Geochemical signatures of organic matter associated with gas generation in the Pohang Basin, South Korea

Abstract

We investigated the source, maturity, and biodegradation of organic matter (OM) deposited at two core sites (PSG-02 and PSG-03) of the Pohang Basin by analyzing stable isotopic compositions of bulk organic matter and specific lipid molecules (nalkanes). In contrast to the lack of substantial variations in bulk carbon isotopic compositions along core depths, a homologous series of saturated alkanes (C-16 to C-35) showed distinct carbon chain-length distribution between both core sites, having predominance in C(20 )to C-25 for PSG-02 and C-29 for PSG-03. Such variation may reflect the different geochemical properties of the deposited OM involved in gas generation. In this regard, various n-alkane indices, for example, carbon preference index (CPI), terrestrial/aquatic ratio (TAR), average chain length (ACL), natural n-alkanes ratio (NAR), percentage of aquatic plants (P-mar (aq)), and pristane/phytane (Pr/ Ph), were applied to understand the OM origins and depositional environments at both core sites. The contribution of thermally petroleum-derived OM is predominant in PSG-02, as shown by the low values of CPI (< 1) and NAR (< 0.6). Conversely, high values of ACL (> 29), TAR (> 2), low values of P-mar aq (< 0.4) and Pr/Ph (-1) may be associated with mixed contributions of marine and terrestrial OM sources in PSG-03. In addition, Pr/Ph ratios suggest that sedimentary diagenesis conditions such as maturity and redox condition should be different between both core sites. Our results highlight useful information regarding the geochemical properties of OM involved in gas generation in the Pohang Basin. This will help us better define accumulated gas systems and reduce the risk associated with future exploration efforts in this area.