Comprehensive Multipore System Analysis for a Better Understanding of Metasediments Reservoir Deliverability: a Case Study from the South Malay Basin Offshore Peninsular, Malaysia
Authors: Gaafar Gamal Ragab , Nagm Ehab Abdul Rahman , Sillapasert Kriangakmon 
Published: May 2016

Fractured basements are one of the more complex types of reservoirs for drilling, assessment, development, and production enhancement. While they represent some of the largest and most productive reservoirs, they decline rapidly and are prone to undesirable fluids breakthrough. When characterizing fractured basements to better estimate reservoir capacity and deliverability, it is important to understand the role that matrix porosity and fracture network play in reservoir performance. This is especially true when the reservoir has been subjected to a variable metamorphic grade during different tectonic episodes, in addition to localized contact metamorphism by intrusions.

Matrix porosity can be estimated from electric wireline and logging/measuring while drilling tools. However, for better estimation and to help minimize uncertainty, identifying the rock-forming minerals using elemental analysis on core or ditch cuttings is essential. The composition of the metasediment can be used to identify the original sedimentary rock, even where it has been subject to high-grade metamorphism and intense deformation. Fracture characterization and porosity and permeability can be estimated using resistivity or acoustic borehole imaging tools. Also, borehole seismic data can help with fracture characterization.

Fractured basement reservoir connectivity can be confirmed using wireline formation testing and sampling (WFT) or drill stem tests (DSTs). In addition, because not all open fractures produce, it is important to identify the critically stressed fractures that will contribute to production using geomechanical analysis. Comparing results from the study area with other fractured basement matrices and fracture porosity and permeability ranges was helpful in validating the analysis and predicting reservoir performance.

This paper presents a proposed workflow for a set of data acquisition and characterization technologies and methods using a local example from an offshore Malay basin. The pros and cons of each technology and methodology proposed are discussed in detail.

Introduction

There are many definitions for basement reservoirs. Landes et al. (1960) state "basement rocks are considered as any metamorphic or igneous rocks (regardless of age) which are un-conformably overlain by a sedimentary sequence."

Basement reservoir-forming rock types vary according to 1) mineralogical composition and texture for igneous rocks or 2) original rock and texture for metamorphic rocks (Fig. 1). Understanding the mineralogical composition is essential for calculating matrix porosity and can provide a better estimation of the expected range. It also indicates the probability of having fractures and what type of filling material is present in the fractures.

In principle, there are many possible sources for hydrocarbon accumulation in basement reservoirs. However, three sources are referenced most commonly (Sircar 2004):Overlying organic rock, from which hydrocarbon was expelled downward during compaction.Lateral, off-the-basement, but topographically lower organic rock, from which hydrocarbon was squeezed into an underlying carrier bed, through which it migrated up dip into the basement rock.Lower, lateral reservoirs, from which earlier trapped hydrocarbon was spilled because of tilting or overfilling.