Distribution, origin, and hydrocarbon-potential of organic matter in sediments from DSDP Leg 66 Holes

As part of an ongoing program of organic geochemical studies of sediments recovered by the Deep Sea Drilling Project, we have analyzed the types, amounts, and thermal alteration indices of organic matter collected from the Pacific continental margin of southern Mexico on Leg 66. The samples were pieces of core frozen aboard ship. Some of them were analyzed by pyrolysis, heavy C15+ hydrocarbons, and nonhydrocarbons to help determine their origin and hydrocarbon potential. Our main objectives were to find out how much organic matter was being deposited; to establish whether it derived from marine or terrestrial sources; to determine the controls of deposition of organic matter; to estimate the hydrocarbon potential of the drilled section; and to compare and contrast organic sedimentation here with that on other margins.

Organic facies of Cenozoic and Cretaceous sediments at DSDP Leg 80 Holes

The organic facies of Cenozoic sediments cored at DSDP Sites 548-551 along the Celtic Sea margin of the northern North Atlantic (Goban Spur) is dominated by terrestrially derived plant remains and charcoal. Similar organic facies also occur in the Lower and Upper Cretaceous sections at these sites. Mid-Cretaceous (uppermost Albian-Turonian) sediments at Sites 549-551, however, record two different periods of enrichment in organic material, wherein marine organic matter was mixed with terrestrial components. The earlier period is represented only in the uppermost Albianmiddle Cenomanian section at the most seaward site, 550. Here, dark laminated marly chalks rich in organic matter occur rhythmically interbedded with light-colored, bioturbated marly chalks poor in organic matter, suggesting that bottom waters alternated between oxidizing and reducing conditions. A later period of enrichment in organic material is recorded in the upper Cenomanian-Turonian sections at Sites 549 and 551 as a single, laminated black mudstone interval containing biogenic siliceous debris. It was deposited along the margin during a time of oxygen deficiency associated with upwelling-induced intensification and expansion of the mid-water oxygen-minimum layer. In both the earlier and later events, variations in productivity appear to have been the immediate cause of oxygen depletion in the bottom waters.

Organic geochemical characteristics at DSDP Holes 76-534A and 44-391C

Pyrolysis assay, bitumen analysis, and elemental analysis of kerogen were used to characterize the organic matter of selected core samples from Hole 534A (Leg 76) and Hole 391C (Leg 44) on the Blake-Bahama Plateau. The organic matter throughout the stratigraphic section appears to be principally of a terrestrial origin. The data from several isolated horizons in the Hatteras and Blake-Bahama Formations imply the presence of significant quantities of autochthonous marine organic matter. However, these horizons appear so limited that they cannot be considered potential liquid hydrocarbon source rocks. All the analyzed samples are immature and have not evolved sufficiently to enter into the main stage of hydrocarbon generation. The temporal and spatial restrictions of strata rich in marine organic matter suggest that they do not represent major expansions and contractions of anoxic bottom-water masses, but represent limited occurrences of anoxic conditions.

Composition of swamp waters from the lower Tom' River Basin

New data on chemical and trace component compositions of acidic and low acidic swamp waters and other types of low mineralized waters are reported in the paper. Special attention is paid to dissolved organic compounds: fulvic and humic acids, bitumen, and hydrocarbons. For the first time detailed data on organic trace components (alkanes, pentacyclic terpenoids, steranes, alkylbenzenes, naphthalenes, phenanthrenes, tetraarenes, etc.) in the swamp waters of the Western Siberia: are reported.

Geochemistry of carbon at DSDP Hole 61-462

Seven quartered sections of Pliocene to Mesozoic (Cenomanian) cores from the Nauru Basin contain primarily marine organic matter admixed with detectable amounts of terrigenous organic matter. The mixture is immature with respect to organic genesis. Chemical properties of this organic matter are compared with properties of other deep-ocean cores from DSDP sites in the central Pacific.

