Unlocking STOIIP of Permian Gharif Clastic Reservoirs in Central Oman

Recep A. Kazdal*, Jamie Doyle, and Suleiman Shukery
PDO
*[email protected]

The Permian Gharif reservoirs of central Oman represent one of the major producing hydrocarbon reservoirs with significant remaining upside potential locked in low-relief structural/stratigraphic traps. The Sadad field, which was discovered in 1986 and brought on stream in 1990, is a low-relief anticline (c. 45 m vertical closure and approximately 100 square km areal closure) located about 30 km to the west of the Mukhaizna field in central Oman. The oil is produced from the Upper Gharif, which comprises a terrestrial succession of arid, fluvial deposits contained within a predominantly muddy floodplain, resulting in a relatively low net/gross of about 30%. The oil is 24^o API with a viscosity of 42 cP. Initial development of the Sadad field until 1998, was based on the drilling of high-angle, deviated wells that aimed to penetrate the reservoir both laterally and vertically. This proved largely unsuccessful and cost-intensive due to the low success rate of finding producible reservoir within the highly heterogeneous Upper Gharif Member. Thus, a new development strategy was instilled, comprising multiple cost-effective, vertical wells (peepholes) that could tolerate the expected low success rate and improve the understanding of the reservoir model to reduce uncertainty in targeting productive sand trends. The first peephole campaign of 20 wells was drilled in 2001 with a success rate of 60% resulting in a production increase. The second peephole campaign drilled another 12 wells and netted a success rate of 66%. Both campaigns achieved the economic and geological success criteria from the peepholes. Several marginal discoveries were also made on similar low relief structures around the Sadad Field in the early 80’s with a single well. Many of these discovery wells were not tested due to poor perceived net/gross, despite good oil shows. Using the Sadad analogue, it could be interpreted that these single wells do not represent the potentially high oil-in-place (STOIIP) of these accumulations, and so warrant further investigation. Additional peephole drilling and possible 3-D seismic acquisition are proposed to support the search for more Gharif oil in central Oman.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain