Contribution of bedding to the petrophysical characterization of naturally fractured reservoirs: Example of the Matachines fields, Upper Magdalena Valley (Valle Superior del Magdalena – VSM) Colombia

The Matachines field, located in the Girardot subbasin, Upper Magdalena Valley (Valle Superior del Magdalena – VSM) basin, has a morphology characterized by antiforms arranged along a N–S axis. These antiforms involve multiphase tectono-sedimentary sequences with half-graben geometries associated with Mesozoic sequences reactivated by at least three Andean phases. Bedding surfaces are mechanically active planes that contribute to the flexural slip that is generated by the folding of multilithologic sequences. In the Matachines field, the bedding planes help to improve the petrophysical characteristics of the reservoirs by incorporating a significant number of discontinuities, which complement the fluid connectivity. Thus, the subhorizontal planes are linked to subvertical fractures preferentially arranged in the ENE–WSW direction, generated by horizontal compressive stress. In particular, these effects are magnified on the western flank and hinge zone of the anticline structure, where they contribute greatly to the connectivity of the production wells. In naturally fractured reservoirs, the correct 4D technical and economic evaluation of the quality and arrangement of discontinuities is essential to determine their actual contribution to improving the petrophysical Contribution of bedding to the petrophysical characterization of naturally fractured reservoirs: Example of the Matachines fields, Upper Magdalena Valley (Valle Superior del Magdalena – VSM) Colombia Contribución de la estratificación en la caracterización petrofísica de reservorios naturalmente fracturados: ejemplo de los campos Matachines (VSM, Colombia) Boletin Geologico 46.indd 23 7/07/20 2:14 p. m. 24 B O L E T Í N G E O L Ó G I C O 4 6 Rossello / Saavedra properties of such reservoirs. Accordingly, many methods often used to study wells from core samples or images identify, along with different types of fractures, the presence of bedding surfaces, although they evaluate them separately, so they overlook petrophysical factors that affect the porosity and permeability of the wells. The present study discusses the genetic and morphological aspects of bedding surfaces that enhance the petrophysical potential of reservoirs, which usually have limited primary values of permeability and porosity, to meet the economic expectations of specific resources in hydrocarbon exploration and/or production activities.


IntroductIon
In naturally fractured hydrocarbon reservoirs, the correct 4D technical and economic evaluation of the quality and arrangement of fractures is essential to determine their actual contribution to improving the petrophysical properties of such reservoirs. Many petrophysical methods that are often used to study wells from core samples of images (Aguilera and Aguilera, 2004;Bratton et al., 2006;Serra, 2008) identify and quantify the presence of bedding surfaces, but they evaluate them in separate statistical analyses and not within the petrophysical model. Thus, by analyzing them separately from discordant fractures, they are overlooked in geo-economics considerations of their reservoirs.
Naturally fractured reservoirs often present a production paradox because their initially high productivity tends to decline rapidly. Many are also responsible for early gas or water penetration. However, they include some of the largest and most productive reservoirs, explaining the growing industry efforts to learn more about them and to more accurately model them.
The Matachines field (03°49' N-74°53' W) is located in the Girardot subbasin, within the Upper Magdalena Valley (Valle Superior del Magdalena -VSM) basin, similarly to the Purificación, Chenche, and Guando oilfields ( Figure 1). The structure of these oilfields is generally characterized by tight and faulted anticlines and by wide synclines with an axis that runs roughly NNE-SSW, related to west-vergence thrust faults that are linked to the uplift of the Eastern Cordillera of Columbia (Campbell and Burgl, 1965;Corrigan, 1967;Anderson, 1972;Butler and Schamel, 1988;Cooper et al., 1995;Barrero et al., 2007 andHiguera, 2012, among many others).
In the present study, the structural aspects that contribute to creating the genetic and morphological characteristics of the bedding surfaces underlying the petrophysical potential of the Matachines field reservoirs are discussed. We focus on sectors where the primary permeability and porosity values are usually too low to meet economic expectations in hydrocarbon exploration or production activities. These analyses and interpretations may be used to evaluate oilfields with similar characteristics that will be subjected to exploration and/or production.

methods
A multidisciplinary study was conducted to correctly evaluate deformation features associated with fractures. This study included the analysis of well core or borehole samples, borehole image logs, and a field study aimed at characterizing the physical attributes of the fractures, such as their geometry, direction, aperture, length, and spacing, to classify them into sets. All discontinuity surfaces analyzed in the present study ranged in size from meters to decimeters.
The seismic data were analyzed on a WorkStation mounted on the Petrel platform to spatiotemporally characterize the deformations by horizon flattening of sequences related to the study area, thereby establishing a relative Rossello / Saavedra criterion to recognize structures before and after the units that make up the oil system of the Matachines field. Core samples were analyzed considering the above, as extensively described by Nelson (1985), Aguilera (2001), and by Kubik and Lowry (1993), among others. The available core samples were tentatively arranged and spatially oriented according to strike and dip data of the corresponding wells. The structures were qualitatively and quantitatively analyzed, organoleptically identifying some of their mineralogical components, using magnifying glasses in more detailed cases.
Ultrasonic borehole images are electronic records of well images that provide information on the slope and azimuth of layers and fractures. These data were tabulated, analyzed, and grouped as a function of slope and azimuth (Ekstrom et al., 1987) using the algorithm proposed by Fisher (Fisher et al., 1987) because, in general, this method yields data that are quasi-normally distributed and requires the mean direction and a concentration parameter for input. The main direction data of the fractures and microfaults were plotted on stereograms (lower hemisphere) to assess the vertical configuration of the fractures in the reservoirs.
This analysis generated a set of characteristic attributes of each fracture pattern, which were tabulated in data-processing software to quantify their occurrence percentages, particularly focusing on the identification of friction surfaces (slickensides), to determine the direction of the movements in the discontinuity surfaces.
Data were collected from two study areas in a structural position analogous to the field to analyze fractures in outcrops, primarily measuring representative fracture sets by microtectonics to contrast them with the structures observed in ultrasonic borehole images and core samples.

GeoloGIcal framework of the matachInes fIeld
From the standpoint of regional tectonics, VSM is bordered to the west, in the Cordillera Central (Central Andes), by the Calarma thrust fault system with eastward vergence, and to the east, in the Cordillera Oriental (East Andes), by faults with opposite vergence, such as the Magdalena fault system (Schamel, 1991;Cooper et al., 1995;Toro et al., 2004).
Morphostructural features derived from surface and subsoil data indicate the multiphase evolution of the Matachines field, which is like that of other regions of VSM ( Figure 2). The main stages of its history, based on our own data and on interpretations compiled from the abundant related bibliography, are listed below (e.g., Irving, 1975;Kroonenberg and Diederix, 1982;Mojica and Franco, 1990;Bayona et al., 1994;Villamil et al., 1999;Sarmiento, 2001;Gómez et al., 2003;Sarmiento and Rangel, 2004;Montes et al., 2005;Mora et al., 2010).

First pre-Andean extensional stage
The tectonic history of the VSM basin started in the Triassic-Jurassic as a retroarc depocenter, which may have been controlled, at least partly, by pre-existing Paleozoic anisotropies (De Freitas, 2001). Half-graben geometries are fairly well preserved, albeit usually modified by the reactivation of at least three Andean phases, which are described below. Then, during the Cretaceous-Early Tertiary, the VSM basin continued as an intracratonic depocenter with the marine sequences of seals of the Caballos Formation (Duarte et al., 2018), generating the hydrocarbons of the Villeta and Guadalupe Groups, resulting in the seals of the Guaduas Formation (Higuera, 2012). Marine transgression and basin subsidence continued during the late Albian until the Santonian, with the VSM basin reaching its maximum extension during the Turonian-Coniacian period (limestones of the La Luna Formation), with the deposition of sediments very rich in organic matter, such as limestones from the Tetuán Formation, pelites from the Bambucá Formation, and pelites and limestones from the Luna Formation, from oldest to youngest (Buitrago, 1994).
Subsequent basin shallowing led to the development of the shallow marine detrital facies of the Guadalupe Group, which is the main producer of several fields (Matachines, Purificación, and Guando) in the Girardot subbasin and of the Monserrate Formation in the Neiva subbasin (Barrero et al., 2007). Marine regression during the Late Cretaceous, with coastline progradation, caused the deposition of fluvio-deltaic sediments of the Guaduas Formation (Late Maastrichtian?-Paleocene), but uplifts during the early-to-middle Eocene eroded these sediments in the highest sections of the basin (Cooper et al., 1995).

Peruvian phase (first stage of compressional
inversion) The Peruvian phase (Cobbold et al., 2007), or the "Mochica event" , has been in general overlooked in Colombia, despite its comprehensive characterization in the rest of the Andes (Pindell and Tabbutt, 1995). In VSM, the Peruvian phase overlaps with the Albian-Cenomanian, as discussed by Jaimes and De Freitas et al. (2006), based on observations by Van der Wiel (1991) in the Garzón massif. De  recognized evidence of this phase in the Balcón region, where they observed half-graben geometries with clear mild inversions, which occurred during the Late Albian-Cenomanian, as shown by thinning and overlap of pelites of the Bambucá Formation. The NNW-ESE direction of the faulting suggests that compression primarily occurs in the WSW-ENE direction, which would transpressionally reactivate the Jurassic faulting in the ENE direction. Thus, they suggested that an early uplift of the Balcón area during this phase may have positively affected the oil system by generating secondary porosity in the Caballos Formation and by controlling subsequent deformations and migration patterns.

Incaic phase (second stage of compressional inversion)
The Incaic phase of Andean orogeny started during the early Eocene (for further details, please refer to Cobbold et al., 2007), with the increase in the speed of convergence between the Nazca plate and the South American plate, a determinant of crustal shortening structures expressed by thrust faults involving the basement, especially in the Arsu Chert Arin areas neighboring or on the western and eastern margins of the Cordillera Central. The continental sediments of the Gualanday Group were deposited from the late Eocene until the late Oligocene. The Incaic phase, the "Early Andean event" , is identified in the Balcón area by Cenomanian horizon flattening (top of the Bambucá Formation) in seismic interpretation, where the angular discordance postdates the most intense deformation pulse of the area. This event is widely distributed in VSM and has been partly obliterated by the most modern Andean tectonic phases. Despite generating controversy, the age of this discordance is most often attributed to the Middle Eocene (e.g., Schamel, 1991;Cooper et al., 1995;George et al., 1997). However, De Freitas et al. (2006), in line with Buitrago (1994), consider that this discordance is younger, with an Oligocene age in the Neiva subbasin because it is immediately superimposed by the Barzalosa Formation (late Oligocene to early Miocene) by the basal section of the Honda Group, attributed to the middle Miocene (Guerrero, 2002). The configuration produced during this phase persists until the late Miocene, when the oil generation peaks, and is thus a key controlling factor of hydrocarbon entrapment (Higuera, 2012).

Quechua phase (last stage of compressionaltranspressional inversion)
Between the late Miocene and the present Quechua phase of Andean orogeny (Cobbold et al., 2007), the deformation and uplift of the Cordillera Oriental led to another compression period, with new thrust faults with westward vergence, which correspond to the Boquerón fault system, and subsequent deposition of continental sediments of the Honda Formation. The Quechua phase, or the "Andean" event of De , caused the main uplifts of the region, including those of the Cordillera Oriental, which through thrust faults such as the Garzón fault superimpose these mountain ranges on VSM and produce the exhumation of Jurassic rocks exceeding 8,000 feet. In the Neiva region, an ENE-WSW compression is suggested by numerous folds, whose axes are arranged perpendicularly, preferentially in the WSW direction. This deformation succeeded the deposits of the Honda Formation, dated to the middle Miocene (Guerrero, 2002). Similarly, a discordance is observed at the base of the Gigante (or Mesa) Formation, which was presumably formed between the late Miocene and the Pliocene and which is gently uplifted in the eastern flank of the basin (De Freitas and Vallejo, 2000). During the Pliocene, strike-slip faults developed along the NW-SE direction, involving the basement and displacing the axes of the folds and of the main faults, due to differential movements of large blocks. Later, from the middle Tertiary, the VSM basin became a marginal foreland basin with continental deposits of the Gualanday Group and Barzalosa Formation, and finally, from the late Tertiary to the present, it became an intermountain basin with the deposits of the Honda Formation (Barrero et al., 2007).
The Neiva and Girardot subbasins of VSM are structural depocenters of the late Cenozoic staggered between the Columbian Cordillera Central and Oriental, which contain three main depositional sequences up to 9,000 m thick on a Paleozoic crystalline basement, which are summarized as follows: 1) Triassic to Jurassic nonmarine siliciclastic rocks and underlying carbonate sequences; 2) Middle Cretaceous to Paleogene marine to nonmarine clastic and carbonate sequences; and 3) a Neogene potent nonmarine molasse-like sequence. According to Beltrán and Gallo (1979), to Buitrago (1994) and to Higuera (2012), the sandstones of the Guadalupe Group (Campanian-Maastrichtian) were deposited in a marine environment from the shallow shelf through the continental shelves to marine-influenced coastal shelves ( Figure 3).
The Guadalupe formation contains the main productive reservoirs with petrophysical characteristics, according to observations of outcrops and core samples available in and collected from the various exploration and production wells. These make it possible to differentiate an Upper Guadalupe or "Arup" alloformation (upper sandstones) and a Lower Guadalupe or "Arlo" alloformation (lower sandstones), separate from the Middle Guadalupe or "chert" alloformation ( Figure 4).
The pelites of the Villeta Group (late Albian to Santonian) are the main hydrocarbon-generating rocks, and the oil extracted from the fields may come from generation areas located in the nearest synclines or from the vertically deepest areas of production structures (Barrero et al., 2007;Higuera, 2012).

structural observatIons of the matachInes fIeld
The Matachines field is located immediately west of the southern end of the Prado syncline and west of the prominent reliefs of the Guadalupe Formation ( Figure 5).
The main hydrocarbon production zone is associated with horizons of the Guadalupe Formation, consisting of structural traps with effective closures located in the hinge area of an asymmetric anticline (with a flatter western flank and a steeper eastern flank) and with a N-S axis, gently dipping towards both ends. The Venganza fault is arranged immediately above the hydrocarbon production zone, differentially bisecting the upper flanks, so that the upper ranges of the Guadalupe Formation (Arup) are partly absent in the eastern section ( Figure 6).
A plan view of the Matachines field shows the asymmetric anticline that generates this field, with a steeper eastern flank than western flank. The South Matachines Field is not a simple structural continuation of the North Matachines Field and is related to the gradual change  (Saavedra, 2013). The westward change in direction of the South Matachines Field in relation to the North Matachines Field results from the action of a structural transfer in the sinistral transverse direction (intra-Matachines transverse zone) to the main fold structure (Figure 7).
In a sublatitudinal section, a differentially eroded asymmetric fold affected by compressional faulting is identified, showing a two-scale model based on the South Matachines Field (Figure 8).
Secondary reverse faults increase the structural complexity of the area of interest in such a way that the Guadalupe Formation is repeated in structural units, as found in well REV 09; thus, other reservoir horizons most likely exist in the area. Based on the interpretation of the well logs, the Guadalupe Formation would have been affected by the second detachment and saturated with oil in the REV 01 well. From a morphological standpoint, the hydrocarbon-producing horizons of the Guadalupe For-  The overlap pattern of the Honda Formation across the discordance and the direction of its layers indicates a clockwise rotation of the compressive stress, now oriented in the NW-SE direction, which would have caused an uplift of approximately 2,500 feet from an intra-Honda Formation reference level . Fault kinematics data collected south of the Balcón area corroborate this direction of transportation (Blanco and de Freitas, 2003). According to the available GPS data on the tectonic movement (Tremkamp et al., 2002;Acosta et al., 2004), the current horizontal maximum principal stress field is arranged sublatitudinally due to the sublatitudinal convergence between the Nazca and South America plates, albeit with variable rates and directions. The maximum horizontal principal effort is oblique to the Caribbean margin, indicating a dextral transpressional component,

naturally fractured reservoIrs
Based on the fracture evidence, the Guadalupe Formation reservoirs can be characterized as "naturally fractured" reservoirs because their timely and persistent fractures may also help to accumulate hydrocarbons. Formation micro-imaging (FMI) and petrographic analyses of limestones of the Villeta Group in the Monicongo-1 well suggest the presence of a good reservoir. In addition, despite the low density of these fractures, their aperture (considering Aguilera, 2001) suffices to reach a permeability higher than 1,000 mD.
As the Guadalupe Formation does not adequately outcrop in the Matachines field sectors, to acquire microtectonic data, surveys were conducted near the access road to the Prado dam, where the wide and clear artificial cut made it possible to perform semiquantitative observations (Figure 9).
The outcrops show fractures that, depending on the granulometric characteristics of the horizons of the affected Guadalupe Formation, develop differential spacing and magnitude patterns (Figure 10 and Figure 11).

