Lower Source Rocks Research Articles

Organic geochemical signatures of source rocks and oil-source correlation in the Papuan Basin, Papua New Guinea

The Papuan Basin is the largest petroliferous basin in Papua New Guinea (PNG), where developed several sets of potential source rocks (SRs). The Jurassic SRs are clay-rich shales, whose organic matter (OM, mainly type Ⅱ 2 -Ⅲ) is predominantly derived from terrestrial higher plants deposited in a more oxic environment with relatively high maturity (0.35% < Ro < 2.03%). The Cretaceous SRs contain mixed inputs of terrigenous and marine OM (type Ⅱ 2 -Ⅲ) formed in sub-oxidized to oxidized depositional conditions with moderate maturity (0.33% < Ro < 1.58%). Whereas the Paleocene-Miocene SRs are predominant marine calcareous shales with subsidiary terrigenous OM inputs. These SRs containing type Ⅲ kerogen are mostly immature (0.21% < Ro < 0.70%). The Papuan Basin oils were classified into three distinct genetic families based on biomarkers and stable carbon isotopes. Family A oils, with heavy carbon isotopes, low Pr/Ph ratios, C 29 /C 27 ααα 20 R sterane <1.0, are likely originated from autochthonic Paleogene-Neogene mature marine OM. Family B oils have the characteristics of high Pr/Ph, moderate carbon isotopes, the dominance of C 29 ααα 20 R sterane, and are predominantly derived from autochthonous Jurassic SRs. Family C oils in the Eastern Papuan Fold Belt (EPFB) can be further subdivided into three subtypes by more subtle differences in specific biomarkers. Type C 1 oils, most likely stemmed from the Lower Cretaceous SRs, are characterized by light stable carbon isotopes, Pr/Ph > 2, the mild dominance of C 29 ααα 20 R sterane and the absence of oleanane ± lupane. Type C 2 oils can be clearly distinguished from Type C 3 by C 19 /(C 19 + C 23 ) tricyclic terpane <0.5, while the latter ones have higher abundances of oleanane ± lupine and diahopanes as well as C 24 tetracyclic/C 23 tricyclic terpane ratios. Type C 2-3 oils are originated from calcareous SRs that deposited in Late Cretaceous or younger age. • The organic geochemical signatures of the Papuan Basin SRs were elaborated. • Biomarkers, stable carbon isotopes, n -alkanes and iso-alkanes were used. • The oil samples in the Papuan Basin can be classified into three genetic families. • The likely origin of diverse oil families in the Papuan Basin were discussed.

Special Distribution of Crude Oil in the Lucaogou Formation in Jimusaer Sag and Genetic Analysis of Its Physical Difference

The shale oil of the Lucaogou Formation in the Jimusaer Sag of the Junggar Basin was divided into two sweet spots for exploration and development. Crude oil in the upper and lower sweet spots comes from the upper and lower source rocks. After years of exploration, it has been found that the crude oil in the lower sweet spot has worse physical properties than that of the upper sweet spot. In this study, through the physical and geochemical analysis of crude oil in the upper and lower sweet spots, combined with the organic petrological observation of the upper and lower source rocks, the cause of the poor physical properties of the crude oil in the lower sweet spot has been identified. n-Alkanes in the saturated hydrocarbons of crude oil in the upper and lower sweet were complete while odd-to-even predominance was evident, indicating that the poor physical properties of the crude oil are unrelated to biodegradation. In addition, the correlation between the biogenic parameters and the physical properties of crude oil was analyzed, finding that the difference in crude oil is mainly related to the composition of biogenic precursors of upper and lower source rocks. Combined with organic petrological observation, the lower source rock was found to be rich in telalginite (green algae), which is therefore the primary reason for the difference in physical properties. In comparing results from the characteristics of crude oil biomarkers from both the upper and lower sweet spots, crude oils in the upper sweet spot are similar to each other, indicating that the enrichment of crude oil has experienced a certain migration. In contrast, the differences in biomarkers between the crude oils of the lower sweet spot were relatively large and changed regularly with depth, suggesting the self-generated and self-stored characteristics of crude oil enrichment. At the same time, it was found that the crude oil in the lower sweet spot is also affected by the maturity of adjacent source rocks under the condition of a consistent parent material source. Overall, it was determined that the lower the maturity of source rocks, the poorer the physical property of the crude oil produced.