4 Sb 191 Veloza g Paper Paleogeografia Mm

Campanian-Maastrichtian paleogeography and reservoir distribution in the Upper Magdalena Valley of Colombia.

VELOZA-FAJARDO G.; DE FREITAS M.; MANTILLA M.

Hocol S.A. Carrera 7 No 116 – 43 Piso 16, Bogotá, Colombia. [email protected]

ABSTRACT. Integration of stratigraphic, structural, petrologic, seismic, well interpretation and biostratigraphic data allow for a new interpretation for the Upper Cretaceous sequence of the Upper Magdalena Valley. Thickness changes among similar depositional environments and between the Neiva and Girardot Sub-basins (from 130-1300 ft respectively) allow us to constrain Maastrichtian deformation, previous to Andean deformation. Eleven stratigraphic cycles were defined based on the observed relations between energetic features and stacking patterns. Two datum levels (Lower and Upper Lidita Fms., Santonian and Middle Campanian respectively) where chosen to tie the correlations, assuming that these units are flooding surfaces (isochronal surfaces). Sequences Kml1-Kml5 represents the interval between the two liditas and are thicker towards the NE area of the basin, getting thinner to the S. Those sequences are composed basically of middle to upper shoreface changing laterally to deltaic deposits in the NE, to lower shoreface and upper offshore environments towards the W and S. The sequences Kmu1-Kmu5, above the Upper Lidita Fm. are thicker in the NW portion of the basin and gets thinner toward the south and east due to the erosion to the south of the Neiva sub-basin. These sequences are composed basically of deposits ranging from lower shoreface in the south, to upper shoreface towards NE. We suggest that the Upper Maastrichtian-Paleocene, Guaduala Fm., rests paraconformably upon the Upper Cretaceous sequence, overlapping progressively distal and older deposits towards the south, where the upper regressive cycles and the Upper Lidita were eroded or bypassed, or both, as a response of the Central Cordillera uplift INTRODUCTION. The Upper Magdalena Valley (UMV) is a intramontane basin bounded by the Chusma Fault system in the W and by the Garzón-Suaza Fault system in the E flank; the northern and southern limits are the Girardot Fold belt and the Altamira Fault respectively. It is comprised by two sub-basins named Girardot (GSB, northern portion) and Neiva (NSB, south) separated by the Natagaima and Pata highs, which exposes the pre-Cretaceous

basement (Figure 1). It is one of the most important basins in terms of hydrocarbon exploration and production, producing approximately 10% of petroleum and 2% of gas per year of the national production from the Caballos, Monserrate and Honda Fms.. Many works, published and unpublished, has focused in the stratigraphy, petrology and palynology of the Upper Cretaceous sequence of this area, principally in the GSB, allowing the development of lithostratigraphic correlations among small areas, where tectonic implications have not been kept in mind, and generalization across all the basin have been made too, principally in the NSB. All those factors motivated us to develop a new stratigraphic interpretation with all the Hocol’s available data. The integration of subsurface and surface data allow us to constrain a model that help us to understand the tectonostratigraphic factors that governed the sedimentation of the Santonian – Maastrichtian deposits, taking in to account the implications for the hydrocarbon exploration. REGIONAL SETTING. The Upper Magdalena Valley (UMV) is an NE-SW elongated intermontane basin separating the Central and Eastern Cordilleras of Colombia (CCC and ECC, respectively, see Figure 1). The UMV extends for nearly 400 km, with an average width of 50 km. Some key references for the understanding of the structure, stratigraphy and hydrocarbon potential of the basin include the following: Corrigan (1967), Beltran & Gallo (1968, 1979), Cediel et al (1981), Kroonenberg & Diederix (1982), Macia et al (1985), Butler & Schamel (1988), Mojica & Franco (1990), Schamel (1991), Van der Wiel (1991), Buitrago (1994), Fabre (1995), Sarmiento (2001). The generalized stratigraphy of the UMV is presented in Figure 2. The UMV basin is underlain by continental crust, which has been interpreted as either the prolongation of the Guayana shield, or an allochthonous, Grenville terrane which collided with South America in the Proterozoic (Kroonenberg, 1982; Forero, 1990). Proterozoic high grade metamorphics make the bulk of the Garzon Massif (Kroonenberg &

