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NE Versus SW Quartersphere Lacustrine Source Rocks: Case Study from Northeastern Asian and South American Basins*
Vlad Andrus1 and Tianguang Xu1
Search and Discovery Article #30109 (2010)
Posted February 12, 2010
*Adapted from poster presentation at AAPG Convention, Denver, Colorado, June 7-10, 2009
1IHS, Houston, TX ([email protected])
Southwestern and northeastern quarterspheres contain a vast portion of the world’s petroleum reserves that originated from lacustrine source rocks. For instance, lacustrine oils in China and Brazil amount to 90% and 95% of their total reserves, respectively, and in the latter the number may rise even higher due to the recent super-giant offshore discoveries linked to lacustrine sources. Studied lacustrine rocks range from Carboniferous to Holocene in age; however, most of them belong to the Cretaceous and Tertiary. These Cretaceous to Tertiary lacustrine source rocks were deposited in diverse basin types.
The objective of this study is to investigate the differences and similarities between lacustrine source rocks deposited in diverse paleolatitudes and paleolongitudes as well as in contrasting hydrological conditions (freshwater vs. saline lakes). Special attention is also paid to organic matter production and preservation. Such a complex approach helps to establish relationships of lakes’ paleogeography, hydrology, organic matter production and preservation with source rocks quality and reserves produced.
Comparative analysis demonstrates that most prolific lacustrine source rocks from South American basins were accumulated under more arid conditions than those from Northeastern Asia (mostly tropical-subtropical). In Latin America, most productive lacustrine source rocks were deposited in the Lower Cretaceous brackish-saline lakes of contemporary Brazil. Those in present China were deposited in the Cretaceous freshwater lakes and Tertiary freshwater to hypersaline lakes. Considered lacustrine source rocks from NE and SW quarterspheres are distinguished mainly with regards to their depositional settings, TOC contents, kerogen types and compositions of generated hydrocarbons.
uAbstract
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Southwestern and northeastern quarterspheres contain a vast and comparable (85-95 Bbo each) portion of the world’s petroleum reserves that originated from lacustrine source rocks. For instance, lacustrine oils in China and Brazil amount to 90% and 95% of their total reserves, respectively, which constantly increase. In Brazil, in particular, reserves may increase significantly by about 40 Bbo due to the recent super-giant offshore discoveries in the Santos Basin linked to lacustrine sources.
Studied lacustrine rocks range from Permian (e.g., Junggar Basin) to Holocene (e.g., Qaidam Basin) in age; however, most of them belong to the Cretaceous (e.g., Campos and Songliao basins) and Tertiary (e.g., Bohai Gulf Basin). These Cretaceous to Tertiary lacustrine source rocks currently recognized in different types of basins.
Lacustrine rocks are distributed unevenly in the sedimentary cover of the South American continent while lacking in its central part (Figure 1). The oldest lacustrine rocks are of Carboniferous-Permian age and are known in some Brazilian, Argentinean and Peruvian basins. Despite the wide age range of these rocks varying from Permian to Pleistocene, proven source rocks are related to the Late Triassic-Late Cretaceous interval, with the most prolific ones affiliated with Early Cretaceous age. All commercial hydrocarbon accumulations known in the southwest quartersphere are situated in rift basins, the overwhelming majority of which originated as a result of Jurassic-Early Cretaceous rifting during the break-up of Africa and South America. During the syn-rift phase, Neocomian lacustrine rocks accumulated at the Brazilian Atlantic margin.
In Northeastern Asia, lacustrine source rocks vary from Permian to Holocene in age, whilst being most prolific in Cretaceous and Tertiary sections. They were deposited in rift/foreland-type basins. Considered lacustrine source rocks from NE and SW quarterspheres are distinguished mainly with regards to their depositional settings, TOC contents, kerogen types and compositions of generated hydrocarbons.
Depositional Setting
Paleoreconstruction modeling demonstrates that the most important (in terms of reserves) lacustrine basins of the SW quartersphere (contemporary Brazil) were located in close proximity to 30º S latitude, thus substantially closer to the equator than basins of the present-day Argentina. The NE quartersphere’s basins were situated north of 30º N latitude in the same, Early Jurassic-Cretaceous time (Figure 2). Lacustrine SR deposition in the NE quartersphere during Cenozoic time occurred closer to the equator (i.e., south of 30º N latitude).
