f A new insight into understanding the middle Cambrian Miqrat reservoir in the North of the Sultanate of Oman: Implication for exploration and development strategies

In the past decade, Petroleum Development Oman (PDO) has been actively drilling deep tight gas wells targeting structural four way dip closures in the Miqrat Formation (middle Cambrian). The Miqrat Formation was widely deposited in an arid to semi-arid continental setting in alluvial and playa/sabkha environments.

The wide distribution of the Miqrat formation and the lack of high resolution seismic together with unavailability of dense well information imposed various challenges in developing accurate subsurface depositional models. The play depositional model was constructed as simplistic as possible assuming a wide-spread playa which tends to be sandy in the south and muddier toward the north. In addition, the previous models were forcing the correlations to fit the entire reservoir subdivisions (Upper, Middle & Lower) to be present within the play domain. This served the regional understanding for prospect identification. However, when zooming into field-scale areas it becomes obvious that the depositional model needs further refining to understand the well data and the flow behaviors in various domains.

Recently, with the arrival of high resolution wide azimuth seismic data and the availability of denser well penetrations, a relook was applied to the previous depositional model by integrating various multidisciplinary inputs including: well log evaluations, seismic inversion for reservoir characterization, well test results and pressure measurements. The data integration revealed that the Miqart reservoir properties and thickness are highly influenced by the paleo structural elevation variations and the accommodation space post the Angudan unconformity (poor properties close to the highs). These variations resulted in lateral change to thinner/poorer units or onlaps on dominant highs, and the existence of capillary trapping ( Figure 1 ). As a result, the tighter and thinner facies retained significant amount of water which did not succeed to get flushed by late gas charge due to capillary barriers. Therefore, wells which are drilled in good reservoir properties in a structurally down dip position show better gas saturation with high gas and low water production rates in comparison to the wells which are drilled up dip in poor properties within the same traps ( Figure 2 ).