Tracing the origin of thermogenic hydrocarbon signals in pockmarks from the southwestern Barents Sea

摘要

The glacially influenced southwestern Barents Sea is a promising area for petroleum exploration. The present study area, which is located in the Loppa High region, exhibits extensive pockmark fields covering an area of several square kilometers and containing thousands of pockmarks in high density. To investigate whether the pockmark formation was driven by petroleum leakage to the surface, acting thus as indicators for an active petroleum system, gravity cores from selected pockmark and reference sites were investigated using an organic geochemical approach. Various biomarkers indicative of thermogenic hydrocarbons such as short chain n-alkanes with a low carbon preference index, alkylcyclohexanes, αβ-hopanes, steranes and diasteranes mixed with biomarkers representing immature organic matter were detected in the pockmark, but also in reference cores outside of the pockmark and even from a non-pockmark area. The abundance and level of thermal maturity of the thermogenic hydrocarbons are comparable between the pockmark and reference sites. Additionally, their distribution with depth is quite similar to those of the immature biomarkers. These observations support the conclusion that the thermogenic hydrocarbon signal in the pockmarks is the result of reworked, eroded mature material mixed with immature organic matter distributed across the entire area, rather than the result of seepage events of thermogenic hydrocarbons through the pockmark system. The data show that in the Loppa High region the pure presence of petroleum-derived hydrocarbons in pockmark areas cannot be used to indicate the presence or leakage of deeper petroleum systems.