Lithostratigraphy determination and characterisation from Logging While Drilling Data: NanTroSEIZE IODP Expedition 314

Completed

Not Energy Related
Fossil Fuels: Oil Gas and Coal(Oil and Gas, Enhanced oil and gas production)

Basic and strategic applied research

ENVIRONMENTAL SCIENCES (Earth Systems and Environmental Sciences)

Not Cross-cutting

Prof M Lovell
Geology
University of Leicester

R&D

NERC

01 December 2007

28 February 2008

3 months

£18,185

East Midlands

Integrated Ocean Drilling Programme

Prof M Lovell, Geology, University of Leicester

Dr S Davies, Geology, University of Leicester

Major aim: This project will create a lithostratigraphy and petrophysical facies scheme using LWD data and borehole images. In addition, to identifying structural and fluid flow features we propose to develop a sedimentological facies scheme that could be tested in subsequent IODP NanTroSEIZE expeditions. Hypothesis: LWD log responses, together with borehole image logs, can identify structural features and fluid flow zones and provide a baseline lithostratigraphy, with defined associated uncertainties. These interpretations can used to develop a regional static geological model. Key Objectives: 1) To obtain information on lithology through analysis of LWD log responses combined with LWD images and create a vertical column representing the lithostratigraphic framework. We will use standa rd techniques to define bed boundaries (e.g. image logs, gamma-ray) and identify major lithological changes related to variations in grainsize, porosity and chemistry/mineralogy in the undisturbed succession seaward of the subduction fault system. Changes in grainsize will be reflected in electrical and possibly acoustic properties while the nuclear properties (neutron/density) will respond toch emistry and mineralogy. Using multidimensional cross-plots it is possible to separate out the effects of porosity and grainsize in addition to the chemistry/mineralogy. The neutron tool is the most susceptible to variations in mineralogy, particularly the clay minerals, but normally the data processing and correction factors used to determine porosity removes this susceptibility. Where possibl e w e will use the raw data from the neutron tools as an indicator of clays and mineralogy and develop this innovative approach further. The basic framework will then provide an initial starting point for applying inversion techniques (using Mineral Solver); this approach enables solid and fluid components to be defined, where the solids can be specific minerals or user-defined proportions, e.g. 'sha le' or mud components. Other Expedition 314 drill-sites will sample more deformed sediments. 2) To use the textural information from high-resolution borehole images, combined with the lithological framework, and a detailed analysisof log response patterns to develop a petrophysical facies scheme. Probabilistic methods will be used to define the petrophysical facies by using a statistical ana lysis for the multi-well quantification of lithology. All the log responses at one depth will be combined into a single, multi-dimensional set, and then subjected to multivariate analysis. The sets can be grouped into populations of numbers which show some internal statistical similarity and can be statistically differentiated from other populations. This innovative approach will supplement the d etail ed analysis of changes in mineralogy (as in 1) using property specific cross-plots 3) To use the petrophysical facies scheme to highlight variability which may be related to sedimentological facies, structural features and potential zones of fluid flow. Information from the lithostratigraphy (1), including interpretations of lithology, mineralogy, bounding surfaces and structures from boreh ole im ages, will be integrated with the statistically defined populations from (2). We will establish a consistent approach to identifying significant structural features and zones of fluid flow and sedimentological facies.

The Integrated Ocean Drilling Programme (IODP) Expedition 314 is part of the Nankai Trough seismogenic zone experiment (NanTroSEIZE) and will use state-of-the-art measurements while drilling to produce continuous data from the strata beneath the seafloor. We propose to maximise these data to interpret the rock properties of strata where no physical sampling will take place. The Nankai Trough is located along the SW margin of Japan, and is partof a subduction zone in the western Pacific where large magnitude earthquakes (>8) have occurred repeatedly. These earthquakes pose a significant threat to people but also allow scientists to study the science of plate tectonics. IODP expeditions (314-316) will drill and sample this active plate boundary fault to (1) observe the fluids and sedimentswithin a fault system associated with the subduction and (2) create an instrumented network to monitor earthquake activity. Sediment is deposited across the Nankai Trough area. In areas seaward of the subduction fault system, mainly undisturbed sediments, fluids and crustal rocks, will be sampled to characterise materials that will eventually be transported into the subduction zone. This sedimentary succession will be drilled at two of six sites, however, it is difficult to collect physical samples and return them to the ship for furtherstudy because many of these sediments are unstable. The continuous measurements from the strata beneath the seafloor are a 'representative' or a 'proxy' for the rock properties. We will use standard interpretation techniques, and also develop new interpretation methodologies, to use these data (electrical, nuclear, images of the rocks along the borehole sides) to deduce rock properties, e.g. rock type, presence and type of fluids and characterof strata. We will be able to identify where the rock types change even where actual samples cannot be retrieved. One project deliverable is to determine how these data can be used to indicate mineralogy. Sediment mineralogy is important for determining when and where the earth's crust will fracture and generate earthquakes. Changes in the mineral content of the sediment deposited will indicate either a change in the source of sediment or in the processes which deliver sediment to a basin. Another objective is to produce a vertical column, based on these proxy data, that represents the succession ofdifferent sediments present at the drill site. This is key to understanding what control the original sediment and fluid exerts on deformation as the sediments are subducted. The project will also interpret other characteristics, such as structures produced by sediment transport or post-deposition disturbance. Even away from the subduction zone, the accumulating sediment may be disturbed by faults, which move during earthquakes, and the sediments can become unstable and 'fail' resulting in transport by slumps and slides. These transport processes will produce different structures in the sediment which will be visible on the borehole image logs. Other planned drill-sites through the fault system will measure more disturbed (deformed) sedimentary strata. We willcompare results from theundisturbed sediments with data retrieved from the deformed sedimentary successions to define lithologies, mineralogies and structures at these sites also. Combining all our 'proxy' data, we will propose an overarching scheme that will group the successions into units with similar data characteristics, identify structural and fluid flow features and enable us to suggest sediment characteristics.These results will inform the planning of subsequent IODP expeditions in late 2007 and early 2008. This academic project is at the interface with applied research: the type of data collected during drilling on Expedition 314 is used by industry in exploration, and its interpretation will become increasingly important in the search for smaller oil and gas fields.

05/09/08