Diederix, 1982). Paleozoic low grade metamorphics, part of a large metamorphic belt that extended to the Santa Marta Massif in northern Colombia, are preserved in parts of the Central Cordillera and the Garzon Massif (Irving, 1975; McCourt et al, 1984). Triassic sediments unconformably overlie the metamorphic basement, and comprise continental siliciclastics (Luisa Fm.) and marine carbonate rocks (Payande Fm., Cediel et al, 1981; Macia et al, 1985). The Jurassic represents a major thermal event and contains vast amounts of intrusives and volcaniclastics (Saldaña Fm.) generated by continental stratovolcanoes and deposited as an infill of rifted blocks in the medial-distal parts of a magmatic arc associated with a convergent margin to the west (Bayona et al, 1994). The Jurassic-Triassic section, regarded as the economic basement for hydrocarbon exploration in the UMV, outcrops along both fronts of the Central and Eastern Cordilleras and in a series of basement uplifts within the basin (Figure 1).

The Cretaceous unconformably overlies the Jurassic in the UMV and records a major marine transgression from the north. In the northeast of the Girardot sub-basin, bordering

the ECC, Cretaceous deposition started in the Barremian. The section gets progressively thinner southwards, containing Aptian fluvial deposits as the older unit in the Neiva sub-basin. In the early stages of this transgression, deposition of fluvial to estuarine sands (Caballos Fm.) created the main reservoir rock of the basin. During maximum extension of the transgression in the Turonian, deposition of the La Luna and equivalent units of Venezuela and Ecuador took place. This represents the principal hydrocarbon source rock in northern South America (Villamil et al, 1999). The Cretaceous ended with a regressive cycle, when shallow marine sands (Monserrate Fm., also a major reservoir rock) were deposited. A major change in sedimentation to continental conditions occurred in the Maastrichtian and is associated with the accretion of the Western Cordillera. The Guaduas Gr. (Maastrichtian to Paleocene) contains mostly coastal plain mudstones (San Francisco Fm.) which grade into siltsones and thin sandstones (Teruel Fm.). Eocene and younger deposited contain alternating sequences of conglomerates and sands capped by claystones and represent molasses from the adjacent rising cordilleras (Beltran & Gallo, 1968; Anderson, 1972; Guerrero, 1993 and others). In the late Cenozoic significant volcanic activity is recorded in volcaniclastic deposits (van Houten, 1976; van der Wiel, 1991 and others). The UMV was part of a Jurassic back-arc rift which has undergone inversion during compressional and transpressional tectonic pulses from Late Eocene to Recent (Schamel, 1991; Cooper et al, 1995; Sarmiento, 2001), resulting in a complex array of compartments with varying structural styles. The existence of previous sets of Paleozoic and Mesozoic faults and weakness zones affecting basement and the pre-Cretaceous section strongly controlled the subsequent structural development during Tertiary compression (Butler & Schamel, 1988; de Freitas, 2001). Zircon and apatite fission-track data suggests that the Central Cordillera presents an uplift of 7-13km since Campanian times, while the Eastern Cordillera has suffer an uplift of at least 3-4km sometime between 65-30 in the Villeta Anticlinorium area (Gomez et al, 2003). Late Cretaceous – Eocene accretion of Western Cordillera led to northward propagation of uplift of the Central Cordillera and to onset of compressional inversion of Mesozoic grabens in the Eastern Cordillera (Gomez et al, 2003).

Actually, four tectonic plates interact in the NW corner of South America. Nazca and Caribbean

plates moves toward the E and SE respectively; South America Plate moves westward and the Cocos Plate moves north-eastward. Focal mechanism determinations indicates that the stress field present in the NW corner of South America has varied since Late Cretaceous principally in two phases, from E-W to SW-NE during Maastrichtian-Late Paleocene to NW-SE since early Eocene-Pleistocene in the southern Middle Magdalena Valley (Cortés et al, 2005), generating oblique reactivation of previous fault sets and en echelon structures. Configuration of the previous extensional sub-basins and the unique position of the UMV between the Central and Eastern Cordillera led to the creation of opposite vergent fold belts and tectonic wedges, arranged in a mosaic of rhomboid blocks separated by NE-SW transfer zones (de Freitas, 2001).

Hydrocarbon exploration in the UMV in the last 50 years resulted in the discovery of 30 oil fields. The petroleum geology of the basin has been reviewed by Buitrago (1994), Fabre (1987), Navarro & Cordoba (2002) and, more recently, by Sarmiento & Rangel (2004).

Petroleum Systems Hydrocarbon accumulations in the Neiva subbasin of the UMV are part of two proven petroleum systems (Buitrago, 1994): the Villeta-Caballos(!) and the Villeta-Monserrate(!). The first straddles along the western portion of the basin, to the west of the San Jacinto fault system, and the second and most prolific, occurs between that fault system and the front of the Eastern Cordillera. Representative fields of these systems are San Francisco and Tello, respectively. Hydrocarbon exploration in the UMV initiated in the late 1940’s and resulted in the discovery of over 30 oil fields to date, of which the San Francisco field, discovered by Hocol in 1985, is the largest (~200 mmbbl of recoverable reserves). The petroleum systems of different sectors of the basin have been reviewed by Buitrago (1994), Fabre (1995), Navarro & Cordoba (2002) and Sarmiento & Rangel (2004). All hydrocarbon discoveries in the UMV are assumed to be source from rocks of the Villeta Group (Albian to Coniacian). Main reservoirs in the basin are the sandstone intervals of the Caballos Fm. (Aptian-Albian), the Monserrate Fm. (Campanian-Maastrichtian), which is the focus of this study, and the Honda Group (Miocene). STRATIGRAPHIC MODEL. Controversy about the stratigraphic nomenclature of the Upper Cretaceous of the UMV has occurred in the past years because of the interpretation of the stratigraphy presented in each area, where regional concepts were applied to local studies. The stratigraphy defined in the GSB differs of the one defined for the NSB (Table No 1), where just one sandstone level is present above the Lower Lidita Fm., where no formal stratigraphic nomenclature has been defined. Eleven informal units or cycles, named from base to top Kml1-5, Upper Lidita Unit and units Kmu1-5 where defined for this time span (from Campanian – Middle Maastrichtian). Units Kml1-5 are deposits time equivalents with the Arenisca Dura Fm. of the Cordillera Oriental area and rests conformably over the limestones of the Lomagorda or Villeta Fm.; and deposits of the cycles Kmu1-5 are equivalents with the deposits of the Arenisca Labor-Tierna of the same area. The equivalent units between GSB and NSB are presented in Table 1. Methodology. Almost 100 wells and 40 surface sections where analyzed and correlated with the purpose of a better understanding of the depositional environments and of the depositional sequences.

All the wells where adjusted to a unique set of logs, then those wells were tilt corrected (when dipmeter data were available) for obtaining the true stratigraphic thickness (TST). Eleven stratigraphic cycles were defined based on characteristics of sedimentary environments and stacking patterns, from one to another minimum energetic levels (marine flooding surfaces). The term “stratigraphic cycles” is used instead of parasequences because the mechanism through they were formed is not absolutely clear, and the genesis of these cycles could be related to different events, rather than exclusively sea level change (e.g. differences in sediment supply because of the uplifting of a new source area). The limits of these cycles are represented by marine flooding surfaces (Nichols, 1999). Two datum levels, Lower and Upper Lidita Fms. were used as regional maximum flooding surfaces (Guerrero et al, 2000), then, we assume that those units are approximately isochronal all around the basin. The facies association for each interval was defined on the basis of cores and lithologs description and observations of the surface sections. Once correlated, the thickness and facies of each interval defined were mapped with the aim of find the special relations between thickness and depositional environments. The wells and surface sections were adjusted with the palynology reported (when available) to define the presence of the regional markers named before, which are well defined horizons of Santonian and Middle Campanian age respectively. The petrographic samples were tied to each defined interval. Four hundred points were counted for each thin section (Veloza, 2005) in order to identify composition of framework and accessory grains , as well as interstitial

material (when plates were available). Detrital modes exclusive of carbonate grains, heavy minerals and authigenic minerals were calculated from point-count results and plotted in ternary diagrams. When no point data were available, estimations made by the author of the report were used to made the sample classification. Litostratigraphy and sedimentary environments. Five principal areas where defined in this study with the basis of similar depositional environments and similar thicknesses (Figure 1). Fusa, Ortega, North Neiva, South Neiva and Altamira, each area has it’s own stratigraphy, depending of the depositional environment and how the tectonic evolution affected the preservation of these deposits. From north to south, in the Fusa area, the deposits of the Kml units represents deposits from upper offshore (Kml1) to middle Shoreface (Kml3) and eventually mouth bar deposits are present. These intervals are represented by three progradational cycles, from the Kml1 to Kml3. The Kml4, Kml5 and Upper Lidita intervals represents the transgression of the sea level, and produced deposits ranging from middle shoreface to offshore, where the last one was deposited. The average thickness of this interval in the Fusa area is 160 ft. The Kmu deposits in this area ranges in environments from middle shoreface to foreshore. Sequences Kmu1-Kmu3 are principally progradationals and Kmu4 and Kmu5 are agradational, representing environments where the accommodation space created was compensated with the sediment influx. The average thickness of this sequence is 70’. In the Ortega area, the depositional environments in the Kml levels range always between offshore and the transition to lower shoreface. These deposits presents an agradational path, and

minimal deflections where observed in the curves; this response of the curves obey to the high silt content of the rock, typical of those environments. The average thickness of this interval is 90’. The Kmu sequence is principally agradational from the Kmu1 to Kmu3 intervals, and became progradational in the Kmu4 and Kmu5 sequences. The Kmu5 represents an abrupt facies shift, from marine to alluvial environments which is the record of the uplifting of the Central Cordillera and that was preserved in this area just when the accommodation space created exceeds the rate of uplift and supply of sediment. This interval has an average thickness of 80’ and increases northward. In the North Neiva area the curves presents a progradational path in all the cycles of the Kml interval. The environments varies from lower shoreface to middle shoreface, and eventually mouth bar deposits appear. Of the Kmu sequence just the 1 and 2 sequences were preserved in the northern portion of the area, in the Cebu-5 well the upper Kmu2 interval was eroded or non deposited. The average thickness is 70’ and 60’ for Kml and Kmu respectively. In the South Neiva area just few wells have all the Kml interval complete. In the northern portion of this area the uppermost level founded is the Upper Lidita, which is in unconformable contact with the Guaduala Group. The depositional environments ranges from lower shoreface to upper offshore, and eventually mouth and mouth bars deposits are founded. The Kml cycles in this area presents principally a progradational path. The thickness of this interval ranges from 40-60’. In the Altamira area just few cycles are preserved, those are Kml1-3. These cycles represents a progradational path, as founded in all the study area. These deposits always has been interpreted as offshore deposits. Our interpretation, based mostly on petrography of surface samples represents energetic, near shore environments (middle to upper shoreface) , which took place far from the input of clastic sediments, allowing the development of biogenic habitats structures. The average thickness of the sedimentary sequence in this area is 50’. Sandstone petrology All the available petrographic samples compiled from various works, petrography of

surface (Osorio & Rodriguez, 2000; Veloza, 2005; Reyes et al, 2003 and others) and wells samples (Atadero-1, Rio Saldaña-2 among others) were plotted in map view for each sequence identified in this job with the aim of identify variations in the source area (Figure 3). Almost all the studied macroscopic and microscopic samples analyzed during the development of this study are compositionally and texturally mature and submature respectively.

Comparison of composition (QFL) and provenance (QtFLt) ternary diagrams (when detailed descriptions were available) from base to top doesn’t shows a principal path between any area, although it’s clear that all the samples falls in the uppermost corner of the triangle, been classified between quartzarenites, subarkoses, and sublithoarenites (the feldspates and lithic fragments ranges from 0-8%, Figure 3). In the provenance diagrams, all the samples falls in the orogen recycled and craton interior areas, which indicates that there are at least two source areas. Those areas are the Guyana Shield, that supplies all the quartz present (supermature sediments), the ancestral Central Cordillera (in a distal position), which supplies the igneous, metamorphic clasts, and a probably third area provides the sediments previously deposited south and westward, and which were eroded and redeposited in the north, evidence of the presence

