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A consistent design of experiments (DoE) based evaluation process was used to assess the magnitude of OOIP uncertainty as well as the relative contributions from uncertainty sources as a function of the historical development of the Jurassic Humma Marrat carbonate reservoir in the Partitioned Neutral Zone (PNZ). Within the Marrat interval, three stratigraphic layers, known informally as the A, C, and E zones, produce oil. Porosity and permeability is best developed in the dolomitized lowermost Marrat E interval. Based on limited data, approximately 80-85% of the current oil production is from the E zone and 10-15% from the A zone. The C zone contribution is 5% or less.

The uncertainty sources used in the DoE-based evaluation were: structure (time-to-depth conversion and overall interpretation uncertainty), original oil-water contact (OOWC), porosity histogram, and oil saturation histogram. All of the uncertainties except structure were evaluated for each of the three stratigraphic zones known to produce oil in the Marrat. High, mid, and low-case values were determined for each of the uncertainty sources listed using well log, core, and analog information available after each well was drilled or as significant new data became available (e.g., reprocessed seismic volume). The time period covered by this historical look-back is from 1998 (pre-drill) to 2005.

The pre-drill P50 OOIP estimate for the Humma Marrat was about 900 million reservoir barrels. Following drilling of the initial two wells, the P50 OOIP estimate was < 400 million reservoir barrels. Subsequent drilling and structure modifications (interpretation and time-to-depth conversion) increased the P50 OOIP estimate to just over 1500 million reservoir barrels in April 2004. The P50 OOIP was dropped to 625 million reservoir barrels after Well F was drilled in mid-2005. The OOIP uncertainty range, defined as the P90 OOIP value minus the P50 OOIP value, decreased from nearly 700 million barrels in 2004 to about 130 million barrels in mid-2005. Analysis of the DoE-based results show that the significant contributors to OOIP uncertainty changed as additional wells were drilled or existing data was re-processed or reinterpreted. However, the structure and/or OOWC uncertainties were generally the largest, though not necessarily always statistically significant contributors to OOIP uncertainty (based on a 95% confidence level). A normalized uncertainty index (UI) derived from the probabilistic OOIP values is used to discuss delineation efficiency and may be useful in delineation well planning.

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