School of Natural and Environmental Sciences

From sediment to rock

From sediment to rock - the role of microbes in early lithification of sabkha sediments

The Holocene to recent evaporite and carbonate sediments along the Abu Dhabi coastline are unique modern day geologic analogues of ancient carbonate deposits and exhibit features observed in core and outcrops deposited many millions of years ago.

Project Leader: Dr Cees Van der Land
Sponsors: SAgE
Partners:
Durham University (Prof Andrew Aplin), The Petroleum Institute UAE (Prof Stephen Lokier)
Start/end dates: 2014 - 2018

Project Details

Such recent sediments thus greatly enhance our ability to interpret the properties of ancient sediments. The Abu Dhabi coastal sediments consist of a combination of microbial carbonate mats, carbonate mud, anhydrite and gypsum.

We aim to use them as a recent analogue to inform our understanding of ancient Jurassic reservoir rocks and possibly other carbonate-evaporite subsurface sequences.

Carbonate rocks form significant hydrocarbon reservoirs and aquifers in the subsurface, but they can exhibit highly varying properties (e.g., porosity, permeability, flow mechanisms) within small sections of the reservoir, making them difficult to characterize.

This has led to the resurgence in the appreciation, use and need for outcrop based studies as analogues and benchmarks for the subsurface. Due to their highly reactive nature, carbonate rocks often display a strong post-depositional overprint.

Diagenetic processes such as cementation, dissolution, compaction and dolomitization begin at the sea floor, continue through deep burial, and extend to subsequent uplift.

The spatial and temporal distribution of carbonate mineralogies, characterized by their different diagenetic reactivity at the time of deposition, is a fundamental factor controlling the subsequent evolution of carbonate reservoir properties.

In microbial carbonates, different mineral precipitation processes (abiotic, biotically induced or controlled) produce a range of textures and pore geometry development styles. Key components of the biologically-influenced mineralisation (organomineralisation) are the microbial metabolism and environmental conditions impacting the calcium carbonate saturation index and an organic matrix comprised of extracellular polymeric substances (EPS), which may provide a template for carbonate mineral nucleation.

Lithification of the microbe-constructed structures is essential for preservation in the sedimentary record and key to the retention of primary porosities within carbonate reservoirs.

The link between the microbial community responsible for initial lithification, the micro-environment in which they live and the volume and types of cements produced and the subsequent reduction in porosity is still not well understood.

We will test the hypothesis that the spatial and temporal distribution of syndepositional mineralisation, associated with microbial communities, is a fundamental factor controlling the subsequent evolution of carbonate reservoir properties.

Research questions:

1) Which depositional rock types are present in the Holocene Abu Dhabi microbial mats and associated sediments and what are their petrophysical properties?
2) What are the microbial communities present and what volume and type of carbonate minerals do they produce?
3) What is the relationship between microbial communities and syndepositional mineralisation?
4) What are the primary depositional features and early diagenetic features of these Holocene deposits?

Publications

  • Dupraz, C., Reid, R.P., Braissant, O., Decho, A.W., Norman, R.S., Visscher, P.T., 2009. Processes of carbonate precipitation in modern microbial mats. Earth-Science Reviews 96, 141-162.
  • Lokier, S.W., Knaf, A., Kimiagar, S., 2013. A quantitative analysis of Recent arid coastal sedimentary facies from the Arabian Gulf Coastline of Abu Dhabi, United Arab Emirates. Marine Geology 346, 141-152.
  • Scherf, A.-K., Rullkötter, J., 2009. Biogeochemistry of high salinity microbial mats – Part 1: Lipid composition of microbial mats across intertidal flats of Abu Dhabi, United Arab Emirates. Organic Geochemistry 40, 1018-1028.

Academic Staff

  • Dr Cees Van der Land
  • Professor Ian Head