Amorphous Marine Research Articles

Thermal maturity assessment of marine source rocks integrating Raman spectroscopy, organic geochemistry and petroleum systems modeling

A core sample set, comprising twenty-eight carbonaceous mudrocks from ten exploration wells was analysed by standard organic geochemical methods (programmed pyrolysis, elemental analysis) and a novel Raman spectroscopic technique. The kerogen of the studied rocks is classified as amorphous marine, sulfur-rich, oil-prone organic matter with predominantly algal precursor material. The sample set covers a wide range of thermal maturation from premature to late mature. Hydrogen index, Tmax and atomic H/C ratios vary from 52 to 708 mg HC/g rock, from 426 to 488 °C, and from 0.58 to 1.35, respectively. Raman spectral features, expressed as Raman band separation (RBS), correlate with geochemical maturity parameters and with results from basin modeling. The combined use of independent maturity parameters, like programmed pyrolysis, elemental analysis and Raman spectroscopy, improves the calibration of a regional basin model in the Eastern Arabian Basin, where direct maturity measurements from vitrinite reflectance are deficient.

CCE V: Primary production, mesozooplankton grazing, and the biological pump in the California Current Ecosystem: Variability and response to El Niño

Predicting marine carbon sequestration in a changing climate requires mechanistic understanding of the processes controlling sinking particle flux under different climatic conditions. The recent occurrence of a warm anomaly (2014–2015) followed by an El Niño (2015–2016) in the southern sector of the California Current System presented an opportunity to analyze changes in the biological carbon pump in response to altered climate forcing. We compare primary production, mesozooplankton grazing, and carbon export from the euphotic zone during quasi-Lagrangian experiments conducted in contrasting conditions: two cruises during warm years - one during the warm anomaly in 2014 and one toward the end of El Niño 2016 – and three cruises during El Niño-neutral years. Results showed no substantial differences in the relationships between vertical carbon export and its presumed drivers (primary production, mesozooplankton grazing) between warm and neutral years. Mesozooplankton fecal pellet enumeration and phaeopigment measurements both showed that fecal pellets were the dominant contributor to export in productive upwelling regions. In more oligotrophic regions, fluxes were dominated by amorphous marine snow with negligible pigment content. We found no evidence for a significant shift in the relationship between mesozooplankton grazing rate and chlorophyll concentration. However, mass-specific grazing rates were lower at low-to-moderate chlorophyll concentrations during warm years relative to neutral years. We also detected a significant difference in the relationship between phytoplankton primary production and photosynthetically active radiation between years: at similar irradiance and nutrient concentrations, productivity decreased during the warm events. Whether these changes resulted from species composition changes remains to be determined. Overall, our results suggest that the processes driving export remain similar during different climate conditions, but that species compositional changes or other structural changes require further attention.

Depositional paleoenvironment and hydrocarbon source potential of the Oligocene Dabaa Formation, north Western Desert, Egypt: A palynofacies approach

A detailed palynofacies study was carried out on 24 shale cutting samples from the Oligocene Dabaa Formation, which was penetrated in the Amana-1X well, Abu El Gharadig Basin, north Western Desert, Egypt. The investigation targeted primarily three main groups of palynological elements: phytoclasts, palynomorphs, and amorphous marine organic matter (AMOM), using transmitted light microscopy. Two major marine palynofacies were recognized: the oldest is palynofacies 1, made up mainly of AMOM (average abundance 47.5%), phytoclasts (31.6%), and palynomorphs (20.9%). A suboxic to anoxic outer shelf to upper slope paleoenvironment is suggested for this palynofacies. Palynofacies 2 is characterized by poorly preserved translucent phytoclasts (average abundance 61.2%) in addition to palynomorphs (24%), AMOM (14.8%), and opaques (

An example of oxidative polymerization of unsaturated fatty acids as a preservation pathway for dinoflagellate organic matter

A palynologically monotypic assemblage from the Eocene Jatta Gypsum Formation has (Pakistan) been analyzed microscopically and chemically. Scanning electron microscopy shows that the palynomorphs are solid to spongeous dinoflagellate (Division Dinophyta) remains embedded in an amorphous ground mass. Unlike dinoflagellate resting and temporary cysts, a resistant outer wall is absent and the fossils are presently interpreted to be the internal contents (“dinocasts”) of a motile thecate dinoflagellate. Chemical analysis shows that the microfossils are highly aliphatic and consist of polymerized free lipids, likely to have been predominantly C 18 and C 16 fatty acids with a predominantly C 9 mid-chain functionality. We suggest that these “dinocasts” have been formed post-mortem by oxidative polymerization of these lipids, derived from cellular membranes and storage vesicles. Additionally, lipids from outside may have contributed, during or after polymerization of the cell contents. Our hypothesis is supported by chemical analysis of autoxidized vegetable oil and contrasts with the generally accepted opinion that algal remains are preserved via the so-called selective preservation pathway only. In the present case, the unique morphological preservation of the “dinocasts” enabled linking of the post-mortem polymerized free lipids to their microalgal source. However, upon regular, less ideal, morphological preservation conditions such linkage is impossible since “amorphous” particles would result. We propose, therefore, that oxidative polymerization of lipids may be responsible for much more of the amorphous marine organic matter in sediments than presently acknowledged.

