ABSTRACT

The Lower Miocene Euphrates and Jeribe formations are significant potential reservoirs in Iraq, with substantial reserves in many fields. A detailed study was conducted to investigate the reservoir characteristics of the Euphrates, Dhiban, and Jeribe formations. This study involves extensive petrographic analysis, formation evaluation, and geochemical analysis in the recently discovered Tawke oilfield and the analogous Peshkabir Anticline. A large amount of data was collected from eight wells, including thin sections, routine core analysis, wireline logs, gas chromatography, and formation water analysis, along with rock samples from two surface sections at the Peshkabir Anticline.  According to petrographic analysis the Euphrates and Jeribe formations contain diverse, mostly dolomitised fauna, with the presence of fossil Borelis indication of Miocene age. The studied formations underwent extensive dolomitisation and dissolution, which largely altered the original fabric and led to the development of secondary porosities and porosity enhancement. Mineralogy, diagenesis, and tectonic activity control porosity and matrix permeability in both formations. Based on dolomite crystal shape, size, and fabric, four dolomite textures (D-I, D-II, D-III and D-IV) were recognised in the Euphrates and Jeribe Formations, with type D-II having the highest contribution to porosity improvement. Five microfacies with two submicrofacies being identified in the Euphrates Formation, with five microfacies and four submicrofacies were identified in the Jeribe Formation. Based on lithology and stratigraphic position of the Dhiban Formaiton, a single microfacies has been suggested to encompass all the lithological units of the formation. Accordingly, an inner platform paleoenvironment was determined for the Euphrates Formation, while semi-restricted lagoon to intertidal-supratidal settings was identified for the Jeribe Formation. The Dhiban Formation is suggested to have deposited in supratidal to intertidal settings.

The log analysis revealed that the Euphrates Formation, with an average shale volume of 14.9%, is classified as shaly. In contrast, the Jeribe Formation can be categorised as clean to less shaly, with an average shale content of 8.3% across the studied wells. The calculated porosity from neutron-density logs indicated that the upper-middle section of the Euphrates Formation is the most porous unit, with a maximum 18% porosity  and 70 mD lambda permeability (Kλ), averaging 10% porosity and 68.9mD Kλ. In comparison, the uppermost part of the Jeribe Formation exhibited the highest porosity values, reaching up to 25% and 10 mD Kλ, averaging 7.4% porosity and 4.8mD Kλ permeability. The secondary porosity index (SPI), calculated from the difference between neutron-density and sonic porosity, indicated prevalence of secondary pores over depositional pores, with average SPI values of 5.4% in the Euphrates and 3.7% in the Jeribe Formation. Additionally, routine core analysis (RCA) results from 16 core plugs showed an average porosity of 20.5% and permeability of 5.62 mD for the Euphrates Formation, indicating good porosity and permeability. Results from 219 core plugs revealed an average He-porosity of 8% and an average permeability of 7.9 mD for the Jeribe Formation. Lithology and RCA revealed that the Dhiban Formation is a non-reservoir rock, with a maximum He-porosity of 2% and a permeability of 0.069 mD, confirming that the formation is insufficient as a hydrocarbon reservoir. The Jeribe Formation contains crude oil of an intermediate gravity and with high sulfur content. According to gas chromatography analysis, the liquid and extracted oil from the Jeribe Formation were originated from a mature marine setting that was possibly algae-rich and has been deposited in a reducing environment. It is believed that these hydrocarbons were sourced from Middle Jurassic Sargelu and Upper Jurassic-Lower Cretaceous Chia Gara formations. The calcium-chloride water class is determined for the produced water in the Jeribe Formation, the element concentration with dissolved salts confirmed that the Jeribe Formation is in contact throughout studied wells of the Tawke Oil Field.