English (pdf)
Article in xml format
Article references
Automatic translation
Send this article by e-mail
Cited by SciELO 
Cited by Google 
Similars in
SciELO 
Similars in Google 
Permalink
Introduction
Numerous studies have been carried out in the Cesar sub-basin. This sub-basin is part of the Cesar-Ranchería Basin, northern side of Colombia (Figure 1). These studies have determined the source rock formations and their hydrocarbon potential, and have also indicated the generation potential, the organic matter contents, and the potential of accumulation as shown in the study from García et al. (2008). Other authors such as Mesa and Rengifo (2011) pointed out that about 50 wells were drilled in the Cesar sub-basin and it was determined a petroleum system in the sub-basin consisting of source rocks: Lagunitas, Aguas Blancas, La Luna, Molino, and Los Cuervos formations; reservoir rocks: with fractured reservoirs of Lagunitas and Aguas Blancas formations; and seal rocks: represented by a thick mudstone sequence of the Molino Formation. The hydrocarbon traps are pop-up structures, and fault propagation folds. The overburden sequence is integrated by Lagunillas to Cuesta-San Antonio formations, and the regional seal on top is the Miocene Cuesta-San Antonio Formation.
Figure 1 Cesar sub-basin, wells, seismic cross lines, surface samples location and additional geochemical Tmax and Ro% and AFT data. Capital letters A, B, C and D represent seismic cross sections (Figure 2).
In the Cesar Basin, the Barco Formation consist of quartz sandstone and claystone, and Los Cuervos Formations consists of claystone, siltstone, and several coal seams, according to Guo et al. (2018). Barco and Los Cuervos formations were deposited during the Paleocene in a continental environment as a product of a slow regression (Hernández, 2003). Los Cuervos Formation corresponds to a sequence of carbonaceous shales, siltstones, sandstones, and coal beds (Mesa and Rengifo, 2011).
Studies carried out by Barrero et al. (2007) and Mesa and Rengifo (2011) show the importance of the exhumation of the Perijá Range (PR) for the Cesar Sub-Basin petroleum system. This is because this exhumation generates the formation of traps and marks the change in the existing conditions in terms of accommodation space. Consequently, it also controls the increase of the lithostatic charge, thus increasing the maturation of the source rocks. Evidence of the time of occurrence of this event was provided by Bayona et al. (2007), who stated that the northern sector of the PR was at the Early Paleocene-Eocene period; contributions were already being made to both the Ranchería sub-basin and the Maracaibo Basin (Bermúdez et al, 2017). Other authors, like Shagam et al. (1984), established that this uplift started from the Cretaceous-Paleogene limit and had successive pulses of uplift throughout the Cenozoic.
This study uses published information of the Cesar sub-basin, 31 data from the maximum pyrolysis temperature (Tmax) and 13 data from vitrinite reflectance (Ro%) from Los Cuervos Formation in the Calenturitas and La Jagua wells; and new geochemical data, 24 samples from Tmax and 14 samples from Ro% from the ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells. This study aims to establish the current maturity state and the potential for hydrocarbon generation (quality and quantity) of the rocks in the Los Cuervos Formation. Furthermore, the goal is to recreate the burial history scenario through numerical modelling with the PetroMod software (version 2012). The modelling is based on geochemical data, the stratigraphic/structural architecture as interpreted by 2D seismic reflection data, and the several uplift events of the Perijá Range that affected the burial history. It also provides insights that would help understand the current state of maturity, as well as the organic transformation ratios (TR) of the Cretaceous formations underneath Los Cuervos and Barco formations. This is fundamental for any understanding of the organic geochemical characteristics of Los Cuervos Formation, as well as how the basin was filled with sediments. This process has direct implications regarding the maturation of organic matter of the units as well as the generation of hydrocarbons.
The interpreted 2-D seismic lines with structural reconstruction, main faults, truncated seismic refractor and on lap-strata terminations of the Cretaceous, Paleogene, and Neogene formations are presented in Figure 2.
