Online Journal for E&P Geoscientists

Zhu, Fuping ¹; Najwani, Hussain ^*1; Harris, Jeremy ¹; Ma'Mary, Talal ¹
(1) Oil North Directorate, Petroleum Development of Oman (PDO), Muscat, Oman.

Permian clastic X reservoirs contribute to a significant portion of PDO reserves. To drill and encounter Upper X (UX) channel sands has been historically challenging. Previous attempts to predict the location and extent of UX sands were discouraging and grid drilling is employed in most fields.

There were three existing wells in the Z area: one with no sand, one with 5 meters of sand and the third with two 10-m-thick sands in UX of100 m gross thickness. Their net to gross ratios (NTG) were all small. Earlier rock property analysis concluded sand and shale couldn’t be separated using seismic data because sands thickness was below the seismic resolution and the sand and the shale had overlapping acoustic properties. However, the three wells showed different characteristics in a 1994 vintage seismic dataset. When a two-sand-layer wedge model was built, the maximum rms synthetic amplitude occurred when the NTG was ~40%. Below this threshold, the amplitude decreased with NTG linearly till 10% NTG and then didn’t change much till 0% NTG. In addition, the seismic trace shape changed with NTG. These observations enabled a decision in 2007 to drill an amplitude hot spot, where good sand presence was predicted. Results from this and further three wells have all confirmed UX sand / shale lithology predictions from seismic data. Wells, synthetic and real seismic data converge on one conclusion: the absence of thick sands eliminates the non-unique correlation between seismic amplitudes and sand thicknesses and makes it possible to predict thin sands! In other words, seismic tuning helps X channel prediction due to its low NTGs.

Extracting value from old data, Field Z’s breakthrough success has not only played a key role in its Field Development Plan study, also rejuvenated PDO’s interest in quantitative seismic interpretation of other X reservoir fields.