M.R. Mello

Geochemical and biological marker assessment of depositional environments using Brazilian offshore oils

Abstract A combined geochemical and molecular characterization of a wide selection of oils from the major Brazilian offshore basins has been undertaken. The elemental (sulphur, nickel and vanadium) and bulk (oAPI and δ13C) properties of each sample been considered, together with its molecular composition determined using liquid and gas chromatography, and quantitative biological marker investigations using gas chromatography-mass spectrometry for alkanes. The results reveal significant differences in the chemical features of the various oils which enable them to be divided into five groups. The distinction of the groups appears to reflect differences in the depositional environment of the source rocks of the oils. Each group is correlated tentatively with source rocks laid down in a specific depositional regime, namely lacustrine freshwater, lacustrine saline water, marine evaporitic, marine carbonate or marine deltaic. The diagnostic features that allow this classification are: the relative abundance and carbon number distributions of n-alkanes; pristane/phytane ratios; sulphur, nickel and vanadium contents; carbon isotope data; the absolute concentrations of hopanes and steranes, and their abundance relative to 4-methylsteranes and, also the occurrence and abundance of several specific biological markers, including 18α(H)-oleanane, gammacerane, β-carotane, tricyclic terpanes, higher acyclic isoprenoids, 28, 30-bisnorhopane and 25, 28, 30-trisnorhopane. This investigation shows the value of a combined geochemical and molecular approach in the assessment of the palaeoenvironment of deposition of the source rocks which gave rise to the oils.

Organic geochemical characterisation of depositional palaeoenvironments of source rocks and oils in Brazilian marginal basins

Abstract An investigation of bulk, isotopic and molecular features of a wide selection of source rocks and oils from Brazilian marginal basins, ranging from Lower Cretaceous to Tertiary in age, provides a number of organic geochemical criteria that characterise and distinguish their depositional palaeo-environments. The methods employed include evaluation of organic carbon contents, Rock-Eval pyrolysis, vitrinite reflectance measurements, determination of carbon isotope ratios, elemental and visual kerogen analysis, assessment of oils and solvent extracts by liquid and gas chromatography, and quantitative biological marker investigations using GC-MS for aliphatic hydrocarbons and UV/Vis spectrophotometry for metalloporphyrins. Selected data were evaluated using principal component analysis. The results enable the recognition and differentiation of seven depositional regimes: lacustrine freshwater; lacustrine saline water; marine evaporitic; marine carbonate; marine deltaic; marine highly anoxic with a predominance of calcareous mudstone lithology, and marine anoxic with predominance of siliciclastic lithology. Diagnostic features in the biological markers from these depositional environments include: the carbon number distributions of n -alkanes, pristane/phytane ratios; absolute concentrations of β-carotane, higher acyclic isoprenoids, C 30 αβ hopane and steranes, and the abundance of the latter relative to 4-methylsteranes; the occurrence and abundance of particular biological markers such as gammacerane, tricyclic and tetracyclic terpanes, 18α (H)-oleanane, 28,30-bisnorhopane and 25,28,30-trisnorhopane, and the relative abundances of nickel and vanadyl porphyrins.

Application of 24-norcholestanes for constraining source age of petroleum

Abstract Two C 26 sterane ratios, the 24-nordiacholestane ratio (NDR) and the 24-norcholestane ratio (NCR), are useful for assessing petroleum source age. Values for these parameters generally increase in the Cretaceous when diatoms were rapidly radiating and further in the Oligo-Miocene due to widespread high-latitude deposition of diatomaceous sediments. Sources with shale lithologies tend to exhibit the highest ratios; while carbonate and terrigenous-rich lithologies generally show lower ratios. On the North Slope of Alaska, elevated ratios are observed for Cretaceous HRZ oils and mixed oils containing Cretaceous input compared to older Triassic and Jurassic derived oils. Tertiary-sourced oils from Romania and Bulgaria have much higher ratios relative to Jurassic-sourced oils in the Black Sea region. Oleanane was observed in comparable levels in some Black Sea Tertiary and Jurassic oils and source rocks. NDR and NCR coupled with other age-diagnostic indicators (such as oleanane and dinosterane) provide powerful tools for differentiating age of source and organic facies for oils from the Black Sea and Sub-Andean Basins of Peru and Ecuador, respectively. Oils derived from sources deposited in low tropical latitudes tend to have little diatom input and as a consequence, do not display high values regardless of age.

