Organic-rich Units Research Articles

Multiple Ba Phases in Marine Sediments elucidate environmental processes and conditions for the Holocene Mediterranean

Mediterranean sediments are known for their major shifts in sedimentary composition associated with dramatic changes in environmental and climate conditions. A typical expression for this in Holocene sediments is the occurrence of a distinctly organic-rich unit, sapropel S1, thought to be related with humid climate conditions. The S1 is accompanied by major changes in Ba content and in other elemental composition, respectively related to biological upper water column productivity and shifts in detrital sediment provenance. Here, we focus on changes in Ba and in its various phases to unravel shifts in environmental conditions around S1.Using selective extractions for sediments in cores MP37PC, MP39PC and MP50PC, respectively at 1908, 1359 and 775m water depth, we distinguish different Ba-phases: detrital Ba (Ba-det), barite Ba (Ba-bar), Mn-oxide-related Ba (Ba-ox), carbonate- and pyrite-associated Ba (Ba-carb and Ba-pyr). The predominant Ba-phase during S1 is Ba-bar (∼70%), whereas before and after S1 it is Ba-det (>90%). All other phases are mostly relatively small but have diagnostic value for environmental conditions and processes.The detrital (Ba/Al) is distinctly lower during S1 than before and after. The (Ba/Al). det variability, and enhanced values for Ba-carb and Ba-ox may have substantial influence on the traditional way to derive barite-Ba or excess-Ba content.The steep gradients in Ba-bar at onset and ending of S1and its high level during S1, clearly identify this period. In addition, and due to the exclusive nature of extraction results, we unequivocally demonstrate for the first time that there is a small gradual ‘early onset’ Ba-bar preceding the major increase in sapropel S1 formation sensu stricto.The ending of S1 formation is relatively abrupt with a well-defined reventilation event (’6 ka reventilation event’). This reventilation is associated with a rapid change in environmental conditions such as oxygenation and related remineralization of organic matter and recrystallization of carbonate. This rapid change and associated diagenetic processes are authentically recorded in Ba-carb and Ba-ox. For the period during sapropel S1 formation, the Ba-carb and Ba-ox results point to different levels of ventilation in the Eastern Mediterranean, i.e. a relatively mixed water mass down to ∼800m, a moderately mixed water mass from ∼800-to ∼1500 m, and a more stagnant water mass below 1500 m.

Marine snowstorm during the Permian–Triassic mass extinction

Abstract The Permian–Triassic mass extinction (PTME) interval is marked by major excursions in both inorganic and organic carbon (C) isotopes. Carbon cycle models predict that these trends were driven by large increases in productivity, yet organic C–rich rocks are not recorded in most PTME shelf sedimentary successions. Anomalous C-rich facies have been reported from rare abyssal plains records now exposed in Japan and New Zealand, where black shales at the PTME are extraordinarily organic-rich units. We examined organic matter at the Waiheke, New Zealand, section, and results show that these deposits are dominated by lamalginites composed of unicellar solitary or colonial phytoplankton produced during algal blooms that falls as “marine snow.” We modeled the impact of ash fall from eruptions in the Siberian Traps large igneous province and argue that they fertilized the Panthalassa Ocean with P and Fe, leading to a marine “snowstorm” and significant C drawdown marking this major biobloom during the PTME.

Deposition of cenomanian – Turonian organic-rich units on the mid-Norwegian margin: Controlling factors and regional implications

On the mid-Norwegian margin, extensive rifting and subsequent deposition of thick Cretaceous and Cenozoic sediments have buried the traditional Upper Jurassic organic-rich shales too deep. Consequently, these organic-rich shales are overmature and spent in the deep basins on the mid-Norwegian margin. The absence of well-control, variable seismic quality and in particularly, the great burial depth, makes it difficult to identify alternative Upper Jurassic and Lower Cretaceous organic-rich units. By combining high-resolution 2D seismic data, well logs, and Rock-Eval data, this study documents the presence of alternative organic-rich units in the Cretaceous succession on the Halten Terrace and the Vøring Basin. Multiple seismic horizons which correspond to regional flooding surfaces and define a series of seismic sequences have been mapped across the study area. The regionally extensive upper Cenomanian horizon is associated with wireline log signals and Rock-Eval parameters which imply the presence of a potential source rock unit. Source rock evaluation indicate that this unit contains mainly kerogen Type III on the Halten Terrace, suggesting an organofacies with significant contribution from terrestrial sources. In the Vøring Basin, the unit is sparsely drilled but appears to be mature, thus displaying a relatively limited potential. One well from the Vigrid Syncline demonstrate somewhat higher potential, with Rock-Eval data indicating a kerogen Type II composition. As such, more prolific units seems to exist in the Vøring Basin, albeit exhibiting a patchy distribution. We speculate that the deposition and preservation of this upper Cenomanian organic-rich unit record the development of an extended oxygen minimum zone attributable to increased primary production and sluggish water circulation, linked to the global Oceanic Anoxic Event 2 (OAE 2). However, local physiographic conditions, such as high sedimentation rates, erosion by gravity flows and periodically oxygenated conditions hindered preservation of a significant quantities of organic matter, thus limiting the thickness and quality of the upper Cenomanian organic-rich unit on the mid-Norwegian margin.

An eolian dust origin for clastic fines of Devono-Mississippian mudrocks of the greater North American midcontinent

ABSTRACT Upper Devonian and Lower–Middle Mississippian strata of the North American midcontinent are ubiquitously fine-grained and silt-rich, comprising both so-called shale as well as argillaceous limestone (or calcareous siltstone) that accumulated in the Laurentian epeiric sea. Although long recognized as recording marine deposition, the origin and transport of the fine-grained siliciclastic material in these units remains enigmatic because they do not connect to any proximal deltaic feeder systems. Here, we present new data on grain size, whole-rock geochemistry, mineralogy, and U-Pb detrital-zircon geochronology from units across Oklahoma; we then integrate these data with models of surface wind circulation, refined paleogeographic reconstructions, and correlations from the greater midcontinent to test the hypothesis that wind transported the siliciclastic fraction to the marine system. The exclusively very fine silt to very fine sand grain size, clear detrital origin, widespread distribution over large regions of the epeiric sea, Appalachian sources, and paleogeographic setting in the subtropical arid belt far-removed from contemporaneous deltaic feeder systems are most consistent with eolian transport of dust lofted from subaerial delta plains of the greater Appalachian orogen and incorporated into subaqueous depositional systems. Delivery of dust that was minimally chemically weathered to Devono-Mississippian epeiric seas likely provided essential nutrients that stimulated organic productivity in these commonly organic-rich units.

Tracing Lower Cretaceous organic-rich units across the SW Barents Shelf

On the Barents Shelf, the northernmost and least explored hydrocarbon province of the Norwegian Continental Shelf, Upper Jurassic organic-rich shales have traditionally been given most attention as these represent the most prolific source rock unit of the region. However, in the western frontier areas of the Barents Shelf, the Upper Jurassic is too deeply buried. By combining high-resolution 2D seismic data, well logs, and digitalized Rock-Eval data, this study documents the lateral distribution and variability of alternative source rock units within the Lower Cretaceous succession on the SW Barents Shelf. Negative high-amplitude anomalies are traced from shallow basins on the platform westward into deeper basins along the western shelf margin. The anomalies are tied to intervals of increased total organic carbon contents in several exploration wells, and we thus establish the presence of four potential source rock units; these are the (1) upper Hauterivian, (2) Barremian, (3) lower Aptian, and (4) upper Cenomanian units. Based on the distribution of the associated seismic anomalies, we infer that the deposition and preservation of these organic-rich units are coupled to localized, fault bounded depocenters, mainly controlled by Late Jurassic – Early Cretaceous rifting and local reactivation events. The lower Aptian stands out as the most significant source rock unit, particularly in the Fingerdjupet Subbasin, where it displays a kerogen Type II composition. The distribution and development of this oil-prone source rock unit is linked to an early Aptian fault reactivation event. Due to increased sediment influx in combination with high subsidence rates during the Albian to Cenomanian, potential pre-Aptian source rock units appear to have undergone too deep burial in the Tromsø and Bjørnøya basins to be presently generative. Furthermore, organic matter dilution due to increased sedimentation rates seems to have reduced the overall potential of the upper Cenomanian unit.

Deposition of Cenomanian – Turonian Organic-Rich Units on the Mid-Norwegian Margin: Controlling Factors and Regional Implication

Deposition of Cenomanian – Turonian Organic-Rich Units on the Mid-Norwegian Margin: Controlling Factors and Regional Implication

The Permian Tunas Formation (Claromecó Basin, Argentina): Potential naturally fractured reservoir and/or coal bed methane (CBM) play?

The analysis of structural discontinuities in the Tunas Formation helps characterize this unit as a potential naturally fractured reservoir. The principal discontinuities are 1) fractures parallel to the bedding planes, 2) fractures coplanar to the axial planar cleavage, 3) dilatant fractures filled by quartz, and 4) subvertical shear fractures. Over a thousand measurements on microtectonic structures help infer a stress field whose maximum principal stress (σ1) has a SW-NE orientation. Based on this structural analysis, optimal orientations for exploration and/or production wells in the Claromecó Basin should be a 130° azimuth. Fracture spacing range between 30 and 100 cm in outcrops of the Tunas sandstones along the Ventana foldbelt (VFB). A similar fractured network is postulated for the subsurface of the basin.Organic-rich levels, ranging from coals to carbonaceous shales, occur in the Tunas Formation in the subsurface of the Claromecó Basin and are a potential shale gas resource play. These beds tend to cap successions of interbedded shales and sandstones. The coals range from semi anthracitic to low volatile bituminous. Vitrinite and fusinite prevail as organic matter types, ranging between 1.3 and 2.4 vitrinite reflectance (Ro). The occurrence within condensate-wet gas to dry gas windows reflect subsurface processes such as strong diagenesis and anchimetamorphism. The coal-rich levels in the Tunas Formation can be correlated with other Lower Permian organic-rich units in the Paraná Basin in Uruguay (Tres Islas Formation) and Brazil (Rio Bonito Formation). All are analogous to the lower Upper Permian Vryheid coals of the eastern Karoo Basin in South Africa with shale gas and coal bed methane (CBM) prospectivity.

Absence of evidence of climate-driven cycles in Carboniferous deposits of Joggins, Nova Scotia, Canada: influence of salt withdrawal tectonics on deposition and pedogenesis

ABSTRACT During the Late Paleozoic Ice Age, the fault-bounded equatorial Cumberland Basin of Nova Scotia experienced rapid subsidence, accumulating kilometer-thick fluvial sedimentary units derived from two highlands to the northwest and southeast. Major variations are recorded in the paleosols exposed at the Joggins Fossil Cliffs, ranging from oxidized and well-drained paleosols with recognizable vertic features to highly reduced organic-rich paleosols. These different soil lithologies suggest alternating conditions between well-drained floodplain environments and water saturation associated with overall poor soil development. Although halokinetic subsidence of the Cumberland Basin is known to have been operative during deposition of these units, previous research favored glacio-eustatic processes as the primary forcing mechanism of sedimentation. A total of 474 fluvial aggradational cycles were identified within a kilometer-thick interval and show a fluctuating accommodation history with a very abrupt nature. The series of fluvial aggradational cycles was used to develop threshold autoregressive models based on 1) their thickness, 2) their paleosol thickness, 3) their sandstone content, and 4) their paleosol-to-sandstone ratio. For each model, results suggest no evidence of statistically significant cyclicity, contradicting the hypothesis that fluvial sedimentation was mainly driven by glacio-eustatic cyclothems. Additionally, a total of 7 lithologies were recognized through 1,655 beds. Evaluation of 8 spherical semivariograms suggests no evidence for cyclicity in the frequency, order, or distribution of the data based on lithologies, although some covariance was found at distances between 550 and 750 m suggesting similar processes controlling sedimentation in the lower and upper Joggins Formation. The Cumberland Basin is known to have been rapidly subsiding, mainly because of ductile deformation of salt deposits in the deeper basinal units. Our results suggest that Joggins records tectonically induced ponding of a part of the sedimentary basin, allowing more extensive preservation of abundant coal and organic-rich units, as well as still-standing fossil forests exposed along the cliffs. These new results suggest that tectonic subsidence of the Cumberland Basin during the Late Paleozoic Ice Age was a more important driver of fluvial sedimentation than previously thought. This novel application of the TAR methodology provides a mathematical description of the sediment accumulation history of terrestrial basins when applied to conformable sedimentary successions, along with the means of linking paleosol development to climatic processes.