Petrography and geochemistry of organic matter in Triassic and Cretaceous sediments from the Wombat and Exmouth Plateau

Triassic (Carnian-Rhaetian) continental margin sediments from the Wombat Plateau off northwest Australia (Sites 759, 760, 761, and 764) contain mainly detrital organic matter of terrestrial higher plant origin. Although deposited in a nearshore deltaic environment, little liptinitic material was preserved. The dominant vitrinites and inertinites are hydrogen-lean, and the small quantities of extractable bitumen contain w-alkanes and bacterial hopanoid hydrocarbons as the most dominant single gas-chromatography-amenable compounds. Lower Cretaceous sediments on the central Exmouth Plateau (Sites 762 and 763) farther south in general have an organic matter composition similar to that in the Wombat Plateau sediments with the exception of a smaller particle size of vitrinites and inertinites, indicating more distal transport and probably deposition in deeper water. Nevertheless, organic matter preservation is slightly better than in the Triassic sediments. Long-chain fatty acids, as well as aliphatic ketones and alcohols, are common constituents in the Lower Cretaceous sediments in addition to n-alkanes and hopanoid hydrocarbons. Thin, black shale layers at the Cenomanian/Turonian boundary, although present at several sites (Sites 762 and 763 on the Exmouth Plateau, Site 765 in the Argo Abyssal Plain, and Site 766 on the continental margin of the Gascoyne Abyssal Plain), are particularly enriched in organic matter only at Site 763 (up to 26%). These organic-matter-rich layers contain mainly bituminite of probable fecal-pellet origin. Considering the high organic carbon content, the moderate hydrogen indices of 350-450 milligrams of hydrocarbon-type material per gram of Corg, the maceral composition, and the low sedimentation rates in the middle Cretaceous, we suggest that these black shales were accumulated in an area of oxygen-depleted bottom-water mass (oceanwide reduced circulation?) underlying an oxygen-rich water column (in which most of the primary biomass other than fecal pellets is destroyed) and a zone of relatively high bioproductivity. Differences in organic matter accumulation at the Cenomanian/Turonian boundary at different sites off northwest Australia are ascribed to regional variations in primary bioproductivity.

Hydrous and anhydrous pyrolysis of DSDP Leg 75 kerogens

The aliphatic hydrocarbon distributions obtained from the natural bitumens of three Leg 75 sediments were compared using computerised gas chromatography-mass spectrometry (C-GC-MS). The kerogens isolated from these sediments were heated in sealed tubes at 330°C using the techniques of hydrous (i.e. heating kerogen in the presence of water) and anhydrous pyrolysis (i.e. heating dry kerogen alone). These experiments were then repeated at a lower temperature (280°C). At 330°C, under anhydrous conditions, considerable destruction of biomarkers in the ancient kerogens (i.e. pre-Tertiary) occurred, whereas with water present significant amounts of hopanes were obtained. However, with more recent kerogens (which contain larger amounts of chemically bound water), both anhydrous and hydrous pyrolysis gave a similar suite of biological markers, in which long chain acyclic isoprenoids (C40) are significant components. Lowering the temperature of pyrolysis to 280°C yielded biological markers under both hydrous and anhydrous conditions for all kerogens. n-Alkenes were not detected in any of the pyrolysates; however, a single unknown triterpene was discovered in several of the hydrous and anhydrous pyrolysates. The results tentatively indicate that the chief value to petroleum research of kerogen hydrous pyrolysis lies in its ability to increase the yield of pyrolysate. High temperature hydrous pyrolysis (280-330°C), under high pressure (2000 psi), does not appear to mimic natural conditions of oil generation. However, this study does not take into account whole rock pyrolysis.

Gas hydrates of the Gulf of Mexico

Gas hydrates were recovered from eight sites on the Louisiana slope of the Gulf of Mexico. The gas hydrate discoveries ranged in water depths from 530 to 2400 m occurring as small to medium sized (0.5-50 mm) nodules, interspersed layers (1-10 mm thick) or as solid masses (> 150 mm thick). The hydrates have gas:fluid ratios as high as 170:1 at STP, C1/(C2 + C3) ratios ranging from 1.9 to > 1000 and d13C ratios from -43 to -71 per mil. Thermogenic gas hydrates are associated with oil-stained cores containing up to 7% extractable oil exhibiting moderate to severe biodegradation. Biogenic gas hydrates are also associated with elevated bitumen levels (10-700 ppm). All gas hydrate associated cores contain high percentages (up to 65%) of authigenic, isotopically light carbonate. The hydrate-containing cores are associated with seismic "wipeout" zones indicative of gassy sediments. Collapsed structures, diapiric crests, or deep faults on the flanks of diapirs appear to be the sites of the shallow hydrates.

