This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 186005, “An Innovative Approach Toward Improving the Relationship Between Flow-Zone Indicators With Lithofacies: A Case Study in a Carbonate Oil Field in the Middle East,” by N.S. Hashim, A.F. Zakaria, and N.A. Ishak, Petronas, prepared for the 2017 SPE Reservoir Characterization and Simulation Conference and Exhibition, Abu Dhabi, 8–10 May. The paper has not been peer reviewed.
The F field in the Middle East currently has more than 40 producing wells in the center of the structure. The uneven well distribution limits the understanding of 3D reservoir characterization, particularly in the flank areas. A fit-for-purpose integrated reservoir-characterization study was carried out. The exercise, outlined in the complete paper, confirms the heterogeneity within B formation (the primary oil reservoir within F field), and it captures the changes in reservoir quality laterally and vertically.
Introduction
F field is located in the northeastern part of the Arabian Peninsula. F field is a low-relief anticlinal structure aligned northwest/southeast, approximately 31 km long and 10 km wide. B formation consists of carbonate-ramp depositional settings, and has an average thickness of 200 m. Multiple development wells were drilled as part of its development plan, and its first oil production was achieved 3 years ago.
Issues with reservoir characterization have arisen because of the uneven well distribution, especially in the flank area. Furthermore, only one conventional core with good recovery was available for Reservoir B, which makes it somewhat difficult to delineate the internal architecture of the carbonate ramp.
B Formation Stratigraphic Interval
B formation subreservoirs include Upper B (UB), Middle B (MB), and Lower B (LB); the MB and LB subreservoirs contain most of the hydrocarbon deposits. UB does not contain any hydrocarbons and is believed to be tight on the basis of log information. Each subreservoir was divided into sub-units (UB-1 and UB-2; MB-1, MB-2, and MB-3; and LB-1, LB-2, and LB-3) on the basis of pressure and fluid information.
One conventional core was taken with a total length of 100 m and total core recovery of 97%. This conventional core covers the Middle B to Lower B subunits. Full-core analysis study was conducted on this conventional core, which includes routine geological analysis, routine core analysis (RCA), special core analysis, and digital rock analysis.
Lithofacies Analysis
The study identified seven lithofacies.
Bioclastic Mudstone (BM). BM consists of a mostly micrite matrix. It is light gray to pale brown and is moderately bioturbated. In terms of reservoir characteristics, this lithofacies is poor, with no visible porosity. BM is usually deposited in an open marine slope.
Bioclastic Wackestone (BW). As with BM, BW also consists of a micrite matrix. Carbonaceous wispy seams, solution seams, and stylolization are also common in this lithofacies. BW is characterized as tight, with no visible porosity. Its measured porosity ranges from 4 to 21%, with average porosity of only 10% and a permeability range between 0.1 and 7.6 md. BW is likely located in the shallow marine middle ramp.
