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Abstract:
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The Late Jurassic Arab D Formation in Saudi Arabia hosts the some of the world’s largest hydrocarbon reservoirs including Ghawar , the world’s largest oil field , and Khurais , the world’s largest supergiant to come into production in the last 5 years . Despite the vast oil reserves within the Arab D , and the central role of this reservoir at Ghawar in making up short -falls in global production , our understanding of the much fundamental characterization work both in terms of modern sequence stratigraphic reservoir frameworks and linked structural /fracture characterization . This study of Arab D reservoir at Qatif , immediately to the north of Ghawar , provides one of the first looks at a modern sequence analysis of this producing interval and illustrates that porosity zonations , and ultimately flow unit architecture may be substantially different than currently in use . The Arab D of the Arabian Plate is a carbonate ramp system of exceedingly low angle ( <1o ) developed during a low -eustatic -amplitude greenhouse Milankovitch setting .
Combined macroscopic and petrographic data analysis allowed recognition of nine depositional facies : 1 ) spiculitic wackestone , 2 ) Planolites -burrowed wackestone , 3 ) bioturbated skeletal -peloidal packstone , 4 ) pelletal packstone , 5 ) peloidal -skeletal grain dominated packstone , 6 ) peloidal -skeletal grainstone , 7 ) skeletal -ooids grainstone , 8 ) cryptalgal laminites and 9 ) anhydrite . The depositional facies defined are used to interpret three facies tracts from deep to shallow across the ramp profile : 1 ) low energy sub -storm wave base (SWB ) dominated facies that may illustrate disaerobic tendencies , 2 ) high energy within -fair -weather -wave -base ramp -crest or mid -ramp facies including foreshore and upper shoreface oolitic and skeletal grainstones that define one of the key reservoir pay zones and 3 ) back -barrier tidal flats consisting of cryptalgal laminites , sabkha -type anhydrites , and salina -type anhydrites .
Three high frequency sequences are defined : QSEQ 1 is asymmetrical , dominated by subtidal lithofacies ; and QSEQ 2 and QSEQ 3 are symmetrical and record a complex history of the fill on an intrashelf basin . Detailed cycle -scale correlations using core -based cycles and wireline log patterns allowed a cycle -scale correlation framework to be established that illustrates a north to south progradation of the Arab D reservoir strata , building landward from the Rimthan Arch .
Diagenetic features observed in the Arab D reservoir include fitted fabric (chemical compaction ) , dolomitization , and cementation . These features play a major role altering reservoir quality properties as they essentially control fluid flow pathways which ultimately alter primary porosity and permeability . |