Hydrocarbon Genesis and Organic Facies in Cambrian Carbonates of the Eastern Officer Basin, South Australia

Heterogeneity of alkaline lacustrine source rocks of the lower Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, northwestern China

“…Ts/Tm ratios of the studied samples are not very high (Figure 5(d)), indicating that the Fengcheng Formation source rocks from the Manan area have not reached the over-mature stage. Moreover, the proportion of mud in the source rocks from the Manan area is relatively high, which precludes the possibility that the clay mineral contents were too low to inhibit the conversion of Tm to Ts (McKirdy et al, 1984). Compared with the source rocks from the Wuxia–Fengcheng area, the Ts/Tm ratios of samples from Wells JL48 and JL49 and some samples from Well MH39 in the Manan area are significantly higher, which may reflect the higher maturity of samples from the Manan area.…”

A new constraint on the antiquity of ancient haloalkaliphilic green algae that flourished in a ca. 300 Ma Paleozoic lake

“…(d) elevated 3β-MeHI values up to 14.2% (mean = 6.5%) pointing to an enhanced methane cycle within the sediments and water column (Rohrssen et al, 2013), and (e) low pristane/phytane ratios (<1.0; mean = 0.6) characteristic of bottom water reducing conditions, as well as small but discernible peaks for C 21 -C 25 regular isoprenoids relative to n-alkanes likely reflecting from halophilic archaeal communities (Figure 3a; Grice, Schouten, Nissenbaum, et al, 1998;McKirdy et al, 1984). This combination of microbial lipid biomarker features has been reported previously from a variety of younger ancient saline alkaline lakes (Collister et al, 1992;Grice, Schouten, Peters, et al, 1998;Horsfield et al, 1994;Ruble et al, 1994;Stüeken et al, 2019).…”

“…Due to the high salinities resulting from evaporative conditions, alkaline lacustrine sediments are typically characterized with major lipid contributions from transported long-chain leaf wax n-alkanes, elevated abundances of acyclic isoprenoids from halophilic and methanogenic archaea, and elevated tetrahymanol/gammacerane from bacteria and/or ciliates indicative of water column stratification (Grimalt et al, 1991;Huguet et al, 2011;Randlett et al, 2017;Romero-Viana et al, 2012;Sarkar et al, 2014). Previous ancient lipid biomarker investigations of saline and alkaline lacustrine rocks from the Paleozoic era are few in number, such as from the Lower Cambrian Observatory Hill Formation in Australia which is characterized by abundant C 21 -C 25 regular isoprenoids (McKirdy et al, 1984) and Middle Permian Lucaogou Formation in China which is characterized by abundant β-carotane (Ding et al, 2020). As such, the microbial community structure of Paleozoic soda lake ecosystems has not been extensively studied.…”

A second type of highly asphaltic crude oil seepage stranded on the South Australian coastline

“…However, the low values of this ratio obtained from the asphaltic tars (0.19-0.27) are inconsistent with other thermal maturity parameters which indicate that these samples originated from the late oil window (discussed in Section 3.3). In addition to thermal maturity, the relative influence of lithology and redox conditions of the source rock on the Ts/(Ts+Tm) ratio is not fully understood, with several studies reporting systematically lower values in oils derived from carbonate source rocks deposited under anoxic conditions (McKirdy et al, 1983(McKirdy et al, , 1984Rullkötter et al, 1985;Price et al, 1987). Aside from this anomalous parameter, the composition of the asphaltic tars appears consistent with their derivation from an anoxic shale.…”

“…• Lateral variability in euxinic conditions across basin • Thermal decomposition of organosulphur compounds in oil to gases (Orr, 1974;Tissot and Welte, 1984) (3) Heavier δ 34 S values • Uptake of sulphur from dissolved sulphates and/or back reaction of H2S during TSR (Orr, 1974;Lillis and Selby, 2013) • Greater relative abundance of Tm attributed to higher carbonate content and anoxic conditions rather than thermal maturity (e.g. McKirdy et al, 1983McKirdy et al, , 1984Rullkötter et al, 1985;Price et al, 1987)…”