the MMVB. Cores used in this study represent the most continuous stratigraphic record of the La Luna Fm. in the central MMVB (Figure 1). Core studies included standard mineralogical and geochemical analyses that tied with detailed core description, biostratigraphical and geochronological dating and extended sets of well logs allowed us improve our understanding of the sedimentology and stratigraphy of La Luna Fm. The objectives of this study are: (1) To describe and characterize the Upper Cretaceous lithologic record for La Luna Fm. in the central MMVB. (2) To establish a sequence stratigraphic framework. (3) By using key dated horizons compare and correlate the lateral distribution of lithofacies within the sequence stratigraphy sense. B

A

Area of Study

C

Figure 1. (A) and (B) Location map of the MMVB and the study area showing regional surface geology of the central MMVB. (C) Schematic cross-section across the central part of the Middle Magdalena Valley Basin.

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In the MMVB La Luna Fm. has been divided into three members from bottom to top Salada, Pujamana and Galembo members (Morales et al., 1958). The lower Salada Member is mainly composed of black, thin-bedded, finely laminated, limy shales and occasional thin beds of black fine-textured limestone. The middle Pujamana Member consists of gray to black color, calcareous, thin-bedded shales. The upper Galembo Member is mainly composed by calcareous shales with interlayering of thin, argillaceous limestone and organic black shales, numerous discoid concretions, and intercalations of thin layers of chert. These facies within La Luna Fm. denote conditions of accumulation in relatively shallow depths, about 200 meters deep with abundant biological productivity and under the influence of anoxic events (Rangel et al., 2000).

Lithofacies & Physical Stratigraphy Compositionally La Luna Fm. is a mixture of hybrid or mixed rocks with variable proportions of terrigenous, ortochemical and allochemical components. Organic content is generally moderate to high regularly higher than 2% TOC by weight. Considering that there is not a great noticeable textural or compositional variability at macroscopic at the scale of core description then we need to rely on a multi-scale approach based upon petrographical and analytical techniques from mm-scale to m-scale that will allow us to add textural or compositional qualifiers to the rock name (Galvis-Portilla et al., 2013). In general, we pay attention to the following description criteria in this particular order: texture, lithology, color, gradations, bed contacts, allochemical constituents, fossils content, physical sedimentary structures, bioturbation, presence and frequency of lamination, and carbonate content. Since there is not a standardized terminology to denominate lithofacies in organic-rich fine-grained rocks, in this study we propose a lithofacies classification grouped in four compositional groups represented by: (1) quartz-dominated lithofacies; (2) carbonate-dominated lithofacies; (3) argillaceous-dominated lithofacies; and (4) mixed-composition lithofacies. Each individual group of lithofacies and their detailed associated modifiers are described and illustrated in Figure 2. In La Luna Fm. the Salada Member (Figure 3) is an organic-rich level, consisting of dark grey to black, laminated, mixed calcareous/siliceous mudstones interbedded with foraminiferal wackestone and packstone. Its lowermost part (70 ft. thick) is dominated by bioclastic wackestone and packstone, recrystallized limestone interbedded with calcareous/siliceous mudstone. Centimeter-sized calcareous concretions (or nodules) are frequent between the middle and the base of the interval. A remarkable characteristic are the numerous occurrences of laminae to thin-beds of volcanic ash deposits or tuffs, which are predominately concentrated towards the base of this interval. Towards its upper contact with its overlying interval (middle member) gradually increased the quartzdominated lithofacies and subtly its argillaceous content. The Pujamana Member (Figure 3) is a monotonous organic-poor level consisting largely of grey to black, laminated, mixed siliceous/argillaceous and non-calcareous mudstones. Moderate to intensive bioturbation and borrows are very common through the entire interval. Locally may occur siltstones and very fine-grained sandstones with glauconite. Some thinly interbedded fossiliferous wackestone are present. The Galembo Member is an organic-rich level consisting of grey to black, laminated, quartz-dominated mudstones, interbedded with mixed calcareous/siliceous mudstone and foraminiferal wackestones/packstones. Towards the top of the interval, phosphatic particles are common. To the middle part a slightly increase in argillaceous/siliceous Lithofacies is present. An important feature is the occurrence of numerous laminae to thin- and medium-beds of volcanic ash depostis (tuffs) scattered throughout the interval. The lithofacies for the Salada Member is the most calcareous interval represented largely by mixed calcareous/siliceous mudstones interbedded with fossiliferous limestones (Figure 3). In turn, the Pujamana Member is the most argillaceous and less calcareous interval, represented by laminated argillaceous/siliceous mudstones. Finally, the Galembo Member constitutes such interval in which wider lithofacies diversity is present, mixed siliceous/calcareous/argillaceous mudstones lithofacies are interbedded with highly calcareous (wackestones and packstones) and highly siliceous mudstones. In a mechanical stratigraphy sense the Salada and Galembo members due to their calcareous and siliceous nature (Ca+Qtz>80%) constitutes competents intervals whereas the Pujamana member due to its argillaceous character (15-45% Clays) may behave as a non-competent interval.

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Symbol

Quartz Dominated Lithofacies

Lithofacies Massive Siliceous Mudstone

Marine suspension settling

Laminated Foraminiferal Siliceous Mudstone


Benthic Foraminiferal Siliceous mudstone


Slightly Borrowed Siliceous Mudstone Siliceous Phosphatic Mudstone

Qtz > 65%

Slightly Borrowed Siltstone with Glauconite

Storm-wave deposits (traction?)

Very fine- to silt-grained sandstone

Storm-wave deposits (traction?)

Marine suspension settling (rhythmic)

Laminated Foraminiferal Packstone


Thinly Interlaminated Foraminiferal Wackestone-Packstone


Wackestone/Packstone with Dolomite

Dolostone Laminated Argillaceous Claystone Silty Claystone Fossiliferous Claystone Clay > 65%

Volcanic Ash/Pyroclastics/Tuff Laminated Siliceous/Argillaceous Mudstone

Mixed Lithofacies Subequal admixture of Clay, Qtz, Carb

Marine precipitates

Laminated Foraminiferal Wackestone

Crystalline Carbonate

Argillaceous Dominated Lithofacies

Winnowing and reworking Storm-wave deposits (traction?)

Calcitic Shelly/Bioclastic Wackestone/Packstone Cal+Dol > 65%

Storm-influence, oxigenated bottom

Laminated to Wavy Fossiliferous Siltstone

Massive Lime Mudstone

Carbonate Dominated Lithofacies

Interpretation

Borrowed Silty Siliceous/Argillaceous Claystone

Insitu reworking of shell material Diagenetically altered Post-depositional modification Diagenetically altered Marine suspension settling Suspension settling, storm influence Marine suspension settling Ashfall deposits Marine suspension settling Storm-influence, oxigenated bottom

Massive Siliceous/Calcareous Mudstone


Siliceous/Calcareous Phosphatic Mudstone

Winnowing and reworking

Slightly Borrowed Siliceous/Calcareous Mudstone

Storm-influence, oxigenated bottom

Laminated Foraminiferal Siliceous/Calcareous Mudstone


Siliceous/Calcareous/Argillaceous Mudstone


Figure 2. Main components of characteristic lithofacies for La Luna Fm. The color code is dependent on the dominant compositional elements.

Sequence Stratigraphy Using principles of sequence stratigraphy and a generalized sequence-stratigraphic model for organic shales (Slatt et al., 2012) (Figure 4) for each well, we first interpret stratigraphic sequences base on gamma ray responses, mineralogy, and TOC. We use both stratigraphic patterns in both gamma ray log and mineralogy/TOC to pick two types of surfaces: Flooding Surface (FS) or Maximum Flooding Surface (MFS) for a major boundary and combined Sequence Boundaries/Transgressive Surfaces of Erosion (SB/TSE). These boundaries record a relative sea level (RSL) cycle beginning with a drop in sea level to form the SB, then early turnaround in sea level to form the TSE over the SB (thus, combining into one SB/TSE surface representing a long interval of geological time), then marine

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transgression to form a Transgressive Systems Tract (TST) which becomes finer-grained and usually more organicrich upward (condensed section or CS is often present in the upper part of the TST); the CS is capped by the FS or a MFS, then a reduction in the rate of relative sea level rise to form the progradational. Highstand System Tract (HST), which within the La Luna Formation is often carbonate rich.

Member

Galembo

Pujamana

Salada

Thickness (ft.)

1600'-1800'

500'-600'

400'-600'

Lithofacies Abundance (Legend in Fig. 2)

Organic-rich, Tuffs, Organic-poor, High Organic-rich, Tuffs, cmPhosphates, meter-sized bioturbation, Nonsized calcareous Other Features calcareous concretions, calcareous, Terrigenous concretions, coarsebenthic foraminifera. organic matter. bioclastic carbonates. Figure 3. Lithofacies distribution for each member of the La Luna Fm. For color code on lithofacies refer to Figure 2.

Typical vertical stacking pattern stratigraphy of a high-frequency sequence within the La Luna Fm. is cyclic, from base to top consisting of (1) an interval of gradual upward increase of gamma-ray values with a SB/TSE at its base (red lines in Figure 5), lithofacies composing this interval exhibit non-laminated, siliceous, less calcareous, and higher Uranium content, the SB/TSE mostly erosive exhibit coarse bioclastic lithofacies related with winnowed of sediments due to the sediment starvation (traction structures) during early stages of sea level rising; (2) a Flooding Surface (FS or MFS) at the highest gamma-ray reading (blue lines in Figure 5), due to the contribution of the Uranium contained in the organic material. Then FS is followed upward by (3) a ‘cleaning upward’ gamma-ray log interval, which usually corresponds to an upward decrease in clay mineral and/or organic matter contents and upward increase in carbonate content due to the biogenic carbonate supply during sea-level highstand. Lithologically the HST record in the La Luna Fm. is represented by carbonate and siliceous dominated lithofacies and higher lamination intensity defined by alternating fossil-rich Wackestone/Packstone and fossil-poor ones, HST within La Luna Fm. in some cases vary to a detrital quartz-silt dominated lithofacies, where wave action becomes notorious by ripple/wavy laminations, also showing more borrows. Though the interpretation of short-term sequences (3 my and 400-800 ft thick) which may be controlled by glacio-eustatic variations in a sub-regional scale (Vail et al., 1991). Figure 6 summarizes our long-term sequence-statrigraphic model based on the interpretation of key geological proxies tied with well logs. In total we identified three long-term sequences. Typically one sequence of this type is represented from base to top by: (1) a sequence boundary and/or transgressive surface (SB/TSE) which is characterized by erosional bed contacts at the base of its lithofacies associated; lithofacies immediately overlying this surface are predominantly shelly bioclastic carbonates in chaotic assemblages. (2) TST represented by upward increase in the GR responses, organic content and Uranium values; lithofacies composing the TST compositionally are admixtures of carbonate and Quartz, lithofacies look less laminated and massive. Following upward occur (3) a MFS/FS, which is marking the organic richest interval due to the maximum shelfward extent of marine conditions. (4) a HST gradually upward decreases its organic content, while increasing frequency of argillaceous-dominated lithofacies (high Th+K), bioturbation and terrigenous organic matter increase, as well as high frequency of fossil laminations are common in the HST deposits (Figure 6).

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Small scale ripple features of silty-sized detrital quartz (HST). Small scale ripple features of foraminifera (HST).

A

Landward

B

HST Seaward

CS

MFS/FS

SB/TSE

TST

GR

GR HST MFS CS

SB

One RSL cycle

Highly organic-rich mudstone (TST).

Bioclastic limestone, reworked material (TSE).

Figure 4. Generalized sequence stratigraphic model and the corresponding gamma ray log responses, with examples from La Luna Fm. (Modified from Slatt et al., 2012)

Stratigraphic Correlations After high-frequency stratigraphic interpretation of each well, we tried to correlate the strata between wells. However, this was a difficult part of the analysis because well cores examination shows that many of the highfrequency cycles look very similar in their gamma-ray pattern. Therefore, we rely on the long-term stratigraphic sequences in Figure 6, which are superimposing the high-frequency ones, thus providing us a view of the more general and larger scale variations and cycles displayed by the logs and lithofacies. Initial correlations between wells were then made based upon these general logs trends, besides that we use the HIRES method for time-based correlations described by Kauffman et al., (1991), which showed to be useful within epeiric successions (e.g. USA Western Interior Basin). We identified various events, which constitute key markers for regional chronostratigraphic correlations, such as physical events (tuff beds, storm deposits-skeletal limestones), chemical events (nodules/concretions, phosphate-rich deposits, organic richness, carbonate content and type of organic macerals), and biological events (benthic/planktonic ratios). We are not correlating every high frequency because it is unlikely that over long distances between wells, that all of the high-frequency sequences will appear the same or some may have stratigraphically pinched-out. However, the important point is that enough low-frequency SB/TSE and MFS/FS surfaces were correlated to show the consistent, general trends of the La Luna Fm. in the central MMVB.

