Organic Facies Research Articles

Biomarker transformations as constraints for the depositional environment and for maximum temperatures during burial of Opalinus Clay and Posidonia Shale in northern Switzerland

Abstract Samples from two argillaceous formations (Opalinus Clay and Posidonia Shale) of near-identical maturity from northern Switzerland were subjected to a geochemical characterisation of organic matter and to confined-system pyrolysis experiments. Throughout the study area, the characteristics of organic matter are similar, indicating a spatially homogeneous sedimentary facies. Posidonia Shale contains marine organic matter deposited in a reducing environment, while a predominantly terrigenous source and a more oxidising environment of deposition was identified for Opalinus Clay. In the western and central parts of the study area, organic maturity is close to the onset of oil generation. In the easternmost part, a higher maturity has been reached due to a deeper burial below thick Tertiary Molasse deposits. Isothermal pyrolysis experiments were conducted at temperatures between 250 and 390 °C over 24 h. Bitumen yields increase along similar pathways for both Opalinus Clay and Posidonia Shale, but the maximum values are displaced by 10–20 °C. Data pertaining to maturity were determined from GC–MS analyses of saturated hydrocarbons, and specific attention was given to C 29 -sterane and C 32 -hopane isomerisation ratios. The evolution of these parameters with rising temperature is slightly different in the two formations, which is attributed to the contrasting organic facies. The pyrolysis data, together with literature data from natural basins, were used to calculate kinetic parameters for C 29 -sterane and C 32 -hopane, assuming a single-step isomerisation scheme according to the Arrhenius law. The resulting values based on pyrolysis data alone are very similar to those based on the combination of pyrolysis and natural data. Activation energies are similar in both formations, while the frequency factors are up to one order of magnitude higher for Posidonia Shale when compared to Opalinus Clay. For the Benken site, maximum temperature during Cretaceous burial was calculated on the basis of the kinetic data, using the TTI approach. The resulting temperatures of 75–80 °C are 5–10 °C below those derived in the literature from apatite fission-track analysis, vitrinite reflectance and basin modelling.

Controls on alkylphenol occurrence and distribution in oils from lacustrine rift basins in East China

Oils from two lacustrine rift basins in east China are thoroughly investigated using geochemical method to understand controls on alkylphenol occurrence and distribution in oils. Oils in the Lujiapu Depression, Kailu Basin are derived from the Cretaceous source rocks, and those in the Dongying Depression, Bohai Bay Basin, from the Tertiary source rocks. All oils are experienced relatively short distance of migration and have similar maturity in each basin. Differences in homologue distributions from different oilfields are most likely caused by organic facies variation of source rocks. The oils in the Lujiapu Depression are characterized by high proportion of C3 alkylphenols (prefixes refer to the number of alkylcarbons joined to the aromatic ring of the phenol molecule) and low proportion of cresols and C2 alkylphenols compared to oils from the Dongying Depression. Alkylphenol isomer distribution is possibly affected by depositional environment especially for C3 alkylphenols. Dysoxic freshwater environment is favorable for the formation of propyl or isopropyl substituted C3 alkylphenols, while highly reducing saline water is more suitable for trimethyl substituted C3 alkylphenols. Variations in alkylphenol concentrations within a petroleum system are controlled mainly by secondary migration processes with alkylphenol concentrations decreasing along migration direction. Interestingly, coupled with geological factors, a subtle change of alkylphenol concentrations can be applied to differentiate carrier systems. When oil migrates through sandy beds, concentrations of total alkylphenols decrease dramatically with migration distance, while such change is less significant when oil migrates vertically along faults. However, most isomer ratios potentially related to migration distance are not as effective as those alkylcarbazoles in migration diagnosis due to complicated affecting factors.

Fácies orgânica: conceitos, métodos e estudos de casos na indústria do petróleo

The concept of organic facies, as well as the definitions and means of the different facies became a very important tool to petroleum exploration. The application of this concept is the best way to integrate microscopy and geochemical techniques to study kerogen contained in sedimentary rocks. Thus, palynofacies analysis and bulk geochemical methods are used to characterize the total particulate organic matter. Palynofacies analysis involves the integrated study of all aspects of the kerogen assemblage: identification of the individual particulate components, assessment of their absolute and relative proportions and preservation states. The correlation between palynofacies and geochemical data provides the organic facies models that point out the depositional environmental conditions and hydrocarbon source rock potential.

