NEERAJ KUMAR

CANACOL ENERGY, CALGARY, NKUMAR@CANACOLENERGY.COM

ARIJIT CHATTOPADHYAY

CANACOL ENERGY, CALGARY, ARIJITCHATTOPADHYAY25@GMAIL.COM

ANDREW WILLIS

CANACOL ENERGY, CALGARY, AWILLIS@CANACOLENERGY.COM

https://doi.org/10.66733/DJCP6730

Introduction

The Lower Magdalena Valley Basin (LMVB) of northwestern Colombia represents a mature hydrocarbon-producing province with a prolonged exploration and production history, marked by the first recorded gas discovery in 1943 (Arminio et al., 2011). Subsequent exploration programs have acquired more than 24,000 km of 2D seismic and approximately 7,000 km² of 3D seismic data, along with the drilling of more than 270 exploratory and development wells (Willis et al., 2017). These integrated exploration efforts have resulted in the identification of at least 20 major gas fields, with estimated reserves ranging from 20 to 435 BCF, besides numerous smaller hydrocarbon accumulations, several of which contain associated liquid hydrocarbons (Willis et al., 2017).

Tectonically, the LMVB is developed along a convergent margin, associated with the interaction between the Caribbean/Pacific domain and the South American plate (Barrero et al., 2007; Arminio et al., 2011). The basin is underlain by continental crust and bounded to the west by the Sinú–San Jacinto accretionary prism, with deformation characterized by a combination of subduction-related processes and strike-slip tectonics. This structural framework has exerted a first-order control on basin architecture, accommodation development, and sediment routing (Barrero et al., 2007; Arminio et al., 2011; Willis et al., 2017) (Figure 1).

Figure 1: Study area is in the San Jorge sub basin of Lower Magdalena Valley Basin (LMVB) (after Arminio et al., 2011; Willis et al., 2017)

The primary reservoir interval within the basin is the Cienaga de Oro (CdO) Formation, comprising of a thick (locally up to ~5000 ft) succession of continental to marginal-marine siliciclastic deposits of Oligocene to Early Miocene age, interpreted to have accumulated within an actively subsiding, transtensional setting directly over basement (Arminio et al., 2011). Regionally, the CdO Formation is overlain by the Porquero Formation, consisting of thick marine shales that form an effective regional top seal. The absence of significant hydrocarbon accumulations above the Porquero interval underscores its sealing capacity (Willis et al., 2017).

The present case study on the CdO Formation in the southern part of the LMVB provides a good opportunity to explore the depositional architecture from a seismic-sequence stratigraphic approach. At the same time, the available seismic and well data challenge the scaling relationships between structured seismic facies (defined by 3D interpretation and attributes) and the stacked depositional facies observed at wells through borehole image logs and associated petrophysical data.

General Stratigraphy

The CdO Formation represents the principal reservoir interval within the LMVB of northwestern Colombia, a structurally partitioned fore-arc basin developed along the convergent margin between the Caribbean and South American plates (Flinch, 2003; Arminio et al., 2011). This tectonic framework exerted a first-order control on sediment routing, accommodation development, and stratigraphic architecture during deposition of the CdO. Within this syn-tectonic framework, the CdO Formation was deposited during the Oligocene to early Miocene (Barrero et al., 2007), marking a phase of enhanced siliciclastic influx and accommodation development (Figure 2).

Figure 2: Idealized lithostratigraphic column for LMVB (after Barrero et al., 2007; Willis et al., 2017) showing the ages for the main formations deposited from Oligocene to Plio‐Quaternary.

The CdO Formation comprises of repetitive successions of quartz-dominated sandstones with subordinate conglomeratic intervals, interbedded with shale, siltstone, and coal, reflecting deposition within fluvial to deltaic and marginal-marine environments under high sediment supply conditions (Arminio et al., 2011). Depositional systems evolved within an actively subsiding, structurally segmented basin, where syn-depositional faulting governed the distribution of depocenters, facies variability, and stratigraphic stacking patterns (Barrero et al., 2007; Arminio et al., 2011). Thickness variations and wedge geometries indicate progressive onlap onto structural highs and enhanced accommodation within fault-controlled lows (Arminio et al., 2011). From a subsurface perspective in the area, the CdO interval forms a vertically and laterally heterogeneous reservoir system (thickness varies from 1000 to 5000 ft), and overlain by fine-grained marine shales of the Porquero Formation that provide an effective top regional seal (Barrero et al., 2007; Arminio et al., 2011; Willis et al., 2017). Gas accumulations are typically associated with fault-bounded structural closures, while intra-formational heterogeneity, driven by facies transitions and stacking patterns, plays a critical role in reservoir connectivity and seismic attribute expression (Arminio et al., 2011). Torres et al. (2012) demonstrated the use of seismic attributes, specifically amplitude, spectral decomposition and curvature, to identify deltaic deposits in the CdO Formation in the LMVB. Ganguly et al. (2018) demonstrated the importance of Amplitude-Versus-Offset (AVO) to delineate the reservoir within the same area mentioned in this case study.

