--> Abstract: The Giant Lacustrine Source Rock Systems of the South Atlantic Realm:Implications in the Deep and Ultra Water Exploration, by Marcio R. Mello, Nilo C. Azambuja Filho, and John M. Moldowan; #90078 (2008)

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The Giant Lacustrine Source Rock Systems of the South Atlantic Realm:Implications in the Deep and Ultra Water Exploration

Marcio R. Mello1, Nilo C. Azambuja Filho1, and John M. Moldowan2
1HRT Petroleum, Rio de Janeiro, Brazil
2Biomarker Technology, Sebastopol, CA

During the opening of the South Atlantic, from Late Jurassic through Aptian, a giant lacustrine rift system was developed. Based on sedimentological and palinological data seven major climatic fluctuations were identified; four of them representing changes from arid to semi-arid and three from semi-humid to humid conditions, both linked with higher frequency Milankovitch oscillations. The integration of these data with high resolution geochemical technology, involving biological markers, diamondoids and carbon isotopic data of biomarkers (CSIA-B) and diamondoids (CSIA-D) of oils and lacustrine organic-rich sediments of rift stage allowed the differentiation between (i) a deep, relatively large, lacustrine system raging from fresh to brackish water (Early Neocomian to Aptian), (ii) a shallow brackish/saline water system (Late Neocomian), and (iii) a shallow hypersaline system (Late Neocomian). Within those lacustrine settings, a successive expansions and contractions of the water bodies, caused variations in the water column salinity and, hence, development of a lacustrine fresh to a hypersaline water depositional environments. Such conditions are clearly reflected by, for example, the type of organisms that thrive in each specific niche. In the same succession, the 13C values of the oils and bulk organic matter also reflect differences in water column chemistry, indicating lower degrees of depletion of 13C under restrictive, increased saline, conditions. In all the lacustrine environments studied, the organic-rich rocks are always associated with the periods that occur just after major lake transgressions; in which more anoxic bottom water conditions were established. This implies that in deep and ultra deep water regions less siliciclastic inputs and better source rock properties were developed, therefore increasing the hydrocarbon source potential in these areas.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas