Tar Modelling Offshore Abu Dhabi with the Arab Formation as Active Source Rock
Authors: Grimmer Jorg O.W. , Alaminos Antonio , Al Jawhari Mohamed , Buhindi Hani , Al Harbi Haifa , Cajas Laura , Peralta Edmundo 
Published: October 2022

This study demonstrates first time in Abu Dhabi the capacity of combined 3D tar and petroleum systems modelling, in an operating oil field offshore Abu Dhabi, indicating locally the tar presence in the Arab formation. It is intended to give insights on the most likely process of tar formation and allows to a certain extent a prediction of tar presence away from well control.

The tar modelling is calibrated through core observations on vertical wells, which define the thickness of the initially assumed constant and homogeneous tar mat. An extensive data set using adapted geochemistry, petrographic analyses, fluid inclusion analyses and inclusion PVTX-modelling is used to analyse the charge history of the oil field and its tar in detail. The analysed tar occludes the pore space in the reservoirs of the lower Arab Formation in the oilfield offshore Abu Dhabi. The petrographic analyses indicate the presence of tar particles even up to the upper Arab. Geochemistry and petrography show that there are two different tar types. The classical reservoir filling black tar in the upper most part of the Lower Arab is identified as APE (asphaltene precursor entity after Wilhelms & Larter, 1994) tar which is caused by a flocculation process at a certain temperature and pressure regime in the reservoir. This concept has been successful modelled and can even explain the observed fine tar particles up to the upper most Arab. The second type of initially called "tar" is analysed and observed in the top Diyab and lowest part of the Arab, in a micritic limestone facies environment. Previous concepts struggled to justify the black tar deposition in the dense micritic carbonate mudstones. The initial porosity in this micritic mudstones was already very low and therefore a tar flocculation process or gravity segregation in such an environment urges for other explanations. Our analyses indicate that the micritic mudstone acts as a source rock at the top of Diyab and the lower most Arab subunit, where the early heavy oil and asphaltenes (POA=pre-oil asphaltenes) did not leave the rock and stayed in-situ as bitumen/black tar. This has been modelled with a tar specific kinetics, differentiating in an early heavy oil component (POA), that is generated in-situ and an asphaltene component (APE) expelled within the oil and transported into the reservoir.

Acceptable tar modelling result have been reached by reconstructing the charge history of the field. It shows that Diyab oil entered the lower Arab reservoir at approx. 105/95 Ma. The tar modelling through time shows that first tar deposited at 78 Ma (+/- 5 Ma) in the southern part of the oil field. The charge modelling indicates the lower Arab seal failure at approx. 58/53 Ma in the past. The shallower reservoir units of the lower and middle Arab up to the upper Arab are subsequently filled with asphaltene rich oil. Then at 48 Ma the asphaltenes reach a flocculation peak. Finally at 47/34 Ma the whole oil field with the already flocculated tar (APE in the reservoir) and the asphaltenes in the source rock (POA) received a paleo heat shock of at least 140°C, which transformed the tar into pyrobitumen and caused the today surprisingly high API (around 40°API) in the oil field by oil-to-gas cracking.