Fácies orgânica: conceitos, métodos e estudos de casos na indústria do petróleo

The concept of organic facies, as well as the definitions and means of the different facies became a very important tool to petroleum exploration. The application of this concept is the best way to integrate microscopy and geochemical techniques to study kerogen contained in sedimentary rocks. Thus, palynofacies analysis and bulk geochemical methods are used to characterize the total particulate organic matter. Palynofacies analysis involves the integrated study of all aspects of the kerogen assemblage: identification of the individual particulate components, assessment of their absolute and relative proportions and preservation states. The correlation between palynofacies and geochemical data provides the organic facies models that point out the depositional environmental conditions and hydrocarbon source rock potential.

Palynofacies patterns, acritarch diversity and stable isotope signatures in the Lower Muschelkalk (Middle Triassic) of N Switzerland: Evidence of third-order cyclicity

The Lower Muschelkalk of N Switzerland is characterised by a relatively monotonous succession of marly and dolomitic limestones with siltstone layers and thin sandstone beds. The studied Leuggern, Weiach and Benken wells comprise completely cored sections of the Lower Muschelkalk (Anisian). Due to the lack of characteristic marker beds and vertical facies changes, cyclic interpretation based on sedimentary features proves to be difficult. Therefore, palynofacies patterns, acritarch diversity and stable isotope signatures are used to detect eustatic signals related to relative sea-level changes. The major flooding phase is recognised in the Middle Wellenmergel (Pelsonian). It is characterised by maximum abundance of marine plankton and the highest amount of refractory opaque phytoclasts. Transgressive deposits are identified by a significant increase of marine constituents and a relatively high number of translucent woody fragments of the phytoclast group. Highstand deposits show a high terrigenous input of blade-shaped, opaque plant debris. Palynofacies of late highstand deposits is dominated by bisaccate pollen grains. This sequence stratigraphic break down allows correlation of the Swiss wells and their correlation with Anisian deposits of other parts of the Germanic Basin. Acritarch diversity is analysed by a morphometric study on vesicle size, process length, process bifurcations, and the number of processes. These parameters correlate with the eustatic evolution during Anisian times as interpreted from palynofacies data. Vesicle size, process length and the percentage of specimens with bifurcated processes are seen to increase during transgression and intervals of maximum flooding. They are decreasing within highstand phases. The number of processes shows the opposite trend and is high during the late highstand. Stable isotope curves both of δ13C and δ18O point out two intervals of maximum flooding. These geochemical signatures correlate with the palynological data. The example from the Middle Triassic of N Switzerland highlights the value of using organic facies and plankton associations in an integrated approach for reconstructing the eustatic history of an epeiric sea characterised by monotonous depositional series lacking significant sedimentary features.

GEOCHEMICAL CHARACTERIZATION OF POTENTIAL JURASSIC/CRETACEOUS SOURCE ROCKS IN THE SHUSHAN BASIN, NORTHERN WESTERN DESERT, EGYPT

Some 180 core and cuttings samples of shales and limestones from the Middle Jurassic – Late Cretaceous succession (Khatatba, Masajid, Alam El‐Bueib, Alamein, Kharita, Bahariya and Abu Roash Formations) were collected from wells Ja 27–2, Tarek‐1 and Jb 26–1 in the central, structurally‐low part of the Shushan Basin and from well Lotus‐1 in the structurally‐elevated western part of the basin. All samples were screened for total organic carbon (TOC) content. Selected samples were then analyzed by Rock‐Eval pyrolysis, and extracted for biomarker analyses. Visual kerogen analysis and vitrinite reflectance measurements were also undertaken and oil ‐ source rock correlations were attempted. The results indicate that the thermal maturity of the samples can be correlated closely with burial depth. Samples from the central part of the basin are more mature than those from the west. Samples from the central part of the basin (except those from the Albian Kharita Formation) have reached thermal maturities sufficient to generate and expel crude oils. Extracts from the Middle Jurasic Khatatba and Early Cretaceous Alam El‐Bueib Formations can be correlated with a crude oil sample from well Ja 27–2.In well Lotus‐1 in the west of the basin, four distinct organic facies can be recognized in the Jurassic‐Cretaceous interval. One of the facies (“facies 4”) has a sufficiently high TOC content to act as a source rock. Thermal maturities range from immature to peak oil generation, and the top of the oil window occurs at approximately 8000 ft.

Source Rock Characterization of the Tertiary Units in Havsa-Edirne Area: Thrace Basin/Turkey

Oligocene Osmancik formation, upper Eocene-lower Oligocene Mezardere formation, and Eocene Hamitabat, Soğucak, and Ceylan formations encountered in the Habiller-2 and Havsa-1 wells drilled in the Thrace basin are composed of shale, limestone, tuff, and sandstone. On the basis of organic geochemical and organic facies observations, organic components of the Oligocene Osmancik formation, upper Eocene-lower Oligocene Mezardere formation, and Eocene Hamitabat, Soğucak, and Ceylan formations are generally herbaceous, algae/amorphous material (Type II and Type III kerogens). In the Habiller-2 well, organic material contents of Oligocene Osmancik formation, upper Eocene-lower Oligocene Mezardere formation, and Eocene Hamitabat, Soğucak, and Ceylan formations are between 0.02 and 1.38 wt% and that of the upper Eocene-lower Oligocene Mezardere formation in the Havza-1 well ranges from 0.31 to 1.06 wt%. On the basis of these values, Eocene-Oligocene units in both wells show weak to good source rock potentials. According to Tmax, vitrinite reflection and Spore Color Index evaluations, Eocene-Oligocene units in the Habiller-2 and Havsa-1 wells are at immature and mature oil zones.

Fácies orgânica da Formação La Luna, Bacia do Valle Superior Del Magdalena, Colômbia: implicações para a geração de petróleo e caracterização paleoambiental

Upper Cretaceous pelitic rocks generate most of the oil reserves in Colombia. In Del Magdalena Upper Valley (MUV), the existence of good quality outcrops and thermal evolution levels that vary from immaturity to the end of the oil generation "window" makes it one of the most appropriate regions for the study of primary migration and generation of oil in Upper Cretaceous. The main objectives of this work were to study the organic facies of La Luna Formation in an outcrop section called Riacho Bambuca (Bambuca Creek), determine the oil generation potential and characterize the paleoenvironment. For this research, 24 samples were used in palynofacies analysis (characterization of the kerogen components assembly) and in organic geochemistry analysis (Total Organic Carbon - TOC analysis and Rock-Eval pyrolysis, provided by Colombia Oil Institute - ECOPETROL). The palynofacies analysis included the use of microscopy techniques (transmitted white light and incident blue light). Furthermore, a Spore Coloration Index (SCI) analysis was made in order to determine the thermal maturation stage. The studied material was composed mainly of an amorphous organic substance, with its fluorescent coloration varying from yellow to bright orange. The palynomorph group is represented by spore morph and dinoflagellate cysts. The ligneous material are present in very low percentages; they are mainly opaque phytoclasts. The results of geochemical analysis showed TOC contents ranging from medium to high, excellent potential for the generation of hydrocarbons (PG >; 4 mg HC/g of rock), hydrogen index ranging from 329 to 589 mg HC/g, and low values of oxygen index, characterizing type II kerogen. The low degree of thermal maturation was determined by Spore Coloration Index values ranging from 2,5 to 3,0, Tmax values below 440ºC and Hydrocarbon Production Index inferior to 0,2. According to the organic facies data, La Luna Formation deposited in a marine environment with facies from disoxic-anoxic distal shelf and suboxic-oxic distal basin; there was kerogen of excellent quality, and its quantity was adequate for the generation of oil.

Developmental modes of the Neoproterozoic-Lower Paleozoic marine hydrocarbon source rocks in China

Based on the researches on rock type, the mode of occurrence, diagnostic minerals and creatures, the sedimentary geochemistry and organic facies of Chinese marine source rocks from wells and outcrops, and on the research findings of developmental modes of foreign marine source rock, the authors consider that it is impossible to objectively make clear the formation mechanism of hydrocarbon source rock with high organic matter abundance by either single mode of preservation or high organic matter productivity. According to the Chinese geological features, the formation mechanism of the Neoproterozoic-Lower Paleozoic marine source rock is generalized into four modes, namely, thermal water activity-upwelling flow-anoxic; carbonate gentle slope-upwelling flow; xerothermic climate-brine euxinic milieu; and humid climate-retained euxinic milieu; as the Lower Cambrian undercompensation basin organic facies (the Tarim Basin, South China and southwestern margin of North China), carbonate gentle lime mud bound organic facies (the Upper Ordovician in Tazhong region of the Tarim Basin and the Lower Silurian in the Upper Yangtze Platform), the Middle Cambrian evaporation laggon organic facies (the Tarim Basin and the Upper Yangtze Platform), enclosed undercompensation terrigenous bay organic facies (the Middle-Upper Ordovician in the west of the Tarim Basin, the Lower Silurian Long-maxi Formation in the Upper Yangtze platform). Chinese marine sedimentations with lower organic matter abundance are generalized into two modes of consumption-dilution mode of open epicontinental sea and depletion-dilution mode of supercompensation basin.

Organic facies variation from well data on the Cudi Group, the eastern part of SE Turkey

South Turkey is situated on the transition zonebetween the Arabian–African plate and Alpine–Hima-layan orogenic belt. The eastern segment of South Tur-key is located at the northern margin of the Arabianplate (Fig. 1). This is the only tectonostratigraphic unitin Turkey where significant petroleum production istaking place. The region includes rock units, ranging inage from Cambrian to Miocene Derik, and HaburGroup strata are the basement of the autochthonousrock units of Southeast (SE) Turkey (Fig. 2).The Middle Triassic–Lower Jurassic Cudi Group(Figs. 2, 3) units are composed mainly of evaporiticdeposits in the west and marine carbonates in the eastpart of SE Turkey. During the detailed geological stud-ies, the geological structures and the tectonostrati-graphic relations that have developed at different geo-logical periods of the southeastern Anatolia are rela-tively well known and have been documented innumerous publications [1–16]. Also source rock poten-tial has been reported from several sedimentary unitswhich was deposited in the western and eastern part ofSE Turkey [17–34]. By contrast, only a few studieshave reported the organic facies in the Cudi Group. Inthis study, six geochemical logs from Cilesiz-1, B.Yolacan-1, G. Dincer-1, Camurlu-101, Camurlu-102, andYolacan-3 wells, which were drilled by the TurkishPetroleum Corporation in the same area, were used.The purpose of this work is to characterize the organicgeochemistry of the Middle Triassic to the LowerJurassic Cudi Group of the eastern part of SE Turkey, todetermine their kerogen type, and to assess theirorganic facies properties.Southeast Anatolia was situated in the southernhemisphere as a part of the Gondwanaland during thePaleozoic. The Arabian Plate and its northern edge (SEAnatolia) were affected by the epirogenic movementsof Caledonian and Hercynian orogenic events [35–38].All sediments were deposited on the continental shelf

On the geobiological evaluation of hydrocarbon source rocks

Hydrocarbon source rocks are characterized by the hydrocarbon discharge, and the alteration and variation in organic compositions and organic content due to the enhanced thermal maturation. These variations throw constraints on the application of the conventional inversion evaluation of hydrocarbon potential by assessing the residual organic matter left in source rocks. Geobiology, probing the interaction between the life system and the earth system, provides new principles in deciphering the whole dynamic processes related to the organic evolution history from living biomass to organic burial. Geobiological subdisciplines, including molecular geobiology, geomicrobiology, geoecology and biogeochemistry, offer new methodology and techniques to estimate the paleoproductivity, depositional organics and organic burial capacity and their components. Geobiofacies, newly proposed herein, is terminologized to define the geobiological dynamic processes through the combination of biofacies with organic facies and sedimentary facies, and expressed by the biohabitat types, paleoproductivity, depositional and preserved organics. Geobiofacies is identified as a useful means to create the geobiological evaluation system, which in turn rectifies the conventional evaluation system for the marine source rocks.

