Geochemical and petrographic investigation of Triassic and Late Miocene organic-rich intervals from onshore Cyprus, Eastern Mediterranean
Résumé
In order to improve the understanding of potential petroleum systems in the still underexplored Eastern Mediterranean Sea, two field campaigns were performed to investigate potential source rocks onshore Cyprus. Elemental data (total organic (TOC) and inorganic carbon (TIC), total sulfur (TS), Fe, Ni, V), Rock-Eval® pyrolysis, biomarker and microscopic analysis (organic petrography, random vitrinite reflectance (VRr), palynofacies) of the obtained Mesozoic to Cenozoic samples have proven the existence of organic matter (OM) rich deposits within the following two geological formations:(1) The Triassic Vlambouros Formation (Fm.) of the Mamonia Complex comprises frequently occurring, few centimeter thin clay layers interbedded between meter thick sandstone units. The clay layers of this formation are characterized by TOC contents of about 1 wt% representing a Type III kerogen showing hydrogen index (HI) values of 48 mg HC/g TOC on average. VRr values of 0.5 to 0.6% as well as several biomarker ratios indicate low thermal maturity. The formation was deposited at the northern margin of Gondwana and similar strata may be present in the basement of the Levant Basin and the Eratosthenes Seamount. There, thermal maturity would be higher and such rocks might contribute to thermogenic gas systems.(2) In the Miocene Pakhna Fm., OM-bearing intervals are mainly present in the upper part of the formation in outcrops east of the Troodos Mountain, while organic-rich intervals are very scarce to absent in the south and southwest of the island. The eastern outcrops show abundant fine-grained, (marly) mud- to wackestones interbedded with coarser-grained carbonates. The fine-grained intervals have TOC contents of about 4 wt% representing mainly Type II-III kerogen with HI values of 238 mg HC/g TOC on average. The OM is immature as indicated by VRr values between 0.3 and 0.5%. TS/TOC ≥ 2.8 and several biomarker ratios indicate oxygen-depleted, even anoxic conditions during deposition. A negative correlation between the TOC and TIC indicates that such conditions were reached during enhanced primary bioproductivity controlled by local and regional clastic derived nutrient input. Similar conditions may have been present along the southeastern margin of the Eratosthenes Seamount, south of Cyprus. Presence of abundant organic matter and high sedimentation rates during the Miocene were favorable for microbial gas generation in the deeper parts of the basins.
Domaines
Sciences de la Terre
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