Geochemistry of Heavy Oil in the T Block, Oriente Basin and its Origin Mechanism

Abstract

: Exploration and development experience show that there is obvious oil gravity difference between the southern part and northern part of the “M1” reservoir in the Fanny oil field in the slope of the Oriente Basin, Ecuador. The American Petroleum Institute Gravity (API) values of oils in the northern part are higher than the one in the southern part of the Fanny oil field, with the values of 20° and 10.0°–13.0°, respectively. So the primary purpose of this study was to analyze the heavy oil characteristics of biodegradation and the oil–oil correlation according to the biomarker data and the δ13C signature of oil samples from T block. The results of the hydrocarbon gas chromatography fingermark and the inversion attribute characteristics indicated that there are fluid compartments between the “M1” reservoir of Fanny south. Finally, the models of oil–gas accumulation under the control of multiple‐activities of complicated fault systems, as well as the origin of heavy oil, are contended. The early stage oils from the western part of the basin were biodegraded heavily in varying degrees in the whole basin, and the later stage oils were derived from the southern part in a large scale and were mature and lighter. Generally, oil mixing is the primary control of net oil properties, such as API gravity in Oriente Basin. We therefore predicted that the API gravity variation of oil pools radically depends on the injection amount of the later stage oil. Because of the shale barrier in the “M1” reservoir of Fanny south, the later stage hydrocarbon could not pass through and contribute to increase the oil API value.

Characterization and Origin Analysis of Heavy Oil in the Western Sag of the Liaohe Basin

“…With the intensification of biodegradation, the changes in various crude oil biomarkers reflect the anti-biodegradation ability of different compounds: n-alkane < single branched alkane, alkyl cyclohexane, and alkyl cycloalkane < isoalkane < C 14 –C 16 bicycloalkane < normal sterane < rearranged sterane < pentacyclic triterpane < tricyclic terpane. 47 , 48 This represents the biodegradation series of weak degradation → medium degradation → severe degradation.…”

“…It can also be verified from Figure 2 that with an increase in depth, the colloid and asphaltene content is reduced, and the density and viscosity also decrease. 45 , 48 Furthermore, the figure illustrates how temperature/depth controls the degree of biodegradation. Water 51 environment: Some research results show that the Western Sag has the characteristics of vertical flow.…”

“…With an increase in depth, the formation temperature also increases. It can also be verified from Figure that with an increase in depth, the colloid and asphaltene content is reduced, and the density and viscosity also decrease. , Furthermore, the figure illustrates how temperature/depth controls the degree of biodegradation. Water environment: Some research results show that the Western Sag has the characteristics of vertical flow. When near the surface, the surface water is continuously injected, mixing with formation water. , Other findings suggest that fault and fracture are the direct passage between surface water and groundwater, which results in high oxygen content in the water environment and promotes the growth and reproduction of a large number of aerobic microorganisms .…”

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

“…Previous studies of the petroleum geology of the Oriente Basin have focused on the sedimentary and tectonic history and on source and reservoir rocks together with processes of oil accumulation and migration (e.g. Smith, 1989;Dashwood and Abbotts, 1990;Marksteiner and Amelan, 1996;Spikings et al, 2001;Gaibor et al, 2008;Hu et al, 2010;Xie et al, 2010;Ma et al, 2014bMa et al, , 2017Liu et al, 2014;Wu, et al, 2015). However there has been comparatively little detailed study of petroleum geochemistry, and the reasons for the presence of oils with different APIs in the same reservoir are still not clearly understood.…”

“…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). The oils were biodegraded during or after migration, resulting in the occurrence of low-API heavy oils in Block T fields (Hu et al, 2010). However few studies have investigated the charge history of Oriente Basin oilfields and the presence of oils with wide-ranging API values in the same reservoir has not been explained satisfactorily.…”