The Miqrat Formation (Cambrian) of North Oman is a deep, tight (low permeability) gas reservoir that was deposited in an arid to semi-arid continental setting. It is dominated by finely interbedded, red-brown shales interstratified with muddy, micaceous, fine-grained sandstones and siltstones. These were deposited in alluvial and playa/lacustrine to sabkha environments with minor aeolian intervals. The formation has a variety of geological, petrophysical and engineering challenges, largely related to its depositional setting and burial history.
Geoscience challenges centre on reservoir productivity identification, involving a combination of depositional and diagenetic features, and ‘sweet-spotting’, particularly the pre-drill prediction of good quality reservoir facies (e.g. aeolian sandstones). Petrophysically the Miqrat is difficult, not only because of its tight nature, but also because it commonly contains low resistivity pay and is thin-bedded, making contact identification, porosity and saturation estimation challenging. Additionally, net pay determination is hampered by the thin-bedded nature of the reservoir and its high feldspar content, such that gamma ray logs consistently read high values, and, typically, neutron-density separation is poor. In terms of petroleum engineering, stimulation strategies are challenging, especially the choice of zones to stimulate, and the absence of barriers to fracture height growth means that hydraulic fracture treatments are performed across multiple non-pay intervals in order to have coverage for the interspersed pay intervals and to achieve the desired fracture length (penetration geometry) for a target interval.
The Miqrat Formation tight gas play is a difficult and challenging play that requires close co-operation between geoscience, petrophysics and petroleum engineering disciplines to overcome these challenges and optimally exploit it as a gas reservoir. Only through such an integrated approach can the full value of the Miqrat Formation be delivered to stakeholders.