A PRELIMINARY EVALUATION OF JURASSIC SOURCE ROCK POTENTIAL IN KUWAIT

Jurassic sedimentary rocks in Kuwait are generally assigned to the Marrat, Dhruma, Sargelu and Najmah Formations (mainly limestones and calcareous shales) and the overlying Hith and Gotnia Formations which are composed of anhydrites and evaporites. This paper reports the results of organic‐geochemical analyses of Jurassic carbonate and shale samples recovered from ten wells in Kuwait. Analytical techniques included TOC analysis, elemental analyses of kerogen, density separation and petrographic analyses. The thermal history of Kuwait was modelled and calibrated with maturity indicators from the studied wells.The analytical results point to the presence of marine kerogen between Types II and III. Generally, the formations show amorphous rich sapropelic organic matter with high H/C ratios and low densities. Biodegradation of some samples resulted in elevated O/C ratios. The results of maturity studies indicate that most of the Jurassic Succession is mature, maturity differences being due to depth variations. Oil generation began in Late Cretaceous to Eocene time when structural traps had already been formed. Jurassic source rocks may therefore have supplied reservoir units in Kuwait. In particular, the Najmah Formation includes well‐preserved amorphous marine algal type organic matter. The high TOC values and thermal maturity of this formation make it one of the most important potential sources of oil in Kuwait.

Minerogenetic mechanism of the Songxi silver-antimony deposit of northeastern Guangdong—Ore-controlling role of organic matter

Organic geochemistry and comparisons of characteristics of the organic matter in wall rocks of the ore-controlling strata and ores of the Lower Jurassic Songling black shale formation and the related Songxi silver-antimony deposit of northeastern Guangdong have been studied in this paper. The results show that the Lower Jurassic Songling shale formation is a suite of biologic-rich and organic-rich ore-bearing marine sedimentary rocks. Micro-components of the organic matter in the Songling black shale formation consists primarily of algae, amorphous marine kerogen, solid bitumen, and pyrobitumen. The thermal evolution of organic matter is at the over-maturity stage. There is a general positive correlation between total organic carbon (TOC) and metallogenetic elements such as Ag and Sb in the black shale formation. Organic matter in the host rocks in the Songxi ore deposit played a role in controlling the silver-antimony depositing environment during the forming process of the black shale ore-bearing formation. In the absence of vitrinite, the relative level of thermal maturity calculated by solid bitumen reflectance indicates that the ore-forming temperature of the Songxi silver-antimony deposit was about 150–170°C, which was considered as an epithermally reworked ore deposit. The roles of organic matter in the formation of the Songxi ore deposit are a primitive accumulation of the metallogenetic elements (Ag, Sb) in the sea-water cycle system for ore source and a concentration of metals by ion exchange or chelation as well as reduction of the oxidized metals.

The influence of mineral matter on the separation of amorphous marine kerogens using density gradient centrifugation

Three relatively immature amorphous marine kerogens were subjected to density gradient centrifugation (DGC). The density fractions obtained were analyzed by Curie-point pyrolysis in combination with gas chromatography (Py-GC) and with gas chromatography-mass spectrometry (Py-GC-MS). Despite the variations in density, no significant variations were observed in chemical compositions of the flash pyrolysates of the density fractions of each kerogen. The variations in density were due to varying amounts of inorganic matter, in particular pyrite, which remained after chemical demineralization procedures and was insufficiently removed during the sample processing. This was substantiated by the ash content of individual density fractions, which is linearly correlated to the densities of the kerogen density fractions. Further micronization and cryogenic treatment followed by a sink-float procedure of the Posidonia kerogen prior to DGC resulted in only a partial removal of the inorganic phase, as indicated by a shift of the average density towards lower densities. These results demonstrate the substantial effect of mineral matter, in particular pyrite, on the separation of amorphous marine kerogens into chemically distinct entities due to an intimate association of the organic and inorganic phase.

