Source Characteristics and Lateral Migration of Middle Cretaceous Petroleum System in Western Abu Dhabi, Insight from Biomarker and Reservoir Investigation

Abstract

In the western Abu Dhabi, oil fields have been discovered from the Mishrif reservoir (Cenomanian/Turonian). These fields are found in shoal/reef related stratigraphic prospects and composed of grainstone or packstone around platform margin setting. Regional understanding believes that the in-situ Shilaif mudstone is not mature enough to source the overlain Mishrif. So, it is inferred that the Mishrif reserves were charged from Shilaif kitchen of regional syncline through over 100km lateral migration from south to north. However, this accumulation pattern seems presumptive as little solid evidence was reported and little geochemistry measurement have been done previously. To enhance understanding of the Shilaif source characteristics and figure out Shilaif-Mishrif accumulation rules, cuttings, cores from Shilaif and oil samples from Mishrif & Tuwayil were taken. Subsequently, geochemistry assay, oil-source correlation and related research were carried out. The results shows the in-situ Shilaif has abundant organic content with high TOC, HI and bitumen "A" with low maturity. Tmax of Shilaif mudstone yields an average value of 430°C and calculated vitrinite reflectance about 0.6, which means an early oil generation for in-situ Shilaif source. Oil-source correlation indicates a higher maturity of crude oil in the Mishrif & Tuwayil than the in-situ Shilaif. Furthermore, similarities of biomarkers between Mishrif oil and the Ghurab syncline Shilaif source are proved. All these evidences reveal the lateral hydrocarbon migration and accumulation northwards. For the migration pathway, it is inferred the regional unconformity of TopMishrif plays an important role to transfer the hydrocarbon. In Western Abu Dhabi, truncation or depositional hiatus around TopMishrif are not as pronounced as Eastern Mishrif platform, but obvious exposure as well as leaching evidence could be observed from uppermost Mishrif and Tuwayil through core investigation. Besides, almost all the oil shows in Mishrif concentrate around the interface, which further proves the role of the unconformity. The study on Shilaif source characteristics and hydrocarbon migration mechanism enhances the insight into accumulation rules of Cretaceous Petroleum system in Western Abu Dhabi and would guide the further exploration activity.

Sequence Stratigraphy, Palaeogeography and Petroleum Plays of the Cenomanian – Turonian Succession of the Arabian Plate: An Updated Synthesis

“…The following factors should be important when considering karst and its impact upon reservoir quality: duration of exposure; relief; magnitude of relative sea-level change; nature of the exposed substrate; climate; depth to water table; the presence and fill of incised valleys (e.g. sand or shale); clay infiltration of caves, fissures and dolines; grain mineralogy and propensity to dissolution (aragonite vs high Mg vs low Mg calcite allochems); differential cementation; salt movement; and whether soils or coaly swamps are present (Burchette and Britton, 1995;Grelaud et al, 2006Grelaud et al, , 2010Ghabeishavi et al, 2009;Cross et al, 2010;Sharp et al, 2010;Burchette et al, 2010;Hollis, 2011;Deville de Periere et al, 2017;Assadi et al, 2018;Xiao et al, 2020).…”

“…Recent discoveries reported in stratigraphic traps such as the Eridu field, SE Iraq (Lukoil, 2017), and the Mishrif discoveries on the western margin of the Shilaif intrashelf basin in the UAE (Lü et al, 2018;Jaya et al, 2020;Xiao et al, 2020), highlight the remaining potential within the stratigraphic play. At the Eridu field, the limited information available (Lukoil, 2017) suggests that the Mishrif reservoir comprises an isolated rudist biostrome, and this appears to be similar to the stratigraphic trapping mechanism at the Dujaila field (Sadooni, 2005;Khawaja et al, 2021) as illustrated by Mahdi and Aqrawi (2014).…”

“…The charging mechanism for the discoveries on the western margin of the Shilaif Basin in the UAE relies on lateral migration (>100 km) from the thermally mature Shilaif kitchen (Xiao et al, 2020). A karstified exposure surface between the Mishrif Formation reservoir and the Tuwayil Formation topseal (corresponding to the Tu1 SB) may have acted as a carrier bed for the laterally migrating hydrocarbons (Xiao et al, 2020). This emphasizes the importance of characterizing the extent and nature of karstified surfaces which may potentially act both as carrier beds and thief zones to breached traps.…”