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AAPG Geoscience Technology Workshop

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Sedimentary Evaluation of Sandstone Reservoir by Using Image Logs, Case Study From North Africa

Abstract

The oil and gas have been accumulating in sedimentary rocks during geological time. The sandstone rocks are one of the very popular oil and gas reservoirs. To predict and find, extension and basin modeling of sandstone reservoir, the palaeocurrent setting and palaeogeography modeling are considered as very important case study for oil companies. By acquiring these data in the main petroleum basin from core and fullset logs of exploration wells and also seismic data, the exploration geoscientists can guide or even predict how the future drilling wells should be focused on which parts of basin map and at the same time the reservoir geologist can explain the main reservoir zones for better designing the completion wells and surface facilities. In the sandstone rock units or series, which contain petroleum reservoir potential, finding the cross bedding zones are very important task for geologists and petrophysicists. The cross bedding intervals have the most porosity and permeability in comparison with the siltstone and contaminated sandstone intervals with clay and shale. The calculations of palaeocurrent directions are very important in this well. The palaeogeography, palaeoslope and current direction are very vital for facies interpretation. The oil base mud image log (or induction resistivity image log) was run in one of the exploration well in this basin. After processing these logs, it has been observed that the cap rock which is evaporitic limestone has been overlay with an unconformity over reservoir rock which is sandstone. In sandstone interval four cross bedding zones or facies have been recognized. The cross bedding interval thicknesses from deeper to shallower depth or from older to newer ages are increasing. After applying structural bedding correction or removing structural tilting, every cross bedding azimuthal facies/zones are plotted in separate and independent rose diagram (with 30 degrees piece intervals). The convention is to plot palaeocurrent azimuthal data in a “current-to” sense. So that, pattern of rose diagram of azimuthal data indicates sedimentary environment. In this well two deeper cross bedding zones intervals have the unimodal azimuthal patterns which indicate one direction palaeocurrent. This unimodal of azimuthal rose diagram in this sandstone is depicting fluvial environment or meandering river. In continuous observation toward upward, the two shallower cross bedding zones intervals have the polymodal and bimodal azimuthal patterns. These azimuthal patterns depict two and three palaeocurrent directions in every shallower zones which are stating the deltaic or marine shelf environment in the thicker cross bedding younger intervals. On the whole view of sandstone formation/reservoir from deeper to shallower on image logs, four cross bedding zones intervals are thickening upward and at the same time their azimuthal patterns on rose diagrams are changing from unimodal to bimodal and polymodal palaeocurrent. Here it can be concluded a transgression of sea level over landward which meandering rivers have been overlaid by delta or marine shelf. This single well image log study has provided valuable geological, petrophysical and reservoir engineering information. It is highly recommended to run more image logs in future drill wells in this basin. By combining multi wells studies of image logs with structural and sedimentary interpretation of seismic data, a clear and detail of palaeogeography and palaeo sedimentary model of this basin would be presented which can provide critical information for future well planning and better indicating of porous borehole zones of completion wells.