Biomarker distributions and depositional environments of continental source rocks in Sichuan Basin, SW China

Abstract

In order to study the distributions of the biomarker of the continental source rocks in the Sichuan Basin, 71 source rock samples were collected from the Upper Triassic-Lower Jurassic strata in different regions. The n-alkanes, isoprenoids, terpane, sterane, sesquiterpenes, caranes and aromatics in the extracts were analyzed in detail. GC-MS analysis has been conducted to analyze the biomarker of the continental source rocks. The results of GC-MS analysis indicate that the Upper Triassic source rocks are high in the content of extended tricyclic terpanes, pristane, phytane, gammacerane, C28 regular sterane and carotene. However, they are low in content of rearranged compounds. The ratio of Pr/Ph is less than 1, with the characteristics of tricyclic terpane C21 > C23. The Lower Jurassic source rocks are extremely low in content (even zero) of extended tricyclic terpanes, pristane, phytane, gammacerane, C28 regular sterane and carotene, and high in content of rearranged compounds. The ratio of Pr/Ph is more than 1, with tricyclic terpane C21 > C23. These characteristics are still preserved after maturation. Moreover, during the sedimentation of the source rocks of T3x2–T3x3 members, the supply of continental plants was low (TAR < 1, with regular sterane C27 > C29, 1-MP/9-MP < 1). The source rocks of T3x5 member were low in salinity (slightly low content of gammacerane and carotene), being different significantly from the other Upper Triassic source rocks. In addition, during the sedimentation of the source rocks of J1dn Member, the supply of continental plants was also low (regular sterane C27 > C29, 1-MP/9-MP < 1), being quite different from that of J1l member. Through analysis of the difference in biomarkers, it is indicated that the sedimentary environment had changed from anoxic and brackish water during the Late Triassic to oxygen-rich and freshwater during the Early Jurassic in the Sichuan Basin. During this process, the types of organic matters had changed for several times.

Molecular and petrographical evidence for lacustrine environmental and biotic change in the palaeo-Sichuan mega-lake (China) during the Toarcian Oceanic Anoxic Event

“…7). This phenomenon has been reported previously for the Da'anzhai Member in Zhu et al (2007) on oils from different wells and in Huang et al (2020) on TLE from shales, which was interpreted as related to the high maturity and the enriched clay content of this source rock, deposited under oxic to suboxic and slightly acidic conditions. Diahopanes (15Me-27-nor hopanes) are most likely derived from the same precursor molecules as the normal hopanes but have undergone oxidation in the D-ring and rearrangement by clay-mediated acidic catalysis (Peters et al, 2005).…”

Redox conditions and ecological resilience during Oceanic Anoxic Event 2 in the Western Interior Seaway

“…12-13) when compared to biotic crises documented from more open ocean settings, such as the end-Triassic mass extinction (~0-0.03: Kasprak et al, 2015), and Toarcian OAE (0.01-0.04, French et al, 2014). Values are more in line with those found during the CTB at other WIS sites, such as west-central Alberta (0.07-0.21 : Furmann et al, 2015) and central Texas (up to 0.58 : French et al, 2019), alongside those values for lacustrine deposits (0.04-0.42 (Chen and Summons, 2001;Huang et al, 2020;Rizzi et al, 2020;Yin et al, 2020). This suggests that the WIS experienced particularly stratified conditions during the CTB, although local variation exists (e.g., values <0.03 in Utah; Boudinot et al, 2020).…”

Maceral and Organic Geochemical Characteristics of the No. 6 Coal Seam from the Haerwusu Surface Mine, Inner Mongolia, China

“…F, DBT, DBF and their alkylated homologues are important PACs and widely distributed in different types of sediments and coals. Their relative abundances in different depositional environments vary distinctly (Huang et al, 2020; Hughes et al, 1995; Zhang et al, 2012). The relative abundance of F in the three compounds (F, DBT, DBF) from the Haerwusu No.…”