Alteration of organic matter in the Viburnum Trend lead-zinc district of southeastern Missouri

Organic petrography and elemental analyses were used to study alteration of organic matter in the Bonneterre Formation dolostones. Reflectance measurements and petrographic characteristics of organic particles define an alteration zone about 20 km wide, centered on the Viburnum Trend ore deposits. Within this zone, bitumen has been generated from kerogen. In contrast, no evidence of local bitumen generation was observed in samples located more than 10 km from the Viburnum Trend. This could indicate a level of thermal maturity significantly greater than the regional background level.The organic matter in the Bonneterre Formation consists primarily of amorphous marine kerogen, algal remains (alginite), and solid bitumen. In the absence of vitrinite, relative levels of thermal maturity were determined from the reflectance, fluorescence, and state of preservation of fossil unicellular algae. Within the alteration zone the alginite has been impregnated with bitumen, and there is convincing petrographic evidence that the bitumen was locally generated. Outside of the alteration zone, the algae are well preserved and exhibit the bright yellow fluorescence characteristic of immaturity. There is also petrographic evidence of increased solid bitumen reflectance due to interaction with pore fluids, both within and outside the alteration zone. Elemental analyses suggest that organic matter has been oxidized throughout the study area.Although there is no direct evidence linking the organic maturation and alteration with ore deposition, the association of the alteration zone with a major Mississippi Valley-type deposit permits the hypothesis that the regional flow of hot brines associated with the deposit was the cause of the anomalous heating, and that the evolution of methane and other gases during this process played a role in the ore deposition.

Generation and migration of oil in the Maturin Subbasin, Eastern Venezuelan basin

Regional geochemical evaluation of the possible Cretaceous and Tertiary source rocks and oil-oil and oil-source rock correlation studies, have identified the calcareous shales and limestones of the Upper Cretaceous, Querecual and San Antonio Formations of the Guayuta Group, with amorphous marine type II organic matter, as the source rocks for oil in the Maturin Subbasin. The deposition of the source rocks took place on a passive continental margin whilst the generation and migration of oil occurred in a foreland basin during Miocene-Recent times. The oils found in the northern part of the subbasin in different reservoirs (Cretaceous, Eocene, Oligocene, Miocene and Pliocene formations) are of mature, marine type with or without bacterial alteration. Geochemical data indicate that some of these oils were mixed with the highly altered residue of a previously accumulated oil. Based on maturity and migration modelling, it appears that the oils essentially migrated from active, Upper Miocene-Recent, kitchens in the Cretaceous source rocks below the main thrusts. The highly altered residue was formed during the Upper Miocene by the biodegradation of oil migrating during the Middle and Upper Miocene. Large areas of mature to overmature Cretaceous source rocks (now exposed in the Interior Mountain Front) were in active oil kitchens during the Lower and Middle Miocene before being involved in thrusting around 12 Myr BP. Long distance (150–300 km) southward lateral migration of oils during the Middle Miocene gave rise to the altered moderately mature marine oils in the shallow Miocene stratigraphic reservoirs of Cerro Negro (Orinoco Oil Belt) in the southern fringe of the subbasin.

Oil source bed recognition and crude oil correlation, Trinidad, West Indies

Potential source rocks capable of generating hydrocarbons in Trinidad were identified using routine geochemical parameters. Tertiary formations have a poor oil source rating due to low organic carbon contents (TOC < 1.0%) and because they contain immature, gas prone type III kerogens. Oil prone organic facies, characterised by relatively high TOCs (2.2% average) and amorphous marine type II kerogens, in the Upper Cretaceous Naparima Hill and Gautier Formations, suggest good to excellent oil source potential. Produced oils were classified into genetic types on the basis of gas chromatography and gas chromatography-mass spectrometry fingerprinting. Only one genetic oil type, sourced from predominantly marine organic matter, was recognised. Biodegradation is evident in several oils analysed. Chromatographic characteristics and gross geochemical properties of produced oils correlate best with rock extracts from the Naparima Hill and Gautier Formations, indicating a probable genetic association.

