Carl Fredrik Gyllenhammar from CaMa GeoScience AS
Volume of clay (Vcl) have been calculated in detail for more than 1000 exploration wells on the Norwegian and UK shelf. The primary input logs have been the gamma ray (GR), density and neutron logs. Vcl was first calculated in the consolidated section of the well using the neutron density cross-plot. A wet 100% clay point was established using wells with Vcl calculated from the Spectrometry of induced gamma rays. Schlumberger introduced the Elemental Capture Spectroscopy (ECS) in the late 1990s. Since then, this service is offered by most vendors. According to Schlumberger the Vcl from Spectrometry of induced gamma rays compares with mineral analysis from electron microscope such as QemScan. Spectrometry of induced gamma rays will not differentiate between different clay minerals as QemScan analysis does, but the Vcl calculated is about 90% correct. Comparing that Vcl with the Vcl calculated form the neutron density cross-plot suggest that it is possible to use constant 100% clay point at 2.58 g/cc and 0.5 pu for most wells.
The GR clean and 100% clay boundaries are then set to make the GR calculated Vcl, read the same Vcl. Where there is only GR log, the clean and 100% clay cut-off boundaries are changed accordingly, but the already established cut-off boundaries can often be migrated up and help setting its limits.
It is possible to calculate the Vcl from the density – sonic cross-plot the same way as done from the neutron-density cross-plot. But the 100% clay point is varying a lot.
It this case the already calculated Vcl was used as input, and the 100% clay point on the sonic-density cross-plot was adjusted to calculate the same Vcl.
The acoustic impedance is only a function of velocity and density. We can therefore only extract a pseudo-sonic and pseudo-density curve from seismic data. To invert a seismic section to a sand-shale section, the Vcl can therefore only be calculated form sonic and density data.
We used maskin learning, trained an AI program to reproduce the sonic and density curve from the seismic reflection. At about 200 well locations with good sonic and density log coverage trained the AL program to extract a sonic and density curve from the nearest seismic trace. Then calculated a pseudo-sonic and density-curve at each shot point on a seismic merge made of all released 3D seismic surveys on the Norwegian part of the North Sea.
We calculated the Vcl using the sonic – density cross-plot. As if was ordinary well logs. The best output would have been using the moving 100% clay point at each interpreted well. To save time we established an equation moving the 100% clay point as a function of seismic travel time. The method has given very encouraging result.
This project has been supported and paid for by the Norwegian Research Council (NFR) and the oil company Pandion.
Dr. Carl Fredrik Gyllenhammar is a professional geoscientist consulting in the oil and gas industry. He started his academic (1975) career-studying math, physics and chemistry at university of Grenoble in France. He has an MSc in Applied Geophysics, with the title; Seismic stratigraphy of the Essaouira basin, offshore Morocco. After 13 years in Conoco he joined a joint industry research project GeoPoP, at University in Durham. The focus was the compaction of clay and the development of pressure in the clay. He completed his PhD; “A critical review of currently available pore pressure methods and their input parameters. – Glaciations and compaction of the North Sea sediments.” In 2001 and joined BP research department in Sunbury and worked with compaction and pore pressure developments in claystone. His PhD is being used as a textbook and guide by several Pore Pressure engineers worldwide. Including Schlumberger’s office in Houston. The last 10 years he has worked with prospect evaluation, interpreting electrical well logs and seismic data. Exploring for oil and gas in the North Sea (UK, Norway and Denmark), Morocco, Tunisia, Azerbaijan and Kazakhstan.