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Abstract: Origin of Chinese Lacustrine High-Wax Oils and Organic Petrology of its Source Rocks

WANG FEIYU, PING HE, SHISHENG HAO

Most of Chinese lacustrine crude oils are characteristics of high wax content, typically in Songlia basins and BiyangNanyang and Damington depressions. It is generally assumed that wax in crude oils, mainly long chain (>C22) n-alkane, originated from higher plant organic matter, especially cuticular waxes, cutinite and sporinite. However, The source rocks kitchens of Chinese lacustrine crude oils are located in deep and semi-deep lake environments, where organic matter usually corresponds to kerogen type I or II. A throughout organic petrology studies of Chinese lacustrine source rocks revealed very high liptinite contents, frequently more than 70% of total macerals identified were lamalginite, telalginite and sapropelinite, and only less than 8% were cutinite and sporinite. Transmission electron microscopy observations indicated amorphous organic matters or sapropelinites were rich in ultralaminae, which derived from the selective preservation of the algaenas occurring in the very thin outer walls of various freshwater algae, some amorphous organic matters, such as from Oligocene Hetaoyuan Formation of Biyang depression, and from Cretaceous Qinshankou formation of Songlia basins, were predominately composed of ultralaminae accumulation. The source rocks in Oligocene Hetaoyuan Formation of Biyang depression are mainly pelagic organic-rich calcareous mudstone, in which kerogen are nearly exclusively composed of lamalginites and ultralaminae derived from Pediastrum of Chlorphyceae. Hydrous pyrolysis of immature Pediastrum kerogen in the temperature range of 280-320 resulted in the almost exclusive generation of n-alkanes, including abundant long chain(>C22) n-alkane. The distribution and carbon isotope composition of these artificially generated hydrocarbons are similar to that of saturated hydrocarbons of naturally occurring high-wax crude oils. Therefore, the main precursor for Chinese lacustrine high-wax oils are some freshwater algae.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky