Carbonate sedimentology and sequence stratigraphy
Carbonate sedimentary rocks are biogeochemical precipitates that are deposited in specific environmental conditions, usually shallow, warm and clear seawater or, less commonly, lakes. Their composition has evolved through time as a function of changes in the Earth’s climate, seawater chemistry and evolution, making them unique proxies for global environmental change in deep time. They also have high economic value; carbonate rocks host over 50% of the World’s oil and gas reserves as well as large volumes of low-temperature minerals such as lead and zinc sulphides, barite and fluorite.
Limestone is commonly quarried for building stone, road-stone and cement production, may be associated with salt deposits and forms important aquifers for fresh- and geothermal water. Finally, carbonate rocks are highly reactive, and can dissolve to form caves and sink holes that form a distinctive natural topography with unique ecosystems. Sometimes, sinkholes created by dissolution of limestone and evaporite minerals collapse, creating localized geo-hazards and engineering challenges.
For enquiries in this area, contact Cathy Hollis. To find more details of researchers across the Basins group, see the People page.
The carbonate research group in the School of Earth and Environmental Sciences is led by Dr Cathy Hollis, with Dr Stefan Schroeder and a team of academic staff working in the field of carbonate sedimentology and diagenesis. The team supports over 10 post-graduate and post-doctoral researchers who are working on multi-scale, interdisciplinary projects that consider the processes governing:
- the presence, distribution and architecture of carbonate platforms, and how these sediments provide a proxy for palaeo-environmental change through geological time;
- post-depositional modification of carbonate rocks as a result of physical and chemical processes, such as compaction, cementation and lithification, dissolution and fracturing;
- the presence and distribution of pore types and how the volume, topology and distribution of those pores affects rock-physical properties, flow behaviour and economic value.
In order to conduct these studies, our group integrates traditional geological field techniques with state-of-the-art digital outcrop modelling, seismic interpretation, core characterisation, geophysical log interpretation, petrographical and geochemical techniques with high-end visualisation and analytical tools such as X-ray CT analysis.
Atlantic margin rift systems and continental carbonates
Unusual lacustrine carbonates in the lower Cretaceous of Angola and Brazil offer a fascinating glimpse into the interaction between tectonics, fluid flow, carbonate deposition and microbial activity during the early phases of continental rifting. Hydrocarbon discoveries in these carbonates have highlighted the oil and gas potential of the southern Atlantic. Translation of these discoveries into viable commercial success requires that the associated risks of frontier exploration and petroleum system prediction are reduced in an area with limited data.
We are currently studying the geological evolution of the South Atlantic at several scales of observation: from process-based reservoir and diagenetic studies through to the links between the petroleum systems and tectonostratigraphic evolution of the conjugate margins. Our integrated approach combines observations on structural data, stratigraphic relationships, chronostratigraphic records, facies analysis, diagenesis and petrophysics, fluid geochemistry and fluid flow.
Work has also expanded into modern analogue systems, studying interaction between microbes and minerals in saline lakes and how this drives early carbonate diagenesis and pore network evolution.
The work builds on the extensive basin analysis expertise available at Manchester, together with regional knowledge in Angola and Brazil. It integrates with the PD3 consortium and the North Africa Research Group studying the central Atlantic conjugate margins. We are working closely with researchers in Spain on outcrop and petrophysical characterization of modern continental carbonate analogs. Work in Angola is supported through data and sample donations by BP and Equinor.
Basin-scale controls on carbonate sedimentation
Carbonate sediments are principally deposited in marine basins, usually in clear, shallow water. Today, these conditions typically occur in sub-equatorial regions, such as the Caribbean, SE Asia and Eastern Australia, away from the influence of large siliciclastic delta systems and often on discrete, topographically high areas, in shallow water. Through geological time, however, the size, morphology, latitudinal extent and architecture of carbonate platforms has varied in response to changes in tectonic plate configuration, regional tectonics, evolution and extinction, global climate and seawater chemistry.
We are currently studying outcrops from a range of sedimentary basins and stratigraphic ages to understand how structural processes, such rifting, influence carbonate platform growth. By comparing platforms of different stratigraphic ages, we can also consider the influence of palaeoclimate, seawater chemistry, siliciclastic sediment flux, and changes in carbonate-secreting organisms through time.
Current and recent projects include platforms from the Precambrian, Cambrian, Devonian, Mississippian, Triassic, Jurassic and Cretaceous of UK and Europe, North and South Africa, North America and the NW Australian Shelf. Understanding this variability helps us to determine the processes that govern the occurrence, size and demise of carbonate platforms in time and space. From this we can not only optimise the natural resource potential of carbonate rocks, but also use them as proxies for palaeo-environmental change.
Processes governing carbonate petrophysical properties
Carbonate rocks are highly reactive, dissolving in a weak acid, such as when CO2 dissolves in water. This results in complex pore networks, often varying in size over several orders of magnitude within a single hand specimen. At the smallest scale, pores may be only a few nanometres in diameter, yet at their largest they comprise caverns and caves that form networks of 10s or even 100s of kilometres. Although the processes by which pore space forms in carbonate rocks is well-studied, there is still much to be learnt about the chemistry and process of dissolution in particular environments, particularly when sediments are buried beneath the influence of surface fluids. Once these pores are formed, they are critical to the storage of water, gas and petroleum, as well hosting mineralization. Therefore, being able to predict the size, distribution and connectivity of porosity is of paramount importance to the supply of water and energy, management of groundwater contamination and mining. Our projects use petrophysical and petrographical analysis, high-resolution X-ray imaging and geochemical analysis to both describe the shape, size and connectivity of porosity and determine the controls on its origin and preservation.
Processes governing dolomitization of carbonate platforms
Dolomite (Ca.Mg.CO3) is a common carbonate mineral, which can replace limestone to form dolostone, a crystalline rock with a similar chemistry to limestone (CaCO3) but very different rock physical properties. The process of dolomitization, by which limestone is replaced by dolomite, has been studied for centuries, but there is still no cohesive understanding of how this occurs. It is particularly enigmatic because modern oceans are enriched in Mg, but dolomite does not precipitate directly from seawater in the same way that calcite does. Dolomitization is known to require a supply of reactive, aqueous fluid, enriched in Mg, and it is facilitated by an increase in temperature. However, the specific processes by which dolomitization occurs – and the size, texture and rock physical properties of the resultant dolostone body – are still not well understood. Our team are currently using field-based studies of dolomitized strata in Europe, North America and North Africa, to map the shape, size and location of dolostone bodies. We then use petrographical and petrophysical techniques to describe the rock texture and properties, and geochemical proxies to unravel the fluids that dolomitized the rock. From this, we can interpret the processes that controlled dolomitization, allowing us to predict where dolomitization is likely to occur. Current projects are evaluating the relationship between dolomitization processes and rock properties in mature and emerging petroleum basins. We are also evaluating the controls on the termination of dolostone bodies, since the contact between dolostone bodies and limestone provide an archive of ancient reaction fronts.
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