Figure 2 Interpreted 2-D seismic lines in two-way time (TWT) with location of the wells; A. CV-1979-03 seismic line; B. CV-1979-11 seismic line; C. CV-1979-04 seismic line; D. CV-1979-08 seismic line.
Data and methods
Geochemical analysis was used to establish the type and quality of organic carbon and other geochemical properties of rocks, which contributed to the evaluation of the potential production of conventional or unconventional hydrocarbons in a prospective play. Geochemical data were obtained using core samples from Los Cuervos Formation of two wells, ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells. Vertical sampling was not performed at equal intervals (Figure 3). The selection criterion was based on the visible presence of organic matter using an ultraviolet lamp.
Figure 3 Lithologies, samples site (+) with Ro%, Pyrolysis Rock-Eval and petrophysical analysis for ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells.
The geochemical analysis was carried out at UIS Petroleum geochemistry laboratory (PGL). Geochemical analysis of Rock-Eval pyrolysis (HI, OI, PI, S1, S2, S3, Tmax, PY), total organic carbon (TOC), and total sulphur content (TS) (Table 1), Ro% (Table 2). Additional existing geochemical data (Tmax and Ro%) used were obtained from Hernández and Jaramillo (2008); Mora et al. (2007); Ayala (2009) and Guo et al. (2018). Thermal maturation data of the Cesar A-1X, Compae-1, and El Paso-3 wells was obtained from Aguilera et al. (2010), Ayala (2009), Mora et al. (2007) and Guo et al. (2018) including Ro% and Tmax data of surface samples. Petrophysical analysis including permeability and porosity of sandstones and mudstones (514 samples) were performed at the UIS Petrophysical analysis laboratory (Supplementary Materials Table S1). The Colombian Agencia Nacional de Hidrocarburos (ANH) and the Universidad Industrial de Santander (UIS) have granted access and approved the use of all data for this study.
Table 1 Geochemical analysis results of Los Cuervos Formation from core samples in ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells. TOC and TS (%); HI (mg HC/g TOC); OI (mg CO2/g TOC); PI (TOC%); S1, S2 and PY (mg HC/g rock); S3 (mg CO2/g rock) and Tmax (°C).
Table 2 Measurements of maceral materials of Los Cuervos Formation from ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells. Ro% (minimum, maximum and mean), TOC and TS.
The TOC analysis was carried out in 38 samples to quantify the amount of organic matter present in the sample. In the first instance, following the usual methodology reported in (Schmacher, 2002), the content of carbonates present in the sample was eliminated by using HCl to dissolve inorganic CaCO3. After that, the total carbon was determined using the Leco SC-144DR automated analyzer. It also allowed us to calculate the TS. Likewise, to evaluate the quality of this organic matter, a Rock-Eval pyrolysis analysis was performed in each of the 24 samples with Rock-Eval 6 technology equipment. This technique presented by Barker (1974) and Behar et al. (2001) uses programmed heating to a small amount of rock (70 mg) or coal (30-50 mg) in an inert atmosphere (helium or nitrogen) in order to determine the quantity of free hydrocarbons present in the sample (S1 peak), as well as those that potentially can be released after maturation (S2 peak). This procedure permitted us to know the maximum pyrolysis temperature (Tmax) and the current state of thermal maturation. The TOC had to be determined along with the mineral carbon content (MinC), the volume of free hydrocarbon generated before pyrolysis (S1), the amount of hydrocarbon generated during pyrolysis (S2), and the amount of emitted carbon dioxide (S3). Derivations of these parameters such as hydrogen index (HI = S2/ TOC x 100), the oxygen index (OI = S3/TOC x 100), the production yield (PY = S1 + S2), and the production index (PI = S1/(S1 + S2)) are calculated.
The vitrinite reflectance (Ro%) that indicated organic matter content, was measured in 14 samples using Zeiss imager microscopy. Vitrinite reflectance was measured according to the method proposed by the ASTM D7708-14 (2014). Sample chips or sidewall core samples were cleaned to remove drilling mud or mud cake and then crushed using a mortar and pestle to a grain-size of less than 3 mm. These samples were mounted in cold-setting resin and polished "as received", so that whole-rock samples were examined, excluding concentrates of organic matter. The core samples were mounted and sectioned perpendicularly to the bedding. Vitrinite reflectance measurements were made using immersion oil of refractive index 1.518 at 546 nm at 23°C and at spinel and garnet standards of 0.42, 0.917, as well as 1.726% reflectance for calibration. After calibrating the equipment an average of 28.4 Ro% readings were taken for each polished section. During optical analysis the vitrinite macerals were identified on the polished surface, taking into account that the standard deviation of the 28.4 readings of Ro% per polished section must be less than 0.003. A correlation of TS concentration vs Ro% was performed to determine the reflectance of suppressed vitrinite according to Mukhopadhyay (1994), who indicated that the suppression occurs between values of 0.2 to 1.4, this suppression takes place in vitrinite due to excess sulphur of more than 1% [w/w] according to Carr (2000). This last author established that in some situations, vitrinite reflectance values may be either 'suppressed' or 'enhanced' and said that low vitrinite reflectance or vitrinite suppression may be caused by perhydrous-vitrinite, impregnation of vitrinite with hydrogen rich amorphous material, and in coals with high sulphur content. Fluorescence-mode observations were made on all samples, providing supplementary evidence concerning organic matter type, exinite abundance, and maturity. For fluorescence-mode, a 3-mm BG-3 excitation filter was used with a TK400 dichroic mirror and a K490 barrier filter. The data correlation of the average Ro% versus Tmax is presented in Figure 4 to show the change in thermal maturity of Los Cuervos Formation in the La Loma-2 and ANH-CR-Los Cerezos-1X wells.
Petrophysical analyses were performed in addition to the organic geochemistry analysis to give greater certainty of the thermal evolution of the Rancheria Sub-Basin. Petrophysical analysis, porosity and permeability in ANH-La Loma-2 Well were determined for 262 and 55 samples of sandstone and mudstone, respectively; and in ANH-CR-Los Cerezos-1X Well were determined for 141 and 56 samples of sandstone and mudstone, respectively. Porosity and permeability were measured at UIS Petrophysical analysis laboratory, using conventional core analysis practices from American Petroleum Institute (1998). Petrophysical sample analysis consists of two stages. First: sample preparation that comprises cutting, profiling, cleansing and drying plugs; and second: determination of petrophysical properties (porosity and permeability). Petrophysical information was plotted to visualize its distribution on profiles using free software PanPlot2 as described by Sieger and Grobe (2013). Petrophysical data allowed establishing and characterizing the thickness of the sequence deposited, during the early Eocene, in the interval from the late Eocene to the early Miocene. This sedimentary sequence was lost in the geological record due to regional tectonic events (see Figure 2). This lost sequence managed to exert a lithostatic charge that increased the maturation of the source rocks and decreased the porosity of reservoir rocks. The 2D seismic lines in two-way time (TWT) used in this research were CV-1979-03, CV-1979-11, CV-1979-04 and CV-1979-08; the interpretation of the lines was used for the purpose of defining the cutting relationships between lithological units present in the basin. Additionally, the seismic lines had the geographic and stratigraphic positions of the wells used for this study. One-dimensional modelling was generated using the PetroMod software in order to reconstruct the burial history and thermal maturation of the geological units present in the wells. This modelling used the vitrinite thermal maturation model (EASY Ro%) proposed by Sweeney and Burnham (1990), based on the Arrhenius first-order parallel-reaction approach with a