Petroleum system modeling and risk assessments of Ad’daimah oil field: a case study from Mesan Governorate, south Iraq

Abstract

Oil biomarkers and isotope values of the analyzed samples from the Sa’adi and Yamama Formation reservoirs of Santonian and Valanginian ages, respectively, from well Da-1 as well as Yamama Formation extracts are plotted on sterane triangle, hopane diagram, tricyclic terpane diagram, pristane to phytane ratio with canonian variables, δ13C saturate, aromatics, and pristine-phytane diagrams and have indicated a source environment and lithology of marine algal type II that are nonbiodegraded and deposited in anoxic environments of carbonate and shale. Source maturation at the time of the oil generation is assessed by plots of oil, and the analyzed values of the Yamama source rocks on the pristine-phytane diagram indicate these to be mature with Tmax equivalent of 435 °C based on plots of methyldibenzothiophene ratio (MDR) and vitrinite reflectance of Ro = 0.70–0.85 based on methylphenothrene ratio (MPI). Source age assessment is taken from δ13C (%) and the calculated C28/C29 sterane ratio indicates early Lower Cretaceous age. The Yamama Formation in this study showed excellent hydrocarbon potential with extracted organic matter of up to 16,674 ppm with a total organic carbon (TOC) equivalent of 16.64 wt%. Correlation of the analyzed data from Ad’daimah field with other oil fields in the Mesan Governorate indicates the same characteristics and revealed this to be clustered in one place in all the diagrams, but the source age shows two oil families where the first is sourced from Upper Jurassic-Lower Cretaceous as in the case in the Ad’daimah field while the second is sourced from Middle and Upper Jurassic as in the case in the Halfaya field. Source-oil correlation is confirmed in this study by plots of values from the analysis of Sa’adi and Yamama oil of well Da-1 with the extracts from the Yamama-Sulaiy formations in isotopic δ13C saturate and aromatics, calculated C28/C29 sterane ratio diagram, sterane triangle, and MPI versus Ro. These showed grouping of the oil and extracts of the analyzed data into one place of depositional environment, maturation, and age characteristics that indicate a very good correlation with excellent hydrocarbon generation from the Yamama Formation that measures up to 16,674 ppm of extracted organic matter. PetroMod software basin modeling has confirmed hydrocarbon generation from the Yamama Formation under burial thermal temperature of 114–120 °C with 285–300 mg HC/g TOC for 95 % hydrocarbon transformation from the disseminated organic matters with critical point of hydrocarbon migration and accumulation at 10 million years ago. Palynomorphs have indicated ages of Valanginan-Berriassian for the Yamama Formation and Tithonian-Berriassian for the Sulaiy Formation. Marking places of source rocks and oil reservoir on seismic section across well Da-1 for the Jurassic and Cretaceous strata have shown model assessments of the hydrocarbon generation places of the Yamama Formation, migration pathways and accumulation site, as well as places of enriched oil to assess reservoir sites and suggest for risk assessments of drilling sites for hydrocarbon production from the Yamama and Sa’adi formations. On the other hand, seismic sections in Halfaya and Buzargan fields showed hydrocarbon migration from the Sargelu Formation.