Abstract

The reservoir characterisation of lacustrine carbonates is currently challenging, as historically they are not common as major carbonate reservoirs and therefore have been less studied in terms of reservoir characterization. The giant discoveries of the Pre-Salt of the South Atlantic Margin in Brazil have introduced lacustrine carbonates as a major target for exploration and development and are driving new research.

Lacustrine carbonates are formed in lakes which are particularly sensitive to salinity, pH, water level and sediment supply variations. This variability results in a wide range of primary fabric types and associated porosity types, likely formed by the interaction of microbial and chemical processes. Lacustrine grainstones can be formed from spherulites or coquinas and stromatolites and stromatolite bioherms can form cavities and laminated pore systems. Shrub morphologies are found that can form complex pore networks. These fabric types can be affected by a series of diagenetic processes that can both enhance and occlude porosity.

In this paper, we review how porosity in these systems may be characterised in thin sections and using micro-CT and how permeability and resistivity might be determined in the laboratory or by 3-D modelling. We also address how rock types may be determined by analysis of capillary pressure data, to determine relative permeabilities for different rock types. This paper addresses some classical reservoir characterisation issues that are likely to be important in these reservoirs. The approach considered here emphasises an integrated link between petrography, facies analysis (outcrop studies) and porosity description in a petrophysically-based rock typing framework appropriate for geomodelling, upscaling and calibration with dynamic data. This reservoir characterisation workflow methodology is applicable to all carbonates, however, it is recognized that lacustrine carbonates present some special challenges.

Introduction

Carbonate reservoirs are strongly controlled by the nature of the porosity distribution. To understand the main primary controls on porosity we need to go back to basics. There are four main constraints on primary porosity (Moore, 2011):

  1. Framework (created within and between the dominant depo-structural features such as stromatolites),

  2. Boring (created by various bio-erosional agents),

  3. Detrital (intergranular porosity between grainstones or interbedded clastics) and

  4. Cement (early cements occurring during the earliest phases of diagenesis).

Secondary processes such as karstification (epi- or endokarst) can create new porosity largely through dissolution. Burial can result in the development of microporosity generally in the fine grained carbonate rocks and/or dolomitisation (either through reflux from marine water mixing) or hydrothermal activity often associated with faults and fractures. Lacustrine carbonates can also be associated with exotic clays and minerals through evaporitic influences.

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