The origin, type and hydrocarbon generation potential of organic matter in a marine-continental transitional facies shale succession (Qaidam Basin, China)

Abstract

This organic-rich shale was analyzed to determine the type, origin, maturity and depositional environment of the organic matter and to evaluate the hydrocarbon generation potential of the shale. This study is based on geochemical (total carbon content, Rock-Eval pyrolysis and the molecular composition of hydrocarbons) and whole-rock petrographic (maceral composition) analyses. The petrographic analyses show that the shale penetrated by the Chaiye 2 well contains large amounts of vitrinite and sapropelinite and that the organic matter within these rocks is type III and highly mature. The geochemical analyses show that these rocks are characterized by high total organic carbon contents and that the organic matter is derived from a mix of terrestrial and marine sources and highly mature. These geochemical characteristics are consistent with the results of the petrographic analyses. The large amounts of organic matter in the Carboniferous shale succession penetrated by the Chaiye 2 well may be due to good preservation under hypersaline lacustrine and anoxic marine conditions. Consequently, the studied shale possesses very good hydrocarbon generation potential because of the presence of large amounts of highly mature type III organic matter.

Geochemical and Petrographic Characterization of Marine-Continental Transitional Facies Shale (Qilian Basin, China): Origin of Organic Matter Input, Depositional Environments and Hydrocarbon Generation Potential

“…From maturity, the shales of marine and marine-continental transitional facies shale in Sichuan Basin and South China, from maturity, were in the high-over mature stage (Table 3; Ma et al, 2012;Tuo et al, 2016;Wang et al, 2018;Dai et al, 2020). Similarly, the shales in the major regions of the United States (for example, Barnett, Haynesville, and Woodford) were also in the high-over mature stage (Dai et al, 2020).…”

“…Approximately 150 g of ground material from each shale sample was subjected to Soxhlet extraction using chloroform for 72 h at a constant temperature of 70 • C. The extracts from the shale samples were deasphalted by precipitation with n-hexane and filtration. The deasphalted maltenes were fractionated into saturates and aromatics via column chromatography using activated silica gel and aluminum oxide (v: v = 3:1) with n-hexane and methylene chloride, respectively (Wang et al, 2018). Saturates and aromatics were then analyzed by the GC-MS system.…”

“…From the point of view of TOC, the abundance of organic matter of shale is high (average > 1.6%) (Table 3) in marine-continental transitional facies shale from Qilian Basin and shale of marine, continental and marine-continental transitional facies in China and the major regions of the United States (for example, Barnett, Haynesville, and Woodford) (Ma et al, 2012;Tuo et al, 2016;Wang et al, 2018;Dai et al, 2020). The minimum TOC of marine-continental transitional facie shale in Qilian Basin was higher than marine-continental transitional facie shale in other regions of China (Table 3), which indicates that the percentage of high-quality shale in the Qilian Basin shale is higher.…”

“…The type of organic matter for marine-continental transitional facies shale in Qilian Basin and Major regions of China were all Type III (Table 3), which were quite different from the marine and continental shale in China and the shale in the major areas of the United States (such as Barnett, Haynesville, Woodford) (Ma et al, 2012;Tuo et al, 2016;Wang et al, 2018;Dai et al, 2020).…”

“…In terms of biomarkers of shale, the marine-continental transitional facies shale in Qilian Basin and the marinecontinental transitional facies shale in Qilian Basi and the marine shales and the continental shales in China were in the mature stage (Wang et al, 2011(Wang et al, , 2018Ma et al, 2012;Zhang et al, 2013;Tuo et al, 2016;Guo et al, 2019;Huang et al, 2020). This conclusion could be enables by various geochemical parameters, such as OEP and CPI and Sterane C 29 -20S/(20S + 20R) and Sterane C 29 -ββ/(ββ + αα), and hopane C 31 -22S/(22S + 22R) and hopane Ts/Tm (Table 4).…”