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Origins of Fluid Compositional Variation in Northern Iraq, NE Syria and SE Turkey

Abstract

The Northern Iraq area - including NE Syria and SE Turkey - has seen exploration by international oil companies in the last decade. Following this phase a number of discovered fields are under development and a number discoveries remain undeveloped.

Reservoir fluid viscosity, GOR and oil gravity vary widely both on an areal and stratigraphic basis. Reservoir fluids can be dense with high viscosity, reflecting a low API gravity stock-tank liquid, low solution GOR or a combination of the two. These variations arise from a number of factors. At reservoir depths as shallow as 300m and at cool temperatures, biodegradation does not impact oil gravity. Reservoir paleo-temperature analysis shows this is because reservoirs were ‘pasteurized’ by deeper burial before uplift and erosion on the anticlines.

Rather, the causes of low API gravity are: (1) Organofacies: all source rocks from Triassic to Tertiary are Organofacies A, generating high sulfur, dense stock tank liquids, except where expulsion occurred at advanced maturity levels; (2) maturity: early expulsion of dense liquid occurred due to high Organic Matter quality (HI), exacerbating the behavior of the A Organofacies; (3) local removal of solution gas from the reservoir fluid by aquifer water.

We performed a regional study using new basin modeling tools by Belmont Technology, beginning by mapping the Ultimate Expellable Potential of the main source rocks: Sargelu ~170MY; Naokelekan ~ 161 MY; and Chia Gara (144 MY). We created a full-transient lithosphere model to predict the thermal history at all major and minor source rock levels, incorporating lateral variations in basement radiogenic heat production (RHP) that induce important lateral variations in heat flow & geothermal gradient.

We then modeled the expulsion kitchen at each level and calibrated the model using Tmaxas a paleo-thermal stress indicator which records the thermal stress levels at the anticlinal locations prior to exhumation.

Extreme examples of these reservoir fluid variations are seen in the Jurassic in Shaikan, where low GOR, early-expelled fluid with 14OAPI stock-tank gravity fluid arises from a relatively shallow foreland burial in an area of low crustal RHP. On the other hand, an Albian reservoir fluid at Taq Taq that was originally likely a volatile oil with several 1000 scf/stb GOR, has seen major solution gas dissolution in the aquifer: producing almost gas-free oil with a gravity of ~48OAPI. This area had greater foreland basin burial and higher crustal RHP.