Organic Geochemical and Biological Marker Characterization of Source Rocks and Oils Derived from Lacustrine Environments in the Brazilian Continental Margin
M. R. Mello, J. R. Maxwell, 1990. "Organic Geochemical and Biological Marker Characterization of Source Rocks and Oils Derived from Lacustrine Environments in the Brazilian Continental Margin", Lacustrine Basin Exploration: Case Studies and Modern Analogs, Barry J. Katz
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Geochemical studies, together with paleogeographical and geological evidence, suggest that most of the organic-rich Neocomian to Aptian rift-stage succession in the Brazilian continental margin was deposited in lacustrine environments. It is possible to differentiate two lacustrine systems responsible for about 85% of Brazilian oil discovered to date—a relatively large, deep, fresh-water type of basin, ranging in age from early Neocomian to Aptian, and a shallow lower to upper Neocomian saline system.
Fresh-water systems are characterized by thick beds of dark-gray to black shale (TOC <6%). In the oils and source rocks abundant high molecular-weight n-alkanes, low S and V/Ni values, low <513C values, high Pr/Ph ratios, absence of dinosterane and C30 desmethyl steranes, and low concentrations of steranes and porphyrins characterize the fresh-water depositional environment.
Saline systems are composed of thick beds of calcareous black shales (TOC up to 9%). The oils and rocks, in addition to showing diagnostic evidence of a nonmarine environment, are characterized by features typical of deposition under saline conditions. These include higher V/Ni ratios, presence of β3-carotane, high concentration of C30 αβ-hopane, moderately abundant gammacerane, Ts/Tm < 1, and high d13C values.
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Lacustrine Basin Exploration: Case Studies and Modern Analogs
Lacustrine environments are a major contributor of petroleum source rocks. Lacustrine source rock prediction is, however, influenced by numerous, complex variables governing lake sedimentation. Current predictive capability can be improved by attempting to map essential climatic variables to limit in space and time the area of lacustrine source rock exploration. Climatic characteristics that govern lake occurrence and the potential for stratification have been investigated with a General Circulation Model of the atmosphere for the present and for the mid-Cretaceous. In this analysis, the distribution of areas with a positive water balance first is used as an indicator of the distribution of areas conducive to lake formation. Second, the distribution of areas that experience large annual climatic variations is used as an indicator of the distribution of lakes that are less likely to be stratified and, hence, less likely to be sites of high organic-carbon preservation. Four factors used to define large climatic variations include (1) seasonal temperature cycle in excess of 40°C; (2) seasonal temperature extreme of less than 4C°; (3) average seasonal differences in precipitation minus evaporation balance in excess of 5 mm/ day; and (4) distribution of mid-latitude winter storms. Evidence is presented to support the capability of climate models that add insight into lacustrine source rock prediction by simulating geographic regions conducive to lake development and to stratification and organic-carbon preservation