Microscopic and Fractal Characterization of Organic Matter within Lacustrine Shale Reservoirs in the First Member of Cretaceous Qingshankou Formation,Songliao Basin,Northeast China

Understanding the occurrences of different fractions of organic matter in shale reservoirs is very important for the evaluation of shale oil.Lacustrine organic-rich shale samples from the first member of the Cretaceous Qingshankou Formation in the Songliao Basin were analyzed with confocal laser scanning microscopy(CLSM)and fluorescence spectral analysis.The results show that the occurrences of different organic components are related to the fabric of samples and vary with depth.The bulk content of light components is significantly higher than heavy components and exhibits a positive relationship with quartz and feldspar contents.Both light and heavy components are distributed parallel with the lamination in microscopic view in laminated samples,and randomly in massive samples.The maximum radius of light components in most of the samples is larger than 20μm,while it is smaller for heavy components,indicating the micro fractionation from clay/organic lamina to quartz/feldspar lamina.The depth of enrichment of light components corresponds to the oil maturity of organic matter.Both the distribution of light and heavy components fits the same fractal model,with fractal dimensions ranging between 2.2 and 2.5.The CLSM results confirm that it can be a reliable tool for the"sweet spot"exploration.

A new method for evaluating the oil mobility based on the relationship between pore structure and state of oil

An accurate evaluation of the shale oil mobility is crucial to its cost-effective exploitation.This study presents a method to assess shale oil mobility by integrating the pore structure and oil states distributions.First,a set of three discrete organic extracts(EOM-A,B and C)were obtained by sequential extraction.The relationships among the EOMs and the oil states were inferred from the group compositions and fluorescence properties of the produced shale oil(free state).The results showed that EOMs A and B represent free oil in the open and closed pores,respectively,while the EOM-C represents adsorbed oil.Then,NMR T_(1)-T_(2)map is used to determine the T_(2-cutoff)values that indicate the pore size ranges of different oil states.Free oil resides mainly in larger pore space(T_(2)>0.5 ms),while the adsorbed oil in smaller pore space(0.2 ms<T_(2)<0.5 ms).Finally,the ratio of free to adsorbed oil(F/A)>0.5 and T_(2-cutoff)>1.0 ms suggest that the free oil in connected pores has the highest mobility.This work can provide a reference for evaluating the shale oil potential and prospectivity in other regions.

Differences in hydrocarbon composition of shale oils in different phase states from the Qingshankou Formation, Songliao Basin, as determined from fluorescence experiments

The phase state of shale oil has a significant impact on its mobility.The mineral and organic matter in shale reservoirs play an important role in oil phase.This study attempts to evaluate the properties of shale oils in different phase states and to investigate how these differences are related to initial shale composition.Samples from the first member of the Qingshankou(Q1)Formation were analyzed using X-ray diffraction,total organic carbon content,rock pyrolysis solvent extraction and group component separation.Subsequently,fluorescence techniques were used to quantitatively determine the content and properties of the free oil(FO),the adsorbed oil associated with carbonate(ACO),and the adsorbed oil associated with silicate and clay-organic complexes(AKO).The results showed that non-hydrocarbons and asphaltenes are the primary fluorescing compounds on shale grain.FO is the dominant phase in the Q1 Formation.The quantitative grain fluorescence on extraction(QGF-E)and total scanning fluorescence(TSF)spectra of ACO and AKO show a significant redshift compared to the FO.The TSF spectra of FO have a characteristic skew to the left and a single peak distribution,suggesting a relatively light hydrocarbon component.The TSF spectra of ACO show a skew to the right and an even,double-peaked distribution.The TSF spectra of AKO show a single peak with a skew to the right,indicating that ACO and AKO hydrocarbons are heavier than FO hydrocarbons.In summary,enrichment of carbonate minerals in shale may result in mis-identification of“sweet spots”when using QGF.The normalized fluorescence intensity of QGF-E and TSF are effective indexes allowing oil content evaluation.As an additional complicating factor,hydrocarbon fractionation occurs during generation and expulsion,leading to a differentiation of oil composition.And FO has high relatively light hydrocarbon content and the strongest fluidity.