Integrated geochemical characterization and geological modeling of organic matter-rich limestones and oils from Ajeel Oilfield in Mesopotamian Basin, Northern Iraq

The current study focuses on the organic geochemical and basin modeling of organic matter-rich limestones and oil samples from the Ajeel Oilfield. The objective was to characterize the organic matter and its hydrocarbon-generating potential, and its relevance to future hydrocarbon exploration in the northwestern Mesopotamian Basin. The organic biomarker results of the extracted bitumen samples indicated that the Sargelu, Naokelekan, and Chia Gara limestones received abundant planktonic-bacterial organic matter input, with a limited terrestrial contribution. Lipid-rich organic matter that originates from phytoplankton and bacteria with low amounts of terrestrial organic matter suggest a high abundance of Type-II or mixed-II/III kerogen as the original organic matter at time of sedimentation.Bulk property and geochemical analyses of the recovered oil samples, with mediumAPI gravities between 31 and 33°, and relatively high sulfur concentrations of up to 3.95 wt%, suggest that examined oil samples were derived mainly from Type II-S kerogen at moderate maturity stages. Biomarker ratios and parameters, as well as carbon isotopes, further indicated that these oils were derived from marine carbonate-rich source rocks deposited under reducing environmental conditions. These biomarker fingerprints and carbon isotopes show a genetic link between the oil examined samples and the extracted rocks of the Sargelu, Naokelekan, and Chia Gara source rocks and represent a mix of these carbonate-rich source rocks. Therefore, these source rocks contained kerogen Type II-S with high sulfur, and generated commercial oil at relatively low thermal maturity of early to moderate stages, as demonstrated from maturity parameters such as Tmax and PI as well as several biomarker ratios.Burial/maturity models with kerogen Type–II–S kinetic parameter revealed that the Sargelu, Naokelekan, and Chia Gara intervals began to generate oil at slightly lower thermal maturity (maturity levels = 0.55–0.79 EASY%Ro). As a result, about 10%–50% of the organic-matter was converted to oil during the Late Paleocene. The basin models also suggested that the organic-rich units including Sargelu and Naokelekan source rocks reached higher maturity levels, with TR of 71%–79% in the Early Miocene (20 Ma) and continued to the present. Trapping large quantities of oil lead to the high pressures and expulsion of these oils. The high conversion ratios further suggested large oil volumes accumulated in the northwestern Mesopotamian Basin were mainly originated from the Middle-Late Jurassic Sargelu and Naokelekan source rocks.

Timing of hydrocarbon entrapment in the eastern foothills of the Eastern Cordillera of Colombia

The eastern foothills in the Colombian Eastern Cordillera have been an important oil-producing region since the discovery of the Cupiagua and Cusiana fields. Three organic-rich units are considered to be the main source rocks. The Aptian Fómeque and the Cenomanian-Coniacian Chipaque Formations comprise a siliciclastic to locally carbonate shallow marine shelf succession with type-II kerogen, whereas the Paleocene Los Cuervos Formation consists of marginal marine to nonmarine siliciclastic rocks with type-III kerogen. We modeled the petroleum systems of these three source units to characterize the hydrocarbon generation-accumulation processes within the basin. The structural record of the Eastern Cordillera shows that the most important tectonic event began in early Oligocene with contractional deformation along the Soapaga through Guaicaramo faults during early Miocene, culminating during the Pliocene with the Cusiana and Yopal faults. These variable rates of burial and exhumation resulted in contrasting time-temperature histories for each of the source rocks. The Fómeque Formation reached the oil window during the Paleocene in the south and the Eocene to the north. In contrast, the Chipaque Formation generation started during Early Oligocene in the south and by Late Oligocene to the north. Conversely, maturation for the Los Cuervos Formation was uniform along the foothills, reaching the oil window during Late Oligocene. Charge history modeling suggested that the Albian sandstones reservoirs were filled between Oligocene to Miocene. In contrast, the proven reservoirs in the area (the Upper Cretaceous, Paleocene, and Eocene sandstones) were filled by late Miocene, with a second episode of recent charge in the Eocene reservoirs, and perhaps active, from the Los Cuervos Formation. The results of this work proved that petroleum system modeling is useful not only to characterize generation-migration processes but it also can be used as a prediction tool in structurally complex areas such as the Colombian foothills.

Comparing interglacials in eastern Australia: A multi-proxy investigation of a new sedimentary record

The widespread formation of organic rich sediments in south-east Australia during the Holocene (Marine Isotope Stage [MIS] 1) reflects the return of wetter and warmer climates following the Last Glacial Maximum (LGM). Yet, little is known about whether a similar event occurred in the region during the previous interglacial (MIS 5e). A 6.8 m sediment core (#LC2) from the now ephemeral Lake Couridjah, Greater Blue Mountains World Heritage Area, Australia, provides insight into this question. Organic rich sediments associated with both MIS 1 and 5e are identified using 14C and optically stimulated luminescence (OSL) dating techniques. Also apparent are less organic sedimentary units representing MIS 6, 5d and 2 and a large depositional hiatus. Sediment δ13C values (−34 to −26‰) suggests that C3 vegetation dominates the organic matter source through the entire sequence. The pollen record highlights the prevalence of sclerophyll trees and shrubs, with local hydrological changes driving variations in the abundance of aquatic and lake-margin species. The upper Holocene sediment (0–1.7 m) is rich in organic matter, including high concentrations of total organic carbon (TOC; 20–40%), fine charcoal and macrophyte remains. These sediments are also characterised by a large proportion of epiphytic diatoms and a substantial biogenic component (chironomids and midges). These attributes, combined with low δ13C and δ15N values, and C:N ratios of approximately 20, indicate a stable peat system in a swamp like setting, under the modern/Holocene climate. In comparison, the lower organic rich unit (MIS 5e-d) has less TOC (5–10%), is relatively higher in δ13C and δ15N, and is devoid of macrophyte remains and biogenic material. Characterisation of the organic matter pool using 13C-NMR spectroscopy identified a strong decomposition signal in the MIS 5e organic sediments relative to MIS 1. Thus the observed shifts in δ13C, δ15N and C:N data between the two periods reflects changes in the organic matter pool, driven by decompositional processes, rather than environmental conditions. Despite this, high proportions of aquatic pollen taxa and planktonic diatoms in the MIS 5e–d deposits, and their absence in the Holocene indicates that last interglacial Lake Couridjah was deeper and, or, had more permanent water, than the current one.

The estimation of CO2 storage potential of a gas-bearing shale succession at the early stage of reservoir characterization: a case study from the Baltic Basin (Poland)

Estimation of the CO 2 storage potential of gas-bearing shales in the Lower Paleozoic Baltic Basin is at an early stage of reservoir exploration and production, based on data from one vertical exploration borehole, supplemented with some information from adjacent boreholes. The borehole section examined is 120 m long and comprises three intervals enriched with organic matter separated by organic-poor intervals. In our approach, the storage capacity is represented by: (1) sorption potential of organic matter, (2) open pore space and (3) potential fracture space. The potential for adsorbed CO 2 was determined from Langmuir isotherm parameters taken from laboratory measurements and recalculated from CH 4 adsorption curves. The pore space capacity was estimated in two ways: by utilizing results of laboratory measurements of dynamic capacity for pores >100 nm and using results of helium porosimetry, the first of these being considered as the most relevant. Due to the low permeability of the shale matrix we have adopted the standard assumption that the CO 2 is able to reach effectively only 10% of the theoretical total sorption and pore volume. For hydraulic fracture space, the theoretical maximum opening of vertical fractures in the direction of minimum horizontal stress was considered, decreased by the expected portion of fracturing fluid flowback and by partial fracture closure by burial compaction. The effectiveness of three CO 2 storage categories for the individual organic-rich and organic-poor shale units shows an obvious positive correlation of TOC content with the storage efficiency by sorption and within pore space, and a negative correlation with the storage efficiency in hydraulic fractures. It was estimated that sorption, over the maximum storage interval (120 m thick), is responsible for ~76% of total storage capacity, pore space accounts for 13% (for the most relevant porosity model) while the contribution of fractures is about 11%. In the minimum storage interval (35 m thick, including the best quality shales) the estimated proportions of sorption, pore space and fractures in the total storage capacity are 84, 10 and 6% respectively. Finally, the result for the best quality storage interval (35 m thick) was compared with the Marcellus Shale of similar thickness (average ~38 m) and with other options of CO 2 storage in Poland. The most organic-rich units in the area studied have a CO 2 storage capacity efficiency (i.e. storage capacity per volume unit of shale) only slightly less than average for the Marcellus Shale, because sorption capacity – the dominant component – is comparable in both cases. However, the open pore space capacity in the Marcellus Shale appears to be far higher, even if the potential fracture space calculated for the borehole studied is taken into consideration, probably because the free gas content in the Marcellus Shale is far higher than in the Baltic Basin. CO 2 storage in depleted shale gas wells is not a competitive solution compared to storage in saline aquifer structures or in larger hydrocarbon fields.

