As conventional hydrocarbon plays have become highly mature after more than fifty years of exploration, Petroleum Development Oman LLC (PDO) is increasingly focusing towards unconventional plays. The Amin gas play is an example of such a play. The play concept consists of high pressure and high temperature alluvial sandstone reservoir with a very high netto-gross (N/G) but relatively low porosity and permeability. Due to these properties, the reservoir is classified as a tight reservoir.
Several wells in North Oman have encountered Amin reservoir showing low permeability and overpressures. One of the key wells within the area of interest has encountered high overpressures with good porosity and gas pay. These findings have triggered a quantitative interpretation (QI) study to pursue a possible correlation between seismically derived properties such as acoustic impedance or velocity with overpressures and/or good porosity to pinpoint sweet spots for drilling.
The QI study was split into two parts: Phase 1 consists of an amplitude versus angle (AVA) forward modelling including cross plotting of the rock properties and Phase 2 comprises an AVA inversion. The purpose of Phase 1 was to assess the feasibility of predicting reservoir quality or/and pore pressure by integrating well log data, VSP's and seismic data. The results from this phase were encouraging and it was decided to proceed with the AVA inversion.
The AVA inversion was carried out on the state of the art wide azimuth seismic data that was acquired in 2009. Using the results from the inversion and the knowledge obtained from Phase 1 enables us to identify sweet spots for future exploration drilling. In this paper the QI workflow is described which has been applied to identify sweet spots for this tight gas play. After this study was completed one additional well was drilled and is used as a blind test well.
The main conclusions from this study are that porosity variations at Amin level can be related to acoustic impedance at well resolution and at seismic resolution. AVA behavior shows that "higher" porosities in Amin give an increase in amplitude with increasing angle while "low" porosities show a polarity flip. Due to the limited availability of well data and their clustered distribution in the south-east corner of the survey, the inversion results are heavily biased by the derived low frequency model. Hence, only a porosity map and no volume has been calculated and is used to guide the porosity distribution in the static model.