Wang, Feiyu1, Pingan Pang2, Shuichang Zhang3
(1) Petroleum University of China, Beijing, China
(2) SKLOG, Guangzhou institute of Geochemistry, CAS, Guangzhou,
(3) Research Institute of Petroleum Exploration and Development, CNPC, Beijing, China
ABSTRACT: Early Cambrian Oceanic Anoxic Event in China and Petroleum Geological Implication
Black shale formation of early Cambrian age are widely distributed in the south (Yangtze area) and the northwest part (Tarim and marginal area) of China, the distribution area about 97´104Km2, and 50´104Km2 respectively in the Yangtze plate and Tarim plate. The early Cambrian organic-rich sequences are typically 100 to 300m thick, and are composed of dark-coloured, laminated shales and interbedded limestone layer and phosphorite concretions, Typically containing 0.70 to 20.0% TOC, maximal 65%TOC. Organic petrologic analysis indicated that the precursor of organic matters mainly originated from plankton algae, minor from zooclast(Tunicata). Sedimentological, geochemical and paleoecological investigation suggest that two genetic models of early Cambrian black shale formation, the first scenario is upwelling-induced oxygen minimum zone (OMZ ), the second scenario is anoxic bottom water (ABW), occurring in two distinct palaogeograghic regime: continental margins ocean and the epicontinental seas. The present organic maturity of early Cambrian black shales are mainly overmature, vitrinite reflectance equivalence(VRE) range from 1.8% to 4.2%. Early Cambrian black shales are most oldest widespread organic-rich source rocks in China, which generated abundant oils and gases in the Paleozoic and Mesozoic era, The present natural gas and solid bitumen in Yangtze area and gigantic Silurian tar sandstone and gas in Tarim cratonic basin were originally derived from Cambrian anoxic organic-rich source rocks. Aryl isoprenoids and gammacerane widespreadly detected in oil inclusions and asphaltene pyrolysate of Silurian tar sandstones in Tarim cratonic basin, which also indicate that the cambrian source rocks deposited under euxinic condition with sulfate and sulfide-rich water bodies.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.