Oil shale potential of the lower cretaceous Jiufotang Formation, Beipiao Basin, Northeast China

Lacustrine oil shale is an important unconventional energy source in China. Previous exploration has mainly focused on stable deep lake deposits, whereas lacustrine oil shale layers in unstable sedimentary settings, which are widely distributed in Cretaceous strata, having received little research attention. The present paper is focused on the Aptian Jiufotang Formation in the Beipiao Basin. In order to study the impact of gravity flows and volcanism on oil shale formation, petrology, organic geochemistry, molecular geochemistry and elemental geochemistry were used to characterize the oil shales, which are very rich in well-preserved fossils (“Jehol Biota”). The predominant macerals in the high quality oil shales are lamalginite and telalginite, suggesting organic matter was derived mainly from phytoplankton and aquatic algal-bacterial organisms. A ternary diagram of C27-C28-C29 steranes showed increasing contribution of land plants in organic-lean units. Element indices such as Sr/Ba, Ca/(Ca + Fe), V/(V + Ni), δCe, Cu/Zn, Th/U, Sr/Cu, Fe/Mn, Ti/Al, TOC/S, as well as biomarker ratios (pristane/phytane, gammacerane index) and the index of compositional variability (ICV), show that high-quality oils shales were deposited under anoxic freshwater conditions. Terrigenous detrital input was low, and a warm and humid palaeoclimate, as well as a stable tectonic setting, likely prevailed. At times, low-density turbidites were accompanied by the input of terrestrial detritus, promoting a greater degree of lake oxygenation, thereby reducing oil shale potential. Volcanic eruptions triggered the occurrence of low-density turbidites due to emission of greenhouse gases which facilitated a drier climate. Periods of high-quality oil shale formation can therefore be linked to times of weak volcanism and tectonic activity, in which the organic matter was deposited under conditions of warm and humid paleoclimate, with enhanced preservation due to anoxic bottom waters in the lacustrine system. The organic-rich unit B within the Jiufotang Formation seems suitable for the application of in-situ oil shale conversion.

A high-TOC shale in a low productivity world: The late Mesoproterozoic Arctic Bay Formation, Nunavut

The latest Mesoproterozoic Arctic Bay Formation (Borden Basin, Nunavut, Canada) is up to ∼1130 m-thick and contains a significant proportion of unusually organic-rich black shale (up to 12.3 wt% total organic carbon). Insofar as increased biological productivity is related to organic matter burial, this organic-rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the Proterozoic. To better understand the conditions leading to development of this organic-rich unit, we explore the redox geochemistry of the Arctic Bay Formation using a multi-proxy approach (nitrogen isotopes, iron speciation, total organic carbon, total sulphur, and trace metal abundances). Redox proxy data support a stratified water column, with oxic surface waters underlain by intermittently euxinic waters, which are in turn underlain by persistently ferruginous deeper waters. The highly alkaline, restricted marine basin in which the Arctic Bay Formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals, resulting in an ‘excess’ of sulphur that resulted in sulphurisation of organic matter. Estimates for organic matter burial rates during deposition of the Arctic Bay Formation suggest that they were perhaps ∼5–6 times mid-Proterozoic average values (although there are permissible scenarios in which it was extremely productive), underscoring that such organic-rich sedimentary rocks could be produced in a low productivity world.

Effects of petroleum retention and migration within the Triassic Chang 7 Member of the Ordos Basin, China

The effects of petroleum retention within and expulsion from five intervals within the Triassic Chang 7 Member in the Ordos Basin in China have been demonstrated by deciphering a large suite of petrologic, organic petrographic, and organic geochemical analyses on 106 core samples.Organic properties and lithological heterogeneities control the amount and chemical composition of retained petroleum. Enrichment of aliphatic hydrocarbons in the silty, organic-lean intervals versus enrichment of aromatic fluids in the clayey, organic-rich units are probably caused by compositional fractionation occurred during primary and short-distance secondary migration. Migration of excess petroleum from organic-rich units to those organic-lean counterparts was further corroborated by the negative expulsion efficiencies calculated using a mass-balance model in the organic-lean intervals. Capillary pressure difference induced by the contrast in pore throat size of the clayey, organic-rich versus the silty, organic-lean intervals is postulated to be the major driver for the migration.A significant implication of this study is that the hydrocarbons in the first and fourth intervals constitute potential petroleum exploitation targets, given (i) improved oil quality caused by enriched aliphatic compounds versus viscous aromatic and polar compounds, (ii) lower organic matter sorption affinities, (iii) relatively high contents of brittle minerals in these two intervals. Despite its high content of retained hydrocarbons, the organic-rich fifth interval is not the best target, which is mainly due to the high sorption affinity of the organic matter structure, thereby causing low oil mobility and producibility.

