Why Use Hydrocarbons? Home

Over the years I have had the opportunity to use many geochemical tools and observe the various conditions in which they are used. Most of the tools are termed "indirect" meaning that they cause an alteration anomaly but are not related to the seeping hydrocarbons themselves. The "direct" methods are termed so because they are a direct measurement of the hydrocarbons seeping from the reservoir.

The survey I ran on geotech.org title "What methods do you use?" indicated that most explorationists prefer the "direct" methods. My question is why. From what I have observed over the twenty plus years I have interpreted geochemical anomalies, the primary anomalies I was looking for were related to geochemical alteration. There is little proof that the "direct" measurements are actually measuring "seepage" related hydrocarbons and alot more evidence that these direct methods are simply measuring altered organic material within the near surface environment.

Looking at the vast number of complex organic materials present in the near surface, what are the chances that the hydrocarbons being detected are actually the hydrocarbons from the reservoir. I am not saying the hydrocarbons don't seep but rather how do we know that the numbers we report are those from the reservoir and not from near surface byproducts of seepage alteration, and that these byproducts are not present in soil gas or sample headspace.

For instance, benzene is sometimes used as a seepage indicator and is said to be only present where petroleum reservoirs are located. However, when reported there is still a statistical background and a statistical anomaly indicating that benzene is ubiquitous. Benzene, as well as other ring structures, is present in the near surface environment and can be artificially concentrated through thermal alteration of surface organics. If this can happen to a complex structure than it certainly will happen to the simpler alkanes. Based on mass spec analyses of surface hydrocarbon samples the number of hydrocarbon species present is overwhelming.

Therefore, I ask why are we so reliant on the direct methods? I feel its based on the comfort level. If it smells like a ham sandwich if must be a ham sandwich. But why pay so much more for a hydrocarbon analysis when a simpler one will do. If indeed all we are looking for is alteration related to hydrocarbon seepage (which may or may not be directly detectable) than why not use a more accurate and reproducible lower cost method.