Improved Understanding of Minor Reservoirs Optimizes Full-Field Productivity
Authors: Carpenter Chris 
Published: March 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 21225, “Defining Heterogeneity and Compartmentalization Predictions of Minor Reservoirs in Fluvial Environments: Geological and Dynamic Context,” by Nur Azah Zulkifli, Lisa C. Chisholm, and Amy M.M. Yusoff, Petronas, et al. The paper has not been peer reviewed. Copyright 2021 International Petroleum Technology Conference. Reproduced by permission.

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Reservoirs in the MN field offshore Malaysia consist of predominantly fluvial delta deposits. Several modeling studies have been performed for major reservoirs. After more than 20 years of production, a project for minor reservoirs has been created. The primary objective of this study was to improve the understanding of the uncertainties affecting well performance and reservoir connectivity and to identify potential infill opportunities.

Introduction
From the initial analysis of a 2017 full-field review (FFR), the team decided that a detailed subsurface evaluation was necessary. This study was completed with the integration of subsurface data to ensure that hydrocarbon volume and geological knowledge have been calibrated with production performance.
The field is 160 km north-northeast offshore peninsular Malaysia in a water depth of 69 m. This field is divided into five areas: Main, West, North, South, and Southwest. The depositional environment of this field is fluvial coastal plain and consists of multistacked oil and gas reservoirs with an average net thickness of 2–20 m, interbedded with shale and coal in a coastal plain setting.

Geological Concept and Overview
Reservoir X is one of 26 major and minor hydrocarbon-bearing units in the field. Twenty-two minor reservoirs were analyzed for the FFR with an integrated study work flow.
Geological analysis concluded that these reservoirs were deposited in a fluvial environment consisting of meandering channels, point bars, overbanks, and abandoned channel-sand mouth bars with deltaic influence (Fig. 1). Seismic data have been used extensively to tackle key uncertainties with either quantitative or qualitative approaches, depending on the seismic detectability limit of the reservoir. The use of seismic in this case was challenging because seismic attributes are not optimal and mostly are used as soft data guided by primary well data. Additionally, several reservoirs required stratigraphic compartmentalization to be introduced.

Heterogeneities in the Fluvial Environment
Capturing and preserving the different level of heterogeneities was complex because of varied data resolution, grid definitions, and hierarchies of impact. This field has relatively large net-to-gross (NTG) ranges from 20 to 98% with individual channel width averaging from 20 to 100 m. Because the existing dynamic models struggled to replicate field performance, the model created in this study had limited predictive capabilities. The four levels of fluvial heterogeneities of megascopic, mesoscopic, macroscopic, and microscopic scales were used to better understand the heterogeneities of these minor reservoirs. This has helped to compartmentalize the reservoir based on interpretation of the dynamic data without compromising geological understanding.