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