As the super-giant fields in the area continue to produce their hydrocarbon resources, there is a natural decline in production as the easy to access resources are going to be depleted in a few decades. Finding additional oil and gas resources to replace the produced hydrocarbon is representing major challenge. The hydrocarbon saturated tight layers, found in the super-giant onshore field, could represent an example for finding additional hydrocarbon volume that accumulated in a difficult trap style. The central onshore field tight layers were found in the successions extended from the Lower Cretaceous, Thamama Group down into the Upper Permian Khuff formation. The high hydrocarbon saturation found in the Thamama tight layers was previously interpreted as residual oil left behind during the vertical oil migration from the Upper Jurassic Hanifa source rock into the Thamama reservoirs. However, the current charge model assumes a lateral westward hydrocarbon migration from east onshore Abu Dhabi through the Thamama reservoirs and towards the major Abu Dhabi central onshore fields. The charge filling started from the top of the structure and down into the spill point and filled both, the porous and tight layers (figure 3). The differences are in the reservoir properties, which by using the technology improvement in the horizontal drilling and hydraulic fracturing, the tight zones hopefully will produce a significant volume of hydrocarbon. The Lower Jurassic and Triassic formations were not previously considered as exploration objectives. Only one well penetrated these formations and it was targeting the deeper Khuff reservoirs. Several attempts were made to test some of these formations, but they were found to be completely tight. The water recovered from such testings was most probably drilling fluids. The Khuff reservoirs high gas shows while drilling, the high hydrocarbon saturation of its interpreted logs do not match its negative testing results. The cause of failure is most probably related to the formation damage and the classical tools and methods used in the production test. In addition to that, the cement jobs were not quite successful, which will allow for the water leakage behind the casing.

Exploration drilling for gas potential in Khuff Formation started more than 40 years ago and wells across multiple assets in offshore Abu Dhabi showed the presence of sizeable reserves. With increasing recent importance on gas production, there is a plan for moving towards development for those Permian tight gas structures to address the nation's gas needs. This paper will try to address the lessons learned from the recent appraisal wells in Khuff, the uncertainties and the success criteria. There have been multiple wells drilled through the Khuff Formation in Offshore UAE in the last two years and have yielded a wealth of information from downhole well logs, mud logs, well test results and core data. The downhole logs acquired in these wells ranged from basic triple-combo and mud logs to image and dipole sonic logs, along with formation testing and sampling measurements, followed by well tests across the zones of interest. Interpretation of all these data have revealed some interesting lessons learned. The shallow marine sequence of the Khuff Formation is generally characterized by poor porosity and low matrix permeability; however, the diagenetic overprint has resulted in a significant heterogeneity development, which controls the present-day porosity and permeability development at the wells. The well test results show variations in terms of 2 or 3 orders of magnitude at the same interval, which highlights potentially problematic development strategies. We have observed significant differences in terms of lithology, porosity and other reservoir quality indicators between wells, located a kilometre apart. Optimization of logging suite to minimize petrophysical evaluation uncertainty will be discussed. Characterising this heterogeneity is key for future economic success of this play. Possible role of fractures influencing flow from the Khuff have been discussed in older publications, however no conclusions were arrived at, with certainty. Presence of fractures and faults beyond the immediate vicinity of the well might be something to look at, in terms of understanding the potential productivity of those intervals. A big step for developing Khuff Formation might be in terms of deciding the optimal stimulation strategy and this is something, which remains to be studied extensively in UAE. Closing the loop of interpretation of the acquired logs with the final well-test results and production logs gives us the advantage of hindsight and helps us in separating out the key productivity indicators as well as bring out the uncertainties in formation evaluation, which affect these tight gas reservoirs.

Recent appraisal drilling undertaken by ADNOC in offshore Abu Dhabi has focussed on evaluation of the Middle Jurassic to Permian Deep Gas reservoir sequences. These formations are characterised by low porosity and permeability and typically contain either dry gas or gas condensate fluids. These appraisal activities form part of a larger program leading to development of these resources. Principal uncertainties addressed by appraisal drilling include determining fluid characteristics, reservoir properties and ultimately well deliverability. This paper uses one such recently drilled (typical) appraisal well as an example of the workflow employed. Exploration drilling of the Middle Jurassic to Permian reservoirs in this field dates back to 1984 and utilised available logging tools and techniques of the time. The current appraisal drilling program built on the results of this work and utilizes the latest available technology and interpretation techniques to both quantify reservoir and fluid properties and minimise subsurface development uncertainties. Typical data acquisition programs includes: conventional coring, advanced mud log data acquisition, triple-combo wireline data, borehole image data, elemental spectroscopy, azimuthal dipole sonic data and formation pressure measurements/samples. The formation evaluation program involved careful analysis and integration of this data to decide at first on formation sampling points and then subsequently testing zones. This approach necessitated the involvement of multiple stakeholders (end-users as well as people performing the interpretation) and required close communication to facilitate rapid, informed, decision making at key stages of the project execution. These different types of data become available at differing times during the course of drilling a well with the earlier acquired data informing the decision-making process on subsequent data acquisition. The first data to come in were the "mud logs" which includes drilling parameters (such as Rate of Penetration) and gas chromatography. This data provides an initial indication of potential zones of interest, along with fluid type. Following acquisition of wireline data, a "quicklook" formation evaluation was integrated with earlier geological analysis to determine the formation pressure and fluid sampling points. Combined together, these results formed the basis of an integrated reservoir and saturating fluid interpretation leading to the selection of perforation intervals for well testing. Effective implementation of this work flow requires a collaborative approach combined with ongoing data integration. This process of consultation across multiple subsurface disciplines and stepwise evaluation guiding future data acquisition requires a certain degree of evaluation flexibility but ultimately results in better decisions. The philosophy of integrating multiple data sources and disciplines in a collaborative evaluation and decision-making work flow is an essential enabler for the exploitation of the Deep Gas resource in Abu Dhabi. In the example well, test results are in accord with the log interpretation work and provide a path forward towards field development and gas production.