Hydrocarbon Generation And Accumulation Effect Of Saline Lacustrine Strata Bearing Gypsum-salt Rock In Dongpu Sag,Bohai Bay Basin

Gypsum-salt rocks were extensively developed in the North Dongpu Sag,Bohai Bay Basin,where is the most important petroliferous province discovered.The aim of this study is to reveal the impact of gypsum-salt rock on potential source rocks properties,hydrocarbon generation and accumulation as well as secondary alteration of hydrocarbon within reservoir,which are still unclear enough.Gas chromatography-mass spectrometry?GC-MS?combined with fourier transform ion cyclotron resonance mass spectrometry?FT-ICR MS?and compound specific carbon isotope analysis are utilized for 31 crude oils and 22 rocks on the background of basic geological investigation.The results show that the property and hydrocarbon generation progress were strongly influenced by gypsum-salt rocks in the area.It was observed that organic matter abundance and types of the potential source rocks in gypsum-salt strata are much better than those in strata without gypsum-salt,and the quality of the source rocks exhibit great heterogeneity.Dark grey and brown shale are recognized as the excellent source rocks in area studied with TOC content over 1.5%,dominated by type I and type II₁ organic matter.Gypsum-salt obviously influences hydrocarbon generation and evolution of the source rock.It was found that oil generation window would be expanded if the thickness of gypsum salt is over 50 m in the Wenliu area,and both the oil and wet gas window would be enlarged within certain increase of salt rock thickness.However,oil generation window would be narrowed while wet gas window would be further enlarged once the thickness of salt rocks over 400 m,which is ascribe to relatively high thermal conductivity of gypsum-salt rock.A hydrocarbon generation mode for the potential source rocks bearing gypsum-salt in the Wenliu area was established based on a comprehensive study of geology and geochemistry.It was suggested that the chemical compositon and relative distributiaon of source rock extracts and related oils in the area were strongly influenced by gypsum-salt rocks.The content of organic sulfur compounds in the oils?4.07%¹0.50%?derived from source rocks in gypsum-salt bearing strata is much higher than those?<4.00%?from fresh water depositional environment.Abounant organic sulfur compounds like S₁?S₂?N₁S₁?O₁S₁?N₁O₁S₁?O₁S₂?O₂S₂ were detected by ESI FT-ICR MS in the crude oils from the Wenliu area with S₁ class species dominanted,followed by O₁S₁ class species.The distribution pattern of S₁ class species were predicted to be controlled by several factors like maturity.Saline sedimentary environment and the activity of sulfate reducing bacteria in early stage of diagenesis is essential for the enrichment of organic sulfur compounds in rock-extracts and crude oils.Significant predominance of phytane over pristine,abnormal high abundance of nC₃₇,nC₃₈ and gammacerane were detected in the samples collected,which were the typical characteristics of saline lacustrine oils.The pattern of compound-specific carbon isotopes for oils appears as sideling linear shape,a little different from some saline lacustrine oils reported previously.Besides of abnormal high content of sulfur compound?8.95%¹0.50%?analyzed by GC-MS were detected in the oils from Pushen 18 well-area with gypsum-salt rock developed in the reservoir,we also detected abundant S₁ species sulfur compounds with DBE values of 3⁸ by ESI FT-ICR MS,which is a kind of compound with a relative low thermal stability usually occurred in immature oils.We suggest that it is a result of thermochemical sulfate reduction?TSR?,currently occurring or occurred before for a relatively high thermal maturity level of the oils(C₂₉-sterane???/????+????ratio:0.60⁰.69).It was also suggested that gypsum-salt rocks have strong controlling on several other aspects such as hydrocarbon distribution,reservoir quality,PVT,formation water and hydrocarbon migration in the Wenliu area after our comprehensive investigation.This study would be helpful for further unraveling hydrocarbon generation and accumulation mechanisms in the area,and indicative for further petroleum exploration and exploitation.