Organic geochemical studies of DSDP Leg 64 Holes

Twenty-six core samples from Leg 64, Holes 474, 474A, 477, 478, 479, and 481A in the Gulf of California, were provided by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) Advisory Panel on Organic Geochemistry for analysis. The high heat flow characteristic of the basin provides an opportunity to study the effect of temperature on the diagenesis of organic matter. The contents and carbon isotope compositions of the organic matter and bitumen fractions of different polarity, isoprenoid and normal alkane distributions, and the nature of tetrapyrrole pigments were studied. Relative contents of hydrocarbons and bitumens depend on the thermal history of the deposits. Among other criteria, the nature and content of tetrapyrrole pigments appear to be most sensitive to thermal stress. Whereas only chlorins are present in the immature samples, porphyrins, including VO-porphyrins, appear in the thermally altered deposits, despite the shallow burial depth. Alkane distributions in thermally changed samples are characterized by low values of phytane to 2-C18 ratios and an odd/even carbon preference index close to unity. The thermally altered samples show unusual carbon isotope distributions of the bitumen fractions. The data also provide some evidence concerning the source of the organic matter and the degree of diagenesis.

Organic matter from black shales collected in DSDP Holes 41-367 and 41-368

It is shown that microscopic algae dominate in source material of organic matter of black shales, and admixture of residues of organisms and terrestrial humic material is contained. The main direction of source material transformation during syngenesis and sedimentogenesis is associated with jellofication resulting to formation of organic matter of significantly sapropelic type. Low reflectance of vitrinite and alginite from organic matter refer to the primary and secondary lignite stages of its carbonification. Significantly sapropel type of organic matter and low stage of carbonification are reliable criteria for assigning black shales to the category of potential oil source strata.

Nature, origin, and petroleum source potential of organic matter at DSDP Leg 56-57 Holes

As part of a continuing program of organic-geochemistry studies of sediments recovered by the Deep Sea Drilling Project, we have analyzed the types, amounts, and thermal-alteration indices of organic matter in samples collected from the landward wall of the Japan Trench on Legs 56 and 57. The samples were canned aboard ship, enabling us to measure also their gas contents. In addition, we analyzed the heavy C15+ hydrocarbons, NSO compounds, and asphaltenes extracted from selected samples. Our samples form a transect down the trench wall, from Holes 438 and 438A (water depth 1558 m), through Holes 435 and 435A (water depth 3401 m), and 440 (water depth 4507 m), to Holes 434 and 434B (water depth 5986 m). The trench wall is the continental slope of Japan. Its sediments are Cenozoic hemipelagic diatomaceous muds that were deposited where they are found or have slumped from farther up the slope. Their terrigenous components probably were deposited from near-bottom nepheloid layers transported by bottom currents or in low density flows (Arthur et al., 1978). Our objective was to find out what types of organic matter exist in the sediment and to estimate their potential for generation of hydrocarbons.

(Table 1) Geochemical and geothermometry data from fibrous calcite vein fills in the Dabashan Fold Belt, China

Fibrous calcite veins with organic inclusions have been widely considered as indicators of oil and gas generation and migration under overpressure. Abundant fibrous calcite veins containing organic-bearing inclusions occur in faulted Lower Paleozoic through Triassic hydrocarbon source rocks in the Dabashan Foreland Belt (DBF). d13CPDB and d18OPDB values of the fibrous calcite range from - 4.8 to -1.9 to per mil and - 12.8 to - 8.4 per mil respectively, which is lighter than that of associated carbonate host rocks ranging from - 1.7 to + 3.1 per mil and - 8.7 to - 4.5 per mil. A linear relationship between d13CPDB and d18OPDB indicates that the calcite veins were precipitated from a mixture of basinal and surface fluids. The fibrous calcite contains a variety of inclusions, such as solid bitumen, methane bearing all-liquid inclusions, and vapor-liquid aqueous inclusions. Homogenization temperatures of aqueous inclusions range from 140 to 196° with an average of 179°. Salinities of aqueous inclusions average 9.7 wt% NaCl. Independent temperatures from bitumen reflectance and inclusion phase relationships of aqueous and methane inclusions were used to determine fluid pressures. Results indicate high pressures, elevated above typical lithostatic confining pressure, from 150 to 200 MPa. The elevated salinity and high temperature and pressure conditions of the fibrous calcite veins argue against an origin solely from burial overpressure resulting from clay transformation and dehydration reactions. Instead fluid inclusion P-T data and geochemistry results and regional geology indicate abnormally high pressures during fluid migration. These findings indicate that tectonic stress generated fracture and fault fluid pathways and caused migration of organic bearing fluids from the DBF during the Yanshan orogeny.