Terrestrial Organic Matter Input Research Articles

Geochemical characterization of steranes and terpanes in certain oils from terrestrial facies within the South China Sea

Using the TSQ 70 GC/MS/MS mass spectrometer, the author analysed and studied steranes and terpanes in certain oils from terrestrial facies within the South China Sea. It was found that C 29 sterane is of the highest relative abundance within the range of C 27C 29 in these crudes, that 4-methyl steranes (in the range of C 28C 30), particularly C 30-4-methyl sterane, are unusually abundant, and that most of the crudes are also commonly rich in rearranges steranes. As for the terpanes, hopanes are dominant, while γ-lupane and oleanane are generally detectable, but gammacerane is absent. Of the terpanes of lower molecular weight, C 24 tetracyclic terpane is comparatively abundant, in company commonly with C 15C 16 bicyclic sesquiterpane. These terrestrial crudes produced in the South China Sea exhibit very similar distribution pattern of steranes and terpanes to that of Eocene source rock in the area which had predominant input of terrestrial organic matter. Furthermore, the source rock is a lacustrine-paludal deposit in which a favorable reducing environment was present with vigorous growth of bacteria. These crudes are worthy of commercial exploration as they have an extensive occurrence in the South China Sea.

Adriatic deep water formation during the Holocene: Implication for the reoxygenation of the deep eastern Mediterranean Sea

Under modern conditions, the reoxygenation of eastern Mediterranean deep water is the result of the formation of dense bottom water in the southern Adriatic Sea. Oxygen isotopic records of planktonic foraminfera used as a paleosalinity tracer show that Po River and/or the alpine glaciers meltwater are not directly responsible of the inhibition of the deep water formation in the Adriatic, during sapropel (S1) formation at 9,000 years B.P.. Sedimentological evidence indicates that fully oxygenated conditions in the deep Adriatic occurred only by 1,650±100 years B.P., a long time after the total reoxygenation of the deep eastern Mediterranean. Organic carbon content and isotopic ratio indicate that persistence of a reduced sediment phase until 1,650 years B.P., does not result from increased primary productivity or large input of terrestrial organic matter.

Depositional Mechanisms and Organic Matter in Mowry Shale (Cretaceous), Wyoming

Four lithofacies, which accumulated under different depositional conditions, can be distinguished in the Lower Cretaceous Mowry Shale of Wyoming. Type and amount of organic matter in these lithofacies are governed largely by sediment transport mechanisms. Nearshore silt and mud containing terrestrial organic matter were deposited in a prodelta environment by tractive processes and from suspension. These sediments, along with terrestrial organic matter, were redistributed by waning bottom flows over a dysaerobic slope; pelagically deposited muds containing marine-derived organic matter accumulated between flow events. Muds farthest offshore accumulated in dysaerobic and anaerobic water by pelagic settling in a region unaffected by bottom currents; these muds contain predom nantly marine-derived organic matter. Petroleum source potential increases in a southeastward direction across west and central Wyoming. This trend results from differential input of marine and terrestrial organic matter, clastic dilution, and postdepositional biodegradation.

Geochemical Relationships of Petroleum in Mesozoic Reservoirs to Carbonate Source Rocks of Jurassic Smackover Formation, Southwestern Alabama

Algal carbonate mudstones of the Jurassic Smackover Formation are the main source rocks for oil and condensate in Mesozoic reservoir rocks in southwestern Alabama. This interpretation is based on geochemical analyses of oils, condensates, and organic matter in selected samples of shale (Norphlet Formation, Haynesville Formation, Trinity Group, Tuscaloosa Group) and carbonate (Smackover Formation) rocks. Potential and probable oil source rocks are present in the Tuscaloosa Group and Smackover Formation, respectively. Extractable organic matter from Smackover carbonates has molecular and isotopic similarities to Jurassic oil. Although the Jurassic oils and condensates in southwestern Alabama have genetic similarities, they show significant compositional variations due to differences in thermal maturity and organic facies/lithofacies. Organic facies reflect different depositional conditions for source rocks in the various basins. The Mississippi Interior Salt basin was characterized by more continuous marine to hypersaline conditions, whereas the Manila and Conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter. Petroleum and organic matter in Jurassic rocks of southwestern Alabama show a range of thermal transformations. The gas content of hydrocarbons in reservoirs increases with increasing depth and temperature. In some reservoirs where the temperature is above 266°F (130°C), gas-condensate is enriched in isotopically heavy sulfur, apparently derived from thermochemical reduction of Jurassic evaporite sulfate. This process also results in increased H2S and CO2 in the gas, and depletion of saturated hydrocarbons in the condensate liquids. Thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature, because some deep (18,000 ft or 5.5 km) and hot (320°F or 160°C) Smackover and Norphlet reservoirs contain low-sulfur petroleum.

Late neogene laminated and opal-rich facies from the Mediterranean region: Geochemical evidence for mechanisms of formation

The mechanisms for the formation of laminated and opal-rich sediments from southern Italy (Bianco and Vrica) and southern Sicily (Gela) are investigated using a geochemical approach. The Vrica laminates are relatively rich in organic carbon while the Gela laminites are not. Carbon/nitrogen ratios, carbon isotopic composition and lipid content of the organic matter at Vrica indicate that the laminates represent periods of increased input of terrestrial organic matter and enhanced preservation. Based on oxygen isotopic studies of planktonic foraminifera, a basin stratification model is presented to explain the formation of laminites at both Vrica and Gela. Biogenic silica and carbonate content, and the carbon isotopic composition of benthic foraminifera suggest that the Bianco diatomites represent periods of increased productivity. The isotopic composition of planktonic foraminifera from Bianco indicates that the increased productivity was a result of basin eutrophication brought on by continental runoff. The low organic carbon content of the Bianco diatomites is attributed to continuous bottom water ventilation. The results of laminites studies indicate that they are not always rich in organic matter, as generally assumed. Moreover the formation of organic-rich laminites can be the result of enhanced preservation, not increased productivity. Results from Bianco indicate that preservation plays a primary role in controlling the organic carbon content of sediments.

Selected organic matter source indicators in the Orinoco, Nile and Changjiang deltas

Carbon isotopic composition, carbon to nitrogen ratios, carbon preference ( n-C 23 to n-C 32), and microscopic examination of selected samples were compared and contrasted as organic matter source indicators. The Orinoco, Nile, and Changjiang deltas were sampled and analyzed. The δ 13C of organic matter increased with distance offshore at all three locations. Organic carbon content decreased seaward on the Orinoco and Changjiang deltas but increased on the Nile delta. C/N ratios decreased regularly with distance offshore on the Orinoco delta, ranged from 10 to 25, and correlated positively with δ 13C OM. Estimations of the relative input of marine versus terrestrial organic matter to the sediments was dependent on the parameter analyzed. These discrepancies were primarily due to the definition of end-member compositions and the fraction of the total organic matter that is measured by a given parameter. Multiple-source indicators must be measured and integrated and end-members must be properly defined in order to obtain realistic input estimates.

Radiocarbon dating of sediments by accelerator mass spectrometry

Accelerator mass spectrometry allows the measurement of the abundance of carbon isotopes in much smaller samples (1 mg of carbon in the form of graphite) than conventional methods of radiocarbon dating. The small sample requirement widens the potential for sediment dating. In this study, fractions of the sedimentary organic matter, for example, humic acid, lipid, macrofossils, were isolated and each fraction dated to yield a suite of dates for one sediment sample. Owing to the complexity of most sedimentary environments, a conventional radiocarbon date reflects the input of sediment from different sources with different carbon reservoirs which may combine to alter the date of the event that is required, i.e., the date of sedimentation. An estimate of the date of sedimentation may be made by taking specific fractions of the sedimentary organic matter which can be characterized and dated, for example, the lipid fraction, which can be sub-divided for analysis to yield information on the relative input of terrestrial and aquatic organic matter. Results have been obtained for five peat samples, the dates of the more water soluble components of the sedimentary organic matter tending to be younger than the lipid samples.

Molecular fossils from chemical degradation of macromolecular organic matter

Hydrogenolysis with rhodium-on-charcoal was found to be an effective method for degrading different types of macromolecular material of geological origin. Between 6 and 70% of coals, kerogens and humic substances were transferred into low-molecular-weight soluble materials. The reaction products contained a series of monomeric and dimeric lignin-derived compounds which strongly suggest intact fossil lignin as their source. The substitutional patterns of the released phenols reflect the type and amount of terrestrial organic matter input and diagenetic alterations. In addition to the lignin constituents, a complete suite of hydrocarbons differing from those of the low-molecular-weight fraction was obtained after hydrogenolysis of the sample material. Hydrocarbons released by the hydrogenolytic degradation technique were attached to the polymeric matrix as monoor polyethers. Deuterium was used in the degradation experiment to evaluate the sites of chemical bonds by which 4-methylsteranes and hopanes are attached to the kerogen matrix. These experiments suggested a linkage of the released molecules with ring A and the side-chain, respectively.

Biologically-derived compounds of significance in the saturate fraction of a crude oil having a predominant terrestrial input

A saturate fraction of a crude oil having a heavy input of terrestrial organic matter was analysed by gas chromatography/mass spectrometry g.c.-m.s. Data has previously been reported concerning the occurrence of dicyclic and tricyclic hydrocarbons in this oil. Here, data is presented concerning the occurrence of other compounds which may be indicators of terrestrial input to the sediment which ultimately yielded the reservoir of oil. Several tetracyclic compounds based upon the perhydro-cyclopenta [a] phenanthrene and perhydrochrysene skeletons are assigned tentative structures. It is shown that these may be novel terrestrial biomarkers. The tentative identifications of lupanes in the oil geochemically date the time of sediment deposition which yielded the oil as Tertiary.

Petroleum Source Beds: Environment of Deposition and Stratigraphy: ABSTRACT

Measurement of organic carbon content, alone, is insufficient to identify potential oil source beds because terrestrial OM (Organic Matter), oxidixzed planktonic OM, or reworked OM from a previous sedimentary cycle can create misleadingly high levels of organic carbon in marine sediments. Consequently, the presence of an organic facies, as identified End_Page 2465------------------------------ by kerogen typing, is essential to establish oil source rock character. Kerogen is mainly dependent on the origin of the precursor plant remains (whether planktonic or terrestrial) and on the oxidizing (oxic) or reducing (anoxic) character of the early depositional environment. Oxygen depletion, sufficient to arrest or minimize bioturbation at the benthic boundary, enhances oil source bed deposition, because it leads to almost entirely anaerobic microbial reworking of planktonic remains. This of bacterial reworking favors the preservation and concentration of lipids in the residual OM, leading to the formation of oil-prone koergens (types I and II). Marine oil source bed deposition and occurrence are controlled mainly by factors relevant to qualitative organic matter preservation during early sedimentation, rather than by planktonic productivity in the shallow euphotic zone. Many areas of high planktonic productivity, today, do not correspond with zones of high organic enrichment in bottom sediments (e.g., Grand Banks of Newfoundland, Antarctica, Australia, Northwest Shelf, Northeast Brazilian Shelf) because of oxic conditions, commonly in combination with low sedimentation rates. Conversely, whenever zones of high productivity, such as those present in some coastal upwellings, are underlain by anoxic water layers, then prolific oil source bed deposition does occur. Zones where deep ventilation and thus oxic conditions prevail at sea bottom are much more common than zones of oxygen depletion. Persistent oxic conditions at the benthic boundary lead to deposition of gas-prone, type III, to non-source, type IV organic facies, depending on sedimentation rate and amount of terrestrial organic matter input. Such unfavorable organic facies, resulting from past oxic conditions, have been commonly recognized as stratigraphically widespread under continental margins and in cratonic basins, regardless of past water depth. Thus, prolific oil source beds, in terms of relative rock volumes, are the exception in most sedimentary basins. These observations are compatible with the functioning of the carbon cycle: efficient organic matter recycling through mineralization, rather than enhanced preservation, is the most common and most probable fate of dead organic matter in the environment. Geochemical-sedimentologic evidence suggests that potential oil source beds are and have been deposited in the geologic past in four main anoxic settings as follows: 1. Large anoxic lakes: Permanent stratification promotes development of anoxic bottom water, particularly in large lakes which are not subject to seasonal overturn, such as Lake Tanganyika. Warm equable climatic conditions favor lacustrine anoxia and nonmarine oil source bed deposition. 2. Anoxic silled basins: Only those landlocked silled basins with positive water balance tend to become anoxic. The Baltic and Black Seas are examples. In arid-region seas (Red and Mediterranean Seas), evaporation exceeds river inflow, causing negative water balance and well-oxygenated bottom waters. Silled basins should be prone to oil source bed deposition at times of worldwide transgression, at high and low paleolatitudes. Silled-basin geometry, however, does not automatically imply the presence of oil source beds. 3. Anoxic layers caused by upwelling: These develop only when the oxygen supply in deep water cannot match demand owing to high surface biologic productivity. Examples are the Benguela Current and Peru coastal upwelling. No systematic correlation exists between upwelling and anoxic conditions because deep oxygen supply is commonly sufficient to match strongest demand. Oil source beds and phosphorites resulting from upwelling are present preferentially at low paleolatitudes and at times of worldwide transgression. 4. Open-ocean anoxic layers: These are present in the oxygen-minimum layers of the northeastern Pacific and northern Indian Oceans, far from deep, oxygenated polar water sources. They are analogous, on a smaller scale, to worldwide anoxic events which occurred at global climatic warmups and major transgressions, as in Late Jurassic and middle Cretaceous times. Known marine oil source bed systems are not randomly distributed in time but tend to coincide with periods of worldwide transgression and oceanic anoxia. Recognition of the proposed anoxic models in ancient sedimentary basins helps in regional stratigraphic mapping of oil shale and oil source beds. Furthermore, explanation and prediction of the most favorable zones for widespread and prolific oil source bed occurrence can be achieved by paleogeographic reconstructions (plate tectonics, paleoclimate, and paleo-oceanography) conducted in conjunction with seismic stratigraphy and regional geochemical studies. End_of_Article - Last_Page 2466------------