Online Journal for E&P Geoscientists

Heister, Lara E.^*1; Villa, Danielle E.¹; Grabowski, George J.¹
(1) ExxonMobil Exploration Company, Houston, TX.

We are analyzing fluid-inclusion volatiles (FIV) in samples from wells across Iraq that span from the Miocene to the Ordovician. Oil stains and bitumen in cuttings and cores are evidence of oil migration through the rock column. However, FIV data provide more quantitative and detailed information about seal competency, migration timing and pathways, and hydrocarbon compositions.

Micron-scale fluid inclusions form in cements and fractures within minerals. As inclusions form, they record the temperature, pressure, and composition of migrating hydrocarbons and fluids, thereby preserving these “paleo-fluids” after pore fluids have changed. In this way, inclusions provide a snapshot during the evolution of a hydrocarbon system. Bulk FIV analyses from cuttings and core samples taken at every ~20 ft provide a detailed volatile chemical stratigraphy for each well. In stratigraphic intervals where multiple migration episodes are suspected, we employ optical FI characterization, in which different generations of fluid inclusions may be detected and the conditions and composition of fluid may then be estimated.

Our data show that sealing rocks for most hydrocarbon columns are generally robust, as they separate reservoirs with strong FIV oil and gas signals. Absence of FIV signals indicates that migration to shallower reservoirs has not occurred by cross-stratal flow within the sedimentary rocks. Multiple migration mechanisms are required to explain discrepancies between the FIV compositions and present-day reservoir fluids.

Oil occurs in Miocene and Cretaceous reservoirs in southern Iraq. There are no FIV indications of hydrocarbons in the intervening interval in Zubair Field, suggesting that oil did not migrate through the rock column to the younger reservoirs in the wells studied. At Nahr Umr Field, FIV response indicates an active hydrocarbon system but only local sealing and minor accumulations. Well data from northern Iraq show that the Paleocene Aaliji Fm. and Miocene Fat'ha Fm. are robust seals that prevent vertical migration of hydrocarbons. At Atshan 1, there is evidence for vertical migration from Paleozoic, Triassic, Jurassic, and Cretaceous sources.