Petrography and source rock potential of Chia Gara Formation (Late Jurassic–Early Cretaceous) in Northern Iraq and Kurdistan Region

摘要

The petrographic characteristics and petroleum potential of Chia Gara Formation (Late Jurassic–Early Cretaceous; Tithonian–Berriasian) in Northern Iraq are studied. In order to find out the disparity of Chia Gara Formation in different tectonic zones, two outcrops (Banik and Barsarin sections) and well-Aj-12 were used for this study. In general, the Chia Gara Formation is lithologically composed of black shale, thin-bedded brown argillaceous limestone, thin to medium yellowish limestone. Petrographic study shows that the matrix is mainly composed of micrite, and various diagenetic processes had influenced limestones of Chia Gara Formation, such as neomorphism, dissolution, calcification, cementation and compaction. Fracture, vuggy and moldic are common types of porosity that were observed in this study. However, porosity was later destroyed by some diagenetic processes. The destruction of porosity resulted in considerably low permeability. In addition to petrography, the source rock potential of Chia Gara Formation was also studied. The average values for Total Organic Carbon in percentage (TOC%) of the formation were 1.70, 3.40 and 0.89 for Barsarin, Banik and Aj-12 sections, showing good, very good, and fair organic carbon content, respectively. The pyrolysis executed for studied samples revealed that kerogen type of Chia Gara Formation in Banik section and well Aj-12 belongs to type-II and type-III kerogens, whereas Barsarin section belongs to type-III and type-IV kerogens. The plot of TOC% versus generation potential (S1?+?S2) indicated poor to very good potential of hydrocarbon generation for studied samples. The values of the Tmax (the temperature that results in the maximum release of hydrocarbons (S2) during pyrolysis program) suggested that the organic matter in Banik and Barsarin sections is in mature stage, whereas it is thermally immature (or late immature) in well Aj-12. Vitrinite reflectance (R%) also indicated Oil Window to Early Gas Window in Barsarin, Oil Window in Banik, and Early Oil Window in Aj-12. This study revealed that the studied sections from different tectonic zones show slightly different stages of thermal maturity. The high thermal maturity in Barsarin area can be resulted in different type of kerogen compared to the other sections.