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Geochemical Characterization of Organic Matter from the Eagle Ford Formation and Indications of Thermal Maturity and Depositional Redox Conditions

Abstract

Characterization and quantification of residual oil abundance and saturate, aromatic, rinse, and asphaltene (SARA) fractions in organic-rich rocks is the basis for assessing oil quality and oil mobility under the high temperature and pressure conditions of the subsurface. Core samples from four wells within the Eagle Ford Formation (EFF) were used for this study. The Eagle Ford samples cover a range of burial depth from outcrop to 9200 ft. Geochemical analysis results show that the organic matter of the Eagle Ford Formation is dominated by type II kerogen. The ternary plot of SARA fractions indicates that oil quality is mainly determined by thermal maturity of the source, which, in general, corresponds to the present burial depths of the samples. Biomarkers in the Eagle Ford shales are sensitive indicators of organic matter thermal maturation. The C29 Sterane isomerization ratio is sensitive in the early oil-generation stage. Ts/(Ts+Tm), tricyclics/ 17a-hopane and diasteranes/steranes ratio are sensitive maturity indicators in the peak oil-generation stage. Once the oil begins to crack, most of the biomarkers in the saturate fraction undergo thermal decomposition and can no longer be treated as reliable maturity indicators. In contrast, the distributions of methylhomologs of phenanthrene are controlled by their thermal stability. With increasing maturity 2-methylphenanthrene and 3-methylphenanthrene become more abundant. The determined thermal maturity level for the most mature source (∼9200 ft burial) is about 1.2% Ro. Lower maturity samples provide an opportunity to investigate contrasts in organic matter conversion between the lower and upper intervals of the EFF. The diagram of Pr/C17 and Ph/C18 suggests that marine algal organic matter deposited under reducing conditions is the main source. The lower EFF was characterized by lower oxygenation at deposition than was the upper. Isomerization at C-22 in the C31-C35 hopanes (in the saturate m/z 191 stream) and isomerization at C-20 in the C29 5a, 14a, 17a (H)-steranes (in the saturate m/z 217 stream) biomarker reactions represent the early conversions of bitumen to oil within the 50 m thick lower EFF. Compared with the lower conversion rate in upper EFF, this rapid organic matter conversion may reflect the involvement of the organosulfur compounds. A greater relative abundance of dibenzothiophenes in the lower EFF suggests more reducing environments favorable to organic matter preservation.