Evidence of paleogene saline lake basin in the 3rd and 4th members of Shahejie Formation in Jiyang Depression and geological significance of shale oil and gas

Jiyang Depression is one of the most prolific conventional oil and gas depressions in the Bohai Bay Basin, having also withnessed a significant breakthrough in the comprehensive strategic exploration of shale oil. Within the Paleogene Shahejie Formation, two primary sets of source rocks, namely the Upper Submember of the Fourth Member (ESS4) and Lower Submember of the third Member (ESX3 ), are prevalent, with their sedimentary environment being controversial. Evidence from core well petrology, element geochemistry, isotopes and inclusion data suggests that the shale systems in ESS4 and ESX3 originated in a salinized lake basin under climate influences. The main evidence are as follows: ESS4 and ESX3 are predominantly characterized by carbonate-rich laminar shale, with an average carbonate mineral mass fraction of 43%, primarily occurring in laminar and lenticular forms. NaCl, BaSO4 and SrSO4 which are typical salty lake basin minerals have been identified in shale samples using back-scattered scanning electron microscope, indicating relatively high water salinity during the shale deposition period. The trace element composition of the shale reflects a salty environment, with the w(Sr) / w(Ba) exceeding 1 and Sr mass fraction surpassing 500 μg / g, while B element mass fraction exceeds 75 × 10-6 , with equivalent B mass fraction ranging mainly from 300×10-6 to 700×10-6 . The w(87 Sr) / w(86 Sr) of shale falls between 0. 710 7 and 0. 712 4, significantly higher than that of contemporaneous seawater. Positive anomaly characteristics of corresponding rare earth elements, such as Ce, suggest that the water conditions during the sedimentary period were influenced by the basin environment rather than transgression events. Carbonate minerals δ18 O range from -11 to -3, while δ13 C values range from 0 to 6. Large δ18 O and δ13 are related to a relatively dry climate in the lake basin and enhanced evaporation of lake water during deposition periods. The homogenization temperature of gas-liquid two-phase primary brine inclusions in shale ranges from 32. 5 ℃ to 45. 3 ℃, corresponding to salinity ranges of 19. 21% ~ 27. 78%. This suggests high formation fluid salinity during the early diagenetic stage. The saline water environment under climatic influence played a pivotal role in the formation of carbonate-rich shale, laying the groundword for the enrichment and high yield of shale oil in ESS4 and ESX3 of the Paleogene in Jiyang Depression. © 2024 University of Petroleum, China. All rights reserved.