Modelling the Deep Biosphere over Deep Geological time at the Nankai Trough, Japan

Recent work has demonstrated that microbes living within deep ocean sedimentary rocks can be descended from the organisms present within the rock's depositional environment. This implies that the microbes experienced a long geological history and in many cases the microbes experienced significant changes in their environment as the sediment was buried and converted to rock.
Computer models of the conversion of sediments into sedimentary rock have been adapted to model the life and death of individual biomes within their host geological formations. However, to date, this has only been done for examples of sub-seafloor cases where there is little movement of groundwater or other fluids between geological formations. However, there are many instances where the transfer of fluids between formations occurs. Examples include oil and gas migration, as well as the catastrophic movement of fluids associated with earthquakes and tectonic activity. The Nankai trough is one such place where both process may occur and subsurface microbes have been detected at depth. At the moment current models of microbes living within sediments can not accurately model the transfer of microbes between geological horizons, and this would prohibit the modelling deep subsurface biomes in these important environments.

An IODP expedition to the Nankai trough will explore the subsurface to investigate the temperature limits of life. This expedition will provide an opportunity to acquire data that will be used to develop better models of the deep biosphere that incorporate the subsurface movement of cells. To achieve this it is necessary to have access to important data characterizing the microbial populations found in the Nankai trough. It is also essential that the boundaries between geological formations, and migration pathways for microbes within the subsurface are recorded and described.

To accurately model past temperatures experienced by microbes as their host sediment is converted into rock, it is necessary to make geochemical measurements that record past episodes of thermal alteration. The type of measurement needed depends on the temperatures concerned, and in this case measurements must span a considerable temperature range including mild temperatures experienced in the near surface, as well as the higher temperatures that generate oil and gas. This later group is common practice for scientists the former is not.

Observations of the boundaries between geological formations must be parameterised; e.g. converted into numerical inputs that can be put into a model. Two things need to be parameterised: 1) Basic information describing how the boundaries permit the passage of fluids. These are routine measurements that are easily made. 2) How different rock textures interact with cells as they move through rocks. This is not common information and will be determined by experiments that take account of specific circumstances appropriate to the Nankai trough sediments; e.g. the presence of gases and fluids, and specific rock textures.

Models built using this data can then be used for addressing fundamental science questions concerning life in the deep subsurface, life on other planetary bodies and the evolution of life over geological time. They will also have practical applications for predicting petroleum occurrences and forward modelling microbial enhanced oil recovery and contaminated ground remediation.

The international Scientific Drilling Community and resource scientists working in subsurface environments will benefit from improved capability to model subsurface biomes. Both sectors face the same problem; that prior to drilling, most drilling targets have a number of unknown parameters, that it would be beneficial to know, but can't be found without drilling. To overcome this site characterizations are performed primarily using geophysical methods, and then modelling to predict such things as formation pressure etc.
Currently, subsurface microbial communities can only be predicted by relatively crude methods - e.g. extrapolations based on temperature, cell concentrations at shallower depths etc. The modelling approach developed by this research would permit subsurface cell counts and biomes to be predicted in a similar way to formation pressure, allowing drill holes and drilling operations to be better planned.

Within the scientific drilling community specific benefits will result from a better understanding of how microbial contamination of formations and geological samples can occur at the pore scale, and the capability to predict if a given formation is likely to host an abundant or weak microbial community, and where extra care should be taken (e.g. in the design of mud weights or if setting casing).

The petroleum sector may benefit in two additional contexts: 1) improved capability to predict biodegraded petroleum occurrences and 2) improved understanding of how microbes move through geological formations. Ahead of drilling, the likelihood of oil having been biodegraded and hence heavier and less valuable is predicted as a risk, primarily based on temperature. The modelling approach developed in this research would permit the processes involved to better modelled as a function of geological history and environmental factors. Using this approach exploration targets that previously appeared risky may no longer appear so. This is particularly important for a mature petroleum province such as the UK, where a number of such undrilled targets may exist. Providing a means to better asses them ahead of drilling would help the potential of these smaller assets to be realized and potential prolong jobs dependent on North Sea Petroleum Exploration and Production .

The data generated within the proposal concerning the transmissibility of microbial cells within different petrographic textures, and the experimental capabilities developed for measuring the transmission of cells through cores could be used for planning microbial enhanced oil recovery. In a mature petroleum provenance such as the UK, methods of enhanced oil recovery are particularly important for prolonging the life of assets and optimizing the use of a nation's non-renewable natural resources.