oa Exploration for Hydrocarbons in a Half-Billion-Year-Old Chert, the Athel Story Continues

In 1989 and 1995, two oil fields were was discovered in South Oman in Athel ‘Silicilyte’ reservoirs,<br>comprising Proterozoic to Early Cambrian chert slabs encased in Ara salt. The reservoir units have good<br>porosity but very low permeability and only flow light oil if massively fracc’ed.<br>When discovered it was thought that the chert layers might be present throughout the region but with<br>limited drilling success in the 1990’s it became clear that the reservoir was more restricted in<br>occurrence. A plethora of possible models exist for the origin of the chert and, therefore, it’s distribution.<br>The play requires that a porous slab of laminated chert be encased in salt, charged with light oil and<br>that overpressure is maintained. The permeability must be sufficient to maintain long-term flow after<br>fracc’ing. These constraints together with the reservoir, seal and charge distribution maps help define<br>the play sweet spot. In order to identify and evaluate likely prospects, high quality 3D seismic is<br>needed and an extensive seismic acquisition, reprocessing and analysis program is in progress in PDO’s<br>Block 6. Re-mapping and prospect analysis is being used to update the lead and prospect portfolio and<br>help de-risk the play. The play-based approach provides a balanced geological and geophysical derisking<br>scheme with the intent of providing drill worthy opportunities by 2011. This paper presents a<br>new reservoir distribution model based on integration of previous work, regional well and seismic data<br>analysis, study of global chert analogues and outcrops in Oman.