A Comprehensive Rock Atlas for Documenting the Petrographic and Petrophysical Character of a Giant Mid-Cretaceous Reservoir in Oman

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This reference is for an abstract only. A full paper was not submitted for this conference. Abstract: The poster introduces a new style of rock-atlas which comprehensively documents the petrographic and petrophysical properties of reservoir rocks from a Giant Mid-Cretaceous carbonate reservoir in Oman. The atlas integrates diverse datasets from sedimentologists, petrophysicists and reservoir engineers in a specially designed one page per sample format to allow comprehensive and easy access to rock data. It provides an overview in digital format of available high quality data for easy visual screening and sits at the top of a digital data base which contains the individual data categories. This giant carbonate reservoir in northern Oman has been producing for over 40 years under primary depletion and secondary development schemes. While cores were taken from some 30 wells over the years, overall coverage and recovery did not provide an adequate data base for a comprehensive and quantitative evaluation of the porosity and permeability system. Consequently, an extensive coring campaign was executed in the years 2003 to 2005 which resulted in excellent overall recovery of above 90% (1810 meters of new cores were taken with about 1700 meters of core recovered). For the first time, not only complete coverage of all Natih formation stratigraphic intervals was achieved, but the availability of cores from multiple wells also provided lateral and some up-dip/down-dip control. This effort has generated extensive geological and petrophysical data and subsequent studies have revealed a multi-scale heterogeneity system. The starting point for analyzing the porosity and permeability system from cores was to systematically cut three plugs per meter of core for routine porosity/permeability and MICP measurements. This resulted in over 5100 routine core samples. Using the results of a subsequent core description and facies analysis a representative subset of 510 plugs was selected for further analysis using petrographic thin sections. Nearly all of these thin sections were subjected to additional porosity quantification using modal analysis (point counting of mineral components and porosity on thin sections) and digital image analysis. After a review of thin sections, MICP data and CT-scans, a further subset with permeability greater than 1mD was selected for special core analysis (SCAL). Porosity and permeability data derived from conventional core analysis on plugs, modal analysis (MA), image analysis (IA) and mercury injection capillary pressure (MICP) measurements are recorded for each sample. Substantial differences in porosity and permeability values have been observed from the different techniques used. Samples provide a representative, descriptive and numerical data set of depositional facies, facies associations and rock types covering the stratigraphy and lateral variations recognized from core descriptions. At this point, the data set has been reduced from an initial starting point of about 5100 potential measurements to some 231 representative samples. These were submitted to a comprehensive SCAL analysis program (relative permeability and capillary pressure by centrifuge for oil-water and gas-oil, wettability (Amott), wettability (USBM), overburden stress, saturation exponent "n", cementation exponent "m" and NMR). Results from the extensive SCAL program were received from the vendor and incorporated in this atlas. However, failure during SCAL experiments (sample collapse and/or erratic data) reduced the number of samples with quality controlled and reliable SCAL data further to 166. While a reduction from a starting point of 5100 plug samples down to 166 samples might sound excessive, it is actually an unusually large and high quality data set not commonly available in carbonate reservoirs. The atlas combines the following data on to one single A3 size page format for each sample: sample source data with facies and rock type information, thin section description, thin section photomicrograph, CT-scan, porosity and permeability values from different measuring techniques, porosity-permeability cross-plot, MICP plots, modal analysis data, SCAL plots, SCAL QC status and SCAL numerical data. By integrating data from different sources into one comprehensive document, this atlas has provided subsurface engineers with an easily accessible reference document which increases understanding of the reservoir properties and heterogeneity and safeguards an expensive data set for future use. Example from 1 sample is provided below.