distribution of activation energies. EASY Ro% were calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. The ages and minimum and maximum thicknesses of the Rio Negro, Lagunitas, Aguas Blancas and lower part of the Molino formations reported for the sub-basin by Mesa and Rengifo (2011) were used. For Barco, Los Cuervos and upper Molino formations, we used the data obtained in ANH-La Loma-2, ANH-CR-Los Cerezos-1X, La Jagua and Calenturitas wells. The eroded thickness in the sequence was estimated considering the maximum thickness reported and the thickness present in the wells. The heat flow was established according to Mesa and Rengifo (2011): the syn-rift stage goes from 200 to 116 million years ago (mya) and the post-rift stage from 116 to 20 mya (Figure 5A). The paleo water depth (PWD) for La Luna and Molino formations was established considering Patarroyo et al. (2017) see Table 3 and Figure 5B. Sediment water interface temperature (SWIT) was calculated based on Wygrala (1989), assuming a current position in the northern hemisphere on the continent of South America at a latitude of 10° (Figure 5C) as obtained by Montes et al, (2019). Multiple kinetics based on lithology were used: Lagunitas, Aguas Blancas, La Luna and Lower Molino Formations, the kinetics IES-TIIBrownLimestone, for the Middle Molino Formation the kinetics IES-TIIToarcianShale, for Los Cuervos Formation the kinetics Handil_MahakamDelta-TIII and IES-TIII_Tertiary_Coal_2C according to DiPrimio and Horsfield (2006).
Table 3 Paleo-batimetry conditions (PWD) vs time used to generate the geochemical modelling. PWD for La Luna and Molino formations (Patarroyo et al., 2017). 0 m deep during the early rift stage (120 mya) according to Mesa and Rengifo (2011).
| Age(Ma) | PWD(m) |
|---|---|
| 0 | 0 |
| 50 | 0 |
| 70 | 15 |
| 79 | 100 |
| 92 | 200 |
| 103 | 100 |
| 116 | 100 |
| 120 | 0 |
The burial history and exhumation event of the Cesar Sub-Basin was obtained from Hernández and Jaramillo (2008). The burial depth of Los Cuervos Formation and the thickness of Eocene age formation, were obtained using the relationship between porosity and depth, known as 'compaction trend' (Lee et al., 2019), and the one-dimensional geochemical model obtained.
Results and discussion
In this section we summarize the results obtained, corresponding to geochemical and petrophysical analysis for ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells. The Figure 3 shows a stratigraphic column with samples collected for vitrinite reflectance (Ro%), pyrolysis Rock-Eval and petrophysical analysis. The Figure 4 shows a correlation between Tmax versus Ro% for the samples analyzed from ANH-La Loma-2 well (depths of 95.2 to 668.6 m) and ANH-CR-Los Cerezos-1X well (depths of 513.7 to 751.9 m), respectively. Figure 4 shows a good correlation between data to illustrate the change in thermal maturity of Los Cuervos Formation in the ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells. Figure 5 shows the basin modelling boundary conditions (heat flow, paleo water depth, SWIT). Multiple scenarios cases for erosions events allow us to discriminate the best calibration for each well (Figure 6 and 7). The burial history diagrams for ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells, and for Calenturitas Mine and La Jagua Mine wells, respectively (Figure 8 and 9). The transformation ratios (TR) are shown at the Figure 10. The transformation ratio (TR), as defined by Tissot and Welte (1984), is the ratio of the petroleum (oil and gas) actually formed by the kerogen to the total amount of petroleum that the kerogen is capable of generating. It depends on the origin of the organic material (kerogen type) and the maturation history associated to the geological history of a basin. Figure 11 and 12 shows the variations in depth of permeability and porosity for ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells, respectively. Figure 13 shows permeability vs porosity correlation in ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells, respectively. These results of porosity and permeability were used to determine the depth to which the lithologies were buried (Figure 14 and 15).
Figure 4 Data correlation of maximum pyrolysis temperature (Tmax) versus vitrinite reflectance (Ro%), to indicate source rock thermal maturity in Los Cuervos Formation of the ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells.