Igneous petroleum systems in the Malargüe fold and thrust belt, Río Grande Valley area, Neuquén Basin, Argentina

Abstract The Rio Grande Valley (Neuquen Basin, Argentina) is a large oil field where voluminous igneous sill complexes are emplaced in different levels of the sedimentary sequence in a fold and thrust belt setting. These sills are predominantly emplaced in organic-rich units such as Vaca Muerta and Agrio Formations and are critical elements of the defined petroleum system, as they affect the thermal maturity of the surrounding source rocks, and also act as naturally fractured reservoirs. In this contribution, we integrated structural sections, borehole data, core descriptions, petrophysical laboratory tests and organic geochemistry measurements to characterize the impact of the magmatism in the petroleum system of the Rio Grande Valley oil field. Especially, we focused in evaluating the impact of the sill-complexes in hydrocarbon generation, migration and accumulation. Our data shows that: (1) intrusions are preferably emplaced in organic rich units; (2) a large number of intrusions is observed in areas intensely deformed by imbricated thrusts; (3) an increase in the number of intrusions drives stronger and more extensive maturation processes in the Vaca Muerta formation, enhancing notably the cracking of kerogen to hydrocarbon within this unit; (4) where the number of intrusions increases, the concentration of total gas and methane also increases, given that all sills are suitable reservoirs; (5) igneous rocks provide a low resistance path for fluids flow and act as the main reservoirs in the study area; (6) the reservoir quality of igneous intrusions is dominated by the combination of matrix and fracture framework properties; (7) zeolites are the most common type of cement in these reservoirs; (8) deep sills located in the Vaca Muerta formation produce larger volumes of hydrocarbons relative to shallower intrusive reservoirs. The Rio Grande Valley oil field offers a world-class example of the implications of igneous complexes in hydrocarbon generation, migration and accumulation in a compressive tectonic setting of fold and thrust belt.

Field gamma-ray patterns and stratigraphic reinterpretation of offshore well-log data from Lower Jurassic organic-rich units of the Lusitanian Basin (Portugal)

Abstract Lower Jurassic (Sinemurian–Pliensbachian) successions cropping out in onshore areas of the Lusitanian Basin provide detailed information of three well characterized organic-rich intervals: i) the Unit F from the Coimbra Formation, ii) the Polvoeira Member of the Agua de Madeiros Formation, and the iii) Marly Limestones with Organic-rich Facies (MLOF) Member of the Vale das Fontes Formation. Despite decades of research, these source rocks have not yet been properly identified and characterized in terms of extension and thickness in the basin's offshore. This paper compares detailed gamma-ray (GR) profiles from onshore outcrops (Coimbra, Montemor-o-Velho, Figueira da Foz, Monte Real, Sao Pedro de Moel and Peniche regions) with seven offshore wells. Our objective is to recognise Lower Jurassic organic-rich intervals in offshore areas of the basin, highlighting their lateral extension and improving the overall stratigraphic definition of the Lower Jurassic in the Lusitanian Basin. GR field measurements indicate that the highest GR values and U contents are consistently observed in the organic-rich intervals, reaching up to 134 API and 9.0 ppm of U. In contrast, the lowest API and U contents were determined in dominantly calcareous units, ranging between 5-34 API and 0.1-1.4 ppm, respectively. Wireline gamma-ray logs show a good correspondence between field patterns and offshore well logs, recognizing the organic-rich intervals in offshore areas. The highest GR values are interpreted to be linked with the 2nd- and 3rd-order maximum flooding intervals previously recognised in the Sinemurian–Pliensbachian successions in the western Iberian margin. Detailed GR maps linked to maximum flooding intervals highlight the variation of the vertical and lateral distributions of the organic-rich units in offshore areas of the Lusitanian Basin.

