Lacustrine Environment and Organic Matter Enrichment Mechanism for Source Rock of the Third Member of the Shahejie Formation in the Raoyang Sag, Bohai Bay Basin, Eastern China

Abstract

The third (Es₃) member of the Eocene Shahejie Formation is the main hydrocarbon source rock interval in the Raoyang Sag of Bohai Bay Basin. The controlling factors of source rocks formation are investigated with geochemical methods of 60 samples from 3 wells to reconstruct lacustrine environment during the deposition of Es₃. The clay assemblages of source rocks defined on the basis of X-ray diffraction analyses exhibit the significant kaolinite accumulation in the lower Es₃ and the illite accumulation in the upper Es₃. This phenomenon indicates that the paleoclimate gradually changed from humid to arid during the deposition of Es_3. The indicators of water salinity such as Sr/Ba, B/Ga and gammacerane index suggest freshwater lakes with unstable thermocline during the lower Es₃ deposition, saline lakes with salinity stratified water column during the deposition of the upper Es₃. Carbon isotopic signatures of organic matters and carbonates (δ¹³C_om and δ¹³C_carb), elements P and Ba are used as paleoproductivity proxies show that the paleoproductivity level had no clear periodical variation during the Es₃ deposition. North American shale composite-normalized rare earth elements (REEs) patterns display sloping LREE trends and flat HREE trends with no obvious depletion of Ce (δCe ranges from 0.72 to 0.92, with average of 0.85) and Eu (δEu ranges from 0.93 to 1.05, with average of 0.98), thus the results can be taken as indicative of the anoxic depositional environment and low sedimentary rate of source rocks during the deposition of Es₃.Furthermore, Pristane/Phytane ratio, enrichment factors of trace elements Mo and U display obvious variation along with the strata changes, and distinguish the redox gradation of environmental conditions. The Pr/Ph ratio for the lower Es₃ varies greatly from 0.46 to 1.41 with average of 1.01, a relatively low and narrow range of 0.1 to 0.36 with average of 0.19 for the upper Es₃. And, the enrichment of Mo and U occurs in the samples of the upper Es₃. These redox proxies indicate that the organic matters preservation conditions of the upper Es₃ are more ideal than those of the lower Es₃. On the other hand, a positive correlation between Δδ¹³C (difference between δ¹³C values of carbonates and organic matters) and hydrogen index values proposes that enhanced anoxia triggered formation of the source rocks during the deposition of Es₃.This study may have important implications for source rock prediction in other lacustrine fault basins.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018