Paleosalinity assessment and its influence on source rock deposition in the western Pearl River Mouth Basin, South China Sea

Abstract

Paleosalinity is an important environmental feature but it is difficult to evaluate. In the present study, paleosalinity was assessed during the deposition of three sets of source rocks in the western Pearl River Mouth Basin, South China Sea, utilizing four different kinds of methods, i.e., saturated hydrocarbon biomarkers, strontium abundance, non-pollen microalgae assemblies, and carbon-sulfur relationships. Results show that the second member of the Eocene Wenchang Formation (E2w2) was deposited in a freshwater environment and the Oligocene Zhuhai Formation (E3z) was deposited in a shallow marine environment. The Oligocene Enping Formation (E3e), which was believed to be deposited in a freshwater environment, was actually deposited in a brackish water environment. Mechanisms of salinity increase during the non-marine E3e deposition were mainly deep hydrothermal fluid input through the south boundary fault and episodic marine transgressions, not evaporation. The effect of salinity on organic matter content and type was investigated. Results show that salinity has no significant influence on total organic carbon (TOC) and hydrogen index (HI) of the E2w2, which was caused by the balance between freshwater algae and euryhaline algae. TOC and HI decrease with increasing salinity for samples from the E3z, which is contrary to the conventional hypothesis that marine transgressions promote source rock deposition. The decrease of TOC with carbon/sulfur ratios for samples from the E3e actually reflect the influence of thermal maturity but thermal maturity only plays the second role in HI. The effect of salinity on HI during the E3e deposition can be divided into two stages. During the first stage, the increase of salinity was mainly caused by deep hydrothermal fluid input without an oxygen level increase. HI values, therefore, remained relatively stable. During the secondary stage, the increase of salinity was mainly caused by marine transgressions which increased the oxygen level and as a consequence, HI decreased sharply with increasing salinity. This study provides a long-term salinity evolution of the western Pearl River Mouth Basin and suggests that salinity is an important factor controlling source rock deposition. In addition, this study presents an example that goes against conventional wisdom that marine transgressions promote source rock deposition in a shallow marine environment. This study also suggested that marine transgressions had already begun at the end of the early Oligocene.

Algal-Microbial Community, Paleoenvironment, and Shale Oil Potential of Lacustrine Organic-Rich Shale in the Upper Cretaceous Nenjiang Formation of the Southern Central Depression, Songliao Basin (NE China)

“…The nearly equal δ 13 C values (−34 to −30‰) are present in Pr and Ph, arguing for a similar origin (i.e., chlorophyll) and against major contributions of Pr and Ph from tocopherols and phytanyl ether lipids, respectively. 5 The GI and ETR values in most of the LORS are close to the third member of Heotaoyuan Formation in the Biyang Depression, 42 the first member of Shahejie Formation in the Bohai Bay Basin, 27 and the Enping and Zhuhai Formations in the Pearly River Mouth Basin, 41 which were deposited in saline to brackish environments (Figure 6C). A minor proportion of LORS (3 samples) displays lower GI and ETR values, similar to the Dongying Formation and the third member of Shahejie Formation in the Bohai Bay Basin, 27 and the second member of Wenchang Formation in the Pearly River Mouth Basin, 41 which were deposited in freshwater environments.…”

“…SLB = Songliao Basin, BYD = Biyang Depression, BBB = Bohai Bay Basin, and PRMB = Pearl River Mouth Basin. Data of BYD, BBB, and PRMB is from refs , and . (D) Cross-plot of phytane/C 18 n -alkane ratios vs pristane/C 17 n -alkane.…”

“…Several biomarker parameters, including GI, ETR, and Pr/Ph ratios are used to reveal the paleoenvironment (paleosalinity and paleoredox conditions) in this study. The precursor of gammacerane is considered to be tetrahymanol (gammacerane-3β-ol), which is derived from bacterivorous ciliates thriving at or below the chemocline in stratified water columns. , Great abundance of gammacerane is generally associated with evaporite or hypersaline environments. ,, ETR has been applied successfully to reflect the paleosalinity in lacustrine basins. ,− A positive correlation is commonly present between ETR and GI, therefore high ETR is corresponding to high paleosalinity (Figure C). The Pr/Ph ratio is a widely used redox indicator, with ratios of <1.0, 1.0–3.0, and >3.0 are indicative of anoxic, dysoxic, and oxic environments, respectively .…”

“…The GI and ETR values in most of the LORS are close to the third member of Heotaoyuan Formation in the Biyang Depression, the first member of Shahejie Formation in the Bohai Bay Basin, and the Enping and Zhuhai Formations in the Pearly River Mouth Basin, which were deposited in saline to brackish environments (Figure C). A minor proportion of LORS (3 samples) displays lower GI and ETR values, similar to the Dongying Formation and the third member of Shahejie Formation in the Bohai Bay Basin, and the second member of Wenchang Formation in the Pearly River Mouth Basin, which were deposited in freshwater environments. The three samples with low GI and ETR values were collected from wells in the north-western margin of the study area, which was one of the source areas during deposition of the studied interval .…”

The influences of sedimentary environments on organic matter enrichment in fine-grained rocks of the Paleogene Shahejie formation in Nanpu Sag, Huanghua Depression, Bohai Bay Basin

“…Shale oil and gas mainly occur in organic-rich fine-grained sedimentary rocks, and the abundance of shale oil and gas is closely related to the enrichment degree of organic matter (OM). OM enrichment is largely controlled by the depositional environments and benefits from high primary production and strong preservation conditions (Demaison and Moore, 1980; Doner et al, 2019; Pedersen and Calvert, 1991; Quan et al, 2019). Previous studies have focused on the controlling effects of environmental changes on OM enrichment at different scales.…”