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Total Gamma-Ray API (0-200) 50

0 ft 11800

7

Uranium Thorium PPM (0-25) PPM (0-50)

100 150 200 0 5 10 15 20 25 0 0 10 20 30 40 50 11800 11800 11800

10’ 11810

Potassium % (0-3) 1

2

3

Lithofacies

TOC wgt%

Core Hand Specimen Thin Section Microphotography

1.3

11810

11810

11810

20’ 11820

11820

11820

11820

30’ 11830

11830

11830

11830

11840

11840

11840

HST 2.0

D

2.1

0.5 mm

FS 40’ 11840

3.0

TST 50’ 11850

11850

11850

11850

3.6

C 60’ 11860

SB 11860

11860

11860

70’ 11870

11870

11870

11870

2.2

1.5 11880 80’

HST

11880

11880

11880

11890

11890

11890

B 11890 90’

4.8

FS 11900 100’

3.0

11900

11900

11900

5.3 11910 110’

TST 11910

11910

11910

4.6 11920 120’

11920

11920

11920

A

Figure 5. Typical vertical stacking pattern of a high-frequency sequence consisting from base to top of a (1) lower, high gamma ray interval with a SB/TSE at its base (red lines) and a flooding surface (FS or MFS) at the highest gamma ray reading (blue line) followed upward by a ‘cleaning upward’ gamma log pattern. In the right column there are examples for each sequence coming from the core and thin sections.

Conclusions: This study demonstrates the possibility of identifying, cyclical, high-frequency sequence stratigraphic framework for the La Luna Formation in wells and to correlate major surfaces between the central part of the MMVB. A high frequency sequence consists of a basal sequence boundary/transgressive surface of erosion SB/TSE followed upward by a fine-grained Transgressive Systems Tract (TST) with an (often) organic-rich Condensed Section (CS) at the top of the TST and capped by the Flooding Surface (FS/MFS), then a progradational Highstand Systems Tract (HST), that is often calcareous, more fossil-laminated and bioturbated. On a Gamma Ray log, the basic pattern is one of a basal high-gamma ray response (corresponding to the TST/CS), with either a sharp or gradational base (corresponding to the SB/TSE), followed upward by an upward-decreasing gamma-ray response (corresponding to the HST). The fact that many of the surfaces and low-frequency sequences could be correlated between the wells is an indicator of long-distance lateral continuity. Thus, if proper hydrocarbon target zones are selected for testing and/or horizontal drilling, the zones should be laterally continuous unless offset by small faults or the occasional stratigraphic pinch-out. Prior studies suggest that many unconventional resource shales can be characterized geomechanically as consisting of brittle-ductile couplets (Slatt et al., 2012). High gamma ray, organic rich horizons of the Transgressive Systems Tract are ductile, and provide the organic material for generation and storage of hydrocarbons; thus they constitute a ‘hydrocarbon source rock’ at a small scale. Lower Gamma Ray, calcite rich

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horizons of the Highstand Systems Tract are brittle, thus they constitute the reservoir rock into which hydrocarbons can flow when the rock is hydraulic fractured. Thus, we favor horizontal drilling into the brittle part of a brittleductile couplet. In the La Luna Formation, the brittle-ductile couplets are analogous to each high frequency cycle shown in Figures 5 and 6. Total Gamma-Ray Lithofacies API (0-200)

Uranium PPM (0-25)

Thorium PPM (0-50)

Potassium % (0-3)

High TOC Values

Bioturbation Intensity

Erosional Bed contacts

Bioclastic W/P

Terrigeous Organic Matter

Sequence Stratigraphy Model

? HST FS TST

Upper Member 1600-1800 ft

SB

HST

FS TST

Lower Member 400-600 ft

Middle Member 500-600 ft

SB

HST

FS TST

Figure 6. Low-frequency sequence stratigraphic interpretation of the La Luna Fm. with the associated spectral gamma ray logs (Uranium, Thorium, and Potassium).

Mineralogical composition data all indicate the La Luna Fm. contains relatively small amounts of clays and approximately subequal amounts of quartz and calcite. Thus, it falls within the ‘brittle’ range of formations, even though there are organic-clay rich intervals in the La Luna Fm., as there are in the other comparable, productive formations. The mineralogy further supports good potential for the La Luna formation because of its high proportion of ‘fracturable’ rock. High frequency sequence stratigraphic units were recognized by a lower, massive to slightly laminated, clayorganic, gray-black shale (TST/CS) grading upward into a lighter-colored, laminated calcareous interval (HST). Sometimes an erosion surface (SB/TSE) could be identified at the base and/or top of a sequence and sometimes a flooding surface (FS) could be identified by a relatively sharp contrast between the two facies. Most of the La Luna Fm. deposition occurred within a low-gradient marine shelf dominantly below the storm wave base, except for the middle and uppermost part of the La Luna Fm. which show evidence of storm wave deposition (small-scale ripples and traction structures). Most of deposition in the lower and upper La Luna Fm. occurred in an anoxic to dysaerobic environment; while the middle La Luna Fm. has deposits in which trace fossils (and low organic richness) indicate a more oxygenated environment.

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Acknowledgments We would like to express thanks to the Unconventional Resources Program of Ecopetrol S.A. for permission to present these results and for its support in conducting this study. We also thank the Colombian Petroleum Institute (ICP-Ecopetrol) for providing numerous lab analyses which were imperative for this study. References ANH, 2007, Colombian Sedimentary Basins: Nomenclature, Boundaries and Petroleum Geology, a New Proposal, Bogotá, Colombia. Bernal, L.A., 2009, Caracterización Estratigráfica y Petrográfica de la Formación La Luna en el Sector de El Tablazo, Valle Medio del Magdalena. Bogota, Universidad Nacional de Colombia. Undergraduate Thesis. Burlg, H., 1962 Sedimentación cíclica en el geo sinclinal Cretáceo de la Cordillera Oriental de Colombia, Bogotá Instituto Geológico Nacional, Boletín Geológico v.1-3, p.85-118. Galvis-Portilla, H., Marfisi, N., Higuera-Díaz, I.C., Ballesteros, C., Cespedes, S., Marin, M.P., 2013, Integrated Multiscale Approach for Lithological and Mineralogical Characterization of Unconventional Shale Plays: Understanding the Early Cretaceous rocks in La Luna-1 well, MMVB, Colombia. Poster presentation at AAPG International Conference & Exhibition, Cartagena, Colombia. Jarvie, D.M., R.J. Hill, T.R. Ruble, and R.M. Pollastro, 2007, Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment: AAPG Bulletin, v.91, no. 4, p. 475–499. Kauffman, E.G., Elder, W.P., Sageman B., 1991, High Resolution Correlation a New Tool in Chronostratigraphy in: Einsele, G. Ricken, W. and Sailacher, A. eds, Cycles and Events in Stratigraphy, Berlin Springer Verlag, p.795819. Macellari, C.E., 1988, Cretaceous Paleogeography and Depositional cycles of Western South America: Journal of South American Earth Sciences, v.1, p.373-418. Morales, L.G, and the Colombian Petroleum Industry, 1958, General Geology and oil ocurrences of Middle Magdalena Valley, Colombia. In: Habitat of Oil Symposium. American Association of Petroleum Geologists, p. 641-695. Rangel, A., Parra, P., Niño, C., 2000, The La Luna formation: Chemostratigraphy and Organic Facies in the Middle Magdalena Basin. Organic Geochemistry, v.31-12, p.1267-1284. Slatt, R.M., and Rodriguez, N.D., 2012, Comparative Sequence Stratigraphy and Organic Geochemistry of Gas Shales: Commonality or coincidence?, Journal of Natural Gas Science and Engineering, v.8, p.68-84 U.S. Energy Information Administration, 2013, Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States, U.S. Department of Energy, Washington, DC. Villamil, T., 1998, Chronology, Relative Sea Level History and A New Sequence Stratigraphic Model for Basinal Cretaceous Facies of Colombia, in: J.L. Pindell and C. Drake, eds., Paleogeographic Evolution and Nonglacial Eustasy, Northern South America: Society for Sedimentary Geology (SEPM), Special Publication v.58, p.161–216. Vail, P.R., Audemard, F., Bowman, S.A., Eisner, P.N., and Perez-Cruz, C., 1991, The stratigraphic signatures of tectonics, eustasy and sedimentology–an overview. In: G. Einsele, W. Ricken and A. Seilacher Eds., Cycles and Events in Stratigraphy, p. 617-659. Berlin, Springer-Verlag.

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Wheeler, O.C., 1929, Report on the Palmira Series with notes on Stratigraphy of the Umir, Lisama and La Paz formations near the Eastern part of De Mares Concession. Internal Report Empresa Colombiana de Petroleos.