HYDROCARBON GENERATION POTENTIAL AND DEPOSITIONAL ENVIRONMENT OF SHALES IN THE CRETACEOUS NAPO FORMATION, EASTERN ORIENTE BASIN, ECUADOR

Xiao-Fa Yang^*+, Yin-Fu Xie^*, Zhi-Wei Zhang^*, Zhong-Zhen Ma^* , Yu-Bing Zhou^*, Ya-Ming Liu^*, Dan-Dan Wang^* and Yong-Bin Zhao^*

^* PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China.

+ corresponding author, email: ouyangxiaofa@petrochina.com.cn

Marine shale samples from the Cretaceous (Albian-Campanian) Napo Formation (n = 26) from six wells in the eastern Oriente Basin of Ecuador were analysed to evaluate their organic geochemical characteristics and petroleum generation potential. Geochemical analyses included measurements of total organic carbon (TOC) content, Rock-Eval pyrolysis, pyrolysis -- gas chromatography (Py–GC) , gas chromatography -- mass-spectrometry (GC–MS), biomarker distributions and kerogen analysis by optical microscopy. Hydrocarbon accumulations in the eastern Oriente Basin are attributable to a single petroleum system, and oil and gas generated by Upper Cretaceous source rocks is trapped in reservoirs ranging in age from Early Cretaceous to Eocene.

The shale samples analysed for this study came from the upper part of the Napo Formation T member (“Upper T”), the overlying B limestone, and the lower part of the U member (“Lower U”). The samples are rich in amorphous organic matter with TOC contents in the range 0.71–5.97 wt% and Rock-Eval T_max values of 427–446°C. Kerogen in the B Limestone shales is oil-prone Type II with δ¹³C of -27.19 to -27.45‰; whereas the Upper T and Lower U member samples contain Type II–III kerogen mixed with Type III (δ¹³C > -26.30‰). The hydrocarbon yield (S₂) ranges from 0.68 to 40.92 mg HC/g rock (average: 12.61 mg HC/g rock). Hydrogen index (HI) values are 427–693 mg HC/g TOC for the B limestone samples, and 68–448 mg HC/g TOC for the Lower U and Upper T samples.

The mean vitrinite reflectance is 0.56–0.79% R₀ for the B limestone samples and 0.40–0.60% R₀ for the Lower U and Upper T samples, indicating early to mid oil window maturity for the former and immature to early maturity for the latter. Microscopy shows that the shales studied contain abundant organic matter which is mainly amorphous or alginite of marine origin.

Extracts of shale samples from the B limestone are characterized by low to medium molecular weight compounds (n-C₁₄ to n-C₂₀) and have a low Pr/Ph ratio (≈ 1.0), high phytane/n-C₁₈ ratio (1.01–1.29), and dominant C₂₇ regular steranes. These biomarker parameters and the abundant amorphous organic matter indicate that the organic matter was derived from marine algal material and was deposited under anoxic conditions. By contrast, the extracts from the Lower U and Upper T shales contain medium to high molecular weight compounds (n-C₂₅ to n-C₃₁) and have a high Pr/Ph ratio (>3.0), low phytane/n-C₁₈ ratio (0.45–0.80) with dominant C₂₉ regular steranes, consistent with an origin from terrigenous higher plant material mixed with marine algae deposited under suboxic conditions. This is also indicated by the presence of mixed amorphous and structured organic matter.

This new geochemical data suggests that the analysed shales from the Napo Formation, especially the shales from the B limestone which contain Type II kerogen, have significant hydrocarbon potential in the eastern part of the Oriente Basin. The data may help to explain the distribution of hydrocarbon reserves in the east of the Oriente Basin, and also assist with the prediction of non-structural traps.

Key words: marine shales, source rock, Cretaceous, Napo Formation, Oriente Basin, Ecuador, hydrocarbon potential.

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