ADVANCES IN UNDERSTANDING THE EVOLUTION OF DIAGENESIS IN CARBONIFEROUS CARBONATE PLATFORMS: INSIGHTS FROM SIMULATIONS OF PALAEOHYDROLOGY, GEOCHEMISTRY, AND STRATIGRAPHIC DEVELOPMENT

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Abstract

Carbonate diagenesis encapsulates a wide range of water rock interactions that can occur within many environments and act to modify rock properties such as porosity, permeability, and mineralogical composition. These rock modification processes occur by the supply of reactant-laden fluids to areas where geochemical reactions are ther- modynamically and kinetically favoured. As such, understanding the development of diagenesis requires an understanding of both palaeohydrology and geochemistry, both of which have their own complexities.However, within geological systems, both the conditions that control fluid migration and the distribution of thermodynamic conditions can change through time in response to external factors. Furthermore, they are often coupled, with rock modification exercis- ing a control on fluid flow by altering the permeability of sediments. Numerical methods allow the coupling of multiple complex processes within a single mathematical formu- lation. As such, they are well suited to investigations into carbonate diagenesis, where multiple component subsystems interact. This thesis details the application of four sep- arate types of numerical forward modelling to investigations of diagenesis within two Carboniferous carbonate platforms, the Derbyshire Platform (Northern England) and the Tengiz Platform (Western Kazakhstan).Investigations of Derbyshire Platform diagenesis are primarily concerned with ex- plaining the presence of Pb-mineralisation and dolomitisation observed within the Di- nantian carbonate succession. A coupled palaeohydrology and basin-development sim- ulation and a series of geochemical simulations was used to investigate the potential for these products to form as a result of basin-derived fluids being driven into the platform by compaction. The results of these models suggest that this mechanism is appropriate for explaining Pb-mineralisation, but dolomitisation requires Mg concentrations within the basin-derived fluids that cannot be attained. Geothermal convection of seawater was thus proposed as an alternative hypothesis to explain the development of dolomitisation. This was tested using an advanced reactive transport model, capable of considering both platform growth and dolomitisation. The results of this suggests that significant dolomi- tisation may have occurred earlier on in the life of the Derbyshire Platform than has previously been recognised. An updated framework for the development of diagenesis in the Derbyshire Platform is proposed to incorporate these new insights.The Tengiz platform forms an important carbonate oil reservoir at the northeastern shore of the Caspian sea. The effective exploitation of any reservoir lies in an under- standing of its internal distributions of porosity and permeability. Within carbonate systems, this is critically controlled by the distribution of diagenetic products. A model of carbonate sedimentation and meteoric diagenesis is used to produce a framework of early diagenesis within a sequence stratigraphic context.The studies mentioned above provide a broad overview of the capabilities and ap- plicability of forward numerical models to two data-limited systems. They reveal the potential for these methods to guide the ongoing assessment and development of our understanding of diagenetic systems and also help identify key questions for the pro- gression of our understanding in the future.

Keyword(s)

Carbonate Diagenesis; Hydrology; Numerical Modelling; Reactive Transport; Sequence Stratigraphy

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