Online Journal for E&P Geoscientists

Al-Ameri, Thamer K.^*1; Al-Nagshbandi, Srood ²; Al-Ahmed, Ahmed A.³; Zumberge, John ⁴; Pitman, Janet ⁵
(1) Geology, Universty of Baghdad, College of Science, Baghdad, Iraq. (2) Geology, University of Salahuddin, Erbil, Iraq. (3) Chemistry, University of Nahrain, Baghdad, Iraq. (4) Geomark Research Ltd., Houston, TX. (5) US Geological survey, Denver, CO.

Structured organic matter of palynomorphs, mainly dinoflagellate cysts, were analysed to determine the precise ages of petroleum source rocks in the Middle Jurassic through the Upper Jurassic and Lower Cretaceous stratigraphic section in northern Iraq. In addition, spore species Cyathidites australis and Glechenidites senonicus were used to evaluate the thermal maturation of this succession. The palynological study consisted of 320 core and cutting samples from twelve oil wells; selected samples were collected from three outcrops.

Terpane and sterane biomarker analysis as well as stable isotopic analysis were performed on selected oils and source-rock extracts to determine oil-to-source rock correlations. Two subfamily oil types (one of Middle Jurassic age (Subfamily A ) and the other of Late Jurassic to Early Cretaceous age (Subfamily B)) were identified. A third oil family that is Triassic in age also was identified based on multivariate statistical analysis (HCA & PCA). Subfamily A oils in well MK-2 from Demir Dagh oil field near the city of Baiji correlate well with rich, marginally-mature carbonate source rocks of the Middle Jurassic Sargelu Formation. In contrast, subfamily B oils have a greater proportion of R28 steranes, indicating they were generated from Upper Jurassic and Lower Cretaceous carbonates similar to those at Gillabat oil field north of Mansuriyah Lake. Oils from Gillabat field display a lower degree of correlation with source facies from the Sargelu Formation than do oils from Demir-Dagh field.

Microscopic and geochemical analysis of the rock samples from exploratory wells and outcrops in northern Iraq indicate that limestone, black shale, and marlstone within the Middle Jurassic Sargelu Formation and Upper Jurassic and Lower Cretaceous Chia Gara Formation contain abundant oil-prone organic matter. The organic matter in both formations comprises principally brazinophyte algae, dinoflagellate cysts, spores, pollen, foraminifera test linings, and phytoclasts. The kerogen was deposited in a distal, suboxic to anoxic basinal regime and compares closely with kerogens classified microscopically as Type A and Type B and chemically as Type II and Type IIS. The timing and extent of petroleum generation in these source rocks was modeled for key wells using one-dimensional petroleum-system software.