The seismic and well log data from a producing gas field in the San Jorge sub-basin of the LMVB were the cornerstone of this case study. This field produced gas from Mid Porquero as well as from prolific CdO sands.

Seismic Interpretation and General Structural Set Up

The producing gas field is a three-way structural closure due to a tilted fault block against a NE-SW strike slip regional fault (Figure 3). The seismic line in Figure 3 shows most of the reservoir (CdO Formation) section. Seismic is zero phase with SEG normal polarity (acoustic impedance increase is positive on seismic). A soft, bright reflector just below the top of the CdO is due to gas-filled reservoir in the field. The CdO is overlain by low-amplitude and continuous reflector package of the top seal and source rock in the area: the Porquero shale. This anomalous reflector, corresponding to the top gas-charged CdO sands, shows a significant downdip amplitude decrease, absorption of frequencies, with a hint of flat spot, and conformable with the structure (Figure 3 and Figure 4). The absorption of higher frequencies due to gas in the crestal area of the structure can be seen on the CdO top horizon extractions with low (10Hz) and higher (40 Hz) spectral cubes (Figure 4). However, such bright amplitude with conformance is not observed in the deeper gas-charged CdO reservoir sands. This is due to interference of seismic reflectors and a weaker acoustic impedance contrast within CdO units as compared to the contrast between Porquero shale unit and Top CdO.

Figure 3: Minimum amplitude map extracted at CdO top reservoir (a). NW-SE oriented seismic line along the regional dip (b).

Figure 4: Frequency absorption due to gas at the crestal part of the structure.

Depositional Architecture of the CdO Formation

Seismic Pattern Recognition of Clinoforms

The entire CdO section is more than 250 ms (TWT) thick. It consists of a few continuous internal reflectors, with non-continuous reflector package between them (Figure 3b). In Figure 5, a top CdO flattened seismic section is shown to understand the geometry of non-continuous reflectors and truncations between the continuous ones. Between the CdO top to CdOS8 and CdOS8 to CdOS10 intervals, the shape of the clinoforms is sigmoidal to oblique, with well-defined topsets, foresets and bottomsets. However, within the S10–S15 and S15–S20 intervals, the seismic character of these inclined reflectors is more shingled.

Figure 5: Prograding features along with possible sequence boundaries in seismic line XY of Figure 3(b) in dip direction.

Figure 6 is showing other geometric attributes such as phase and variance to delineate clinoforms.

Figure 6: Phase and Variance attributes were helpful to map these clinoforms in the area.

In the strike direction (Figure 7), the strata-bound inclined reflectors corresponding to the individual prograding packages truncate each other too, suggesting lateral discontinuity of seismic reflectors and hence corresponding seismic facies.

Regionally, the CdO paleogeography reveals SE to NW sediment fairway systems, controlled by fore-arc tectonics. Locally the syn-depositional fault trends also affected the accommodation space creation and hence guided the local sediment routing patterns differently than the regional trend.

Figure 7: Prograding features along with possible sequence boundaries in a seismic line in the strike direction.

The individual thickness of the inclined reflector package is in the range of 40 –60 ms. Average velocity of the CdO in the area is about 2800–3000 m/s, hence, the approximate thickness of these recognizable prograding seismic packages should be in the range of 60–80 m.

Amplitude maps extracted along the bounding surfaces of the prograding seismic packages show evolving orientations of the geobodies defined by the seismic amplitude from S15 (deeper) to CdO top (younger) section. The average strike orientations of the depositional units estimated from the image log interpretations are placed alongside the amplitude maps as inset rose diagrams. The strike orientations of the depositional units (rose diagrams) demonstrate the similarities between seismic facies and well data. In deeper section, S15-S17, amplitude-driven geobodies are oriented almost N-S direction (Figure 8), whereas towards the younger section (S10 to CdO top), gross orientation of the geobodies slowly changes from oblique to almost parallel (Figure 9, Figure 10 and Figure 11) to the bounding NE-SW fault.

