Origin of overpressure and pore pressure prediction in carbonate reservoirs of the Abadan Plain Basin

Abstract

This thesis analyses overpressure throughout the Abadan Plain Basin and evaluates pore pressure in this basin using conventional petroleum industry methods, as well as two new proposed pore pressure prediction methods. Overpressures in the Abadan Plain Basin are primarily exist within carbonates, whereas most previously published overpressure analysis has been undertaken in shale-dominated clastic rocks. Overpressure in this basin is encountered in two main zones, primarily the Gachsaran and Gadvan/Fahliyan formations. South-west to north-east oriented thickening and shortening, as result of Arabia-Eurasia collision, has affected the pressure regime within the Gachsaran Formation, but seemed ineffectual to the Gadvan and Fahliyan overpressures. In order to analyse overpressure origins and test conventional pore pressure prediction methods, a discrimination scheme was applied to remove the impact of lithology on the log recordings, resulting in isolating the minor shale interbeds within, and as a representative of, the carbonate sequences. Disequilibrium compaction was identified as the primary origin of overpressure in the Abadan Plain Basin. Eaton’s (1972) pore pressure prediction method was applied on the filtered shale data with an exponent of 1.0 for sonic velocity, 0.1 for resistivity, and 5 for density data. Bowers’ (1995) method was also tested and, while it accurately predicted pore pressure in the Gadvan and Fahliyan formations, it underestimates pore pressure in shallower formations. This thesis also introduces a new ‘compressibility method’ for pore pressure prediction, developed by the author, that uses porosity-compressibility correlations. This new ‘compressibility method’ provided reliable pore pressure prediction results in the studied wells. Alternatively, overpressure as a result of sediment compaction is also estimated using Biot’s (1941) general theory of three-dimensional consolidation. A generalised compaction model was constructed, and the resulting modelled pore pressure provides a reasonable estimate of observed pore pressure.