ABSTRACT: Reflectance of vitrinite-like maceral derived from benthic multicellular algae as organic maturity indicator for pre-Silurian marine strata

Wang, Feiyu , Petroleum University of China, Beijing, China

The organic macerals resembling vitrinite were found in pre-Silurian marine strata, famous in Cambrian-Ordovican Alum Shales of Southern Scandinavia (Kisch, 1980, Buchardt et al. 1986, 1990), Buchardt et al(1990) suggested the reflectance of vitrinite-like macerals can be used as thermal maturity index for pre-Silurian rocks, but origin of vitrinite-like macerals are still controversial. The organic maturity assessments of Cambrian-Ordovician and Neoproterozoic source rocks in Tarim Basin and north China indicate that only reflectance of vitrinite-like maceral derived from benthic multicellular algae can be used as good organic maturity indicator for pre-Silurian marine strata. Megascopic carbonaceous fossils shaped like leaves exist widely in Chinese Neoproterozoic and lower Paleozoic marine source rocks, the oldest fossil records found at 1700MABP Tuanshanzi Formation of the uppermost Paleoproterozoic Changcheng Group (1600-1800MABP) in north China, which reflect the emergence of multicellular organisms. The typical megascopic leaflike fossils consists of foliates, parastem and even rhizoid, such as Lonfengshania, Chuaria, Tawuia, Thallophyca, Fascicullella etc. According to paleobiological analysis, they are benthic, multicellular algal fossils similar to modern kelp or seaweed. Comparison microscopically organic maceral with megascopically fossill forms indicate all leaflike fossil organic matters occur as nonfluorescent, hydrogen-poor, gas-prone vitrinite like macerals. Fluorescent alginite and nonfluorescent vitrinite-like maceral coexist in mature Ordovician and Neoproterozoic source rocks in Tarim Basin and North China, which suggest that these nonfluorescent vitrinite-like macerals mostly originate from primary hydrogen-poor benthic multicellular algae, derived from organic remains rich in polysaccharides. The reflectance relationship of vitrinite and vitrinite-like macerals has been established as VRo=0.533VLRo+0.667 through organic petrological analysis of natural maturation sequence and artificial pylolysis experiments. The organic maturity of Cambrian-Ordovician marine source rocks in Tarim Basin have been discussed as an example based on reflectance of vitrinite-like macerals and other marine derived organic macerals including graptolites and conodont.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia