Hydrocarbon Generation Potential and Depositional Environment of Shales in the Cretaceous Napo Formation, Eastern Oriente Basin, Ecuador

Abstract

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 Tmax 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 Ba...

Geochemical Characteristics and Charge History of Oil in the Upper Cretaceous M1 Sandstones (Napo Formation) in Block T, Oriente Basin, Ecuador

“…Baby et al (2013) proposed that Cretaceous Napo Formation source rocks may locally be mature in the eastern slope zone of the basin. Yang et al (2017) proposed that Napo Formation source rocks may be early to peak mature over larger areas of the slope zone based on source rock and maturation analyses.…”

“…2) (Dashwood and Abbotts, 1990;Baby et al, 2014). Recent studies (Yang et al, 2017;Ma et al, 2020) have demonstrated that there is a close geochemical relationship between Napo Formation calcareous shale source rocks and oil Canfield et al, 1982;Smith, 1989;Dashwood and Abbotts, 1990;White et al,1995;Barragán et al, 2005;. The approximate positions of the oil samples and the sandstone core samples from the Napo Formation M1 Sandstones are shown in the chronostratigraphic profile to the right (red and blue circles).…”

“…1b) (Ma et al, 2017). These discoveries make Block T a significant area for hydrocarbons exploration not only in the Oriente Basin but also within the entire sub-Andean basin system (Ma et al, 2014a;Yang et al, 2017;Yang et al, 2019).…”

“…samples from fields in the slope zone in the east of the Oriente Basin. Within this basin, mature source rocks are mainly confined to deeply-buried kitchen areas in the foredeep area (Baby et al, 2013) although Yang et al (2017) suggested that the Napo Formation may also be thermally mature over large areas of the slope zone to the east. Upper Cretaceous source rocks are thought to have entered the oil window in the early Miocene (Dashwood and Abbotts, 1990;IHS, 2015a;IHS, 2015b) with oil and gas subsequently migrating laterally updip to sandstone reservoirs (Xie et al, 2010).…”

A Study of Architecture of Thin Interlayer within Meandering River Monosandbody of Thick-Bedded Oil Layers

“…Along with profound exploration and exploitation (Figure 1), the oilfield at the No.1 area Yaojia formation reservoir in Daqing has come into the ultra-high water-containing phase (Fan & Ma, 2011;Ma et al, 2015;Lin, Shan, & Chen, 2017b;Yang et al, 2017). Therefore, hunting for remaining oil, enhancing oil and gas output to attain the full potential are urgent problems to be solved.…”