Geochemical Characterization of Natural Gas: A Physical Multivariable Approach and its Applications in Maturity and Migration Estimates

Gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons. In this paper, graphical representations of physico-chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified. These diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data. From 11 chemical and isotopic ratios, a statistical analysis (PCA) yields two very important parameters. The first parameter, using mainly the C2+ fraction of the gas, relates to maturity and the second parameter, involving the proportions and d13C values of methane, indicates mainly segregative migration. Positive values of the second parameter indicate that gases accumulated far from their source, whereas negative values correspond to residual gas pools after leakage of a part of the fluids. A tentative reconstruction of the gas history has been performed in two Brazilian basins: the Espirito Santo basin and part of the Reconcavo basin. The Espirito Santo basin is located on the passive continental margin of the Atlantic Ocean, and the Reconcavo basin corresponds to an intracontinental aborted rift. In both cases, the source rocks are mainly lacustrine, with thermal maturities ranging between the oil window and the beginning of the gas window. Results show that in the Reconcavo basin, a major fault (the Mata Catu fault) acts as a drain for hydrocarbon migration at the basin scale, associating a major isotopic fractionation to the gas migration with a clear correlation between isotope fractionation and the distance of migration. In the Espirito Santo basin, this segregation appears in the platform sediments to a lesser extent and is absent in the gas pools located in the paleocanyons filled with turbidites. This long-distance migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore.

Extended tricyclic terpanes in sediments and petroleums

An extended series of tricyclic terpanes was identified in sediments and petroleums from Brazil. The identification of the tricyclic terpanes was based on their chromatographic behavior and characteristic MS/MS transitions from molecular ions to m/z 191. We identified tricyclic terpane homologs up to C54. The presence of this extended series, from C19 up to C54, varies considerably with depositional environment, being prominent in saline lacustrine and marine carbonate settings. This suggests that the biological precursor organism lived in moderate conditions of salinity.

Late Cretaceous anoxic events in the Brazilian continental margin

Abstract A combined geochemical and micropalaeontological study of Cenomanian to Maastrichtian pelitic sediments from the Brazilian continental margin has been undertaken. The organic carbon contents, Rock-Eval pyrolysis results and biological marker distributions, together with the taxonomic selection and distribution patterns of microfossil assemblages indicate that intermittent anoxic events occurred in the Brazilian marginal basins during the Cenomanian-Santonian with deposition taking place in progressively deeper waters from neritic to bathyal. Such events are characterised by the following: 1. (i) a predominance among the benthonic foraminifera of low-diversity, small-sized specimens (mainly composed of gavelinellids, buliminids and nodosariids) indicating “dysaerobic” bottom conditions; 2. (ii) levels with virtually no benthonic foraminifera and a well-developed planktonic biota (foraminifera, radiolarians and diatoms) indicating “anaerobic” bottom conditions with oxygenated surface waters; 3. (iii) the occurrence of organic rich calcareous mudstone sediments; 4. (iv) features in the biological marker distributions such as high concentrations of 28,30-bisnorhopane and 25,28,30-trisnorhopane (up to 130 ppm), a predominance of phytane over pristane, and medium to high concentrations of nickel and vanadyl porphyrins (up to ca 5700 ppm). The widely recognized Cenomanian-Turonian and Santonian “oceanic anoxic events” (Schlanger and Jenkyns, 1976; Ryan and Cita, 1977 and Jenkyns, 1980) are therefore manifest in the Cassipore, Ceara, Sergipe/Alagoas and Campos basins within the Brazilian continental margin, as is the presence of such events in the Coniacian. In contrast, the Campanian-Maastrichtian appears to be a time interval when deposition under oxygenated conditions produced sediments with low organic carbon contents and poor hydrocarbon source potential, containing highly diversified and abundant calcareous and agglutinated benthonic foraminifera.

Origin of Oil in the Sureste Basin, Mexico

Geochemical and biological marker analyses of oils and rock samples from the Sureste basin of Mexico were effective in identifying and geographically limiting four major oil families related by age and source rock depositional environment: Oxfordian, Tithonian, Early Cretaceous, and Tertiary. The source rocks giving rise to the Jurassic and Cretaceous oils are associated with marine carbonate environments. In contrast, the source rocks giving rise to the Tertiary oils are associated with a marine deltaic siliciclastic depositional setting. Biomarker and isotope differences observed in the oils derived from marine carbonate environments can be interpreted in terms of salinity, clay content, and oxygen depletion variations. These differences provide diagnostic criteria for recognizing and differentiating five distinct organic-rich depositional regimes as the sources for these oil types: an anoxic hypersaline marine-carbonate environment associated with a narrow and shallow semirestricted sea (Oxfordian age, family 1 oil); an anoxic marine-carbonate environment associated with a silled basin geometry (Tithonian age, family 2 oils, subtype 2a); an anoxic marine-carbonate environment associated with a shallow, gentle, broad marine-carbonate ramp in a distal position (Tithonian age, family 2 oils, subtype 2b); a clay-rich suboxic/anoxic marine-carbonate environment associated with a carbonate platform in a proximal position (Tithonian age, family 2 oils, subtype 2c); and an anoxic marine-evaporitic environment (Early Cretaceous age, family 3 oils). The Tertiary oils (family 4) are derived from bacterially reworked terrigenous and marine organic source materials deposited in a marine-deltaic environment. The Tithonian-related oils in the Mexican southern side of the Gulf of Mexico accumulated both offshore and onshore and throughout the stratigraphic column from Kimmeridgian to Pleistocene reservoirs, suggesting vertical pathways as the principal secondary migration mechanism. The lateral variations of these oils can be interpreted to reflect the Tithonian paleogeography in the area and could be useful in predicting differences in the oil compositions.

The upper Aptian-Albian succession of the Sergipe basin, Brazil: An integrated paleoenvironmental assessment

A combined micropaleontological, geochemical, and sedimentological study of the upper Aptian-Albian succession from the Sergipe basin, northeastern Brazil, has been undertaken. The paleoenvironmental evolution of the basin from the late Aptian to late Albian can be subdivided into three major depositional phases: (1) late Aptian, (2) early to middle Albian; (3) late Albian. A shallow carbonate compensation depth within upper mesopelagic depths (c. 300-500 m) is inferred for the late Aptian-Albian. Intermittent anoxic events, associated with salinity-stratified water masses, occurred in the basin during the late Aptian to Albian. An oxygen minimum (dysaerobic to anoxic conditions) occurred during the late Aptian to earliest Albian, in middle-outer neritic to upper bathyal settings. Waning dysaerobic to oxic conditions are apparent in the late Albian. The foraminiferal assemblages recovered from the upper Aptian marine deposits have a characteristic Tethyan affinity. The microfaunal evidence suggests that this area of the northern South Atlantic had at least some surface water exchange with low-latitude central North Atlantic-western Tethyan Provinces, even possible at intermediate (epipelagic to mesopelagic) water depths. Contribution of microfaunal elements coming from high-latitude northern (Boreal Realm) and/or southern (Austral) regions is also apparent, although of less significance.

Micropalaeontological and geochemical evidence of mid-Cretaceous dysoxic-anoxic palaeoenvironments in the Sergipe Basin, northeastern Brazil

Abstract Combined micropalaeontological, geochemical and sedimentological studies of the mid-Cretaceous (upper Aptian to Turonian) succession of the Sergipe Basin, northeastern Brazil, indicate that intermittent dysoxic-anoxic events occurred in the basin, most likely associated with the interplay of processes such as restricted physiography in the deep basin, salinity-stratified water masses, increased epipelagic primary productivity and high sea levels. Three maxima in oxygen depletion are recorded in the succession from middle neritic to upper bathyal environments: the late Aptian-earliest Albian, the early Cenomanian, and the latest Cenomanian-earliest Turonian. Improving oxidizing conditions (dysoxic to oxic) are apparent during middle-late Albian and early-middle Turonian times. The existence of a rich and diverse planktonic biota throughout the mid-Cretaceous succession suggests widespread oxygenated epipelagic layers of variable thickness in space and time in the basin.

3-Alkyl and 3-carboxyalkyl steranes in marine evaporitic oils of the Potiguar Basin, Brazil

Abstract The geochemical characterization of marine evaporitic oils from the Fazenda Belem oil field, Potiguar Basin, Brazil showed good correlation between both acidic and neutral fractions, and led to the identification of several biomarkers including the homologous series of 3-alkyl- and 3-carboxyalkyl-5α(H) steranes and also the new series of 3-alkyl- and 3-carboxyalkyl-5β(H) steranes. These compounds were identified using GC/MS and their structures were confirmed by coinjection with synthetic standards. A minor series of 4-carboxyalkyl steranes was also detected. The analysis of the acidic biomarkers allowed a more accurate evaluation of maturation, biodegradation and migration of the studied oils.