AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration

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Regional Variations in Fluid Composition and Properties in the Kurdistan Region of Iraq

Abstract

After the Kurdistan Region of Iraq opened up for exploration in 2004 DNO was among the first international oil companies to enter the region. The first post Iraq war discovery was then made by DNO with the Tawke-1 Cretaceous oil discovery in 2005. In the subsequent 13-year period, around 280 exploration, appraisal and development wells have been drilled across Kurdistan. This period of intense exploration and appraisal has led to an increase in the regional geochemical fluid and source rock database. With a desire for further exploration, appraisal and development, this database has been used to help identify some of the regional and local geochemical controls on fluid quality variation. It is found that reservoir fluid viscosity, GOR and oil gravity vary widely both on an areal and stratigraphic basis. Fluid GOR varies significantly regionally, with for example low GOR oil at Tawke and occurrence of shallow dry gas at Summail. Another element of the fluid quality story to consider is the presence of hydrogen sulphide and carbon dioxide within oils residing in Jurassic and Triassic reservoirs. On the whole these fluid compositional variations pose a predictive challenge for Petroleum Systems Analysis. Extreme examples of the reservoir fluid variations are seen in the Jurassic at the Shaikan field, where a low GOR, early-expelled fluid with low API stock-tank gravity arises from a relatively shallow regional foreland burial in an area of low crustal radiogenic heat production. Here, surface measurements of low API and low solution GOR reflect reservoir fluids that are dense and highly viscous. On the other hand, to the south-east, in an area with greater foreland basin burial and higher crustal radiogenic heat production, the Cretaceous reservoir fluid at the Taq Taq field produces almost gas-free light oil. The variation of oil API could be the result of several factors. One suggested mechanism for the presence of heavy oil in the shallow, cool temperature reservoirs in Kurdistan is biodegradation. The geochemical dataset, however indicates that biodegradation does not generally appear to impact oil gravity. Reservoir paleo-temperature analysis shows that this is because reservoirs were ‘pasteurized’ by prior deeper burial before uplift and erosion of the anticline crests. Rather, the causes of low API gravity are proposed to be: (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 Organofacies A. In addition, the cause of the GOR variation is proposed to be via local removal of solution gas from the reservoir fluid by aquifer water, altering in one case what could originally have been a volatile oil with several 1000 scf/stb GOR to a 48 API oil with only minor gas. Finally, it is suggested that the variation in carbon dioxide and hydrogen sulphide regionally could be related to the presence of massive anhydrites.