Abstract
The Cretaceous Era has always been a focus of geologic and palaeoenvironmental studies. Previous researchers believed that the impact of the global carbon cycle represents significant short-term global biogeochemical fluctuations, leading to the formation of a large number of organic rich sediments in the marine environment. During the Turonian, a large number of organic-rich oil shales were deposited in the lakes of the Songliao Basin in the Qingshankou Formation. How the depositional environment affected the formation of oil shales in continental lakes and the characteristics of these oil shales remain controversial. In this paper, through sampling of Qingshankou Formation strata, various testing methods are used to provide a variety of new data to study the characteristics of oil shales and palaeoenvironment evolution history in the Songliao Basin. The research of the sediments in the Qingshankou Formation in the Fuyu oilfield, Songliao Basin, via result analysis revealed that the oil shales possess an excellent oil-generation potential with moderate-high total organic carbon (TOC) levels (0.58–9.43%), high hydrogen index (HI) values (265–959 mg hydrocarbons (HC)/g TOC), high extractable organic matter (EOM) levels (2.50–6.96 mg/g TOC) and high hydrocarbon fractions (48–89%). The sources of the organic matter were mainly zooplankton, red algae and higher plants (including marine organisms). The aqueous palaeoenvironment of the Qingshankou Formation was a saline water environment with a high sulfate concentration, which promoted an increase in nutrients and stratification of the water density in the lake basin. Oxygen consumption in the bottom water layer promoted the accumulation and burial of high-abundance organic matter, thus forming the high-quality oil shales in the Qingshankou Formation. The global carbon cycle, warm-humid palaeoclimate, dynamic local biogeochemical cycling and relative passive tectonism were the most likely reasons for the TOC increase and negative δ13Corg deviation.
Highlights
Cretaceous events have always been a focus of geological and palaeoenvironmental research, characterized by a greenhouse climate and repeated oceanic anoxic events (OAEs) [1]
The results show that there are two pCO2 peaks in the whole Cretaceous period: the first peak occurs near the Cenomanian-Turonian (OAE2), and the second peak is at the S-C boundary
The results show that the Ro value is between 0.47% and 0.63%, which shows that the organic matter of the Qingshankou Formation in the Songliao Basin is mainly in the low-mature stage
Summary
Cretaceous events have always been a focus of geological and palaeoenvironmental research, characterized by a greenhouse climate and repeated oceanic anoxic events (OAEs) [1]. OAEs occurred during the Aptian-Albian (OAE1), Cenomanian-Turonian (OAE2) and Coniacian-Santonian (OAE3). OAEs may be related to an increase in atmospheric greenhouse gas concentration [2], and the increase in atmospheric greenhouse gas concentration during OAEs may have been enhanced by rifting and volcanism [3]. The results show that there are two pCO2 peaks in the whole Cretaceous period: the first peak occurs near the Cenomanian-Turonian (OAE2), and the second peak is at the S-C boundary. After the Early Campanian, the pCO2 level decreased significantly [4,5]. After the Early Campanian, the pCO2 level decreased significantly [4,5]. etc
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