The Cogollo Group and the oceanic anoxic events 1 a and 1 b , Maracaibo basin , Venezuela

Manuscript ID: 30192. Received: 10/30/2014. Approved: 04/08/2015. ABSTRACT: Carbonates of Cogollo Group (Apón, Lisure and Maraca formations) constitute the broader calcareous platform system originated during Aptian and Albian of Cretaceous in north-western South America, Maracaibo Basin, Venezuela. On the shallow shelf, a variety of calcareous sedimentary facies were deposited during marine transgressive and regressive cycles. Some of them developed porosity and constitute important hydrocarbon reservoirs. Due to some major marine transgressions, from early Aptian, the anoxic environment and characteristic facies of a pelagic environment moved from the outer slope and basin to the shallow shelf, during specific time intervals, favouring the sedimentation of organic matter-rich facies, which correspond to the oceanic anoxic events (OAEs) 1a and 1b. The source rock of Machiques Member (Apón Formation) was deposited during early Aptian OAE 1a (~ 120 Ma). The source rock of Piché Member, located at the top of the Apón Formation, was deposited during late Aptian OAE 1b (~ 113 Ma). Finally, La Luna Formation, from Cenomanian, that covers the OAE 2 (~ 93 Ma), represents the most important source rock in the Maracaibo Basin. In this way and based on sedimentological and organic geochemistry results from the determinations performed on 247 samples belonging to six cores in the Maracaibo Basin, we propose these two organic-rich levels, deposited on the shallow shelf of the Cogollo Group, as “effective source rocks”, additional to La Luna Formation, with oil migration in relatively small distances to the porosity facies.


INTRODUCTION
The Cogollo Group is a shallow carbonate platform composed by many sedimentological facies and depositional textures.The system was deposited during all the Aptian and Albian of Cretaceous Period, in the Maracaibo Basin, which is located in the North-West of Venezuela (Fig. 1).It is formed by Apón (Tibú, Machiques and Piché members), Lisure and Maraca formations (Renz 1977;Bartok et al. 1981;LEV 1970LEV , 1997)).
The variety of depositional textures and fossils includes mudstone, wackestone, packstone, dolomite and grainstone with bivalve, echinoderms, benthic foraminifera, red and green algae, oolites, among others (Ford & Houbolt 1963;LEV 1997).Between these shallow facies, two stratigraphic levels of organic rich pelagic and hemipelagic carbonate facies are found, with variable shale and silt content.These high total organic carbon (TOC) levels are the expression in the Maracaibo Basin sedimentary record of two oceanic anoxic events (OAEs): OAE 1a of the early Aptian (~ 120 Ma), represented by the Machiques Member, and the OAE 1b of the late Aptian (~ 113 Ma), represented by the Piché Member, both of them belong to the Apón Formation (Méndez et al. 2008).These two bituminous intervals, whose correlative sections have been studied in many sedimentary basins in Europe, the Mediterranean sea, Florida (USA), Gulf of Mexico, the Pacific and the Atlantic oceans (Sliter 1989;Bralower et al. 1993Bralower et al. , 1994;;Jenkyns & Wilson 1999;Leckie et al. 2002;Dumitrescu et al. 2006;Hofmann et al. 2008;Robinson et al. 2008), represent two effective source rock levels.This implies a new configuration of the Cogollo Group and Río Negro Formation petroleum system.
The oil contained in the production levels of the conglomerate and sandstone reservoir facies of the Río Negro Formation (Barremian), underlying the Gogollo Group and the Piché Member reservoir facies (Apón Formation), was generated by Machiques Member and Piché Member source rock levels, respectively.Therefore, La Luna Formation should not be considered the only source rock in the Maracaibo Basin.(B) The deep of the Maracaibo Basin in the subsurface is ranged between 6,000 and 18,000 feet and is extended over the Zulia state and part of the Gulf of Venezuela.The most important outcrops with large thicknesses of Lower Cretaceous are in Perijá.In the Andes of Venezuela, the thicknesses are smaller but the changes in facies of ancient shorelines of the platform can be observed.Detailed studies on sedimentation, transgressive and regressive stratigraphic cycles, porosity levels, diagenesis, organic and inorganic constituents were performed by macro descriptions of cores from several wells within the basin and a subsequently petrographic and petrological detailed study of thin sections.Once characterized the various sedimentary facies, porosity areas and possible source rock levels in the Cogollo Group platform system, the corresponding intervals to oceanic anoxic events 1a and 1b were correlated by their identification through the use of electrical gamma ray logs in several wells in the basin.
To determine the hydrocarbon generating potential of these two TOC rich stratigraphic levels, a set of samples was taken from the base to the top of these black sedimentary levels in several cores, with the aim of quantify and determine the thermal maturity, quality and source of the organic matter that forms the kerogen, and the redox conditions of the depositional environment.For this purpose, the following experimental methods and techniques were required: Rock-Eval pyrolysis 6, to determine the values of S1, S2, S3, T Max, building kerogen types and organic matter maturation state; Visual Kerogen Assessment, that includes a study of the kerogen macerals and the determination of the Thermal Alteration Index (TAI), the Vitrinite Reflectance and Liptinite Fluorescence.Eventually, a sedimentological model and paleoenvironmental implications for these facies are developed.

GEOLOGICAL SETTING
Before clastic sedimentation of the Río Negro Formation and subsequent Cogollo Group calcareous sediments, a system of grabens was developed in southwest-northeast direction, stretching from Ecuador to the current Gulf of Venezuela (Fig. 2).These grabens were located among the current Magdalena River valley, the valley of the Cesar and the Perija Mountains.Much of the Maracaibo Basin was included in this system, which included the Perijá-Machiques fossa, the Uribante fossa, the groove of the Guajira, the San Lázaro graben and the graben of Lake of Maracaibo (LEV 1970(LEV , 1997)).The grabens represent a part of the tectonic extension and partial rupture during the breakup of Pangea during early Jurassic and the beginning of the opening of the proto-Caribbean during early Cretaceous, when the volcanic activity ceased.A series of volcanic red beds, known as La Quinta Formation, was deposited inside the grabens.This process caused the partial fillings of the grabens.Later, during the Neocomian and Barremian, continental sedimentation and fluvial and alluvial environments dominated the sedimentary processes, and sandstones and conglomerates facies were deposited interbedded with siltstones and shales of the Río Negro Formation of Barremian age.
The sandstones and conglomerates facies overlying La Quinta Formation completed the filling of the grabens, shaping a plain physiographic system.The Río Negro Formation has highly variable thicknesses.In the grooves as Machiques, thicknesses may exceed 1,000 m, whereas on the peneplain and most of Lake of Maracaibo, it has only a few tens of meters (LEV 1997).
Carbonates of Cogollo Group have an origin and belong to the same derivative sedimentation of ancient Tethys Sea, in which calcareous sediments were deposited with similar characteristics and ages in the Gulf of Mexico (calcareous Cretaceous environments of Mexico and southern USA).The calcareous sedimentation took place on a passive margin platform, during the development of the proto-caribbean, as a consequence of the separation that the North American craton and the South American craton experienced by means of a mid-ocean ridge (Ross & Scotese 1988;Pindell & Kennan 2001;Bachmann 2001;Orihuela 2012).Passive margins corresponded to shelf seas.Once the transgression began, these shelf seas penetrated deep into the cratons, developing reef margins and calcareous sedimentary environments behind the margins (Fig. 3).
During the formation of the Gulf of Mexico, previous carbonate deposits, from late Jurassic, were developed on the coastal plains areas of Mississippi, Alabama and Florida (Manzini et al. 2001).Since the late Barremian and early Aptian, one active calcareous sedimentation began around the platforms bordering the Gulf of México as a consequence of an eustatic transgression (Vail et al. 1977;Méndez 1989a;Himes 2000;Lehmann et al. 2000), which was related to a large release of ocean basalts, which caused the development of a thick oceanic crust and the rising of the sea level during Aptian, Albian and most of late Cretaceous (Arthur   al. 1985;Larson 1991;Berner 1992;Jones & Jenkyns 2001;Kuypers et al. 2002;Jahren 2002;Wagreich et al. 2011).From the Valanginian and Hauterivian, carbonates were developed on the shorelines of the northwest coast of South America (now Colombia).

STRATIGRAPHIC AND SEDIMENTOLOGICAL CHARACTERIZATION
The carbonates of Cogollo Group were deposited throughout the Aptian and Albian.This group is formed by three formations: Apón (Aptian), Lisure (Albian) and Maraca (late Albian).The Apón Formation is divided into three members: Tibú, Machiques and Piché.The carbonate sedimentation is interdigitaded with siliciclastic facies deposited mainly in shorelines to the east and south of the old carbonate platform.These carbonate and clastic sediments represent an initial ramp that evolved into a platform (Méndez 1989a(Méndez , 1997)).Sedimentation at the base of the Apón Formation (Tibú Member) was developed on a ramp and the transition to a platform is at the stratigraphic level that corresponds to the Machiques Member.The top of the Apón Formation (Piché Member), and the Lisure and Maraca Formation indicate that the sedimentation occurred in the platform, with regional extension for most facies.A good example is the oolites facies in Lisure Formation and Piché Member, which extend throughout the platform for a specific stratigraphic level and a geologic time.Maraca Formation has similar regional features, because the entire platform has a similar thickness and characteristics in terms of lithology, texture and porosity present (Fig. 4).
The calcareous sedimentation was developed at the beginning of Aptian in the north of South America craton and it overlies the conglomerates and sandstones of Río Negro Formation of late Barremian age.Overlying Cogollo Group is La Luna Formation, deposited since Cenomanian to Campanian, which is one of the most important oil source rocks in the world (Fig. 4).Today, among the subsurface and lifted areas in Perijá and Los Andes Mountains, the basin extends over many states from western Venezuela, covering Zulia, Táchira, Mérida and Lara states, western Falcón state and a part of the Gulf of Venezuela.From the beginning of the sedimentation, the basin covers over 100,000 km 2 , between what is now Venezuela and Colombia.In Venezuela, it currently covers over 70,000 km 2 .Most of the basin lies in the subsurface and partly as outcrops in the Andes and Perijá mountains.The stratigraphy and sedimentation were initially studied in outcrops of the Andes and Perijá (Renz 1977;García Jarpa et al. 1980;Macellari 1988), but in the last 30 years, researches have been focused mainly in core studies (Bartok et al. 1981;Chacartegui 1985;Méndez 1989aMéndez , 1997)), and log analysis (Fig. 4).The Cogollo Group in the subsurface range from 17,000 to 14,000 feet deep, but in some areas it is deeper and shallower than it is in others, such as the La Paz field, where carbonates are found at 6,000 -8,000 feet deep.Thicknesses range between 1,700 feet in the subsurface near the Perijá Mountains Piedmont (west of the basin) up to 900 feet in the central area of the basin.In the Andes (east) and the south of the basin, the thickness of the Cogollo Group decreases to 700 feet.This area represented the coast line with sandstone, siltstone and shale interdigitations.The thickness difference is related to a greater or lesser subsidence that allowed the variation in the sedimentation.In the west, a greater subsidence allowed more sediment accumulation, while in the center, east and south of the basin, the subsidence was lesser.This increased the accumulation of sediment in the Tibú member, which corresponds to the bottom of the Apón Formation, represents a ramp-type depositional environment that subsequently evolved to a platform of shallow depths.In the West, Tibú Member has 700 feet thickness, but in the center, east and south, it has only 40 -30 feet.
The reservoirs facies of Río Negro, Apón, Lisure and Maraca formations are vertically separated by more than 400 feet between them, therefore, it is necessary to estimate more than one source rock, allowing the migration of hydrocarbons to nearby reservoirs.The oil stored in these reservoirs was generated by the anoxic and high TOC facies deposited during the oceanic anoxic events that occurred during the early Cretaceous (OAE 1a of early Aptian, ~ 120 Ma; and OAE 1b of late Aptian, ~ 113 Ma) and late Cretaceous (OAE 2 of Cenomanian-Turonian ~ 93 Ma).The OAEs 1a and 1b are represented in the stratigraphic record of the Cogollo Group by a black pelagic and hemipelagic levels which are present in the Machiques and the top of the Piché members respectively and both are part of the Apón Formation, while the OAE 2 is part of La Luna Formation.
Many controversies have arisen from various studies on porosities and main type facies that may consist in hydrocarbon reservoirs, their origin, main diagenesis related (surface or burial) and fractures as the porosity main forming element (Méndez 1989b(Méndez , 1997(Méndez , 2009;;Kummerow & Pérez de Mejía 1989;Márquez 1997;Montoya & Méndez 2005;Nelson et al. 2000).Usually in the last decades of drilling for Cretaceous in the Maracaibo Basin, the wells were located near major faults and fracture systems looking porosity related to these (Fig. 4).The authors of this study estimate that the major chemical and diagenetic changes in dissolution and cementing carbonates occurred either on the surface or close  to it, during primary diagenesis.However, this controversial and important topic is not covered in this article.

Apón Formation
The Apón Formation represents the onset of carbonate sedimentation overlaying Río Negro Formation.It indicates the gradual cessation of erosion and transport of siliciclastic positive areas in the craton and the onset of marine transgression on the platform.The sedimentation gradually changes, along the entire basin, from sandstones of Río Negro Formation to carbonates and calcareous facies with interdigitaded clastic sediments derived from an incipient ramp, which gradually changes to a shallower platform system.Apón Formation is formed by Tibú, Machiques and Piché members.The typical fauna described by Renz (1977) of Aptian age, consists of several bivalve genera: Ostrea scyfax, Exogyra boussingaulti and echinoids as Toxaster sp.The Choffatella decipiens, a benthic foraminifera that is found always at the base of the Apón Formation, is characteristic of Tibú Member as well as the green algae Dinaric Salpingoporella (Ford & Houbolt 1963).

Tibú Member
It represents the bottom of Apón Formation and it indicates the initial phase of a marine transgression on the platform, mainly forming a ramp as the sedimentation and the transgression progressed on ancient shorelines.All the transgressions (TST) moved the cost line from west to east, but always intercalated with a little regressions until the cost line was established at the actual position of the Andes of Venezuela.The texture of the sediments varies from mudstone to packstone.Wackestone and mudstone predominate with green algae, echinoderms, bivalves and miliolids fragments.Packstone facies are occasionally composed by caprinid fragments (rudist).The Choffatella decipiens is common at Tibú base along the basin.The porosity is too small to constitute reservoirs and it rarely exceeds 2%.The thicknesses are highly variable over 700 feet in the west of the basin to 30 feet near the ancient shorelines, which is typical of a ramp system.

Machiques Member
Machiques Member was formed by a great and fast transgression over the Tibú Member.The transgresion covered almost the entire basin and reached the coast line during previous sedimentation of Tibú Member.It was described as "a possible source rock" by Méndez (1989c) and Méndez et al. (1990).This was based on the observations and descriptions of many cores, which are located in different points of the Maracaibo Basin, and many examples for TOC analysis (2 -5.6%) and Tmax (temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis at the top of S2 peak: 434 -441°C).Alberdi-Genolet and Tocco (1999) proposed the Machiques Member as a "possible source rock" based on geochemical analysis performed on an outcrop in Perijá, Venezuela.With the increasing of depth and the rising in the organic matter content at the top of Tibú Member, a rapid transgression occurred during early Aptian, characterized by an oceanic anoxic event (OAE 1a).From the Cogollo Group platform margins, the transgression resulted in the sedimentation of a pelagic facies that covered most of the basin, which is present at Rosario, Aricuaisa, Machiques Alturitas, San Jose, Alpuf, La Villa, La Paz, Urdaneta West fields, among others (Méndez 2009).The designation of pelagic to this sedimentation does not necessarily indicate a great water depth, since the anoxic event source was determined by physicochemical characteristics of the waters transgression, being oxygenated at the surface and anoxic at the bottom, and these conditions were achieved at only few tens of meters deep.The Machiques Member in some sectors of the fields mentioned above it may reach 100 -120 feet thick.In Perijá region, concretions contain abundant macrofossils, among them fish, molluscs, Inoceramus and Douvilliceras, Drufencia, Deshayesites, ammonites.After petrography studies of the thin sections, undeveloped pelagic foraminifera were identified (LEV 1997).Sutton (1946) describes the following ammonites: Deshayesites colombianus, Chelonieras cornuelianum, and Parahoplites inconstants.Renz (1977) reports Engonoceras genus ammonites at the top.
The top of Machiques is characterized by a massive dolomite facies, which appears on a regional scale to the north and west of the platform and gradually decreases towards the Lake of Maracaibo and the Andes.This dolomite facies presents, in some wells, intercrystalline and vugs porosity, and fractures on the top.These features indicate a rapid marine regression (LSW) that exposed the upper sediments deposited on the platform, which favored an intense evaporation and dolomitization processes.The black shale of Machiques indicates, by its stratigraphic position, a favorable migration to overlying porosity facies at the Piché Member.

Piché Member
When the Piché Member sedimentation started, all the carbonate system was represented by a shelf bordered by a margin (packstone and grainstone of bioclasts, oolite and isolated coral and rudist colonies) and not by a barrier reef.The inner shelf is represented by facies and environments we know today (lagoon, banks, cost line).The Piché Member represents shallow facies and several cycles of lagoon facies (TST) at the base with mudstone and wackestone mainly of green algae and peloids, and packstone and grainstone in banks and bars at the top of the sequence (HST), with progradation in the inner platform.
The facies are composed of fragments of bioclasts (green algae, pelecypods, gastropods, benthic foraminifera, echinoderms, bryozoans), and peloids, ooids, among others, and they formed a series of sequences of small marine transgressions and regressions that partially affected the platform.The Piché Member is represented by several depositional textures, among which mudstone, wackestone, packstone and grainstone are included.These facies indicate banks and bars of calcareous sediments at shallow depths, formed by currents and tidal systems.All of these facies and depositional environments were originated at shallow depths and the most important packstone and grainstone textures were formed during the progradation on the platform.
In the upper zone of Piché Member, the benthic foraminifera Orbitolina is found, which is considered as a biozone and its stratigraphic position is the same for the entire basin.At the top of Piché, organic matter-rich pelagic facies was deposited during a mayor transgression as a consequence of an oceanic anoxic event, corresponding to OAE 1b.These TOC rich sediments generated the oil stored in the middle and near the top of the Apón formation reservoirs.The Piché Member has similar thickness throughout the basin, about 400 feet.This indicates the subsidence along the platform did not vary.

Lisure Formation
The Lisure Formation presents a very similar thickness throughout the basin.The thickness is slightly larger to the east and north.This fact indicates a balance between subsidence and sedimentation rate, but with a small increasing of sedimentation mainly in packstone and grainstone of oolite and bioclasts facies.The deposition of this formation is typical of a progradational facies with the development of banks at shallow depths and prograding sedimentation toward the basin.At the base of Lisure, in the southern area of Lake of Maracaibo, there are two levels of quartz sandstones.These levels are separated by a limestone interval.This indicates two terrigenous events which probably represent a mouth bar sedimentation environment that belongs to Aguardiente Formation, but it is considered an interdigitation of carbonates and clastics that belongs to the base of Lisure (Azpiritxaga 1991).The most representative Lisure Formation limestone facies were developed as bioclasts bars, oolites, pellets and oncolites, among which we can find gap facies of green algae, benthic foraminifera (mainly miliolids) and partial dolomitization events of wackestone, packstone and occasionally grainstone on these platform and lagoon facies.
The Lisure Formation represents a transgressive facies sequences at the base and high levels at the top, with an active sedimentation at very shallow depths.Facies are mainly transgressive and they typically have large amounts of glauconite, which indicates a decrease in overall sedimentation and a partially reducing environment.The facies of oolites, intraclasts and bioclasts are regional and they indicate interdigitated bars mobilized by currents and tides.The oolites of Lisure Formation do not indicate the presence of a margin or close to it, as it is usually indicated in some facies distribution schemes (Wilson 1975).By the time of sedimentation, Lisure developed high levels of intergranular porosity in grainstone facies with oolites and bioclasts.However, the presence of sparite precipitated in pore spaces into many Lisure Formation facies indicates a high rainfall climate in the basin and in the area of terrigenous sediments sources.Sparite formed due to diagenesis in meteoric groundwater and freshwater environments destroyed much of the porosity.At the Urdaneta West field, the Lisure formation has a thickness that ranges from approximately 370 -380 feet.

