Abstract

The giant Dukhan Field is a mature oil and gas field located onshore in Qatar, approximately 80 km west of Doha (Figure 1). The Dukhan structure is a north-south plunging anticline approximately 70 km long by 8 km wide. The field was discovered in 1939 and first production occurred in 1949, with more than 750 well penetrations drilled since then. Major reservoirs are the Upper Jurassic Arab C and Arab D formations, the focus of this study, which contain oil and oil with associated gas, respectively. Oil and associated gas are also produced from the Middle Jurassic Uwainat formation, and the Permo-Triassic Khuff formation contains non-associated gas.

The origin of the Dukhan structure is interpreted to be the shallow expression stemming from subtle reactivation of basement faulting that originated during the initial amalgamation of the Arabian plate in the Late Proterozoic. A genetic model for the evolution of the Dukhan structure was generated that establishes a context for interpretation of the currently poorly imaged faulting at Dukhan, and provides a model of the timing, distribution, and potential reservoir impact of subseismic faulting. In addition, the regional and field scale evaluation of structure integrated with geochemical analysis provides an enhanced approach to interpret structural evolution, charge history, and bitumen distribution for the Jurassic reservoirs.

A variety of geochemical analytical techniques have been applied to both rock and fluid samples from Dukhan, including fluid inclusion volatile (FIV) analysis, high-resolution compositional analysis of core samples from bitumen/tar mat intervals, and high-resolution compositional analysis of produced hydrocarbon fluids. These geochemical analyses provide further support to the interpretation of the field's structural evolution. Several key observations related to the geochemical analyses are:

  • Light hydrocarbon analyses indicate that the Arab C, Arab D, and Uwainat reservoirs share similar sources, but are not in chemical communication. Similar oil composition within each reservoir indicates that there is no lateral compartmentalization.

  • An early charge of oil in the Arab D was partially displaced later by gas.

  • Oil-filled inclusions are present well below the estimated original oil-water contacts (OOWCs), and suggest trap modification after oil migration, or the active migration of oil through these strata.

  • Multiple levels of hard, pore-occluding bitumen occur within the Arab D reservoir and correlate roughly to paleoclosures.

Initially, this relationship was taken to suggest that the bitumen distribution might reflect paleo oil-water contacts that could track the Dukhan trap evolution. Subsequent geochemical evaluation suggests instead that bitumen development is most likely the result of gas de-asphalting and most likely relates to the Late Cretaceous and Tertiary charge of light hydrocarbons and gas.

These observations provide a strong technical foundation for an improved understanding of field compartmentalization and the controls on original fluid distribution. Accurate models for fluid distribution and structural evolution are critical inputs to 3D reservoir models, and also provide support for future field development decisions. As an example of this utility, these models will assist in interpretations of irreducible and residual water saturation distribution across the field.

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