Organic Geochemistry of Triassic Lacustrine Yanchang Formation Shale, The Ordos Basin, North-Central China

Abstract

Mesozoic strata source rock samples from Yangchang Formation in the Ordos Basin were studied to determine the variations in organic matter input, depositional environments and thermal maturities. A series of organic geochemical analyses have been conducted to evaluate the source rock generation potential and thermal evolution. Our results show that the organic matter from Upper Triassic Yangchang Formation is dominated by Type II kerogen for organic-rich shale, which TOC content varies from 4% to 10%. Geochemical signatures of low proportion of isoprenoids to n-alkanes indicate that organic matter source originated from fresh-water lacustrine aquatic organisms and land plants, which generated high paraffinic and waxy hydrocarbons for Yangchang Fm. Furthermore, tricyclic terpanes with abietane skeleton were found in all the samples, indicating gymnosperm higher plant resins and tissue relics. Preservation of organic matter in the lacustrine system is dependent on the formation of anoxic or micro-oxic bottom waters. A consistent reducing depositional environment was beneficial in organic matter accumulated in Yanchang Formation. Moderate Pr/Ph ratios and poorly preserved higher homohopanes, combined with the low gammacerane indexes and low C35 homohopane indexes, suggest that the source rock from Yanchang Formation was deposited under a weakly reducing to sub-oxic, freshwater depositional environment. The maturity of the source rock is in the oil window, and a crude increasing trend with current burial depth appears with exceptions of two samples. The two samples, whose present burial depths are deeper than 1700 m, exhibit maturity levels as low as 0.66 (Rc%), which equal to that of the shallowest sample at the present depth of 407 m. The thermal maturities of these two samples do not correspond to their present burial depths. The inconsistence of burial depth and thermal maturation of these two samples probably related to organic facies variations along the analyzed section, and further reflects the complexity of organic matter input and maturation of lacustrine Yanchang Formation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016