Maraca Formation
This formation marks the end filling carbonate platform, with extensive regional facies found throughout the basin, which were deposited in shallow depths, and probably never exceeded 3 or 4 feet deep.The Maraca Formation has a thickness that ranges from approximately 50 -60 feet all over the basin, indicating a gradational system with progradation at the top of the sequence.At the top of the Maraca Formation, the facies are characterized by large colonies of pelecypods and often rudist mounds are present in the platform.Biostromes that prograded developed regionally at high sedimentation levels.The biostromes were exposed to subaerial diagenesis during a marine regional regression and moldic and vug porosities were developed, providing a level of bivalves with packstone porosities above, that characterizes this formation reservoir.At the top of Maraca Formation, the foraminifera biota is planktonic with pelecypods, indicating the first transgressive developments that eventually gave rise to La Luna Formation.The organisms' content of the facies at the bottom is more heterogeneous, as well as inorganic grains and authigenic minerals.The depositional textures are mainly packstone of bioclast and wackestone generally of ooids, intraclasts and bioclasts with varying proportions of glauconite, pyrite and quartz fragments.

OCEANIC ANOXIC EVENTS
During the Phanerozoic, changes in sea level developed episodes with significant accumulations of organic carbon-rich sediments or source rocks.Hydrocarbon generators sediments are related to rising sea levels and marine transgressions, while coal deposits occurred during low sea levels and marine regressions.During the Middle and Late Cretaceous, globally transgressive events (Fig. 6) coupled with an increase in the organic matter productivity and the diversity of pelagic organisms led to the deposition of sedimentary facies, both carbonate and clastic, organic-rich, which represent the main source rocks worldwide.These events are characterized by a large burial of organic carbon during the Cretaceous and were the cause of isochronous cyclic perturbations of the global carbon cycle, which are known in the literature as the "oceanic anoxic events" (Schlanger & Jenkyns 1976;Baron et al. (1981); Jenkyns 1980Jenkyns , 2010)).The causes that produced these oceanic anoxic events in a global scale were derived from a combination of several factors (Schlanger & Jenkyns 1976;Baron et al. (1981); Jenkyns 1980Jenkyns , 2003Jenkyns , 2010;;Arthur et al. 1990;Bralower et al. 1994;Wignall 1994):  (C) Increase and rapid sea-level transgression on the shallow shelf; increase in the oxygen minimum zone in the basin and the platform margin; transfer of oxygen minimum zone and anoxic waters to the platform by the marine transgression; upwelling currents influence in the margins of the platform, whose wealth of nutrients was propitious for the development of the planktonic biota, contributing to anoxia at bottom waters.Thus, during the Cretaceous, several marine transgressions were able to move the anoxic environments to shallow shelf, causing various organic matter-rich sediment levels.■ Marine transgressions that covered large areas of the continental platforms, causing epeiric or epicontinental seas.■ Greater emission of basalts in ocean ridges and the formation of Large Igneous Provinces (LIP) in various parts of the world that increased the CO 2 concentration in the marine environment and the atmosphere.■ High productivity of organic matter derived from phytoplankton and zooplankton and organic matter developed in large coastal areas (deltas, lagoons), which were incorporated into the platforms and epeiric seas.■ Development of anoxic conditions at the bottom marine waters of the deep basin and the platform due to the consumption of dissolved oxygen in the water column as a consequence of a high primary productivity, which led to the deposition and preservation of large amounts of organic matter.This led to an increase in the concentration of CO 2 in marine environments.■ Expansion of oxygen minimum zone, poor circulation and vertical mixing of the water due to the stratification of the same, based on differences in density of water bodies according to depth.■ Influence of upwelling currents, which locally transported nutrients and led to an increase in the productivity of organic matter and the expansion of the oxygen minimum zone.■ Stagnation of water in enclosed marine basins.

PELAGIC LIMESTONE IN COGOLLO GROUP
During sedimentation Cogollo Group, which represents the facies situated only behind a platform margin, the inner shelf was subject to the normal cycles of transgressions and regressions.As the platform became shallower, a relatively small transgression partially or completely flooded this platform.Thus, the Cogollo Group carbonates are an interdigitation of transgressive and regressive stratigraphic sequences.The transgressive facies, as part of the petroleum system, act as seals, or in some cases, as source rock for porosity facies which were originated and preserved during the regressive events.
In front of the margin platform (slope and basin) an anoxic environment was developed in the background and it had periods of alternate anoxia-dysoxia events with higher and lower intensity.
During some major transgressions, the anoxic environment and characteristics facies of a pelagic environment moved from the slope to the platform, whereby organic matter was developed and preserved on an essentially shallow sedimentary system, interspersed between facies that indicate high sedimentation levels that in some cases developed porosity.Thus, the Cogollo Group platform was able to develop "source rock" levels that generated oil volumes in addition to those generated by La Luna Formation, with relatively small migrations to porosity facies at the same platform (Méndez 1989c(Méndez , 2009)).
These ideas have also been explored in the Persian Gulf, at the platforms of the Middle Cretaceous carbonates (Khatiyah and Mishrif formations), where it is estimated that a high organic matter content in mudstone and wackestone facies of the inner shelf act as the source rock and adjacent porosity level act as reservoir facies (Farzadi 2006).Therefore, the generation and storage of hydrocarbons occurred in adjacent facies.
Porosity zones and oil reservoirs in the Cogollo Group have three different source rocks originated during three oceanic anoxic events displaced from the margin to the inner platform: 1. the source rock of Machiques Member was deposited during the OAE 1a of the early Aptian (~ 120 Ma).Probably this source rock has generated the oil stored in Río Negro Formation reservoirs (Barremian sandstones and conglomerates underlying the Apón Formation) and Piché Member reservoirs, together with those arising from the OAE 1b (~ 113 Ma).It is located at the western areas of the basin, with a decreasing thickness in the area of Lake of Maracaibo; 2. the source rock of the Piché Member is located at the top of this member of the Apón Formation and was deposited during the OAE 1b (late Aptian, ~ 113 Ma).
Probably the oil stored in the Piché porosity facies was generated by this source rock.3. the La Luna Formation, which is deposited from the Cenomanian and is the most important source rock in the Maracaibo Basin, corresponds to OAE 2 (~ 93 Ma).
The reservoirs in the Maraca Formation porosity facies throughout the basin contain the oil generated by the La Luna Formation.

Pelagic limestone in Apón Formation
The Apón Formation has two important pelagic limestones facies (black shale), representing two "effective source rocks".The first, known as the Machiques Member, was deposited during early Aptian OAE 1a.The second one is virtually unknown, but reported as possible "source rock" and corresponds to OAE 1b (Méndez et al. 2008).

Pelagic limestone in Machiques Member -oceanic anoxic event 1a
The Machiques Member is an example of a pelagic limestone.It was deposited on a shallow shelf at the top of Tibú Member, displacing anoxic features from the outer edge of the platform and the basin to the inner shelf by a marine transgression.The Machiques Member is well represented in the western part of the basin, mainly in the Alpuf, Totumo, San Jose, Alturitas, San Julián, Machiques, La Paz, Urdaneta Oeste fields, among others.
In some of these areas, it may be up to 100 or more feet thick.Towards the middle and near the top, the facies are purely pelagic limestones with planktonic foraminifera and, especially toward the top, dolomite crystals are disseminated in the limestone matrix (Fig. 7).Therefore, the Machiques Member represents an oceanic anoxic event that was derived from a marine transgression that led to the deposition of a "source rock" that overlies the shallow shelf carbonates facies.This event corresponds to OAE 1a (Aptian, ~ 120 Ma).
The top of Machiques Member indicates a marine regression that affected most of the basin, mainly in the western areas.The marine regression affected the uppermost platform sediments and it caused a shallow sedimentary environment at the top of the pelagic limestones, increasing the evaporation rate and the Mg/Ca ratio, promoting regional dolomitization.The importance of the pelagic limestones and the Machiques Member "source rock" is the presence of an oceanic anoxic event on the shallow platform itself, which generated the hydrocarbons that migrated easily after its generation to these dolomite facies with intercrystalline porosity and other facies located in the Piché Member.

Pelagic limestone in Piché Memberoceanic anoxic event 1b
The Piché Member of the Apón Formation is a classic example of transgressive and regressive stratigraphic sequences.Some of these are regressive cycles in which moldic porosity and vugs were developed.These cycles are located between transgressive sequences that act as seals for porosity zones.On the top of Piché Member, there are two levels of pelagic limestone that indicate, as in the case of Machiques, an oceanic anoxic event which was displaced toward the internal platform by marine transgressions (Fig. 7).
These levels of high TOC values have generated the hydrocarbons stored into adjacent facies with porosity.Higher thicknesses are found on the top of the member, where the pelagic facies may submit up to 60 feet thick and lay between shallower levels of porosity facies that are often present.It is important to consider that the top of the Piché Member has the best Apón Formation reservoirs.The presence of pelagic limestones, interdigitated between porosity zones, suggests that Piché Member oil reservoirs were not necessarily generated by La Luna Formation, and a more logical view indicates that they were generated by a source rock near or adjacent to the reservoirs facies.We consider that this anoxic level of the late Aptian belongs to the OAE 1b (~ 113 Ma).
The black shale term has been used in the literature for a wide variety of rocks, usually without a specific definition (Wignall 1994).Therefore, the use of this term is descriptive and does not include strictly the mineralogical composition of the rock and its petrological classification.Tyson (1987) defined black shales as a muddy fine-grained, dark colored rock which presents sedimentological, paleoecological and geochemical characteristics associated with sedimentation in anoxic environments.In this paper, the authors consider an organic matter-rich facies as black shale, regardless of the ratio between the content of carbonate and clastic sediments.

ORGANIC GEOCHEMISTRY RESULTS
Sampling covered the stratigraphic intervals of interest in six cores of six wells located at the Maracaibo Basin, and this was done based on the following criteria: ■ Visual estimates as the colour of the rock, lamination degree, grain size and absence of bioturbation.■ Correlation between the depth of the rock intervals of interest in both electric logs and cores (Fig. 8).
A total of 254 samples were taken, that corresponds to the stratigraphic intervals of OAEs 1a (~ 120 Ma, Machiques Member) and 1b (~ 113 Ma, Piché Member).All samples were administered through the Rock-Eval 6 pyrolysis technique, in order to obtain the values of: TOC (% weight), S1 (mg HC/g rock; free hydrocarbons), S2 (mg HC/g rock; hydrocarbons generated from the thermal cracking of kerogen), S3 (mg HC/g rock; CO 2 generated during pyrolysis kerogen), and T Max (maximum temperature reached during the peak of S2; Celsius degrees).Some of the results are shown in Tabs. 1 to 3.
The percentage of TOC average for most the studied intervals is greater than 2% and maximum values that are greater than 7%, values for defining these intervals as a "very good" (C) Laminated mudstone with isolated dolomite crystals and fragments of planktonic foraminifera (OAE 1a).(D) Similar to previous but with major planktonic foraminifera development (OAE 1a).(E, to G) Facies with isolated dolomite crystals and bioclasts fragments, kerogen and/or bitumen are oriented according to the rock lamination (OAE 1b).(H) Similar to the previous but more developed planktonic foraminifera (OAE 1b).source rocks as to the amount of organic matter posing.S2 values are between 5 and 20 mg HC/g rock, representing source rocks values "good" to "very good".The maximum temperature (T Max) reached at the peak of generation S2 lies on an average between 435 and 445°C, which implies that these rock intervals reached the temperature required to cause the thermal cracking of the kerogen and form the bitumen (Tabs. 1 to 3).This allows to state that the source rock levels proposed in Machiques and Piché Members of Apón Formation should be considered as "effective source rocks" as they have generated and expelled hydrocarbons (Hunt 1995;Peters et al. 2005a).
For a total of 42 samples, it was performed a visual kerogen assessment, which was composed of the following studies (Figs. 9 to 16): ■ Determination of the proportions of macerals in the samples, with a predominance of liptinite content (macerals abundant in source rocks composed of type II kerogen and oil generators) of about 80% and the identification of solid bitumen, which indicates in situ hydrocarbon generation and primary migration processes.■ Thermal alteration index (TAI), using a transmitted light microscopy, with results that, according to the colour scale used in this technique, yielded dark colours for the organic matter; that is indicative of a thermal maturity according to the generation of hydrocarbons.■ Vitrinite reflectance (%Ro), with average values between 0.6 and 1.1, that places most consistent samples within the thermal maturity hydrocarbon generation window.It can be seen that the greatest thickness in the Apón Formation is related to the Tibú Member and this occurs in the western areas, which may be associated with a subsidence increase during the deposition, while thicknesses of Piché Member and Lisure and Maraca formations are relatively constant, indicating similar subsidence for the basin.It was also observed that the OAE 1a was deposited in one period of time on the platform, while the OAE 1b can be divided into two levels of black shale facies, separated by a shallow carbonate facies with low organic matter content.Depth is presented in feet.
■ Fluorescence liptinites, macerals that within the studied samples showed a low fluorescence intensity and indicative of thermal maturity commensurate with the oil generation window.In turn, the identification of organic material of amorphous structure in most samples under ultraviolet excitation of blue light is more indicative of the presence of organic material suited for generating oil (Hunt 1995;Peters et al. 2005a).
Visual kerogen assessments complement chemical assessments by recording information from the discrete particles (macerals) that make up the sedimentary organic matter.Vitrinite macerals are particles of sedimentary organic matter derived

Soxhlet extraction
This technique was performed on a set of 11 core samples for the purposes of quantifying the amount of extractable organic matter (EOM or bitumen).The amount of bitumen obtained for each of the analyzed samples is greater than 2,000 ppm (very good), and in 7 samples it is greater than 4,000 ppm (excellent) (Peters et al. 2005a).Some of the results are shown in Tab. 4.

CONCLUSIONS
The Cogollo Group in the Maracaibo Basin (Venezuela) is the largest system of shallow shelf carbonates deposited during the Aptian and Albian in north-western South America.It consists of the Apón Formation, with Tibú Machiques and Piché members, the Lisure Formation and Maraca Formation.This carbonate system underlies the La Luna Formation, which is one of the most important world source rocks deposited from Cenomanian to Campanian.
The calcareous sedimentation begins with the sea level rise in the late Barremian and early Aptian above the conglomerates and sandstones of the Río Negro Formation.In a similar way, most of the carbonate systems began as a carbonate ramp, which evolved to a platform system, gaining stability between regional subsidence and sedimentation.
The Cogollo Group has a wide variety of lithofacies, biofacies and porosity levels, and generally presents regional extension.Some of these facies present porosity and represent oil reservoirs in the Apón and Maraca formations, which have the largest carbonate reservoirs along the basin.
In reference to anoxic events with pelagic limestone sedimentation deposited in the Cogollo Group platform and prior to the massive event that gave rise to the La Luna Formation, we can consider the following: ■ During the sedimentation of the Cogollo Group carbonates, some mayor transgressions were able to move the physicochemical and sedimentological characteristics to the inner shelf and the slope from outer margin and caused the deposition of anoxic pelagic and hemipelagic limestones with high organic matter content.■ The Machiques Member represents a regional event, characterized by a massive OAE with carbonate pelagic sedimentation on the shallow inner shelf.The thickness of this high TOC level is 80 feet or more in some western areas of the basin.■ Towards the top of the Piché Member, there is another high TOC level shifted to the inner shelf and divided by two intervals, the main of which is located closer to the top of the Piché Member and had a longer duration, with thicknesses of over 60 feet, on a regional basis, and it is located between hydrocarbon producing intervals.
According to this, there were two major OAEs derived from marine transgressions on the platform as well as other smaller events with similar characteristics.Major events correspond to Machiques Member (OAE 1a) and the top of Piché Member (OAE 1b).
According to sedimentological, petrographical and organic geochemistry results, the organic matter-rich facies in Machiques (OAE 1a) and Piché (OAE 1b) members should be considered as "effective source rocks".Rock-Eval pyrolysis, visual kerogen assessment, soxhlet extraction, gas chromatography mass spectrometry saturated and aromatic biomarkers results indicate that the thermal maturity of the kerogen in the analyzed samples is in the range between early window and peak oil window generation.

Figure 1
Figure 1.(A) Geographical position of the Maracaibo basin with respect to Venezuela and South America.(B)The deep of the Maracaibo Basin in the subsurface is ranged between 6,000 and 18,000 feet and is extended over the Zulia state and part of the Gulf of Venezuela.The most important outcrops with large thicknesses of Lower Cretaceous are in Perijá.In the Andes of Venezuela, the thicknesses are smaller but the changes in facies of ancient shorelines of the platform can be observed.

Figure 2 .
Figure 2. Position of different grabens formed during the early Jurassic as part of tectonic extension and breakup of Pangaea.This graben system and its sedimentary filling underlie the Cogollo Group (LEV 1997).

Figure 3 .
Figure3.Areas of carbonate platforms during the early Aptian (in green).In Venezuela, these carbonate systems were developed in the Maracaibo and the Orient basins.Part of this system is also found in eastern Colombia in outcrops and the subsurface. et

Figure 4 .
Figure 4. (A) Idealized model for the platform of the Cogollo Group.(B) Stratigraphic chart for the Cretaceous Period in the Maracaibo Basin.(C) Stratigraphic position of the formations, members and OAEs 1a and 1b in the Cogollo Group in a gamma ray well log (well TOT 3).(D) Alignment of several wells (red dots) in the Maracaibo basin, following the pattern of faulting at the large faults systems, looking for porous stratigraphic intervals developed by fractures.

Figure 6 .
Figure 6.Diagram that illustrates the sedimentation of TOC-rich facies on the shallow shelf after a transgressive event due to the expansion of the oxygen minimum zone.(A) Normal oxygenation of the ocean waters in the basin, slope and shallow shelf.(B) Beginning of the expansion of the oxygen minimum zone in the basin and the slope due to the increase in the organic matter productivity and normal oxygenation in the shallow shelf.(C)Increase and rapid sea-level transgression on the shallow shelf; increase in the oxygen minimum zone in the basin and the platform margin; transfer of oxygen minimum zone and anoxic waters to the platform by the marine transgression; upwelling currents influence in the margins of the platform, whose wealth of nutrients was propitious for the development of the planktonic biota, contributing to anoxia at bottom waters.Thus, during the Cretaceous, several marine transgressions were able to move the anoxic environments to shallow shelf, causing various organic matter-rich sediment levels.

Figure 7 .
Figure 7. (A) Contact between the rich TOC mudstone facies and dolomite facies on top of the Machiques Member (OAE 1a).(B) Partially phosphatized mudstone with kerogen and bitumen accumulated in pore spaces (OAE 1a).(C)Laminated mudstone with isolated dolomite crystals and fragments of planktonic foraminifera (OAE 1a).(D) Similar to previous but with major planktonic foraminifera development(OAE 1a).(E, to G) Facies with isolated dolomite crystals and bioclasts fragments, kerogen and/or bitumen are oriented according to the rock lamination (OAE 1b).(H) Similar to the previous but more developed planktonic foraminifera (OAE 1b).

Figure 8 .
Figure8.Correlation of wells with electric logs of gamma ray in western Maracaibo basin.The stratigraphic and sedimentological features observed in the cores are extrapolated to records which allows for an effective correlation throughout the basin.It can be seen that the greatest thickness in the Apón Formation is related to the Tibú Member and this occurs in the western areas, which may be associated with a subsidence increase during the deposition, while thicknesses of Piché Member and Lisure and Maraca formations are relatively constant, indicating similar subsidence for the basin.It was also observed that the OAE 1a was deposited in one period of time on the platform, while the OAE 1b can be divided into two levels of black shale facies, separated by a shallow carbonate facies with low organic matter content.Depth is presented in feet.

Table 1 .
ALPUF 6 core samples.Total organic carbon, Rock-Eval pyrolysis, hydrogen index and vitrinite reflectance results.: total organic carbon; HI: hydrogen index; Tmax: temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis at the top of S2 peak; %Ro: vitrinite reflectance; WR: whole rock. TOC

Table 2 .
Z26 D2 core samples.Total organic carbon, Rock-Eval pyrolysis, hydrogen index and vitrinite reflectance results.: total organic carbon; HI: hydrogen index; Tmax: temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis at the top of S2 peak; %Ro: vitrinite reflectance; WR: whole rock. TOC
TOC: total organic carbon; HI: hydrogen index; Tmax: temperature at which the maximum release of hydrocarbons from cracking of kerogen occurs during pyrolysis at the top of S2 peak; %Ro: vitrinite reflectance; WR: whole rock.