Figure 5 Boundary conditions used for 1D modelling of burial history of lithological units in the ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells: (A) heat flow, (B) paleo water depth, and SWIT.
Figure 6 Best calibration and multiple models for calibration of vitrinite reflectance (Ro%) and Tmax with depth: (A) and (B) for the ANH-La Loma-2 Well, and (C) and (D) for ANH-CR-Los Cerezos-1X Well.
Figure 7 Best calibration and multiple models for calibration of vitrinite reflectance (Ro%) and Tmax with depth: (A) and (B) for the Calenturitas Well, and (C) and (D) for La Jagua Well.
Figure 8 Burial history using modelled temperature variation: (A) for ANH-La Loma-2 Well and (B) for ANH-CR-Los Cerezos-1X Well.
Figure 9 Burial history using modelled temperature variation: (A) for Calenturitas Well and (B) for La Jagua Mine Well.
Figure 10 Hydrocarbon transformation ratios (TR) for the Cretaceous and Paleocene formations: (A) in the ANH-La Loma-2 Well, and (B) in ANH-CR-Los Cerezos-1X Well.
Figure 11 Variations in depth of permeability and porosity from different lithologies for ANH-La Loma-2 Well.
Figure 12 Variations in depth of permeability and porosity from different lithologies for ANH-CR-Los Cerezos-1X Well.
Figure 15 Petrophysical analysis results of porosity and permeability for ANH- CR-Los Cerezos-1X Well.
In the ANH-La Loma-2 well (Figure 3) the following results were obtained for 18 mudstone samples with TOC values with a min. of 0.306%, max. of 5.257%, a mean of 2.595% and a standard deviation (sd) of 1.709%. The sulphur content with a min. of0.016% [w/w] (were [w/w] means percentage concentration by weight), max. of 5.274% [w/w], mean of 1.516% [w/w] and sd of 1.778% [w/w]. A sample of coal with 52.68 TOC and sulphur content of 8.64% [w/w]. The 2 samples of sandstone with TOC values with a min. of 0.04%, max. of 0.20%, a mean of 0.120%. The sulphur content with a min. Of 1.286% [w/w], max. of 3.564% [w/w], mean of 2.425% [w/w].
In the ANH-CR-Los Cerezos-1X well (Figure 3) the following results were obtained for 10 mudstone samples with TOC values with a min. of 0.631%, max. of 15.684%, a mean of 6.112% and a sd of 5.638%. The TS with a min. of 0.166% [w/w], max. of 12.125% [w/w], mean of 2.587% [w/w] and sd of3.465% [w/w]. A sample of sandstone with 0.82 TOC and sulphur content of 0.065% [w/w].
Multiple modellings were made using Tmax and Ro% data and with different thicknesses of the Eocene sequence (e.g. from 2000 to 4700 in 300 m intervals) were made (Figure 6B and 6D). The modelling that best fit for ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells was 3200m and 3500m respectively (Figure 6A and 6C). Figure 7 shows a calibration with Ro% and Tmax for La Jagua and Calenturitas mines using data of Guo et al. (2018).
The burial diagrams obtained for the ANH-La Loma-2, ANH-CR-Los Cerezos-1X, Calenturitas and La Jagua wells (Figures 8 and 9) are consistent with the two major deformational phases affecting the Cesar-Ranchería Basin, first to a Middle Eocene convergent phase related to the accretion of the Great Arc of the Caribbean (Sánchez and Mann, 2015; Cardona et al, 2011; Villagomez et al, 2011); and a second phase in the Miocene age characterized by large-displacement thrusting in the eastern Cesar Ranchería Basin driven by Late Miocene collision of the Panama Arc with South America Plate (Sánchez and Mann, 2015; Taboada et al, 2000; Vargas and Mann, 2013).
