Using Stochastic Inversion for STOIIP Estimation of Sub-seismic Exploration Discovery

The Lekhwair formation is a new play for PDO discovered by serendipity in 2014 after encountering an oil bearing reservoir while drilling a Shuaiba well (L1). The well found oil saturations in the thin formation below Shuaiba (in the Lekhwair). Reservoir was not expected in this formation, originally interpreted as a unit of tight lagoonal limestones. Further, oil was flowed and sampled from the reservoir in an MDT run. A follow up dedicated exploration well, well L2, was drilled in 2015 to prove reservoir productivity and it confirmed a high quality and productive reservoir. The producing unit is only ~8m thick which is clearly below the seismic resolution of ~28m. Nevertheless, the reservoir stands out acoustically from the background tighter units by having significantly lower acoustic impedance (AI) properties. This enabled testing stochastic inversion to de-risk reservoir presence and continuity. Moreover, for the first time PDO used stochastic inversion outputs to calculate the STOIIP of the discovery to support statistical volume estimations.