results
Bedding surfaces are mechanically active planes that can considerably enhance the flexural slip produced by the folding of multilithologic sequences (Jamison, 1997). In the Matachines field, bedding also improves the petrophysical characteristics of its reservoirs by incorporating a significant number of discontinuities, thereby complementing fluid connectivity from subhorizontal positions through planes of subvertical fractures generated by the horizontal compressional stresses, which are preferentially arranged in the ENE-WSW direction. In particular, these effects are magnified on the western flank of the anticline structure and to a large extent participate in the connectivity of production wells ( Figure 12). The images of the available wells of fine sand horizons, in light-gray colors, with rhythmic laminations with bioturbations, show microfractures concurrent with postsedimentary structures, which were used by subsequent faults. Thus, for example, one of the main characteristics of fractures interpreted in the VEN-40 well is that they are conductive, preferentially with an axis along the E-W direction and rarely in the NE-SW direction. The highest fracture density, accounting for 67% of the total data (722), ranges from 0 to 1.6 fractures per foot, distributed between hydrocarbon-producing horizons (Arlo-2, Arlo-3, and Arlo-4). In the other 23 % of the data, fracture density ranges from 2 to 3 fractures per foot. The microfractures have planes with E-W and NNE-SSW directions, on average, and are preferentially located on an upper chert (from 6,970 to 7,021' MD), at a 60° angle (to the axis of the well) and with angles of inclination ranging from 70° to 90°. The main characteristic of the fractures interpreted in the VEN-15 well is that they are conductive, with an axis preferentially in the E-W direction and with rare NE-SW fractures ( Figure 13). The apertures of some fractures were also measured in core samples with simple techniques such as by using gauges. Most apertures were smaller than 2 mm, and most values were approximately 1 mm or less (Figure 14).

Rossello / Saavedra
In the reservoirs that, in addition to their primary characteristics, are considered naturally fractured (Hennings et al., 2000), a correct 4D technical and economic evaluation of the quality and arrangement of discontinui-ties is essential to determine their actual contribution to improving the petrophysical properties of such reservoirs ( Figure 15).  The relationships between the permeabilities and porosities of hydrocarbon-producing horizons of the Guadalupe Formation in some wells derived from the South and North Matachines Fields show fairly regular behaviors, albeit with different dispersions, most likely due to the direction of the intersecting boreholes ( Figure 16).
The relationships between porosity and horizontal permeability in core samples corresponding to the levels of the Arup and Arlo horizons in the VEN-1, VEN-17, REV-1ST, REV-2ST, and REV-6 wells of North and South Matachines clearly show that bedding-lamination produced mechanical boundaries that increased permeability (Figure 17). Permeability was compared between vertical and horizontal wells, considering permeability data in hydrocarbon-producing horizons within stratigraphic units expressing mechanically active boundaries, showing that permeability is higher in horizontal wells than in vertical wells ( Figure 18).
The greater permeability of horizontal wells should have been generated by their location in the top hinge section of the main structure of the field (Figure 12), which in turn has higher flexural slip folding and therefore higher permeability in the stratigraphic mechanical boundaries. For this reason, horizontal wells parallel to the bedding in the hinge areas are more likely to inter-sect dilating sectors with better porosity and secondary permeability characteristics. Furthermore, when they are located in the top sections of the hinges, the locally more extensional conditions favor their development.
Based on the morphostructural features identified in the surface and subsoil data, the Matachines fields shows a multiphase evolution, which may have been controlled, at least partly, by pre-existing Paleozoic anisotropies. The new structural interpretation on the geologic time scale shows a main fault in the WNW-ESE direction in the North Matachines field, which is consistent with the fracture data from the BHI analyses ( Figure 19). Accordingly, many commonly used methods for studying wells from core samples or images (Nelson, 1985;Nelson and Serra, 1995;Rider, 1986;Ortega and Marret, 2000;Laongsakul and Dürrast, 2011) identify different types of discordant fractures, but the bedding surfaces are evaluated separately, which does not statistically con-sider the petrophysical effects that contribute to porosity and permeability (Figure 20). Discontinuities due to bedding planes are highly important because they control the distribution of fluids running parallel to the main axis of folding.

