GEO ExPro

A Long-Overdue Book!

Understanding Oil and Gas Shows and Seals in the Search for Hydrocarbons
This article appeared in Vol. 14, No. 3 - 2017

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Let me start with my conclusion: Mr. Dolson’s book is a long-overdue work, filling a critical knowledge gap that has spanned decades of exploration and has resulted in billions of dollars spent on dry holes. It is not that the information contained in Mr. Dolson’s book was not or is not available, but his approach to pulling it together and explaining the subject in a refreshingly straightforward manner is easy to digest and grasp. His years of experience with major oil companies, as well as teaching and consulting, provide an exceptional perspective on this critically important subject. 

Why So Many Dry Holes? 

Understanding Oil and Gas Shows and Seals in the Search for Hydrocarbons, John Dolson. Springer, 2016 Whatever source you reference the message is consistent: most dry holes are a result of seal failure and/or breached traps. I’ve read that the staggering figure of up to 70% of dry holes industry-wide result from a failure to understand and properly risk seal capacity and trap integrity. I recall my early, and not so early, days at a major oil and gas company where the in-house training emphasis was on reservoir, sequence stratigraphy and structure, including glorious field trips to see reservoir outcrops and magnificent structures in the Rocky Mountains. Other than my own graduate work on a Cretaceous organic shale, which included a read of Schowalter’s 1979 seminal paper on secondary migration, I found that traps and, in particular, seals were only given a skin-deep mention; almost as a side thought. 

In the 1980s and early 1990s the emergence of a probabilistic risk approach to exploration, coupled with 3D seismic becoming more commonly available, led to the expectation that successful exploration would be commonplace and dry holes reduced to a minimum. This was a step change in the way geoscientists and engineers approached risk. But once again, the pitfall was the focus on reservoir, facies and structure risk with an attempt to model source and migration, but only basic seal analysis considered. Seal capacity was largely glossed over - in part, or perhaps mostly, because of the inherent complication of identifying and quantifying shale, or other cap rock, sealing capacity. Team meetings and peer risk review groups spent countless hours debating a <2% difference in the probability of reservoir presence while blissfully assigning a blanket 95 or 100% chance of seal capacity and integrity success with little or no discussion. 

Residual and marginal shows from mud logs and core were seldom given serious analysis or significance especially if they conflicted with electric logs, and although production geologists and engineers were certainly keen to know what they meant, our tools to document remigration were limited. 

A ‘Must-Read’ 

Mr. Dolson analyses the risk relationships of seal, trap and hydrodynamics within the context of a basin-wide perspective while addressing the detailed operational aspects to determining subtle mud log shows and approximating capillary pressures and column height. 

Successful companies today employ a rigorous process that includes a comprehensive approach to basin modelling and incorporates an understanding of hydrodynamics and petroleum fluid flow. Mr. Dolson provides examples and discussion of migration modelling at all levels by using 3D modelling programs that are capable of the interactive modelling of migration with open and closed faults, timing variations and fluid or gas type. Nothing is risk-free but a clear-eyed analysis of all available data in this context will most certainly reduce risk. As Mr. Dolson quotes, all subsurface maps are wrong, it’s just understanding how wrong. 

So, this book is an absolute ‘must-read and keep’ for explorationists at all levels as well as managers and engineers. No more hardcopy textbooks are allowed in my house so I have the e-version on my iPad, making it easy to access and refer to. It serves as a refresher for those more experienced geoscientists and engineers and importantly provides a clear process-oriented approach for entry-level professionals and emerging leaders in the petroleum industry. His early chapters are pretty basic oil and gas stuff but hidden gems and personal stories make it an engaging read and they should not be skipped. 

In short, this book is highly recommended.

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