Prediction of Reservoir Quality & Deliverability of the Tight Barik Formation in Khazzan Field, Oman
Authors: Al Rashdi Khalil Ali , Wells Martin , Clark Nigel 
Published: December 2021

The giant Khazzan gas field, located in onshore Oman, has been under development since 2013 and in production since 2017. The field is currently producing 1 billion cubic feet of gas per day from the Cambro-Ordovician Barik Formation. The 80-metre-thick paralic reservoir is 4.5 kilometers deep and has undergone complex stages of diagenesis, hydrocarbon charge and structural regime changes.

Reservoir quality (RQ) is typically classed as tight (average porosity 6 porosity unit, average permeability 1 Milidarcy) but locally exceeds expectations given the burial history reaching up to 12 pu and 100 Milidarcy. This RQ variability and complexity makes reservoir deliverability (RD) a key uncertainty impacting the field development scheme and ultimately the projected economics. This study aims to create and test hypotheses of RQ and RD controls to reduce uncertainty in production and increase reservoir development efficiency.

In order to better understand the key controls on reservoir quality, an extensive set of core, petrophysical log analysis and production data were integrated with field-wide seismic and outcrop data to update the Barik stratigraphic, structural and depositional frameworks. Extensive analytical techniques, including reservoir quality modelling, petrographic analysis, X-ray diffraction, mercury injection capillary pressure and minipermeameter data were also integrated.

Quartz cementation and compaction are the principal degrading controls on reservoir quality. The controls on quartz cementation are complex and variably inter-related, although in general it is ductile content, proximity to mudstone and feldspar content that are the best predictors of porosity and permeability when convolved.

Minipermeameter data confirms that distance to mudstone, or sandstone thickness, is an important control on reservoir quality. Using normalized gamma ray log data, total and mean individual sandstone thickness were calculated for every Barik well in Khazzan and compared to well dynamic behavior which demonstrated a positive correlation. Areas with high mean individual sandstone thickness and total sandstone thickness frequently equate with relatively high IP30s (average well production at 1100 psi well head pressure for 30 days). In contrast, areas with high total sandstone thickness, but low mean individual sandstone thickness may only have moderate IP30s as those sandstones may be more quartz cemented.

Reservoir deliverability risk maps based on total and mean individual sandstone thickness and IP30 were constructed. These maps give insight into regions of poor and good gas deliverability and have identified areas that may be untested or undeveloped that may have potential upside.

The resultant reservoir deliverability understanding of the Barik formation is consistent with depositional environment, diagenetic understanding and well performance. It is a good example of integrating diverse static and dynamic data to improve reservoir understanding and has direct business impact.