dIscussIon
The action of a compressive stress field is responsible for the structural style of this basin, characterized by the successive action of the Incaic and Quechua phases between the Eocene and recent times, which started with the reactivation of the pre-existing normal fault planes during the distensive tectonics between the late Paleozoic and the early Cretaceous ( Figure 21). The tensional state produced by successive Andean tectonic phases has remained active for a long time, as shown by the differential deformation of sedimentary sequences, which was more intense in those deposited earlier because they have accumulated deformations from this stage to now (overlapping phases). Thus, these earlier (Cretaceous) horizons may exhibit a more pronounced clockwise rotation of fractures than those generated in the later (Neogene) sequences of the study area ( Figure 22). Thus, a simplified scheme highlights discontinuous tectonic features with warped surfaces, suggesting an increase in (clockwise or counterclockwise) rotation with depth ( Figure 23). These sigmoidal warps, regardless of possible stress changes due to localized and variable effective geologic overpressures, would be based on the fact that sequences deposited earlier accumulate more rotational deformation than sequences deposited later within an invariant stress field in space throughout a contemporary span with all sequences involved (Rossello, 2018).
Due to the structuring characteristics of the Matachines field, the boreholes are concentrated in the NNW-SSE quadrant (subvertical fractures), mostly with a slightly plunging direction due to bedding surfaces, so it is more likely for there to be a higher number of potentially open fractures per linear meter of their trajectory in the reservoir horizons of the Guadalupe Formation ( Figure  24).

conclusIons
In reservoirs with primary sedimentological characteristics that are considered naturally fractured, a correct 4D technical and economic evaluation of the quality and arrangement of discontinuities should include the effect of the bedding. Many commonly used methods for studying hydraulic fracturing from core samples or images identify bedding surfaces. However, the bedding surfaces, when evaluated separately, are not sufficiently accounted for in the petrophysical considerations that contribute to porosity and permeability.
The seismic data collected in a WorkStation and mounted on the Petrel platform make it possible to spa- tiotemporally characterize, with a mechanical criterion of deformation, a series of pre-and postdiscordance structures preceding the Honda Group. This tectonic context favors the development of deformations that tend to express clockwise rotations as a function of the time in this stress field. Thus, early generated structures have a higher clockwise rotation rate than structures generated later. The NW-SE transverse zone that separates the two Matachines fields is considered a transfer zone resulting from an initially normal fault subjected to a sinistral strike-slip reactivation that has been timely and efficient to compartmentalize the oil system.
All the available tectosedimentary data on the Matachines field and on the spatial characteristics of the hydraulic fracturing assessed in the wells show that the evolution of the oil system is based on the following processes: 1) extensional tectonics, as shown by the normal growth of the faults that accommodated the deposition of the Lower Cretaceous sequences; 2) the transpressional deformation that occurred during the late Cretaceous, which reactivated pre-existing normal faults; 3) burial due to deposition and erosion from the Tertiary; and 4) the structuring that occurred during the Miocene, which caused the uplift of the area, and the resulting erosion and reactivation of previous structures. Therefore, the most important and efficient traps, in terms of timeliness, are those that formed during the late Cretaceous, before the maturation of the hydrocarbons.
Bedding surfaces are mechanically active planes that have enhanced the flexural slip produced by the folding of multilithological sequences. In the Matachines field, bedding improves the petrophysical characteristics of its reservoirs by incorporating a significant number of discontinuities, thereby complementing fluid connectivity from subhorizontal positions through planes of subvertical fractures generated by the horizontal compressional stresses preferentially arranged in the ENE-WSW direction. In particular, these effects are magnified on the western flank of the anticline structure and near its top hinge, resulting in their strong effect on the connectivity of production wells from their subhorizontal positions, with the action of subvertical fracture planes generated by the compressional efforts preferentially arranged in the ENE-WSW direction.

acknowledGments
The colleagues at Hocol Petroleum Limited are thanked for their valuable discussions throughout this study, which helped us understand the technical and regional aspects of the Matachines field. The meticulous editorial work and thoughtful comments and suggestions from the anonymous referees greatly improved the clarity of this work.