Geochemical correlation of Asmari/Jahrum reservoir of Nargesi oilfield with seeps nearby

Abstract

ABSTRACT In case of maintaining the reservoir pressure by gas injection, connection between the reservoir and oil seeps around the oilfield should be investigated. Existence of probable conduits from reservoir to surface would cause problems such as environmental issues and capital losses. Comprehensive geochemical analyses were performed on Nargesi oilfield and nearby seep samples in order to investigate their geochemical correlation. Biomarker and diamondoid distributions as well as stable carbon isotope analysis on selected samples were determined by GC-MS and Finnigan Delta Plus mass spectrometer, respectively. Two active petroleum systems were identified in the region. A Jurassic petroleum system charged Nargesi oilfield, whereas the seeps appear to be charged by an older petroleum system. Higher maturity of seep samples identified by methylnaphthalenes, trisnorhopanes, and triaromatic steroid ratios implied that the second petroleum system should be older than Jurassic. Biomarker ratios confirmed that Marl-Carbonate source rocks deposited under anoxic to dysoxic conditions in open marine environment were the main oil-generating facies for the oilfield. Diamondoid indexes further support the concept developed by biomarker data and reveal that the seeps should be probably charged by shaly source rock. The negative correlation implies that gas injection would be effective.