The Influence of Volcanic Ash Sediments on the Formation of Lacustrine Organic-Rich Shale in Ordos Basin, Central China

Abstract

The organic-rich shale of the 7th member (Ch7) of Upper Triassic Yanchang Formation is a very important hydrocarbon source rock in Ordos basin, central China, and contains an abundance of volcanic ash sediments (VAS) intervals ranging from a few millimeters to dozens of centimeters in thickness. The organic-rich shale and VAS had been studied extensively in sedimentology, petrology and geochemistry. However, whether VAS influenced the formation of Ch7 organic-rich shale and how it did are still ambiguous. In order to investigate the problem, 55 organic-rich shale and 37 VAS samples from drill cores of Ch7 are collected and their total organic carbon (TOC) content, major and trace elements have been analyzed. The organic-rich shale has very good to excellent TOC content ranging from 0.28% to 36.83%, with an average of 6.75%. P and Fe are important nutrient elements. The averages of P2O5 and TFe2O3 in Ch7 organic-rich shale are 0.39% and 7.22%, respectively. These averages are higher than that of the North America shale, which are 0.13% and 5.66%. In contrast, the averages of P2O5 and TFe2O3 in Ch7 VAS are 0.08% and 1.76%, respectively. They are lower than that of the fresh volcanic ash (15 groups data collected from published papers), which are 0.20% and 5.60%. These data of P2O5 and TFe2O3 indicate that when volcanic ash deposited, P and Fe elements were transferred into the lake water from volcanic ash and kept in the shale formed later. Furthermore, the P2O5 and TFe2O3 contents of dense samples of organic-rich shale which covered over VAS decrease gradually from bottom to top, that is to say, the closer the distance from VAS is, the higher the P2O5 and TFe2O3 contents are. The TOC also has the same trend accordingly and shows an obvious positive correlation with the P2O5 and TFe2O3 contents, which indicates that the VAS influences the distribution of TOC, P2O5 and TFe2O3. In addition, it is found that the contents of TOC, P2O5 and TFe2O3 are higher in VAS-rich areas than those places where VAS are rare, which indicates that the distribution of VAS controlled the P2O5 and TFe2O3contents, and further influenced the TOC of organic-rich shale. In conclusion, VAS could influence the formation of Ch7 organic-rich shale by providing nutrient elements to improve lacustrine primary productivity. Ch7 organic-rich shale is characterized by high contents of P2O5 and TFe2O3 because P and Fe elements were absorbed by phytoplankton and were buried with them into shale.

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