Figure 8: Amplitude extraction (left) and geobodies interpreted in the model (right) at S15-S17. Image log derived average depositional strike orientation are placed as inset. Notice that orientation of geobodies is almost N-S, oblique to the main bounding fault.

Figure 9: Amplitude extraction (left) and geobodies interpreted in the model (right) at S8-S10. Image log derived average depositional strike orientations are placed as insets. Notice that orientation of geobodies is different than the Figure 6 but it is still oblique to the main fault.

Figure 10: Amplitude extraction (left) and geobodies interpreted in the model (right) at S4-S7. Image log derived average depositional strike orientation are placed as inset. Notice that orientation of geobodies is different than the Figure 8 and 9 and now it is almost parallel to the main fault.

Figure 11: Amplitude extraction (left) and geobodies interpreted in the model (right) at S1-S3, upper part of CdO. Image log derived average depositional strike orientation are placed as inset. Notice that orientation of geobodies is different than the Figure 8 and 9 but like Figure 10 and parallel to the main fault. Also, notice the gas effect on the seismic amplitude.

Scale Integration: Well Log vs Seismic Observations

A key objective of this study is exploring ways to correlate the vertical dimension of the depositional sedimentary units observed in well data and the prograding clinoforms observed in seismic data.

Image logs resolve individual facies transitions along a well path and erosional surfaces at the meter scale, while seismic data capture stratigraphic architecture at the tens-of-meters scale. Seismic data reveal that CdO Formation is organized into a series of approximately 60–80 m thick progradational clinoform packages, each defined by inclined seismic reflectors.

The image log interpretation further presents the opportunity to quantify the average strike orientation of the depositional units of CdO across the mapped intervals. These average orientations of depositional units through image logs reveal close similarities with the seismic facies for the upper, mid and lower CdO intervals (Figure 8, Figure 9, Figure 10 and Figure 11).

The thickness range of the seismic clinoforms package closely correlates with the vertical scale of composite depositional packages observed in well logs and image logs. The composite sedimentary packages are enveloped by bedding plane-truncating events at the top and base, which are also in close sync with the seismic reflectors enveloping the observed clinoforms. The strata-bound composite sedimentary units display regular depositional dip-azimuth reversals internally, indicating a switching nature of depositional fairways as the facies stack vertically over time (Figure 12).

Figure 12: Strata-bound composite sedimentary units display regular depositional dip-azimuth reversals and bed truncations observed and interpreted through borehole image log in Mid-Lower CdO.

In seismic-sequence stratigraphic context, the strata (seismic reflector) bound lateral migration of shingled/sigmoidal/inclined seismic facies indicates oblique migration of estuarine bay /back-barrier lagoonal margin with time, truncated by a parasequence boundary or flooding surface. Consequently, oblique seismic packages are interpreted to represent composite marginal marine depositional packages comprising multiple stacked facies, rather than individual ones.

Image Log-Constrained Depositional Architecture

High-resolution image log interpretation of a representative well provides direct evidence of the internal architecture of the CdO succession and establishes the fundamental building blocks of the depositional system. Multiple intervals display cross-bedded and massive sand bodies of fluvial and tidal inlet channels sharply truncating underlying bioturbated shale and coal-bearing units (Figure 12, Figure 13). These contacts are commonly marked by basal coarse-grained lags and abrupt vertical facies transitions. Importantly, these truncation surfaces are not isolated but recur systematically at several stratigraphic levels, including S4/S5, S6, S8, S10, S13, S15 and below: indicating a repeated process of fluvial and tidal depositional cycles rather than localized sedimentary events (Figure 12, Figure 13).

In addition to facies juxtaposition, dip azimuth measurements derived from image logs show clear shifts across these truncation surfaces, in some cases including reversals in dip orientation in stacked systems (Figure 12, Figure 13). This observation provides strong evidence for reorganization of distributary deltaic to estuarine channel or estuarine/tidal bar depositional fairways, rather than simple vertical aggradation. The coexistence of erosional truncation, basal lags, and dip variability supports interpretation of these surfaces as erosional channel base or incision surfaces. In Upper or Mid-CdO, the channelized units, with erosive base, are vertically capped by highly bioturbated fine-grained shale intervals: indicative of estuarine/back-barrier lagoonal swamp or lower-delta floodplain respectively (Figure 12, Figure 13). This repeated motif reflects episodic high-energy depositional events incising into lower-energy background deposits, establishing a hierarchical architecture of channelized elements embedded within a finer-grained matrix.