The geochemical modelling (Figures 8 and 9), assumes a continuous deposition during the Middle Eocene. Since Middle Eocene the accumulated thickness Cretaceous-Middle Eocene begins their erosion, without significant loss of sedimentary record. This erosion event is prior to the deposition of the Cuesta Formation and is more pronounced closer to the Perijá Range (PR). The minimum thickness values of each formation in the Cesar Sub-Basin used to generate model (Figure 8) was obtained from the seismic interpretation (Figure 2B) and Mesa and Rengifo (2011). The model of burial history (Figure 8 and 9), with overlapping values and the different hydrocarbon generation windows were coloured as proposed by Sweeney and Burnham (1990).
These models projected to the economic basement (Río Negro Formation), the Lagunitas, Aguas Blancas and Luna formations are expected to enter the main oil generation window, while the Molino Formation is expected to enter the early oil window in the Middle Eocene just before the loss of sedimentary record.
The geochemical modelling of the Calenturitas and La Jagua wells (Figures 9A and 9B) was achieved using data ofGuo et al. (2018). The modelling permit simulate the evolution in thermal maturation and transformation ratios over time. It was only possible to determine how the sedimentary sequence behaved for the Calenturitas Well using seismic (Figure 2A). But a fault was found just below the synchronization line where Los Cuervos formation is completed. The fault repeats the Cretaceous sequence and cause errors in the determining the thickness units to be modelled. Los Cuervos Formation in the Calenturitas Well, is expected to be immature and the Molino Formation enters the early oil window during the Middle Eocene (Figure 9A). The Molino Formation in La Jagua Well, is expected that reached the early oil window during the same period (Figure 9B).
The diagrams of TR variation in time from source rocks in ANH-La Loma-2 and ANH-CR-Los Cerezos-1X wells, are presented in Figure 10A and 10B). These subsidence model implies that the TR of source rocks do not increase after the Eocene. The units of the Lower Cretaceous (Lagunillas and Aguas Blancas formations) reached the highest TR of the modeled source rocks, obtaining its maximum advance during the Paleocene. The formations of the Upper Cretaceous (La Luna and Los Cuervos formations) reached its maximum advance during the Eocene. Los Cuervos Formation is not expected to have such high TR but around 66%, while La Luna Formation is expected to be hovering around 95% of TR in this area. Likewise, it is evident that Los Cuervos Formation did not advance in the transformation of its organic matter in hydrocarbons. Based on the models, the TR are larger in ANH-CR-Los Cerezos-1X Well followed by ANH-La Loma-2 Well.
The petrophysical analysis results of the porosity and permeability of the sandstone and mudstone lithologies of the ANH-La Loma-2 Well, show a decrease in their values with increasing depth (Figure 11). Sandstones beds with high permeability values are also highlighted. However, these are local and are distributed especially towards the top of the well, while the permeability values in the mudstone remain very low. For ANH-CR-Los Cerezos-1X Well, the porosity and permeability values (Figure 12) also decrease with increasing depth. However, higher values in mudstone permeability are evident, which can be up to 3 times higher in some cases, while sandstones are maintained with permeabilities that do not exceed 2500 mD, unlike those observed for ANH-La Loma-2 that can reach 6000 mD.
The porosity and permeability values show a positive correlation (Figure 13), in ANH-La Loma-2 Well (Figure 13A), a significant number of samples of sandstones are dominated by porosity from 13.16% to 29.62%, with mean of 21.37% and a standard deviation (sd) of 5.52%. For mudstones, a porosity values predominates between 4.86% and 14.60%, with mean of 10.75% and sd of 4.61%. Permeability values range from 0.1 to 560 mD for sandstones, with mean of 461.27mD and sd of 930.54mD. For mudstones permeability values are between 0.01 and 1.27mD, with mean 0.95mD and sd of 1.62mD.
