--> Abstract: Predicting Carbonate Diagenesis Using Reactive Transport Models: Emerging Insights and Challenges, by Fiona Whitaker, Katherine J. Cooper, Anwar Al-Helal, Peter Smart, Sebastian Geiger, and Yitian Xiao; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Predicting Carbonate Diagenesis Using Reactive Transport Models: Emerging Insights and Challenges

Fiona Whitaker1; Katherine J. Cooper1; Anwar Al-Helal1; Peter Smart2; Sebastian Geiger3; Yitian Xiao4

(1) Earth Sciences, University of Bristol, Bristol, United Kingdom.

(2) Geographical Sciences, University of Bristol, Bristol, United Kingdom.

(3) Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, United Kingdom.

(4) ExxonMobil Upstream Research Company, Houston, TX.

Reactive transport models (RTMs) can improve our capability to predict carbonate diagenesis by 1) helping to develop better conceptual models based on chemically and physically realistic scenarios, 2) providing quantitative estimates of rates and distribution of diagenesis and 3) describing diagenetic geobodies which can be used to populate reservoir models. Here we explore elements key to developing meaningful RTMs, using examples from early meteoric and reflux diagenesis.

In the vadose zone flows are essentially vertical and diagenesis can be captured in a 1D model. However, most systems need to be simulated in 2D or 3D, with systematic differences in diagenesis predicted for linear and circular platforms. Model results are only as good as the processes included. PCO2 is a major diagenetic driver and thus it is important to simulate the effects of root and microbial respiration, while temperature-dependent reactions (e.g. dolomitisation) require inclusion of heat transport. Reaction rates in natural systems are orders of magnitude slower than in the laboratory, reflecting the reduction in both reactivity and contact between reactive fluid and mineral surfaces over time. This is seen in comparisons of early meteoric diagenesis in systems dominated by inter-granular porosity and those with significant karst/fracture permeability.

Temporal changes in boundary conditions (e.g. relative sea-level and climate) are a particular challenge for modeling early diagenesis. Thus shallow meteoric alteration occurring over thousands of years is driven by seasonal alternation between periods of recharge and evaporation that cannot be represented by some average condition. The spatial scale of model cells must match the rates of fluid flow and reactions, and also capture key geological variability. Depositional facies and prior diagenesis control both the permeability and effective reactive surface area, and thus the acuity of prediction is a function of the representation of heterogeneities in rock fabric. Spatial variations in sediment texture result in diagenetic geobodies with complex geometries, reflecting a balance between supply of reactive fluids and reaction rate.

When configured correctly to capture key elements of the diagenetic systems RTMs can contribute to better prediction of reservoir quality. The challenge now is to incorporate more sophisticated feedbacks between diagenetic alteration and permeability/reactivity at a range of scales.