A Comparison between different Core Clay Volume Methods. Which is Right?

The scope of this publication, quite contrary to what the title establishes, is not to elucidate which method is better than another in all circumstances. Rather, the aim is to promote a conciliation between different methods to achieve best results, understanding that each one has its strengths and weaknesses through a pleasant and simple reading. Moreover, the use of spectroscopy logs was found to be essential to proper calibration of the log data to core.

First let's see a table that will expose some characteristics of each method

This kind of tables have induced petrophysicists to establish that XRD always provides the best quantitative results and that is not necessarily true. In this example, we wanted to perform a clay volume core-log calibration. Instead of performing the common practice of using the XRD data alone, The clay volumes obtained from XRD Versus results obtained from thin sections were plotted first. A wide difference was found in 4 samples in terms of clay volume (up to 20% difference). Samples 5 and 6 show coherent results, so any calibration/interpretation problem was discarded. Quartz plot also shows these differences, so this discrepancy needed to be reconciled.

One method says that the rock is clean and the other method shows that the clay content is high. The variation in effective porosity, permeability, irreducible and free water saturation will be substantially high depending on the chosen method.

By visualizing thin sections, the high volume of clay present is clearly visible. SEM images also show high volume of detrital clay, without a defined crystalline structure. The above makes it difficult to correctly quantify the volumes by XRD. However, a reasonable doubt exists that results obtained by thin section and SEM only represent an very small part of the rock and XRD results could be more representative at log scale. Likewise it is necessary to validate if the numerical value established by thin section is logical.

At this point it was decided to incorporate data from spectroscopy log taken in this well to solve these questions. A deterministic approach based on the equations proposed by Herron (SPE 77631) was used to calculate clay volume. Clay volume was also determined from Gamma Ray log using Larinov 2.

According to the results obtained in this approach, clay volumes from thin sections are closer than those reflected by the XRD. To complement these results a multimineral evaluation was performed, using as input logs: Gr, RHOB, NPHI, U, Resistivity, Si, Al, Fe and Ca.

These results also reflect clay-volume-XRD-derived underestimate clay volume in this core.

Best practice of using an integrated approach allowed us to save time and resources by identifying the high clay volume present in the formation that XRD method could not identify in this case. Incorporating data from nuclear spectroscopy log using a deterministic approach and subsequently a probabilistic one allowed concluding that the clay volume determined by thin sections was logical.

This publication is based on collaborative work with Hugo Govea

Please consult following references if you want further information:

• IPTC 12849
• SPE 23828
• SPE 144248
• Tavakoli . V. Geological Core Analysis: Application to Reservoir Characterization. Springer Editorial
• Clays and Clay Minerals, Vol. 49, No. 6, 514-528, 2001.