Geochemical Characterization On The Occurrence Of Shale Oil

A suite of 30 lacustrine shale core samples from the Eocene Shahejie Formation of three shale oil exploration well in the Dongying Depression have been chemically(sequential solvent extraction)and thermally(multiple isothermal stages pyrolysis)extracted to characterize the relative content of shale oils in different physical status.Comparative study between these two approaches on the characterization of oil-in-place(OIP)and the variation of molecular compositions were performed.Moreover,constraints on shale oil producibility in the study area are discussed herein.Standard Rock-Eval pyrolysis,Soxhlet extraction and gas chromatography-mass spectrometry(GC-MS)performed on the lower subunit of the E_s3 member(E_s3_x)and the upper subunit of the E_s4 member(E_s4_s)suggests that organic-rich samples present a good material base for shale oil resource.Additionally,these shale strata are in an early to peak oil generation stage.Shale oil physical status was defined as free,adsorbed and residual oils herein,by carrying out sequential extraction on shales with different particle sizes.Quantitative data demonstrate that free oil manifest predominance among the total extractable organic matter(EOM)yields(with an average proportion of 66%).whilst the relative content of adsorbed and residual oils account for 16%and 17%averagely.The content of saturated hydrocarbon fraction shows a predominance among the EOM's bulk compositions and decreases sequentially from free-to residual oils.Conversely,relative content of polar compound fraction increases with the proceeding of extractions.Comprehensive analysis based on the correlation between EOM amounts and petrological factors is carried out herein.TOC and clay mineral content exert marked impact on adsorption ability and retention proportion,whilst brittleness index presents a negative correlation to EOM amounts.Thermal maturity shows critical impact on the total extract amount.Compared with standard Rock-Eval pyrolysis on OIP estimation,the MIS pyrolysis presents more detailed information concerning gross compositions and physical status of shale oils,which may reduce the impact of the frequently-occurred carryover effect to some extent.Moreover,coupling relationship is demonstrated between MIS pyrolysis and sequential extraction on the characterization of the macroscopic OIP and the microcosmic molecular compositions,especially the light vs.heavy n-alkanes.The oil saturated index,which was widely employed in the producibility assessment of marine shale oil systems,are not recommended to numerically applied in the assessement of complicated lacustrine shale oil system.Relatively low maturity coupled with high clay mineral contents are deemed as major constraints on shale oil producibility in the study area.With minor-or non-cracking in this maturity regime,shale oils herein are considered as‘black oil',whose viscosity is high and mobility is quite low.Quality of fluid(i.e.,the mobility)is a key factor in the assessment of shale oil resources as well.Future exploration ought to target at the shale intervals with maturity of late-to condense oil generation stage.