Palynofacies and paleoenvironment of the Upper Jurassic mud-supported carbonates, southern Croatia: Preliminary evaluation of the hydrocarbon source rock potential

Despite the vast research on various aspects of its stratigraphy, relatively little is known about the source-rock potential of the Upper Jurassic Adriatic platform, southern Croatia. Here a case palynological study has been provided to determine the kerogen type, quality, and thermal maturity, and assess its relative hydrocarbon generative potential. Palynofacies characteristics indicate a low-energy, shallow-marine oxygen-depleted setting. The deposited mud-supported carbonates contain organic matter that is almost entirely composed of a fluorescent amorphous organic matter of planktonic and bacterial origin. A high phycomata/dinocyst ratio suggests relative hydrographic stability in an environment favorable for motile-stage dinoflagellate community with reduced production of dinocysts. The lack of anaerobic degradation and very good preservation of lipoid palynomorphs indicates dysoxic conditions; the latter also is suggested by a low proportion of algal-phytoplanktic carbohydrates. The lack of oxygen-deficient zone within the water column, coupled with well-preserved lipoid sediment components, suggests that the boundary between the oxygen-rich and oxygen-poor levels was likely located at/near the sediment-water interface. The palyno-organic facies analysis suggests that the organic matter corresponds to the type II (possibly I/II) kerogen (VRo ∼0.3-0.4%) that was protected from degradation before, during and after sedimentation. Well-preserved lipoid components suggest a good hydrocarbon-generative potential, but the low TOC values (<0.1%) of studied outcropping Upper Jurassic carbonates prevent their designation as source rocks. Given the known source-rock potential of the Upper Jurassic succession in the wider region, further studies of subsurface samples are needed to exclude the Oligocene-recent subaerial weathering as a culprit for the low TOC values.