Monday 15 April 2019

Applying petrophysics to geoscience challenges


Before we get into this topic, it is important to note how I am defining petrophysics. For a full explanation see my previous blog, but in a nutshell I use petrophysics as “a term to express and explain the physical responses of particular rocks and sediment types”. 

Straight off the bat it should be obvious that petrophysics and the associated insights it brings are applicable to a wide variety of geoscience challenges, particularly in the understanding of the subsurface. I would like to offer a few examples below but I should also mention that this article is not exhaustive. These are just a few example applications of data, primarily from the International Ocean Discovery Program (IODP) and its precursor programs, where downhole (well) log data is routinely collected and has been used to answer the scientific challenges of its science plan. 

Evidence for the wide applicability of petrophysical data is its role in the IODP science plan 2013-2023. The science plan is intended to guide multidisciplinary international collaboration by outlining the breadth of questions that IODP aim to tackle. Petrophysical analysis plays a part in answering all these questions covering the fields of: 
  1. Global climate past and present
  2. Deep life and the biosphere
  3. Planetary dynamics and tectonics
  4. Geohazards
Many of the strengths and applications of petrophysical data derive from the nature of its acquisition. Well log data provide an ‘investigation area’ that is relatively unique in the fields of geoscience investigation techniques. Sediment, rock and core samples and their analysis are commonplace, and investigate the nano-to-centimetre scale. Whilst seismic profiling provides basin-scale architecture, at its very best the data have a resolution of 10 metres. Log data covers the 0.1-100 metre scale area of investigation and is one of the few technologies specifically designed to do so. Added to the fact that logs capture continuous data and the in situ properties and, for these reasons, I think logging data should be more commonly utilised and considered when generating, and ground-truthing, geological models. 


Geoscientists can benefit greatly from integrating petrophysical data with their other data generated from direct sampling or observation of sediments or rocks. Petrophysics is geared towards answering why certain rock types exhibit the physical responses that they do, and in doing so producing quantifiable information on chemistry, mineralogy and fluid content (where possible). Invaluable information for sedimentology and petrology. 

The continuous downhole (or more accurately, up-hole) recording of data also makes it ideal for capturing and analysing stratigraphy, cyclicity and other trends. This again can be valuable information for all geoscience disciplines but really add value to geoscientists researching palaeoclimates, paleoenvironments and paleoceanography, including those interested in sediment source. 

Logging tools have different purposes with some aiming to characterise different aspects of the formation than others. Many tools such as electrical resistivity have deep areas of investigation (typically 1.5-2 metres). These tools can provide information on formation structure and fluid content rather than mineralogy and chemistry. Other tools can have extremely shallow depths of investigation such as the various imaging tools (millimetres). Borehole images can be used to analyse millimetre-scale textures and sedimentary structures within rocks and sediments as well as to examine fault orientation, dip and dip direction. 

Imaging tools are deployed as standard during IODP expeditions and are becoming increasingly common elsewhere. The examples of borehole images below cover most of the spectrum. The left image was recorded during IODP Expedition 364 using a slimline acoustic imager [1]. The image has clearly recorded the coarse grained nature of the granite, but has also captured two generations of intrusion and measurable fractures comparable with the corresponding core from this depth. In the right image, wireline electrical resistivity images and Logging While Drilling (LWD) images have captured cross bedding in deltaic sandstones 1600 feet (~490 metres) below the surface in an oil well in Oklahoma [2]. 


The discipline of petrophysics is closely related to geophysics and integration of petrophysical measurements with other types of geophysics data can be of great benefit. It could be argued that the most useful data are those generated by the sonic tool. The sonic log is a continuous, usually high-resolution record of compressional velocity along the well path [5]. These data can ground-truth seismic surveys in the area by establishing the time-depth relationship and, importantly, linking well log to seismic profile and ultimately core to seismic. Morgan et al. [3] demonstrate this in their scientific expedition drilling in the Chixculub impact crater peak-ring. Here the seismic P-wave velocity (km/s) obtained from sonic wireline logging data confirmed that the predominantly coarse-grained, granitic rocks of the peak ring were indeed characterised by the low densities and low seismic velocities suggested by geophysical models based on seismic refraction data. 

When the sonic log is combined with a density log it also becomes possible to calculate acoustic impedance (a property of rock layers and their boundaries that govern acoustic reflection coefficients). Combining these petrophysical log data allows for creation of a synthetic seismogram. Further insights can be made into seismic profiles if shear slowness logs are generated as these can advise on formation fluids. 

Access to fresh water is already one of society’s greatest challenges and will be an increasing concern in to our future. Lofi et al. [4] used a range of data from IODP Expedition 313 including lithology, 2-D seismic profiles, pore-water salinity measurements, porosity measurements, density measured from core, thorium content (from downhole spectral gamma-ray logs) and sonic velocities from downhole logs to determine the geological heterogeneities affecting groundwater exchanges on the New Jersey shelf. Their work revealed evidence for a multi-layered reservoir/aquifer where waters with very low salinities (<3 g/L) were encountered at depths below sea floor exceeding 400 m and fresh and/or brackish-water intervals alternate vertically with salty water intervals on this passive margin.


It is also worth mentioning borehole gravity surveys, and here I must admit that I am a bit out of my area of expertise. Suffice to say though they are to gravity surveys what sonic logs are to seismic surveys [5]. For more information on borehole gravity surveys and their relationship to surface gravity surveys see Martin Kennedy’s book Practical Petrophysics – Chapter 14: Geophysical Applications.

Further applications:


Downhole tools are becoming increasingly versatile with tools for magnetic susceptibility, fluid sampling, magnetisation and borehole imaging. Core-based petrophysics is also a rapidly expanding field with increasing commonality of chemical analysis such as XRF, hyperspectral imaging and near-infrared spectroscopy. Understanding of the data produced by these new tools is of increasing importance to academia and industry. 

Petrophysics and its techniques can also aid in the fields of: 
  1. Contamination
  2. Remote sensing
  3. Soil and sediment science
  4. Geochemistry
  5. Hydrology and hydrogeology
  6. Geotechnical measurements
And finally, what of geological models? Martin Kennedy makes a great point about this in his book Practical Petrophysics. I’ll let his words speak for themselves: 
“The increasing use of software to build detailed 3D geological models [of reservoirs] has meant that petrophysics has to be properly integrated with the other sub-surface disciplines. The model builder needs to know what assumptions have gone into the creation of the petrophysical property curves and the petrophysicist needs to know that their results are being used appropriately. Consequently a working knowledge of practical petrophysics is no longer just a ‘nice to have’.”
For ‘of reservoirs’ read any sedimentary basin, aquifer, impact crater, passive margin, mid-ocean ridge, obducted ophiolite, subduction zone, slow-slip zone – the opportunities are limitless. So if this has given you pause for thought and you are interested in knowing more, why not have a look into how petrophysics can benefit your science?



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