Generation and migration of hydrocarbons in the Maracaibo basin, Venezuela: An integrated basin study

The history of hydrocarbon generation and migration in the Maracaibo Basin, Venezuela, is closely related to its stratigraphic, structural and tectonic evolution during the Cretaceous and Cenozoic times. Geochemical evaluation of the possible source rocks within the Cretaceous and the Tertiary sequences indicates that the argillaceous limestones and calcareous shales of the La Luna Formation (Cenomanian-Coniacian) are the most important source rocks in the basin. These rocks contain organic carbon ranging from 1.5 to 9.6% ( c. original TOC 2.5–10.8%), have fluorescent amorphous marine, type II organic matter (oil prone) and have in the immature stage a H-index of 700 mg HC/g TOC and a H/C ratio of 1.5. The beginning of the oil window in these rocks occurs at a relatively low thermal maturity (Lopatin TTI about 10) and the peak of oil generation corresponds to a TTI value of 35. Geochemistry of oils from the different reservoirs (Cretaceous, Paleocene, Eocene and Miocene beds), and oil-oil and oil-source rock correlation studies based on biomarkers have identified three genetic types of oil in the basin: a marine type essentially derived from the La Luna source, a terrestrial type possibly originating either from the Cretaceous Lisure shales or the Paleocene shales, and a third type derived from the mixing of the first two types. The marine type oil is distributed extensively throughout the basin whereas the terrestrial as well as the mixed marine and terrestrial types are found only in the southwestern part of the basin. The marine type oils have been further subdivided based on their alteration and maturity. Reconstruction of the hydrocarbon generating areas at different geological times using TTI-maturity diagrams indicates that the generation of oil and gas in the basin took place during two periods; the first phase occured during the Middle to Upper Eocene times in the northeastern part of the basin, while the second phase occured during the Upper Miocene to Recent times in the central, western and southern parts of the basin. Based on organic geochemical and sedimentological characteristics, it is proposed that primary migration of oil from the La Luna source rocks took place by expulsion of hydrocarbons through abundant microfractures developed by the pore fluid overpressure during hydrocarbon generation. Mass balance considerations suggest that the expulsion probably occurred as a hydrocarbon single phase and the expulsion efficiency for oil in La Luna source rocks should have been very high (30–50%). Taking into consideration the development of oil and gas kitchens through time, the paleostructures at the time of hydrocarbon generation, and the characteristics of oils, the migration history of the oil and gas generated from the La Luna source has been outlined. Mixing of oils, derived at different times (Eocene and Miocene-Recent) from the La Luna source of different areas, appeared to have taken place in Block IV, Block 11, Lago-Lamar, Motatan and Bolivar Coastal fields.

Organic Facies of Middle Cretaceous Black Shales in Deep North Atlantic

The middle Cretaceous black shales of the deep North Atlantic consist of alternating layers of sediment rich in total organic carbon (TOC) that contains abundant amorphous types of organic matter and sediment poor in TOC that contains mostly woody and coaly types of organic matter. The amorphous materials are derived mostly from marine organisms in the eastern North Atlantic off Africa, in the Caribbean, and on the Demerara Rise off Surinam and from higher land plants in the western North Atlantic and off Europe. Marine amorphous material is important off Africa because this was an area of upwelling and highly productive surface water. Amorphous marine organic matter was deposited across the entire North Atlantic Basin in the Cenomanian in response to an ocean-wide upwell ng event connected with the opening of the equatorial Atlantic. The difference between the TOC-rich and background TOC layers reflects deposition of the former under reducing conditions, which allowed the preservation of labile amorphous materials of marine and terrestrial orgin, and of the latter under oxidizing conditions, when nearly all that was preserved was refractory organic material.

Organic Facies of Mid-Cretaceous Black Shales in Deep North Atlantic: ABSTRACT

The mid-Cretaceous black shales of the deep North Atlantic consist of alternating layers of sediment rich in total organic carbon (TOC) that contain abundant amorphous types of organic matter, and sediment poor in TOC that contain mostly woody and coaly types of organic matter. The amorphous materials are derived mostly from marine organisms in the eastern North Atlantic off Africa, in the Caribbean, and on the Demarara Rise off Surinam, and from higher land plants in the western North Atlantic and off Europe. Marine amorphous material is important off Africa because this was an area of upwelling and highly productive surface water. Amorphous marine organic matter was deposited across the entire North Atlantic basin in the Cenomanian in response to an ocean-wide upwelling event connected with the opening of the equatorial Atlantic. The difference between the TOC-rich and background TOC layers reflects deposition of the former under reducing conditions, which allowed the preservation of labile amorphous materials of marine and terrestrial origin, and of the latter under oxidizing conditions, when all that was preserved was refractory organic material. End_of_Article - Last_Page 790------------

Amorphous marine muds from tropically weathered basalt

Clay-sized detrital sediments of Kaneohe Bay, Hawaii, closely reflect the intense tropical weathering conditions of their basaltic source area. Most of the silica and alumina present is amorphous allophane or in weakly crystalline halloysite. It is suggested that unstable muds of this composition may have been the raw materials for authigenesis of illite found in old sedimentary rocks.