AAPG Middle East Region Geoscience Technology Workshop

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Challenges of the Low-Resistivity Pay Transition Zone of the Shuaiba Formation


Oman Oil Company Exploration & Production LLC (OOCEP) operates the Bisat field onshore Oman, which produces from the Shuaiba formation, a Cretaceous limestone averaging porosity at 28%. It is a heterogeneous carbonate formation comprising facies impacted by diagenetic alteration of the original rock fabric. Conventional open hole measurements were acquired during the first exploration well campaign and include pressure measurements and resistivity logs. The resistivity-based saturation logs identify a water zone, confirmed by a water-like gradient observed on pressure measurements, by contradiction, downhole fluid analysis of appraisal wells identified mobile oil below the derived Free Water Level (FWL). A sedimentological, diagenetic and reservoir quality evaluation of the cored Shuaiba formation has been carried out and has provided greater understanding of the key factors impacting the resistivity measurements from logs. A variation in permeability within the Shuaiba Facies results from a change in the pore system: the uppermost rudist rich reservoir becomes micropore dominated with depth. The combination of microporosity and transition zone is proposed as the primary cause of low resistivity pay in this reservoir. The capillary bound water in the micropores forms a continuous path for the current and creates a low resistivity contrast between water and oil, resulting in an over estimation of water saturation using the Archie method. In parallel, pressure measurements across the oil/water transition zone failed to predict a correct free water level. Numerical simulations for analogues reservoirs (A.J.G. Carnegie, 2007), suggests that wells drilled with a water-based mud (WBM) in a mixed wet environment (wettability decreasing from more oil wet at the top to water wet closer to the FWL) result in significant anomalies in the gradients. Thus, it is difficult to determine the FWL from pressure alone. The reservoir has a long transition zone, which varies in thickness across the field because of reservoir quality variations. One of the main challenges is to build a saturation model for hydrocarbon volume calculation and field development study with a good consistency with testing information. SCAL program is ongoing to obtain a better capillary pressure vs saturation relationship. STOIIP increased significantly as most of the initial oil in place is in the capillary transition zone spreading over an extensive area. This case study demonstrates the benefits of downhole fluid sampling and testing in the characterization of a complex limestone transition zone, where pressure and resistivity logs do not provide the confidence to predict correct contacts.