Michael A. Abrams¹
(¹Department of Earth Science and Engineering, Imperial College London. London UK)
Abstract: Near surface sediment gases are currently used in petroleum exploration to better understand the presence a working petroleum system, petroleum phase (gas versus oil), assist in risking non-hydrocarbon reservoir gases, evaluate basin migration pathways, and regional seal assessments. This information is not only critical for petroleum exploration but important in carbon sequestration and regional baseline environmental studies. There are several methods currently available to evaluate near-surface marine sediment gases. Multiple laboratory and field studies have demonstrated that different sediment collection and analysis procedures will provide different results. Sediment gases reside in interstitial spaces, bound to mineral or organic surfaces, and/or entrapped in carbonate inclusions. The sediment gas extraction methods can be sub-divided based on gas phase (vapor, dissolved or bound) and extraction mechanics (non-mechanical, mechanical or chemical enhancement). Understanding the impact of sample collection (sample location, depth below water sediment interface, and field sample preservation) as well as laboratory procedures will be extremely important to determine the sediment gas origin and geological implications.
A series of laboratory and field studies undertaken as a calibration study provides insight into the best practices for sediment gas sample collection, analysis, and interpretation. Targeted (site specific), collection below Zone of Maximum Disturbance, and interstitial gas analysis will provide the best results to evaluate hydrocarbon and non-hydrocarbon (carbon dioxide, hydrogen, helium, etc.) sediment gases. The headspace interstitial gas extraction method utilizes a high speed shaking process to release the vapor phase interstitial sediment gases into the container headspace for compositional analysis. The traditional bound sediment gas extraction method requires coarse-grained fraction removal by wet sieving the bulk sediment sample followed by heating in phosphoric acid and partial vacuum to remove the bound gas. Laboratory and field studies indicate the bound gases do not provide an accurate sediment gas measurement due to the preparation and extraction protocols.
Sediment gases are not the same as production gases hence the ratios and charts designed for reservoir gas interpretation will work differently with extracted sediment gases hence to be used with great caution. Interpretation of sediment gases require a different set of evaluation tools.
Keywords: Surface geochemistry, sediment gas sampling and extraction.
Author Profile (first or corresponding author):
Dr. Michael A. Abrams is currently a visiting professor at Imperial College London undertaking industry research projects and teaching. Before joining Imperial College, Professor Abrams was the Manager of Petroleum Systems for Apache’s Exploration and Production Technology team, research professor for Energy and Geoscience Institute at the University of Utah, and senior research geochemist at Exxon Production Research Company (now ExxonMobil Upstream Research). Michael’s research interests include near-surface expression of hydrocarbon migration, petroleum systems analysis, unconventional resource play evaluation, and regional petroleum charge analysis. Michael has been the co-convener/chair for 3 AAPG Hedberg research conferences (1994, 2001, and 2019), AAPG GTW workshop (2014), co-convener, Geological Society of London - Energy Group conference (2021) as well as session co-chair for multiple international conferences. Professor Abrams was an Associate Editor special SEG Interpretation volume, Section Editor (Organic Geochemistry) Marine and Petroleum Geology, co-editor Marine and Petroleum Geology thematic volume, and co-editor AAPG Memoir 66, as well as author of many international peer reviewed publications. Michael received his Ph.D. in Petroleum Geochemistry from the Imperial College London, M.S. in Geology from the University of Southern California, and B.S. in Geology from George Washington University.
