Middle Miocene Jeribe Formation hydrocarbon sources and accumulations, Diala district, northeastern Iraq

Abstract

The stratigraphic section between the Middle Jurassic Alan Anhydrite and Middle Miocene Lower Fars Formation contains an important petroleum system in northeastern Iraq. Both the Alan Anhydrite and Lower Fars Formation constitute regional seals. In the Diala District this section includes oil and gas reservoirs in the Jambour, Pulkhana, Qumar and Mansuriya fields. This study focused on the source rocks of this system. Gas chromatography-mass spectroscopy (GC-MS) analysis of the oil in the Middle Miocene Jeribe Formation (especially from the Jambour oil field) identified several molecular fossils (biomarkers) parameters. Pristine/n17 versus Phytane/m18, Ternary sterane diagram, Tricyclic terpane scatters and Pristane/Phytane versus carbonisotope ratios indicated kerogen sources types II and III. The kerogen shows very low biodegradation and high maturity, and consists of marine algae deposited in suboxic-anoxic palaeoenvironments within carbonate sediments. The graphical presentation of other biomarker parameters suggested an Upper Jurassic and Lower Cretaceous age for the source rock: (1) C28/C29 sterane ratio of 0.9; (2) oleanane value of 0.2; and (3) a low value of tricyclic terpane (0.48) versus time. The age of the source rocks was confirmed by oil palynomorphs. Based on kerogen enrichment, type and maturity, rock types and petroleum potential, the source rocks might belong to one or more of the local formations Chiagara, Balambo and Sarmord. Pyrolysis analysis of 232 core rock samples of Jurassic and Cretaceous strata are presented on a Van Krevlen diagram (Hydrogen index versus Oxygen index), Kerogen type and maturity (Hydrogen index versus Tmax) and kerogen conversion and maturity (Production index versus Tmax) have indicated types II and III for all the studied formations. The Chiaga and Balambo formations are within the oil window and zone of generation and expulsion. The Shiranish Formation is immature whereas the Sargelu Formation has no molecular fossil contribution to the oil found in the Jeribe Formation. The Chiagara and Balambo formations have a distal suboxicanoxic deposition as indicated in the Ternary kerogen plot, and hence are correlatable with the oil molecular fossil of the Jeribe Formation. The Jeribe Reservoir has a permeability of 30 mDarcy and good porosity as indicated by electrical logs (10–24%, consisting of intraparticle, fracture, vuggy and channel types). It is sealed by the anhydrites of the Lower Fars Formation and found in predominantly NW-trending anticlinal traps, which formed during the Late Miocene-Pliocene compressional tectonism associated with the closure of Neo-Tethys Ocean along the Zagros Suture. The Jeribe Reservoir closures were therefore sealed before the main phase of oil and gas generation, which continues to the present-day. Gas accumulations occur further northeast in Mansuriya field, near the Diala River, whereas oil accumulation occur towards the north-northeast within the Jambour oil field, near Kirkuk.