of this sediments are the abundant intraclasts fragments. In the southern portion of the basin (Altamira area) the presence of packstone biomicrites and grainstone bioesparites, although calcareous sandstones indicates that highly energetic conditions prevailed during this time for this area, this sediments always has been misinterpreted locating them into distal (offshore or shelf) environments. The presence of subarkoses, sublithoarenites and quartzarenites in all the study area suggests that those source areas were presents since at least Lower Campanian age. Macroscopic samples, principally in the Ortega area from surface section and cores (Ecopetrol-ICP, 2000; Dunia, 2003), presents chert, cuarzite, and pink quartz clasts of 8mm (fine–medium pebble grain size) average diameter. Those clasts doesn’t appear in the petrographic results, but it’s clear that older sedimentary and metamorphic rocks were under erosion because of their presence, principally in the upper Kmu5 cycle (equivalent to La Tabla and Cimarrona Fms.). Then, this cycle represents the principal evidence to suggest the first recorded pulse of the uplifting of the Central Cordillera, and no genetic relations must be done between this interval and the underlying. Biostratigraphy. Integration of biostratigraphic information of many wells, and surface published reports (Ecopetrol-ICP, 2000; Guerrero et al, 2000; Tchegliakova & Mojica, 2001) allow us to develop a chronostratigraphic framework for this study. Almost all the published biostratigraphic information of the UMV has been obtained for the northern portion of the basin, GSB. The intervals used as datum for correlation purposes were the Lower and Upper Lidita Fms. (Santonian and Middle Campanian, respectively) which represents regional flooding surfaces and are the less diachronous surfaces. In the Ortega area biostratigraphic reports from the Toldado Field and Rio Saldaña wells (Ecopetrol-ICP, 2000) suggest an Santonian age for the Lower Lidita level, Lower Campanian for the Kml interval, Middle – Upper Campanian for the Upper Lidita and Upper Campanian – Middle Maastrichtian for the Kmu sequence. This chronostratigraphic framework is retained for the Fusa area, where few samples of the Atadero-1 well were analysed. Although, for this area a subtle

paraconformity was identified on the top of the Guadalupe Gr. (Bayona et al, 2003) where the Middle Maastrichtian time span is absent. In the North Neiva area, private reports from the Cebu-5 well (Robertson, 1984) indicate a Santonian age for the interval identified as Lower Lidita level, and indicate an undefined Campanian-Maastrichtian? age for the overlying layers. The same conclusion was achieved by palynological analysis made on the Yaguará area, in the Los Mangos-60 well (Duque-Caro, 1998), where the Upper Lidita level appear on top of the analyzed interval. In the southernmost portion of the UMV, in the Iskana-1 well, palynological samples where analyzed (Tepma, 2001) and a lack of biozones was determined on the top of the Cretaceous sequence, where the interval defined as Caliza del Tobo represents the Campanian-Early Maastrichtian and the overlying deposits of the Guaduala Fm. yield palynomorphs that suggest a late Maastrichtian age. All this observations are in agreement with the angular unconformity observed in northern Ecuador, where the Cretaceous marine strata of the Napo Fm. (Albian-Campanian) is overlied by the Maastrichtian-Paleocene Tena Fm. (Balkwill et al, 1995; Vaca et al, 2005; Barragán et al, 2005). DISCUSSION In the UMV, a 130-1300 ft thickness variations of Campanian- early Maastrichtian sediments is present form south to north (Figures 4, 5). This change represents the environmental variations and facies distribution of the Santonian-Lower Campanian time span (Arenisca Dura Fm’s time equivalents) from offshore to middle shoreface deposits. The absence of the Labor-Tierna Fms. time equivalents represents the erosion or non-deposition of the sediments of the Upper Campanian-Middle Maastrichtian time span, at least, in the NSB, caused by the uplift of the ancestral Central Cordillera which produces a paraconformity, between the strata of Seca (and equivalent units) and the Monserrate Fm. in this area, shifting depositional environments from alluvial plain to lower shoreface respectively. Evidences that support this hypothesis are: 1) Biozones absences of the middle Campanian- lower Maastrichtian in the Fusagasuga area (Bayona et al, 2003) and in the Altamira area (Tepma, 2001) where detailed biostratigraphic

