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Reservoir characterization of the unconventional Najmah Formation, Sabriyah field, northern Kuwait
Al-Qaod, Moath
Al-Qaod, Moath
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2017
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Abstract
The Sabriyah Field is one of the largest producing elds in northern Kuwait. It is domi- nated by carbonate rocks. The Najmah Formation is considered the hydrocarbon source for most of Sabriyah eld reservoirs. The objective of this research is to characterize the Na- jmah Kerogen Formation to provide a better understanding of the formation, pore structure, permeability and porosity for future drilling plans. The Najmah Kerogen unconventional reservoir has an average thickness of 50 ft. (Acharya et al., 2009) and is laterally continuous from the east to west of Kuwait. It is fractured and has minor subtle faults. The Najmah Kerogen Member of Najmah Formation is an unconventional resource play indicated by its low eective permeability (range from 0.01- 1.5mD). The great potential arises from the excellent total organic content (TOC) that ranges from 2.13-30.5 wt.% with an average of 8.3 wt.% and a vitrinite re ectance (Ro) of 1.28-6.6% (Khan et al., 2014). The Najmah Kerogen Member is mainly composed of calcite, minor dolomite and illite clay indicated by XRD analysis. The methods used for the geological reservoir characterization were both qualitative and quantitative for better understanding of the porosity and permeability of the unconventional reservoir. The qualitative methods included core description, analysis petrographic thin sections, Field emission Scanning electron microscope (FESEM), facies, fossils and sedimen- tary structures. The quantitative methods used were the x-ray diraction (XRD), x-ray uorescence (XRF), source rock analyzer and modied Schmoker equation for TOC mea- surements. In addition, mercury injection capillary pressure and nitrogen adsorption were used to measure the pore size distribution of the samples. The Najmah Kerogen is organic-rich mudstone that is composed mainly of calcite, dolomite quartz and pyrite. The dominant grain size was the mudsize of micrite (<0.02 mm) that reached 0.2 mm (sand size) in other samples determined from the petrographic thin sec- iii tions. In addition, the pore diameters measured from mercury injection capillary pressure (MICP) and nitrogen adsorption analyses were 0.0036-0.007 microns and 0.6-21 nm. The porosity ranged from 2-6% and dominant pore type was the organic matter pores and most of Najmah pore sizes were nanopores that explains the high entry pressure needed for MICP experiment. The texture of Najmah Kerogen Member is mature to over-mature, well sorted and well rounded grains. The micrite texture is subhedral to anhedral due to the secondary diagenesis cementation. The diagenetic processes observed were the dissolution and micriti- zation that enhance the pore development and also the secondary cementation of matrix over-printing and grain overgrowths that close the pores in the matrix. The fossils observed were ostracods, sh bones and shell fragments. The Najmah Kerogen was deposited in an in- trashelf basin that was anoxic during the deposition allowing the organic matter preservation and pyrite framboid formation. Most of the sediments of the Najmah Kerogen have both a biogenic and detrital origin. Pyro-bitumen was observed in all samples with secondary pyrite framboids suggesting the anoxic environmental conditions and conrmed by the XRD and XRF. The entry pressure of the samples in MICP was directly proportional with the depth. The Najmah Kerogen Member has a great potential for development of light oil and gas. This research will help delineate the dierent properties of Najmah Kerogen and its permeability association that will pave the way for the development of unconventional resources in Kuwait.
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