In ANH-CR-Los Cerezos-1X Well (Figure 13B), a significant number of samples of sandstones are dominated by a porosity between 15.25% and 31.02%, with mean of 24.78% and sd of 4.72%. For mudstones, a porosity predominate value is between 8.98% and 23.96%, with mean of 17.64% and sd of 6.68%. Permeability values range from 0.39 to 250 mD for sandstones, with mean of322.54 mD and sd of 565.12 mD. For mudstones are between 0.37 and 6.47mD, with mean 3.89 mD and sd of 6.55 mD.
Bjørlykke (2014) describes two different types of compaction processes (mechanical and chemical). Compaction generates a loss of porosity in siliceous sediments during burial in sedimentary basins. Being the mechanical compaction caused by the effective stress more relevant before exceeding temperatures greater than 100 °C. Chemical compaction begins after 100 °C to dominate the process of loss of porosity, it is based on the chemical reactions that occur according to the lithology (mineralogy). In Cesar Sub-Basin, through modelling with PetroMod (Avendaño-Sánchez et al, 2019), shows that the analyzed lithologies did not exceed this temperature limit, therefore it is considered that the loss of porosity was due to the effect of mechanical compaction. Accordingly, and using the models of Kim et al. (2018) who performed numerical modelling of the decrease in porosity with respect to depth in sandstone, mudstone, and limestone, the following low-end, mean and high-end curves were obtained. The porosities obtained by petrophysical analysis for the ANH-La Loma-2 Well (Figure 14) in both sandstones and mudstones show that these lithologies were at depths between 2.5 and 3 km. While for the ANH-CR-Los Cerezos-1X Well (Figure 15) is expected these lithologies were at depths less than 2.5 km.
The stratigraphic record deposited between the Eocene and the Neogene in the Cesar Sub-Basin is very thin, Hernández (2003) reported 10 meters of slightly conglomerate quartz sandstone in La Jagua de Ibirico syncline, these would be the 10 meters that Mesa and Rengifo (2011) describe in its generalized column of the basin as the Eocene Sedimentitas unit. In Ayala (2009), a paraconformity is proposed between the Eocene Sedimentitas unit and Los Cuervos Formation. Both are considered the product of continuous succession without the presence of erosive contact. In the Ranchería Sub-Basin, Ronderos (1957) reported that Eocene Aguas Nuevas Formation is made up of reddish conglomeratic sandstones, glauconite-containing sandstones, reddish sandstones, and shales. The lack of Eocene age sedimentary record in the Cesar-Rancheria Basin that Kellogg (1984) describes as the result of compressional and transpressional tectonics associated with tectonic convergence along the southern margin of the Caribbean and the northern sector of the Andes during the Cenozoic.
Authors such as Mesa and Rengifo (2011) propose subsidence models for the Cesar Sub-Basin, in which a significant Cenozoic sedimentation occurred during the Paleocene (about 2.1 km) corresponding to the Barco Formation. The formations deposited during the Eocene have been little investigated, with a reported thickness of 300 meters, which is presumed to have been completely eroded. The model also presents a large amount of sedimentation in less than 5 million years (my) time, while sedimentation was scarce for the 20 my of the Eocene. However, authors such as García et al. (2008), and Sánchez and Mann (2015) give more relevance in their models to the formation deposited in the Eocene, assigning a thickness of 1 km and 2 km, respectively. García et al. (2008), and Sánchez and Mann (2015) establishes that its sequence was eroded during the Oligocene-Miocene and Oligocene-Pliocene, respectively. But it is important to highlight that the three models mentioned do not show the calibration with paleo-geo-thermometer data and do not show the boundary conditions used. García et al. (2008) used a variation of the heat flow that reached 100 mW m-2 between 110 and 90 mya. These values differ with those used in our modelling, which do not exceed 60 mW m-2. These values are in the phase with crustal thinning and increased accommodation space that occurred in the Cretaceous.
