Pollen Fossil Research and Geological Significance of Well Y51 in the Gulong Sag, Songliao Basin

The Y51 well is a pre-exploration well located on the southwest limb of the Telaha Syncline in the Gulong Sag of the central depression area of the Songliao Basin. To study the paleoenvironmental and paleoclimatic characteristics of the Qingshankou Formation Member 1 in this area, 25 samples were collected from the core interval of 2171.4 to 2266.86 meters. Analysis and identification revealed that the pollen spores in this interval were predominantly gymnosperm pollen (averaging 82.93%), followed by fern spores (averaging 17.03%), with scarce angiosperm pollen. Among the gymnosperm pollen, Pinaceae and Podocarpaceae had the highest content, followed by Cedripites minutulus, Piceapollis tobolicus, Keteleeriaepollenites davidianaeformis, Parcisporites minutus, P. parvisaccus, and Podocarpidites. In the fern spores, Cyatheaceae had the highest content, almost entirely consisting of Cyathidites minor and C. australis, while Lygodiaceae also contributed a certain proportion. The angiosperm pollen was only sporadically represented by Quercus pollen. The above analysis indicates that the fossil assemblage is characterized by the prevalence of two-celled conifer pollen, the relatively developed Cyatheaceae spores and Phyllocladaceae pollen, and a certain proportion of Lygodiaceae spores. Based on this, the Pinuspollenites-Cyathidites-Cicatricosisporites assemblage was established. This assemblage can be compared with the Cicatricosisporites-Cyathidites-Pinuspollenites assemblage found in the southern borehole of Well Song in the Gulong Sag of the central depression area of the Songliao Basin. Other assemblages roughly comparable to this include the Cicatricosisporites-Cyathidites-Pinuspollenites assemblage in the Qijia-Gulong Sag western Taikang area of the Songliao Basin during the Qingshankou Formation Member 1, and the Pinuspollenites-Cyathidites assemblage in the Niahe section of the Bin County-Wangfu Sag in the southeastern uplift area of the Songliao Basin during the same member. In summary, the current interval corresponds to the Qingshankou Formation Member 1. The vegetation of this period was dominated by coniferous trees (primarily Mesozoic Pinaceae, followed by wetland Podocarpaceae and xerophytic Phyllocladaceae), with a certain proportion of evergreen broad-leaved trees (wetland Cyatheaceae), while shrubs and herbaceous plants were relatively scarce. The vegetation type should be coniferous forest, reflecting a semi-humid subtropical climate. The algal fossil content in this well section is moderate, with poor preservation. Some may belong to the Leiosphaeridia/Granodiscus of questionable origin, while Dinogymniopsis daqingensis of the haptophyte group is sporadically distributed. Botryococcus of the chlorophytes appears in individual samples. This assemblage may indicate a brackish water depositional environment.