Climate in Early Cretaceous time was arid in the contemporaneous Brazilian part (Atlantic margin) and semiarid in the present Argentinean sector (e.g., San Jorge Basin). Hydrological regime in the lakes varied from fresh to hypersaline, though in most productive basins it was brackish-saline. In the NE quartersphere, climate was warm-temperate/subtropical in Early Cretaceous and earlier times, changing to semiarid/tropical in the Cenozoic (Table 1). Freshwater component in these basins persisted during Cenozoic, except for several saline basins (Western Qaidam, etc.) that existed in the arid environment.
TOC Content
Total Organic Carbon (TOC) content of lacustrine rocks in South American basins varies from 0.2% to 13%, being typically in the 2-4% range. In most productive brackish-saline basins, TOC content does not exceed 6%; however, TOC content varies significantly in freshwater to freshwater-saline basins where it reaches 10-13%. In the northeastern Asian basins, TOC content is typically lower, although it occasionally reaches 12-14% (Beibu Gulf and Minhe basins). No correlations are found between TOC content and reserves amount in the basins of both quarterspheres.
Kerogen Type
Type I kerogen (algal) is prevalent in most prolific lacustrine basins of both quarterspheres. Mixed I/II type kerogens are known in the basins where marine transgressions occurred into lacustrine depositional settings (e.g., Espirito Santo, Potiguar, Jizhong, Beibu Gulf basins). Mixture of I/II/III and II/III types of kerogen characterizes lacustrine rocks of more than half of NE Asian basins, including all the major basins.
Hydrocarbon Composition
Lacustrine oils in both quarterspheres are enriched in saturated vs. aromatic hydrocarbons, have elevated isoprenoid ratios, low sulfur content and sterane/hopane ratio. Unlike the SW quartersphere lacustrine oils, those in NE Asia are often waxy, enriched in C29 ααα-20R steranes in freshwater oils and C28 steranes in saline/hypersaline oils compared to C27 regular steranes.
● Comparative analysis demonstrates that most prolific lacustrine source rocks in the SW quartersphere accumulated under more arid conditions than those from the NE quartersphere (mostly tropical-subtropical-semiarid), depending on their paleolatitudinal positions. ● In South American basins, most productive lacustrine source rocks were deposited in the Lower Cretaceous brackish-saline lakes of contemporary Brazil. Those in present China were deposited in the Permian-Cretaceous freshwater/freshwater-brackish lakes and Tertiary freshwater to hypersaline lakes. ● Depositional setting peculiarities resulted in specific distribution of lacustrine petroleum reserves in both quarterspheres, as only 1.8% of South American reserves originated from freshwater/with freshwater component basins, and only 3.5% of Northeastern Asian reserves derived from saline/hypersaline (i.e., without freshwater component) basins (Figure 3). ● Climate and tectonic variabilities predetermined hydrological regimes which in turn defined the type of organic matter and thus composition of generated hydrocarbons. ● SW quartersphere’s lacustrine oil compositions are more uniform compared to that of NE quartersphere which ensued from more diverse depositional environments and, therefore, kerogen types, in the latter.
The authors thank IHS for giving permission to present data from IHS Database, as well as Didier Arbouille for his review and Matt Luangkhot for drafting the SA lacustrine basins location map.
Berman, A., 2008, Three super-giant fields discovered offshore Brazil: World Oil, 229 (2), p. 23-24.
Hu, J., and D. Huang, 1996, Nonmarine Petroleum Geology of China: Petroleum Industry Press, Beijing, China, 270 p.
Katz, B.J., and M.R. Mello, 2000, Petroleum systems of South Atlantic marginal basins - an overview: in M.R. Mello, and B.J. Katz, eds., Petroleum Systems of South Atlantic Margins, AAPG Memoir 73, p. 1-13.
Schiefelbein, C.F., J.E. Zumberge, N.R. Cameroon, and S.W. Brown, 1999, Petroleum systems in the South Atlantic margins: in N.R. Cameron, R.H. Bate, and V.S. Clure, eds., The Oil and Gas Habitats of the South Atlantic, Geological Society, London, Special Publication 153, p. 169-179.
Scotese, C.R., 2000, Paleomap Project. http://www.scotese.com/ecretcli.htm
Wu, C., and S. Xue, 1992, Sedimentology of Petroliferous Basins in China: Petroleum Industry Press, Beijing, China, 484 p. (in Chinese).
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