Palynofacies analysis and sedimentary environment of Early Jurassic coastal sediments at the southern border of the Neuquén Basin, Argentina

This study shows how a multidisciplinary scope, integrating the structure of palynologic assemblages, organic facies, sedimentary facies associations, and the results of basin analysis, clarifies the complexities of environmental and stratigraphic relations. The main objectives include understanding paleoenvironmental dynamics through sedimentologic and palynofacies analysis to reveal how a pronounced geodiversity is conditioning high biodiversity within a near-coast ecotonal system. The Nestares Formation outcrops at its type locality Alicurá dam, northern Patagonia. Sedimentologic and palynofacies analysis reveals a coastal environment of the formation. Lithology and sedimentary structures indicate fluvial conditions in the lower part of the outcropping Nestares Formation with a high input of terrestrial-derived organic material, as shown by palynofacies. The middle and upper parts represent relatively quiet environments of an interdistributary paralic bay with tidal influence (flaser bedding) between deltaic channels, probably surrounded by marshes. Alternating palynofacies dominated by amorphous organic matter of algal origin suggest episodes of restricted deposition conditions in which energy levels change frequently. A marine Late Toarcian palynologic assemblage suggests that the Nestares sequence constitutes the basal stratigraphic unit of the Cuyano Group; thus, the marine transgression of the Neuquén Basin arrived at this southernmost part of the basin as late as the Late Toarcian. Together with the arrival of Callialasporites, the presence of the Parvocysta suite, with Phallocysta, Nannoceratopsis, and (?)Suessiaceae, supports a Late Toarcian age.

Source Rock Evaluation of The Eocene Shales in the Gökçesu Area (Bolu/Turkey)

Hydrocarbon potential of oil shale samples from the Gökçesu basin in Bolu, Turkey, was evaluated on the basis of organic geochemical, organic petrographic, gas chromatography, and thin-layer chromatography techniques. Organic materials were found to be oil-producing type (Type I and II kerogen). Thermal maturities, that indicate diagenetic and early mature oil formation stage, were determined with pyrolysis (Tmax), production index (PI), and spore color index (SCI) analyses. The amount of organic material ranging between 0.21 and 15.89% reveals that the unit has variable source rock potentials from weak to perfect. The productivity of the basin was probably high and organic matter preservation was enhanced by a stratified water column. As the basin began to fill with sediment and become shallower, TOC and HI values declined, as the basin water was better oxygenated and preservation conditions declined. On the basis of geologic data, organic geochemical analyses and organic petrographic studies, organic facies of Eocene shales is of AB and B types.

Temporal resolution of an oil charging history – A case study of residual oil benzocarbazoles from the Gidgealpa Field

Reservoired oil is usually a mixture of multiple discrete petroleum charges. Unravelling the contribution and character of each charge is amongst the most daring challenges in petroleum geochemistry. This paper describes an approach that involves the evaluation of both free and residual oils to reconstruct the history of petroleum accumulation within the multi-reservoir Gidgealpa Field, located in the Cooper and Eromanga Basins of central Australia. Porewall-coating residual oils form by reservoir wetting processes and grow continuously by adsorption of polar compounds, retaining molecular information on the first oil that was present in the pore system. Such sequentially recovered residual oils proved useful in reconstructing the record of oil charges to the Gidgealpa Ridge. Using Araucariacaean markers for source discrimination and the benzocarbazole a/( a + c) ratio (BCR) for identification of the source region, five discrete charging events were distinguished. In contrast to most previous studies, benzocarbazole distributions were found to be independent of lithofacies, organic facies and maturity. However, alkylated carbazoles did exhibit a pronounced facies dependency. The application of alkylcarbazoles as source markers in the study area provides the first evidence of the expulsion of petroleum from Cambrian marine source rocks in the underlying Warburton Basin.

Oil and gas geochemistry and petroleum systems of the Fort Worth Basin

Detailed biomarker and light hydrocarbon geochemistry confirm that the marine Mississippian Barnett Shale is the primary source rock for petroleum in the Fort Worth Basin, north-central Texas, although contributions from other sources are possible. Biomarker data indicate that the main oil-generating Barnett Shale facies is marine and was deposited under dysoxic, strong upwelling, normal salinity conditions. The analysis of two outcrop samples and cuttings from seven wells indicates variability in the Barnett Shale organic facies and a possibility of other oil subfamilies being present. Light hydrocarbon analyses reveal significant terrigenous-sourced condensate input to some reservoirs, resulting in terrigenous and mixed marine-terrigenous light hydrocarbon signatures for many oils. The light hydrocarbon data suggest a secondary, condensate-generating source facies containing terrigenous or mixed terrigenous-marine organic matter. This indication of a secondary source rock that is not revealed by biomarker analysis emphasizes the importance of integrating biomarker and light hydrocarbon data to define petroleum source rocks. Gases in the Fort Worth Basin are thermogenic in origin and appear to be cogenerated with oil from the Barnett Shale, although some gas may also originate by oil cracking. Isotope data indicate minor contribution of biogenic gas. Except for reservoirs in the Pennsylvanian Bend Group, which contain gases spanning the complete range of observed maturities, the gases appear to be stratigraphically segregated, younger reservoirs contain less mature gas, and older reservoirs contain more mature gas. We cannot rule out the possibility that other source units within the Fort Worth Basin, such as the Smithwick Shale, are locally important petroleum sources.

