Integrated reservoir characterization of a Miocene carbonate buildup without the benefit of core data - a case study from Central Luconia Province, Sarawak

Abstract

Central Luconia Province is one of the world's largest carbonate complexes, with over 200 known Miocene build-ups, created by coral reef structures growing on horst blocks. Most build-ups are gas reservoirs, but also include some oil fields. Traps are stratigraphic, as porous carbonates are encased in sealing shales. The main method used to model properties in Miocene carbonate patch reefs is a direct porosity modeling using Acoustic Impedance as a secondary variable. The lack of long cored sections in this block is the main reason for the absence of facies integration in the static model. Another challenge is the limited number of wells per patch reef, which makes the understanding of lateral facies variation very limited. The main challenge to model these reservoirs is to characterize the distribution of the multi-scale heterogeneities, where hydraulic properties are not necessarily directly related to porosity. Accurate rock typing and porosity determination are key components for establishing porosity – permeability relationships based on rock type. The study area is located in Central Luconia Province, offshore Sarawak. The discovery well proved several hundred meters of gas column down to the gas-water contact within a Cycle IV carbonate patch reef. A complete set of logs was acquired, although no cores were available for facies identification and calibration. This case study presents a methodology to populate facies and porosity in 3D through a holistic approach to data integration and using conceptual models from field analogues. Introduction There is not a single industry-wide agreed-upon scheme that defines how to group lithofacies and environmental settings for mapping purposes or for subsequent reef/platform modeling. Within the Central Luconia Province this may be due to complex interactions between diagenesis and depositional facies, particularly within the shallow-water part of carbonate build-ups. Ali and Abolins (1999) proposed six basic groupings of facies based on diagenetic features: chalkified limestone, moldic limestone, tight argillaceous limestone, sucrosic dolomite, moldic sucrosic dolomite, and "overdolomite". These terms are difficult to relate back to recognized lithofacies terminology and certainly do not relate well to depositional settings. Also, no significantly dolomitized carbonates have been observed in any of the cores from the fields in the study area, and the term "overdolomite" was not defined. Therefore this classification doesn't appear to be directly applicable for this case study. Based on field studies over a 5 year period Epting (1980) proposed that diagenetic processes affected the various depositional units selectively. He concluded that Central Luconia carbonate build-ups can be subdivided into four lithological groupings with discrete ranges of petrophysical properties. He referred to these groupings as "rock types". Based on core examination, correlation with logs suites, and an understanding of the biological and depositional zonation found on modern patch reefs, Epting (1980) suggested four environments that could provide useful lithofacies groupings within Central Luconia study area. His classification of carbonate settings includes protected, reefoid, shallow open marine off-reef and deeper open marine off reef. Other summaries have been offered for mature shelf-margin reefs (Longman, 1981), ribbon reefs from the Great Barrier Reef (Vernon and Hudson, 1978) and many others. Definitions for a well-defined, not overly-complex scheme that is well matched to patch reef settings in the Indonesia-Malaysia area by Jordan (1998) include the following settings: a. Middle Shelf, b. Inter-reef Lagoon, c. Reef-Sediment Slope (Base of Slope), d. Reef, e. Reef Flats, and f. Island. This study is based on this latest classification.