Integrated Depositional Model

Besides the image logs and correlatable petrophysical log motifs of the wells (Figure 12, Figure 13) drilled across the field, the biostratigraphic data—dino-cyst, calcareous microfossil, and spore-pollen (Figure 14)—clearly shows increasing open-marine influence from CdO-S5 interval towards CdO-top, and enhanced influence of marginal-marine/marine depositional environment from CdO-S17 onwards and upwards stratigraphically through CdO-S13. Spore-pollen data support mangrove rich intervals between CdO-S17 and CdO-S5.

Figure 13: Examples of composite sedimentary units display regular depositional dip-azimuth reversals and bed truncations, interpreted from Image Log in Upper CdO.

Figure 14: Biostratigraphic data provide insights of increasing marginal marine/open marine influence towards CdO top (modified after Biostratigraphic Report (PetroStrat Ltd.) of a representative well under present study, acquired by Canacol Energy Ltd.).

Integration of image log and seismic observations supports interpretation of the CdO Formation as a fluvio-deltaic to marginal-marine estuarine/tidal depositional system, in which both fluvial discharge and marine processes contributed to distribution and orientation of depositional units. Background sedimentation occurred in relatively low-energy environments, represented by bioturbated shales and coal-bearing intervals, likely corresponding to estuarine, lagoonal, deltaic-floodplain, or interdistributary settings.

Superimposed on this background are relatively high-energy depositional events, represented by channelized sand bodies that incise into and erode underlying strata. These events are responsible for the observed truncation surfaces with basal lags. The cross-bedded units provide paleocurrent signatures and/or relative sea level changes. The repeated alternation between low-energy deposition and high-energy incision indicates a system characterized by episodic sediment delivery and redistribution, rather than continuous progradation alone.

The observed variability in dip azimuth/paleocurrent direction, together with seismic evidence of shifting geobody trends, suggests that the depositional system was highly dynamic, even with strata-bound sediment fairways frequently reorganized in response to changing accommodation space under tectonic influence and sediment supply.

Sequence Stratigraphic Framework

The combined dataset allows identification of a hierarchy of stratigraphic surfaces that define the sequence stratigraphic architecture of the CdO Formation. The most prominent surfaces are erosional truncation surfaces, identified in image logs and biostratigraphic logs and supported by seismic geometries. These occur at multiple levels, including CdO-S1, S2, S4/S5, S6, S8, S10, S13, S15, S17 and are interpreted as periods of progradation in an overall retrogradational depositional environment. In contrast, laterally continuous, shale-dominated intervals that form regionally correlatable seismic reflectors are interpreted as marine flooding surfaces. These surfaces separate sand-rich depositional packages and represent intervals of reduced clastic input and increased accommodation.

The Vertical and Spatial Stacking pattern reveals a systematic organization:

In deeper CdO intervals (S15–S17), the depositional elements are oriented predominantly N–S (Figure 8), suggesting relatively unconstrained sediment routing.

Mid-CdO intervals (S8–S14) show orientations channel/depositional fairways become oblique (Figure 9), reflecting increased variability and reorganization under influence of structural control on accommodation space creation.

Upper-Mid CdO intervals (S4–S7) show frequent erosional surfaces and discontinuous sand bodies indicating increased marginal-marine/estuarine heterogeneity with distributary channelization, tidal bar-forms and back-barrier lagoonal swamps. Depositional elements align parallel to the regional fault trend (Figure 10), indicating stronger structural control.

Upper CdO intervals (S1–S3) are characterized by laterally continuous, sheet-like deposits, indicating relatively stable depositional conditions and limited incision, e.g., upper shoreface or barrier bar depositional environments, aligned parallel to the bounding fault trend (Figure 11).

This architecture reflects an interplay between progradation, episodic incision, and flooding, resulting in a broad retrogradational stratigraphic framework defined by alternating high energy erosional and low energy aggradational phases. Resultant is a transition from a more distributary system to a more structurally-guided depositional framework.

At the same time, the lateral/oblique-dip migration of depositional environments is a combined result of allocyclic and autocyclic responses to sediment delivery and syn-tectonic accommodation space creation.

Implications for Reservoir Architecture

Seismic amplitudes reflect the cumulative response of these heterogeneous facies assemblages and therefore must be interpreted in the context of stacked depositional elements rather than individual reservoirs. This has important implications for predicting reservoir continuity and for integrating seismic interpretation with static modelling.

