Interview with Gil Machado on Salt Biostratigraphy: Science and Technology Showcase

Published
American Association of Petroleum Geologists (AAPG)

Fossils and microfossils can be used to determine information about evaporites as well as carbonates and clastics. In fact it is an area that is growing in importance. Welcome to an interview with Gil Machado, who discusses salt biostratigraphy and its new usefulness.

Gil Machado
Gil Machado
What is your name and your background?

Gil Machado. I'm Portuguese and after several international academic and professional periods abroad, I'm back to Portugal heading a geological consulting company. I took my Geology degree at the University of Lisbon and spent a year as a visiting student at Trinity College Dublin. The PhD was done at Aveiro University (Portugal), with supervisors from the University of Lisbon and the Academy of Sciences of the Czech Republic - what is called an European Doctorate.

How did you get interested in geology?

I was lucky to grow up in Lisbon near one of the last remaining outcrops in the city. The shallow marine Miocene strata had shells, shark teeth and other fossils which got me hooked from early age! A few years later Jurassic Park came along and made my "dinosaur phase", which included hunting for them in Jurassic coastal outcrops north of Lisbon. After entering University the interest shifted from Paleontology (s.s.) to the stratigraphic and paleoenvironmental significance of fossils. I think I'm still in that phase!

How is biostratigraphy used in today's exploration and development?
Salt Biostratigraphy
An Interview with Gil Machado, University of Lisbon

Biostratigraphy, as any other Petroleum Geology discipline, felt the ups and downs of the industry. Many companies had their own Stratigraphy team in house until the 80s. They did most of the microscope and interpretation work and only a few consulting companies existed. During the 80's the expertise was transferred to newly formed consulting companies. The prime use of Biostratigraphy was, and still is, sedimentation age determination. The application of this goes from basin-scale studies to real-time well site biostratigraphy (including biosteering). No other discipline or technique allows to have this information in the cost and time-effective way Biostratigraphy does. Other uses include paleoenvironmental interpretation, which in some cases can be used to refine taxonomy-based Biostratigraphy - such as the uses of eco-biostratigraphy. Palynology in particular, because it studies organic matter contained in rocks can also be used to assess source rock potential and thermal maturation. I think there is still a lot more information we can extract from the biological content of rocks, which hasn't been explored. I'm particularly interested in statistical approaches to biostratigraphic data and I'm now planning to start working on micro-nano porosity of organic particles, an area of great relevance for unconventional resources.

Gathering Mine Samples
Gathering Mine Samples
What is salt biostratigraphy?

Salt biostratigraphy is a methodological approach to evaporites and associated sediments. It's not rocket science. Simply a way to extract microfossils from rocks which were not regarded as sources of data. There are only a few published papers on this subject and although the results were quite relevant to the knowledge of the studied regions, it never picked up. It requires a quite different method, when compared to standard palynology, as it does not use acids to remove the non-organic component of the rocks. What we have done is to refine the method to be applicable to dirty halite, gypsum, salty shales, primary anhydrite and other lithologies. If palynomorphs are present (a pre-condition needed in any kind of rock) they can be observed as any other sample. In most cases phytoclasts (=vitrinite) is also observable, so quantitative thermal maturation assessment is also possible. We will soon start testing for nannofossils as well.

What are some examples of successful application of salt biostratigraphy?
Gathering Outcrop Samples
Gathering Outcrop Samples

To validate the method we tested 5 different locations, each with varied lithologies, from Permian to Miocene in age. These are outcrop, mine and well samples. We were able to extract organic particles the vast majority of them. In some cases the organic particles did not provide biostratigraphically relevant information due to oxidation either during sedimentation or diagenesis. This is common to all types of sedimentary rocks. That is one of the main conclusions - oxidation, more than crystal size, deformation or other aspect is the most relevant variable. Having said that, redish and pinkish halites are also productive. These are the colours one usually avoids as they are indications of oxidative conditions in non-evaporitic sedimentary rocks. Overall we achieved a success rate close to standard samples.

How does salt biostratigraphy demonstrate that it is cost effective?

Salt biostratigraphy can actually be cheaper than normal biostratigraphy. It requires selective sampling and adapted laboratory procedures, but because it does not use acids, it can be run in simpler labs and turn-around time can be reduced. We are already running a first commercial project.

Please recommend a few books

Applied Micropalaeontology edited by J.M. Jenkins is a good way to enter the world of commercial biostratigraphy with many examples of its applications. Microfossils, by H. A. Armstrong and M.D. Brasier is also a good way to explore the several fossil groups used in the industry and its uses, beyond sedimentation age determination. On the non-professional side, the books by Jorge Amado (a Brazilian author) were some of the ones that impacted me the most.

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