Patiño et al. (2019) with new and by compiling thermochrometry data of apatite fission track AFT (Villagómez et al., 2011) and new apatite (AHe) and zircon (ZHe) (U-Th)/He, established that the highest peak of exhumation rates of 0.9 km/my. This event affected the Sierra Nevada de Santa Marta (SNSM) during the late Eocene to early Miocene and resulted in their vertical exhumation of 15-19 km, based on geobarometry of granitoids (Cardona et al, 2011). Bermúdez (2009) from AFT analysis on synorogenic sediments supports the Eocene-Oligocene origin of the Andes Range in Venezuela. This exhumation rates coincides with the deformation front of the Caribbean began a more eastern migration with respect to South America in the late Eocene -Oligocene as proposed by Montes et al. (2019).
It can be concluded that late Eocene to early Miocene tilting event that occurred in the Maracaibo block may have been the cause of the total or partial loss of the deposited sedimentary units between Paleocene to the Middle Eocene in some areas of the Cesar Sub-Basin. The correlation between Tmax and Ro% data which are conditioned by the increase in temperatures, and the porosity data that are conditioned by the compaction of the sedimentary units, excluding the possible effect of cementation, allow to corroborate that it is not necessary to assign higher values in the heat flow within the sub-basin in order to reach the same point of thermal maturation.
The results obtained in the burial modelling carried out in this study contrast with the work proposed by Martínez et al. (2012), where the methodology used to replicate the effect of the Perijá Range (PR) thrusting on the Cesar Sub-Basin. Here, the maturation of the Cesar Sub- Basin rocks is a consequence of the effects of continuous sedimentation until the Middle Eocene, at which time the regional uplift event of the Maracaibo Block interrupted de sedimentation. This model coincides with that proposed by Sánchez and Mann (2015), which also proposes that the thickness of the units deposited in the Eocene is greater in the direction of the Maracaibo Basin.
Conclusions
It can be concluded that late Eocene to early Miocene tilting event that occurred in the Maracaibo block may have been the cause of the total or partial loss of the deposited sedimentary units between Paleocene to the Middle Eocene in some areas of the Cesar Sub-Basin.
The maturation of the Cesar Sub- Basin rocks is a consequence of the effects of continuous sedimentation until the Middle Eocene, at which time the regional uplift event of the Maracaibo Block interrupted de sedimentation.
The correlation between Tmax and Ro% data which are conditioned by the increase in temperatures, and the porosity data that are conditioned by the compaction of the sedimentary units, excluding the possible effect of cementation, allow to corroborate that it is not necessary to assign higher values in the heat flow within the sub-basin in order to reach the same point of thermal maturation.
Using the data geochemical obtained for Los Cuervos Formation (416 -431 °C of Tmax, and 0.46-0.57 of Ro%), it can be established that this formation in the southern area of the Cesar Sub-Basin is thermally immature, near the oil generation window, but this formation presents higher values reported in the La Jagua Well towards the Perijá Range.
Based on the porosity values, it is expected that the sandstones and mudstones of Los Cuervos Formation, have reached depths from 2 to 3 km in the study area. Furthermore, according to the values of Tmax and Ro%, it is expected that Los Cuervos Formation has been at a depth from 2600 to 3800m for ANH-La Loma-2, from 2900 to 4100m for ANH-CR-Los Cerezos-1X, from 2600 to 4400m for Calenturitas Well, and from 3500 to 5300 for La Jagua Well, taking into account that this thickness was considered with the properties of quartz sandstones.
According to the modelling carried out for this study area, the Late Cretaceous formations (Aguas Blancas and Lagunitas) have had their greatest advance in hydrocarbon transformation ratios during the Paleocene, while the Early Cretaceous formations (La Luna and Molino) peaked in hydrocarbon transformation ratios from the early Eocene to the middle Eocene. Similarly, through the modelling of the Lagunitas, Aguas Blancas and La Luna formations, it is concluded that they had transformation ratios greater than 80% while the Molino Formation has transformation ratios around 50%.
