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Inter-well Scale Heterogeneity of the Upper Khuff Carbonate Reservoir Units, Outcrop Analog Approach, Central Saudi Arabia


The reservoir units within Khuff carbonates in the Middle East are known to contain about 30% of the World’s gas reserves. These reservoirs are also known for their complex heterogeneities at the inter-well scale that is obviously beyond the resolution of seismic data. The Upper Khartam Member exposed in central Saudi Arabia represent good outcrop analog for the Upper Khuff reservoirs in the subsurface. The aim of this study is to characterize the horizontal intra-reservoir heterogeneity and quality within the inter-well spacing using outcrop analog. Based on specific criteria, three potential reservoir units within the outcrop of Upper Khartam were selected to assess their heterogeneity and quality. The common criteria between these units are: composed of grainstone texture associated with high porosity and permeability values and they reflect good lateral continuity and spatial occurrence along the whole outcrop. The units are deposited above each other, unit A is at the bottom of the outcrop, unit B in the middle and unit C is at the top. Systematic horizontal sampling have been directed with 5 m spacing between each sample and another for the three reservoir units. A total of 90 samples were collected along these units and thin sections and core plugs were prepared from the whole samples and the porosity and permeability values have been measured. For each unit, several measurements have been estimated and investigated based on different statistical analysis. The techniques that measure the heterogeneity are geostatistical techniques and therefore they provide a single value for the heterogeneity in a dataset. These techniques include; homogeneity index, Lorenz coefficient and dykstra - parsons coefficient. Here, the focus on the Lorenz coefficient and a modified version of it. Based on the measured porosity and permeability values, basic equations were used to investigate about the reservoir quality: Reservoir Quality Index (RQI), pore volume to grain volume ratio (ɸz) and the Flow Zone Indicator (FZI). The porosity and permeability of these units, or in other words; their quality, have been significantly altered by diagenetic processes in a function of their lateral extension. This resulted in dividing each one of the reservoir units into segments with ranges of qualities and therefore a number of flow zones. The outcomes of this study may represent guides and leads for field geologists and engineers if integrated with subsurface data.