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Diamondoid Concentrations in Onshore Surface Sediments as Evidence of Petroleum Seeps and Microseeps


In the early days of the oil industry, oil bubbling from the ground (as seen in the Beverly Hillbillies), asphalts, mud volcanoes, bubbling aquifers, lakes and streams all pointed to the presence of an active petroleum system and possible nearby accumulations. Famous early discoveries including Titusville, Pennsylvania, “Birthplace of the Oil Industry”, and Spindletop, were found based on the presence of nearby surface seeps. Successful location of seepages offshore proved to be equally as valuable. With time, new surface seeps became increasingly difficult to find and explorationists turned to geochemical methods to find microseeps. These methods included extraction and analysis of soil and sediment samples, fluorescence and gas detection. All of these techniques have limitations. Modern organic matter interferes with detection of petroleum in extracts, small molecules do not fluoresce, gas can escape and compounds used to detect petroleum may be absent due to biodegradation. To solve many of these problems, we developed a method for finding seeps and microseeps by determining and mapping diamondoid concentrations in sediments. Diamondoids occur in all oils and condensates, they are relatively easy to analyze, stable in high-temperature fluids which do not fluoresce, occur in gas and fall out as pressure drops, and they are not natural products so their presence is indicative of petroleum. Initial tests of mapping diamondoid concentrations from marine piston core extracts were highly successful and the method is routinely used on many coring campaigns. As a result of this success, we were keen to try mapping diamondoid concentrations onshore. To test the method, we obtained surface soil samples: (1) over a known oilfield, (2) in nearby areas thought to be devoid of oil accumulations and (3) near a prospect. These soil samples were spiked with internal standards and extracted to determine diamondoid concentrations, which were then mapped in each of the three regions described above. The results show that above the oilfield and prospect, soil diamondoid concentrations are considerably higher (in some cases more than an order of magnitude) than above the area thought to be barren of oil, although a diamondoid background existed even in the these regions. In conclusion, we believe that determining diamondoid concentrations in onshore soil samples will prove to be a powerful method for recognizing onshore seeps and microseeps.