In early Aptian times, subtle tectonic movements may have been activated along the NW-SE strike-slip faults and have resulted in a vertical displacement along these faults. The displacement would have allowed the carbonate-producing organisms to colonize along the shallower southern margin and generate well developed reservoir facies. The basinal facies were deposited to the north of the shelf margin, which is known to be the Bab Basin. Significant oil was discovered in the Shuaiba shelf facies. However, the lagoonal and basinal facies have potential for discovering a significant volume of hydrocarbon, especially in the fields that are located in the Upper Thamama hydrocarbon migration pathways. This potential is supported by the absence of an effective seal separating Thamama Zone-A from Shuaiba basinal facies above, which allowed for the Zone-A hydrocarbon to migrate vertically into the Shuaiba basinal facies. In addition, this potential was supported by the hydrocarbon shows while drilling and by the interpreted well logs, which confirm the presence of movable hydrocarbon in the Shuaiba lagoonal and basinal facies. The Shuaiba Formation is comprised of two supersequences (Azzam and Taher, 1995, van Buchem et al., 2010 and Lyndon et al., 2010) that were deposited during the Aptian time. The lower sequence started at the early Aptian time and mainly included Hawar Member, Thamama Zone-A, the Shuaiba shelf facies and its time equivalent Shuaiba basinal facies sediments. The second sequence was deposited after the drop in sea level below the Shuaiba shelf break. This drop caused subaerial exposure of the Shuaiba shelf platform and allowed for the clastics material to influx the fine-grained over the Shuaiba basinal facies area. The clastic influx and the carbonate rich claystone of the Late Aptian sequence sediments may represent the effective top seal for the Shuaiba basinal facies and Zone-A oil accumulation. The Shuaiba basinal facies were deposited in an intrashelf basin that was enclosed by the Shuaiba shelfal facies sediments. This resulted in restricted water circulation, anoxic condition and deposition below the wave base. Such depositional environment is favourable for source rock preservation. Lithologically, Shuaiba basinal facies consist of pelagic lime-mudstone, wackestone and packstone with abundant planktonic microfossils. These facies are characterized by low permeability values, but their porosity can reach up to 20%. The lagoonal sediments consists of a deepening sequence of carbonate sediments, with shallow marine algal deposits at the base and fine hemipelagic to pelagic carbonates in the upper section. The differences between the Shuaiba Shelf and the Shuaiba Basin are mainly in permeability values. By applying the latest technology in horizontal drilling and hydraulic fracturing, the Shuaiba basinal facies will produce a significant volume of hydrocarbon.

The petrophysical evaluation for Sabkha environment is very challenge due to high reservoir heterogeneity and complex mineralogy in addition to existing of multi-lamination that have various thickness ranging from 1 to 20 ft., also these kinds of reservoirs normally have very high formation salinity that impact as well the water saturation calculations. In 2018, a unique worldwide biogenic shallow gas reservoir was discovered in Onshore Abu Dhabi, UAE in Miocene Sabkha deposits while drilling a dedicated well for deeper reservoir, the average reservoir gross thickness is about 4500 ft. with hundreds of intercalations from Clay, Limestone, Dolomite, Marl, Anhydrite and Halite. This paper presents the detailed challenges for such reservoir regarding the petrophysical evaluation (including but not limited to porosity calculations, water saturation evaluation by different methodology, net to gross ratio, log and core data integration, definition of high potential spot for testing, etc..) Moreover, the way to manage and mitigate these challenges in addition to testing technology and results. So far Five dedicated exploration wells were drilled in this reservoir at which a complete set of logs were acquired in addition to collecting of core footage about 4560 ft. from two wells and analysis is ongoing, also Rigless testing was done for 3 wells. In general, based on collecting data the reservoir properties showed very tight reservoir (permeability < 0.01 md) with high mineralogy complexity as well as high formation salinity (+ 300 kppm). Good Total Organic Carbone is also measures across the reservoir that confirmed the ability to produce Microbial gas. As a results of the integrated study, the sweet spots across the reservoir were identified and tested, accordingly placement for horizontal development wells as well as stimulation technology are optimized

Several shallow gas kicks in Miocene have been encountered during drilling in North East Abu Dhabi (Ghantoot area). Gas origin is confirmed to be predominantly biogenic. ADNOC is evaluating subsurface potential as part of its strategy in developing prospective shallow gas accumulation. Tight layers are targeted to unlock potentially high amount of hydrocarbons and to achieve economical production targets. This paper demonstrates effectiveness of a modern reservoir-oriented technique for well and reservoir performance monitoring before and after stimulation jobs. This technique was proved to be effective at exploration stage when cost- and production-effective stimulation methods are analyzed and decided upon. The spectral acoustic logging technique was applied to estimate inflow intervals in the tight gas reservoir, including pre- and post-stimulation monitoring. Spectral acoustic sensors record signals in a wide frequency range from 8 Hz to 60 kHz. Their dynamic range of 90 dB and large scanning radius allow accurate recording of relatively low-amplitude reservoir acoustic signals. Comprehensive analysis of the spectral acoustic data in combination with other logging techniques, such as temperature logging and a heat exchange sensor (a type of heat flow-meter) can be potentially useful for verification of complex, low-permeability reservoir parameters. Shallow tight Gachsaran and Asmari biogenic gas formations are currently under appraisal targeting identification of highly potential zones and screening of production enhancement technics that allow achieving economical gas rates. Different stimulation technics were evaluated while testing of several exploration wells. One of the way to evaluate stimulation efficiency is an integrated logging that includes high-precision temperature logging and broadband high-sensitivity acoustic logging. Several logging campaigns were conducted in exploration wells to evaluate well performance before and after different types of stimulation jobs: routine HCl stimulation, advanced chemical stimulation, mini- and propped hydraulic fracturing. Due to the reservoir tightness, matrix flow is extremely weak and doesn’t allow sustaining the flow with or without nitrogen lifting that exclude the possibility of routine production logging with spinners. Using of High Precision Temperature (HPT) and Spectral Noise Logging (SNL) allows production profile evaluation for tight reservoir when survey is conducted after series of nitrogen lifting. Due to the complexity of reservoir mineralogy (presence of clays, gypsum, anhydrites) HCl routine matrix treatment is found to be inefficient. Due to the reservoir tightness and based on logging and testing results, it was concluded that any types of matrix stimulation would not be efficient production enhancement technic for biogenic gas tight formations. Propped hydraulic fracturing allowed to bring gas to surface in the vertical well; sustainability of the flow needs to be evaluated in the horizontal well with propped stage fracking. Differentiation between matrix and fracture flow was possible while interpreting noise amplitude and frequency; conducting HPT-SNL logging after propped hydraulic fracturing allows identification the direction of fracture propagation and level of containment within the target interval. HPT-SNL logging was proved to be effective at exploration stage when cost- and production-effective stimulation methods are analyzed and decided upon. In tight gas reservoirs with high heterogeneity and mineralogy variation, it is challengeable to select proper enhancement technic allows achieving economical production rates. Selected logging techniques allowed identification of low rate flow intervals in tight gas reservoir and evaluation the efficiency of different stimulation techniques.

Recently there has been a growing interest in gas exploration, much of this focus has been directed toward thermogenic gas derived from cracking kerogen in the highly mature kitchens. However, a significant proportion of the global gas reserve is not thermogenic but of bacterial origin (Katz, 1995). Biogenic gas is an important exploration target because it occurs in geologically predictable circumstances, in areally widespread area, and in large quantities at shallow depths as free gas (Schneider et al, 2016). The recent exploration wells drilled in the northeast onshore Abu Dhabi showed elevated total gas readings during the drilling of the Gachsaran formation. Consequently, mud-weight was increased to control the gas flow. In addition, the recorded wireline logs indicate the presence of relatively high hydrocarbon saturations in several high porous zones of Gachsaran and Asmari formations. To assess the productivity and commerciality of the Biogenic gas potential in Abu Dhabi, several exploration wells are planned to be drilled before the end of 2019. The positive results of these wells will open the door for a new era of sweet gas exploration activities in Abu Dhabi and its surrounding areas. The primary gas reservoirs are thin carbonate and clastics layers in the Gachsaran Formation at a depth that ranges between 1600-5200 feet below sea level. Organic carbon isotopes, Rock Eval analysis, TOC log data and gas shows analysis indicated that the methane gas found in the Gachsaran Formation is of a biogenic origin and sourced mainly from the organic-rich argillaceous limestone of the Middle Gachsaran. Gachsaran formation is comprised of alternating thin layers of anhydrite, limestone, marl and shale sediments in addition to the presence of salt layers in the lower part. This mixed lithology resulted in the reservoirs property deterioration in particular by shale and anhydrite nodules cementation. The biogenic basin areal extent, significant thickness of the Gachsaran in this basin and the organic richness distribution, conclude possible generation of a huge volume of biogenic gas in northeast onshore Abu Dhabi. However, additional work is required to estimate the volume of gas that is accumulated and that can be produced from the Gachsaran and Asmari formations.

The Gachsaran Formation across Onshore Abu Dhabi and possibly across U.A.E poses high potential of generating Shallow Biogenic Gas (mainly methane) and as such has taken the attention to further investigate, understand and evaluate its capability for promising Gas Resources. The paper provides a detailed G&G analysis that has potentially allowed an appropriate characterization of this unique formation that has first time uncovered interesting data responses in differentiating the sweet spot. For the first time in the history of U.A.E., new data was acquired targeting specifically the Miocene, Gachsaran Formation. This includes; 2D Seismic and party 3D Seismic interpretations, thousands of feet continuous core, conventional and advanced subsurface and surface loggings, Formation Pressure, Fluid sampling, Geochemical and Geomechanical labs measurements, stimulations and Frac tests data. The Gachsaran Formation is very challenging due to complex, thinly bedded and intercalated lithological varitions, and tightness provides difficulties in identifying the promising areas of Gas bearing layers. A comprehensive analysis was performed, in the light of regional understanding, by integrating the results of all available data in the form of correlation, cluster analysis, cross plotting and well based rock physics to differentiate the effect of Gas existence within the formation. The potential zones were further tested and results were integrated to confirm the analysis. The Gachsaran Formation has been subdivided into Lower, Middle and Upper Gachsaran Members. The Lower Member is predominantly evaporitic, becoming more argillaceous carbonate and shale –bearing in the the Middle Member with comparatively less anhydrites. The Upper Member contains mainly anhydrites with interbedded shales and carbonates. The potential sequences which represent high Total Organic Carbon and Gas Shows are found within the Middle Gachsaran. Consequently, the Middle Gachsaran Member was analyzed based on the robust data acquisition performed. Several relationships among GR, TOC, Gas shows, Lithology, RHOB, NPHI, Sonic, AI, Vp/Vs, Gradient Impedance and XRD Clay mineralogy have been attempted to check possible identification of Gas existence effect on the data. This has led to identify the sweet spots caused by the existence of any dominant Gas within the Study Area. The potential zones were confirmed by well testing. Furthermore, data variables were distributed within a 3D Grid and based on the analysis performed the area of sweet spots were identified. In the next phase of the study, the results will be integrated with the upcoming Geophysical Seismic Inversion studies to further optimize the possibility of identifying the sweet spot across the Study Area. The robust data acquisition targeting Gachsaran was performed first time in the history of U.A.E. The results are encouraging in establishing the relationship to identify the dominant existence of Gas effects within the area. The estimation of realistic Gas In-Place and its confirmation of commercial discovery will open a new era of Shallow Gas resources within U.A.E.