Marine incursion events in the Late Cretaceous Songliao Basin: Constraints from sulfur geochemistry records

Highlights

• The paper proved saline water conditions for the deposition of K₂qn¹.

• Sulfur geochemical data shows contribution of marine incursion to such condition.

• The marine incursion may have begun earlier than ever suggested.

Abstract

Songliao Basin in NE China developed the most productive oilfield in the world sourced from terrestrial rocks. The main source rock of the basin includes member 1 of the Qingshankou (K₂qn¹) and members 1 and 2 of the Nenjiang Formation (K₂nj^1–2). However the exact reasons for the formation of the source rock, especially the K₂qn¹ are still controversial. Former paleontological and organic geochemical research suggested that organic matter was deposited during marine incursion events of K₂qn¹ but further geochemical evidence is needed. This paper explores the distinct sulfate levels that distinguish marine from fresh waters of the Songliao paleo-lake. We undertook a systematic investigation the sulfur geochemistry of the sediments from top of member 4 of the Quantou Formation (K₁q⁴) to top of member 1 of the Qingshankou (K₂qn¹). The ratio of organic carbon to the pyrite sulfur (TOC/PYS) proved the previously suggested saline conditions during the deposition of K₂qn¹; and the pyrite sulfur isotope indicated that marine incursion may not only have occurred for K₂qn¹, but also for top of K₁q⁴. The exact time for the beginning of marine incursion is to be determined. The marine incursion within K₂qn¹ is not strong and partially controlled by the fluctuation of paleo-lake level. The present study will be helpful for understanding the contribution of marine incursion events to the burial of organic carbon in the Songliao paleo-lake, although further studies are still needed.

Introduction

As one of the largest late Mesozoic–Cenozoic continental sedimentary basins in East Asia, the Songliao Basin in NE China developed an almost complete Cretaceous succession (Chen, 1987, Chen, 2003). During the Cretaceous, the basin was a large rift basin that hosted a long-lived deep lake (Chen, 1987). This history enabled the basin to become the largest oil and gas producing basin in China, with China's largest oilfield, Daqing, being situated in the central part of the basin (Zhou and Littke, 1999).

Most of the source rocks in the Daqing oil field are terrestrial deposits; this may be the most productive oilfield in the world sourced from terrestrial rocks (Yang et al., 1985). However the exact reason for the formation of the mass hydrocarbon source rock, including the member 1 of Qingshankou and member 1 and 2 of Nenjiang Formation deposited in the Songliao Basin, is still unknown (Li and Pang, 2004). Prior studies of the formation of source rock in the Songliao Basin have found that the hydrocarbon source rock of Songliao was deposited under brackish-saline conditions (Hou et al., 2000 and references therein), despite the fact that the Songliao paleo-lake was mainly a freshwater lake during most of its life cycle. Paleontological and organic geochemical evidences suggested that salinity conditions may be due to an influx of seawater during a brief marine connection and lake transgression (Huang et al., 1999, Hou et al., 2000). For example the identification of marine dinophyceae, planktonic foraminifera, shark teeth, and biomarkers such as C30 4-desmethylsteranes in these strata would suggest conditions with a strong marine influence (Zhang and Zhou, 1976, Zhang et al., 1977, Gao et al., 1994a, Gao et al., 1994b, Huang et al., 1998, Gao and Zhao, 1999, Hou et al., 2000, Huang, 2007), however controversy questions the validity of these evidences (Ye and Zhong, 1990, Huang and Huang, 1998, Huang et al., 1999, Li et al., 2000, Ye et al., 2002). New evidences of the marine incursion events are still needed.

Different levels of sulfate distinguish fresh from marine water (~ 10 to hundreds of μM vs. 28 mM). The sulfate concentration of lake water would have increased and the geochemical behavior of sulfur changed if the lake was connected with the open ocean (Holmer and Storkholm, 2001). This effect is best illustrated by the modern Black Sea, which was formerly a fresh-water lake but was connected with the Mediterranean not long ago; sulfidic conditions prevail in the present water column (Yücel et al., 2010 and reference therein). A complete investigation of sulfur geochemistry of the sediments, i.e., the species and isotope ratio would be of help to understand the details of the sulfur cycle in an aquatic system (Holmer and Storkholm, 2001), and thus revealing the potential marine incursion events of this lake. Previous data have shown that the reduced sulfur was relatively enriched in member 1 of the Qingshankou and members 1 and 2 of the Nenjiang Formation deposited in the Songliao Basin, which makes such investigation possible (Huang, 2007). This work concentrates on the isotopic geochemistry of sulfur to elucidate if marine incursion events actually occurred during the deposition of member 1 of the Qingshankou Formation. The present study will also be of value in answering the long-debated question of whether the marine incursion really contributed to the organic carbon accumulation in the Songliao paleo-lake, because the sulfur cycle regulates the redox condition, nutrient status and affects the productivity of both the lake (Fleet et al., 1988, Caroca et al., 1993, Cohen, 2003) and marine systems (Adams et al., 2010).