Determining the Source of Fugitive Dust in Lattimer, Pennsylvania

Fugitive dust samples collected from residential properties in the village of Lattimer, Pennsylvania were analyzed for total concentrations of major and minor elements Si, Al, Ti, Fe, Mg, Na, K, C, and S using energy dispersive x-ray spectrometry. Observed under high magnification, the individual dust particles are irregular-shaped with angular edges and range in size from 20 to 150 microns. Rock samples of gray to black organic-rich mudstone, shale, and siltstone units interbedded with thin layers of anthracite coal were collected next to a rock quarry operation that is suspected as a possible source of the fugitive dust. Major and minor element concentrations in the rock samples are compared to major and minor element concentrations in 150 individual dust particles. The fugitive dust and rock samples that were analyzed have Si concentrations that vary from 0.08 to 43.34 wt.% and C concentrations that vary from 3.36 to 95.68 wt.%. All of the major and minor element concentrations in the rock samples lie within the representative range of element concentrations in the individual dust particles. The Si and C concentrations in the fugitive dust suggest that the particles originated from a carbon-rich silicate rock source. Five of the fugitive dust particles analyzed had C concentrations in excess of 60 wt.% and can be considered coal dust. Results suggest that the chemical composition of the fugitive dust particles is consistent with the chemical composition of the rocks that are actively extracted and crushed at the rock quarry site located adjacent to the village of Lattimer, Pennsylvania.

Concurrent Salinization and Development ofAnoxic Conditions in a Confined Aquifer, Southern Israel.

An ancient, brackish, anoxic, and relatively hot water body exists within the Yarqon-Tanninim Aquifer in southern Israel. A hydrogeological-geochemical conceptual model is presented, whereby the low water quality is the outcome of three conditions that are met simultaneously: (1) Presence of an organic-rich unit with low permeability that overlies and confines the aquifer; the confining unit contains perched horizons with relatively saline water. (2) Local phreatic/roofed conditions within the aquifer that enable seepage of the organic-rich brackish water from above. The oxidation of the dissolved organic matter in the seeping water consumes the dissolved oxygen and continues through bacterial sulfate reduction, with H2 S as a product. These exothermic reactions result in some heating. (3) The seeping water comprises a relatively large portion of the water volume. In the presented case study, the latter condition first developed in the Late Pleistocene following climate change, which led to a dramatic decline in recharge. Consequently, water flow in the local basin has nearly ceased, as evident by old water ages, specific isotopic composition, and nearly equipotential water levels. The continuous seepage from above into the almost stagnant water body has resulted in degraded water quality. Seepages of organic-rich brackish water exist at other sites throughout the aquifer but have limited impact on the salinity and redox conditions due to the dynamic water flow, which flushes the seeping water, that is, the third condition is not met. The coexistence of the above three conditions may explain the development of anoxic and saline groundwater in other aquifers worldwide.

Characterization of the source rocks of a paleo-petroleum system (Cameros Basin) based on organic matter petrology and geochemical analyses

The Cameros Basin is a paleo-petroleum system formed in the north-eastern Iberian Peninsula. The basin formed during the Mesozoic Iberian Rift and was later inverted during the Alpine orogeny. Hydrothermal events took place during the post-extensional and inversion stages, producing an important impact on the thermal history of the basin. In order to determine the source rock of the petroleum system of the basin, organic matter characteristics, the petroleum generation potential and the maturity of the basin infill are determined by means of petrology and geochemical analyses.Several organic rich units of the stratigraphic record of the basin are potential source rocks for hydrocarbons generation, although their characteristics differ depending on their location. Organic matter content in the northern sector is scarce and limited to vitrinite, inertinite and solid bitumen particles. The residual total organic carbon (TOC < 1%), the low hydrocarbon potential (S2 < 0.3 mg HC/g rock, HI < 50 mg HC/g TOC), the mature to overmature thermal stages (%Ro from 1.7 to 4.6) and the presence of solid bitumen particles indicate that hydrocarbons have been already generated by these rocks. In contrast, the southern sector of the basin is characterized by abundant organic matter remnants (TOC from 2 to 17%) and immature to early oil-window thermal conditions (0.38–0.75%Ro), indicating a high hydrocarbon potential for these rocks (S2 from 11 to 123 mg HC/g and HI values from 23 to 715 mg HC/g TOC).The different evolution of the source rocks in the basin is the result of the combination of differential subsidence rates, which determine different thermal histories and of the circulation of hydrothermal fluids in the northern-central sector during the evolution of the basin.