Welcome to Petroleum Geology Forums

First of all, what a fantastic initiative this community is. I'm glad I found it!

Now, the reason I'm posting here is the following. Our company is currently assessing a new petroleum system. Geochem has pointed out we have encountered oils of which no source rock has been drilled. To get insight in the potential age and depositional environment of the SR we have requested a geochemical interpretation. Reports we got back indicate this is most likely an oil derived from a lacustrine source rock. Low sulfur content and a high gammacerane index seem to be the most important parameters for this conclusion. I can understand how low sulfur content indicates a lacustrine source as a hypersaline environment would most likely result in high-sulfur kerogens, because iron (and other metals) content is insufficient to bind all sulfur in these environments and most sulfur is bound to kerogen.

But, what is the gammacerane index and how does this indicate a lacustrine source? I can;t seem to be able to find good information on this.

thanks in advance!

-Mark

Mark. I would be weary of reports that link gammacerane to a lacustrine source. Here's why:

Gammacerane, a pentacyclic triterpanes, is a facies-controlled triterpane. It is most likely released from biosynthesized tetrahymanol in bacterivorous ciliates which produce more tetrahymanol when they live at aerobic-anaerobic interfaces. They are not very mobile and high gammacerane levels therefore can be considered as an indicator for water column stratification (Sinninghe Damsté et al., 1995). Water column stratification can however occur in several different settings. In lacustrine environments the absence of seasonal turnover may cause stratification, while in a marine setting hypersaline conditions may cause stratification. It is thus usually used to point towards one of these depositional environments, in conjunction with other parameters.

However, gammacerane is more resistant to biodegradation than the hopanes (between which the gammacerane index used for interpretation is defined: gammacerane/C30-αβ-hopane) and caution should be taken when using gammacerane to interpret source rock depositional environments when severe biodegradation occurs (Ollivier & Magot, 2005).

However, your low sulphur content may be backing up the gammacerane->lacustrine story as biodegradation usually enriches sulfur due to preferential removal of saturated hydrocarbons (Peters et al., 2005).

You should thus try to identify if your oil is severely biodegraded or is still pretty fresh... What's the depth and temperature it is extracted from? and it's API??