ABSTRACT

Substantial volumes of world hydrocarbon resources occur in interlayered limestone-dolomite reservoirs. Diagenetic variations in lithology and primary depositional texture control the magnitude and spatial distribution of petrophysical properties. The frequency and nature of limestone-dolomite transitions that define flow units and baffles/barriers are critical for understanding reservoir connectivity and optimizing field development.

Existing subsurface predictions are largely based on observation and are occasionally linked to sequence stratigraphy. This approach can be relatively successful at predicting general trends in limestone and dolomite occurrence, but there is considerable uncertainty in predicting and correlating spatial variations in diagenetic styles at the field scale.

Reactive transport models that explicitly couple fluid flow and chemical reactions to facilitate quantitative investigations of limestone-dolomite transitions are a recent addition to the predictive diagenesis tool kit. The approach is illustrated with a generic study designed to investigate the fundamentals of brine reflux dolomitization. Results provide new insights on:

  1. Dynamic propagation of limestone-dolomite fronts by fingering,

  2. The connectivity of dolomite 'fingers' to the 'main body';

  3. Dolomitization and anhydrite occurrence (primary precipitation and cements);

  4. Locating reservoir 'sweet spots' in a dolomite geobody and

  5. The interaction and importance of semi-regional versus local brine reflux flow.

This process-based approach, in combination with available observational data and sequence stratigraphic paleoenvironmental reconstructions, has significant implications for predicting limestone-dolomite transitions, the spatial distribution of petrophysical properties and viable realizations of reservoir connectivity for flow simulation.

INTRODUCTION

Diagenetic transitions between limestone and dolomite are a critical control on reservoir quality in many carbonate reservoirs. The Jurassic Ghawar field in Saudi Arabia and the Permian-Triassic North Dome field in Qatar and Iran are two notable prolific Middle East examples. Dolomite can behave as a reservoir or as a baffle / barrier to flow depending on original depositional texture, style of dolomitization and presence of anhydrite cement [1–3]. High permeability, or so-called "Super K" dolomites with multi-darcy permeability that sporadically occur in thin zones (typically <5 ft) are vital contributors to individual well and overall reservoir performance [2]. Laterally continuous low permeability 'tight" dolomites can also have significant impact on subsurface flow and thus reservoir management strategies. The transition between limestone and dolomite is readily identified in both wireline logs and core and is dominantly observed as a sharp interface described as a "dolomite front" sensu Wilson et al. [4]. Unfortunately, dolomite fronts in carbonate reservoirs are often below seismic detection, particularly those associated with 'thin' beds of dolomite on the order of a few to several tens of feet thick. Predicting the spatial distribution or connectivity of dolomite and associated petrophysical properties between wells and away from well control is thus a fundamental challenge in carbonate reservoir characterization. In contrast to depositional facies there are no defined rules or guidelines for correlating dolomite bodies in the subsurface. Existing correlations of dolomite tend to be lithostratigraphic and contain considerable uncertainty in the degree of interpreted reservoir connectivity (Figure 1).

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