Organic facies and geochemical aspects in Neogene neritic sediments of the Takafu syncline area of central Japan: Paleoenvironmental and sedimentological reconstructions

Abstract Organic petrological observations of kerogen macerals and organic geochemical analyses of carbon isotopes of kerogen macerals and biomarkers were conducted on Neogene neritic sediments of the Takafu syncline area of central Japan. The Senmi, Sakainomiya and Lower Shigarami Formations in that area were deposited at the neritic provinces on the southern edge of the paleo‐Japan Sea during the Late Miocene to Early Pliocene. Sedimentary organic matter in these formations was almost terrigenous in origin. Changes in kerogen maceral compositions reflect sedimentological and tectonic histories evaluated in previous studies from sedimentary facies and paleontology. It was found that carbon isotope ratios (δ13C) of kerogen macerals increased from −28‰ to −25‰ from the Sakainomiya to the lower part of the Lower Shigarami Formations. The cause of that increase was presumably the expansion of C4 plants into southwest Japan. The timing was concordant with that of the expansion of C4 plant grasslands in East Asia. The oxicity (oxic to anoxic) conditions of sea bottoms evaluated from pristane/phytane ratios varied. Particularly, in the lower part of the Senmi Formation, layers in which no steroid biomarkers could be detected were found, and had presumably formed under oxic conditions when strong biodegradation had occurred. Concentrations of regular (C27–C29) steranes and dinosteranes were higher in the Sakainomiya and Lower Shigarami Formations. This indicates that dinoflagellates‐dominant primary productions were higher at those stages. In addition, concentrations of diatomaceous biomarkers such as C26 norsterane increased from the Lower Shigarami Formation, thus adding diatoms to the major producers. Furthermore, similar associations between the increases of δ13C values of kerogen macerals and concentrations of diatomaceous biomarkers were observed in the Takafu syncline area. Thus, the expansion of C4 plants was possibly associated with the high production of diatom in the shallow‐marine areas of the paleo‐Japan Sea during the Neogene Period.

The application of stable carbon and hydrogen isotopic compositions of individual n-alkanes to Paleozoic oil/source rock correlation enigmas in the Huanghua depression, China

The carbon and hydrogen isotopic compositions of individual n-alkanes from the Paleozoic source rocks, oil and oil sand in the Huanghua depression, China, were determined for oil/source correlation purposes. The sedimentary facies of the upper Paleozoic (terrigenous-marine inter-bedded) is in this paper concluded to be different from that of the lower Paleozoic (marine) in the Huanghua depression. As shown by the results of the isotope analyse, the δ 13C and δD (δ 2H) values of the individual n-alkanes of the lower Paleozoic samples range from − 29‰ to − 33‰ and − 110‰ to − 140‰, respectively, while they range from − 27‰ to − 29‰ and − 140‰ to − 170‰, respectively, for the upper Paleozoic samples. The latter is relatively enriched 13C and depleted in D ( 2H). For well Konggu 7 crude oil, the δ 13C and δD values of the individual n-alkanes are − 29‰ to − 30‰ and − 126‰ to − 140‰. However, for well Konggu 4 oil sandstone, the δ 13C and δD values of the individual n-alkanes are − 27‰ to − 29‰ and − 144‰ to − 162‰. This is irrefutable proof that the Konggu 7 oil was sourced from the lower Paleozoic source rocks, while the Konggu 4 oil originated from the upper Paleozoic source rocks. To conclude, data of δ 13C values and δD values of the individual n-alkanes may be used to assess the organic facies of source rocks and depositional environments in addition to being of inverse help for correlating oil/oil and oil/source.