The depositional and stratigraphic framework has direct implications for reservoir characterization. The presence of repeated erosional surfaces leads to vertical compartmentalization, while lateral variability in channel geometry results in heterogeneous connectivity. Sand bodies are not laterally continuous at all stratigraphic levels, particularly within the middle and lower intervals where channelization is dominant.

Summarily, the prograding nature of the multistoried strata-bound seismic clinoforms highlight the seismic-stratigraphic context of CdO Formation. In the study area, CdO Formation is best described as a structurally influenced depositional system, characterized by alternating phases of low-energy sedimentation, high-energy channel incision, and marine flooding. The integration of image log and seismic data demonstrates that depositional architecture is controlled by the interaction of progradation, episodic erosion, and evolving structural influence, resulting in a complex stratigraphic framework that governs reservoir distribution and connectivity.

Bibliography

Arminio, F., Yoris, F., Porras, L., Garcia, E., and Diluca, M., 2011, Petroleum Geology of Colombia, volume 10, Lower Magdalena Basin: Agencia Nacional de Hidrocarburos. Bogotá (ANH).

Barrero, D., Pardo, A., Vargas, C.A., and Martínez, J. F., 2007, Colombian Sedimentary Basins: Nomenclature, Boundaries and Petroleum Geology, a New Proposal. Agencia Nacional de Hidrocarburos. Bogotá (ANH).

Flinch, J. F., 2003, Structural evolution of the Sinu-Lower Magdalena area (Northern Colombia), in C. Bartolini, R. T. Buffler, and J. Blickwede, eds., The Circum-Gulf of Mexico and the Caribbean: Hydrocarbon habitats, basin formation, and plate tectonics: AAPG Memoir 79, 776–796.

Ganguly, N., Rodriguez, L., Willis, A., Hiebert, S., Johnston, S., Umback, K., Teare, M. and Juan A., 2018, Application of AVO in Lower Magdalena Valley Basin (LMV), Colombia: CSEG Symposium.

Torres, E.J., Marfurt, K.J., and Molinares, C., 2012, Seismic characterization of delta front sand bodies in Lower Magdalena Valley basin and their potentiality as reservoirs in Cienaga de Oro Formation, Colombia: 82nd Annual International Meeting, SEG, Expanded Abstracts.

Willis, A., Rodriguez, L., Ganguly, N., Hiebert, S., Johnston, S., Umbach, K., Teare, M. and Juan A., 2017, Clarinete gas field, Lower Magdalena Valley Basin, Colombia- a significant discovery in an established hydrocarbon province: GeoConvention, Conference Abstracts.

About the Authors

Neeraj Kumar has more than 24 years of experience in petroleum exploration and development. He currently serves as the chief geophysicist at Canacol Energy in Calgary, Canada. His previous roles include positions at Cairn Energy, Mubadala and Petronas. His areas of expertise include seismic interpretation, seismic attribute analysis, prospect evaluation, volumetrics, geological risking and project management. He is a member of SEG, AAPG, CSEG and have P.Geo. certification from APEGA.

 

 

Arijit Chattopadhyay is a geologist with over 15 years of global experience in hydrocarbon exploration, reservoir characterization, geomodelling, and field development. Starting his career in 2005, Arijit worked with organizations including Kuwait Oil Company, Gaffney Cline & Associates (Baker Hughes), and Reliance Industries Limited, contributing to multidisciplinary subsurface projects across diverse basins in the Americas, Europe, the Middle East, Africa, and South/Southeast Asia till 2020. Arijit completed his Ph.D. in Sedimentology and Basin Analysis at Queen’s University in Fall’ 2025, where his research was focused on integrating process sedimentology with subsurface and outcrop data to improve reservoir prediction in deepwater systems.Returning to industry, as an independent consultant in Calgary, Arijit contributed to reservoir characterization and geomodelling projects for heavy-oil and gas-producing assets in Southern Alberta and Colombia, respectively. Arijit recently joined AER as a Geomodeller. He has published in peer-reviewed journals and presented at more than ten international conferences. He is also a co-inventor on a U.S. patent on reservoir surveillance. Moving to Calgary, Arijit is honing his volunteering acumen and hiking skills. Arijit’s wanderlust took him to many countries across five continents except South America and Antarctica.

Andrew Willis is currently Chief Geologist at Canacol Energy. He joined the company in 2010 as a Latin America startup and has helped guide them to being the largest independent gas producer in Colombia. Prior to that he spent 22 years working as an exploration geologist with Amoco UK, Home Oil, Total Canada, Rigel Energy, Talisman, Petro Canada and Suncor.