works have been developed. 2) Abrupt change of facies between the Guaduala Fm. and Monserrate Fm. in the NSB and in the southern portion of GSB. 3) Velocity anomalies on top of Monserrate Fm (De Freitas, in press) that suggests subaerial exposure of the sediments. 4) The presence of feldspars (principally microcline), lithic fragments in low amounts indicates a far source area or reworked sediments or both. We favour the last hypothesis because of the observed results in the ternary diagrams of provenance (Fig 3). Reservoir distribution. The distribution of the best reservoir areas all around the basin and in each stratigraphic interval obviously depends of the tectonostratigraphic development of the area, although, some areas with good characteristics were eroded during the late Tertiary uplift (Figure 6). For the Kml sequence the best reservoir areas are present in the Fusgasuga area, where Guando field is present and produces from this interval. Porosities around 15% presents in this area. In the Ortega area the Kml interval almost lack of producer intervals, porosities in this area are around 2% because of the sedimentary environments present. The north and south Neiva areas presents good porosities, ranging between 5-20% and many fields produce from this interval (e.g. Tello and Cebú fields). The Altamira area lacks of producer intervals; this area presents porosities of 0-3%. The absence of producer intervals (at least from primary porosities) obey to the sedimentary environments that prevailed for this area during the Early Campanian times. The Kmu sequence, presented only in the GSB present good reservoir deposits in the Fusagasuga and north Neiva areas, where porosities range from 5-20%. The Ortega area presents low porosities in the Kmu1-4 intervals, although the Kmu5 interval (principally composed of conglomerate levels) presents high porosities ranging from 17-22% and could represent the best reservoir of the basin for this time span. CONCLUSIONS. The best reservoir sands were deposited for the Kml interval toward NE, in the Fusagasuga area and in the North Neiva area, the South Neiva and Ortega areas presents eventually good reservoirs, represented principally by mouth bar deposits, which prograded far from

source area (deltaic deposits near actual Bogotá position) and has averages porosities of 15%. The calcareous rocks deposited in the Altamira area don’t present good reservoir characteristics because the sediments has no primary porosity; although fractured reservoirs could be found, improving the exploration in this area. The Kmu deposits presents good reservoir characteristics in almost all the studied area. In the Ortega area, the sediments of the Kmu5 (equivalent to La Tabla and Cimarrona Fms.) interval could reaches porosities of around 30% and in the Fusagasuga area the same interval presents porosities of 8% (Atadero-1). Locally, the structural expression of the proposed discordance is paraconformably, although the study of stratigraphic cycles shows the presence of a regionally disconformity between Monserrate and Guaduala Fms. The implications of tectonics in the development of best reservoir areas is the redistribution of sediments previously deposited in the NSB. The alluvial fans and rivers deposits of the Cimarrona or La Tabla Fms. are the stratigraphic record of this uplifting (Figure 7), those sediments migrated eastward and northward during the uplift of the Central Cordillera. Figure 8 provides a chronostratigraphic framework for the UMV with almost all the stratigraphic names used in all the area. In this article, we propose that the names of the El Tobo and Monserrate Fms. been preserved because of their extensive use in the literature, but with the correction that those units are approximately time equivalents with the El Cobre Fm., which means that those units have a Early Campanian age. AKNOWLEDGMENTS Thanks to Hocol S.A. for allow us to publish this ideas. GV wants to tanks to Martin and Mario for all the aid, support and the teachings received during this months. Discussion with many geologists improved this document. REFERENCES Anderson, T.A., 1972. Paleogene non-marine Gualanday Group, Neiva Basin, Colombia, and regional development of the Colombian Andes: GSA Bull., v. 83, pp. 2423-2438. Balkwill, H. R., G. Rodrigue, F. I. Paredes, and J. P. Almeida, 1995, Northern part of Oriente basin, Ecuador: reflection seismic expression of structures, in A. J. Tankard, R. Suárez S., and H. J. Welsink, Petroleum basins of South America: AAPG Memoir 62, p. 559-571. Barragán, R., Toro-Alava, J., Jaillard, E., White., H., Toulkeridis., T., Montenegro., J., Medina, G., 2005, Coger Maastrichtian syntectonic sedimentation along the Subandean Zone and its relationship with an accretionary event of an

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