Origin and alteration of oils and oil seeps from the Sinú-San Jacinto Basin, Colombia

Liquid hydrocarbons have been detected in the subsurface as well as in the surface in the Sinú-San Jacinto Basin (northwestern Colombia). The origin of the oils has not been conclusively established especially in the southern part of the basin. The most likely source rocks in the basin are the Ciénaga de Oro Fm. of the Oligocene-Early Miocene and the Cansona Fm. of the Upper Cretaceous. In this study, oil samples, seeps and source rock extracts were analyzed by GC and GC/MS, and δ 13C was determined to identify the source facies. The sulphur content and gravity data were also considered. Two organic facies were identified: one constituted by terrestrial organic matter deposited in siliciclastic sediments in marginal marine to deltaic environments and the other made up of marine organic matter deposited in marine costal shelf to pelagic environments. The oils from the former organic facies have low sulphur contents, whereas the oils from the latter facies have high sulphur levels. Correlation of the oil seeps from the former facies with the Ciénaga de Oro Fm. has not been clearly established. The oil seeps from the latter facies correlate well with the extracts from the source rocks of the Cansona Fm., deposited along the fold belt of San Jacinto (east side). The oil seeps are affected by moderate to severe biodegradation, whereas the oil from the only oil producing well in the Sinú Basin (Floresanto-6 well) has not undergone biodegradation.

From petroleum-type organofacies to hydrocarbon phase prediction

The organic facies concept states that kerogen abundance and composition are relatable to depositional settings. This link has been extended, using open-system pyrolysis methods, to include major petroleum types whose gross compositions are inherently defined by source rock kerogens and, hence, to petroleum composition, which directly controls the physical response of fluids to changing pressure and temperature conditions during secondary migration. The possibility to model petroleum composition during hydrocarbon generation as well as the phase behavior of the fluids during migration is now available in modern basin modeling software. However, whereas pyrolysis methods can accurately reconstruct hydrocarbon gas:oil ratios and liquid compositions, they are inherently incapable of correctly reproducing the gas composition of natural fluids, and because the gas composition dominantly controls the phase behavior of petroleums, multicompound compositional kinetic predictions based on pyrolysis results alone are inappropriate for the prediction of phase behavior. The new PhaseKinetics approach described here combines open- and closed-system pyrolysis techniques to characterize the compositional evolution of the fluids generated as a function of increasing thermal stress, as well as correcting experimentally determined gas compositions and, therefore, overcomes these deficiencies. It has been used to develop compositional kinetic schemes capable of predicting natural petroleum phase behavior for different organofacies types. The methodology uses a compositional description of the generated fluids similar to that used in pressure-volume-temperature (PVT) simulation. Selected studies in the Norwegian North Sea, Brazil, and Mexico demonstrate the quality of the tuned compositional predictions for different organic facies types.

Organic geochemical characteristics of the Paleocene–Eocene oil shales in the Nallıhan Region, Ankara, Turkey

Units of Paleozoic, Mesozoic and Cenozoic age are exposed in the study area which extends along the Sakarya River by the Sarýcakaya town of Eskiþehir at west and Nallýhan district of Ankara at East. Paleozoic is represented with metamorphites consisting of glaucophane schist, muscovite schist and marble alternations and granites cutting them. The Mesozoic composing of Jurassic–Cretaceous Soğukçam limestones at the northern part of the area is in tectonic contact with other units. The Tertiary is characterized by the Paleocene–Eocene Kızılçay Group which consists of the Beyköy, Çamalan and Laçin Formations. The Çamalan Formation is composed of sandstone, limestone, oil shale and locally coal bands. In the area, as a result of intense tectonic activity by the late Eocene, sedimentary sequence that compacted within the Paleozoic metamorphic rocks were folded and fractured. The Paleocene–Eocene shales have adequate organic material content to form potential source rocks. In order to determine hydrocarbon potential of the unit, various organic geochemical analysis techniques were performed. According to pyrolysis results (TOC, S 1 and S 2), the Paleocene–Eocene shales are perfect source rocks for oil. On the basis of Rock-Eval pyrolysis analyses, kerogens are Type I and Type II kerogens. Spore Color Index and T max evaluations indicate that maturity of the unit is between diagenesis and early mature oil zone and at the beginning of the oil-producing stage. Organic facies of the Paleocene–Eocene shales are A, AB and B.