Micropaleontology and palaeoclimate during the early Cretaceous in the Lishu depression,Songliao basin,Northeast China

Diverse and abundant microfossils, such as palynomorphs, algae and Ostracoda, were collected from lower Cretaceous strata of Lishu depression, located in southeastern Songliao basin, and were identified and classified in order to provide relevant, detailed records for paleoclimate research. The early Creta- ceous vegetation and climate of southeastern Songliao basin have been inferred from the analysis of palynomorph genera, algae and Ostracoda of the LSI and SW110 wells. The lower Cretaceous strata include, in ascending stratigraphic order, the Shahezi, Yingcheng and Denglouku formations. Palyno- logical assemblages for each formation, based on biostratigraphic and statistical analyses, provide an assessment of their longitudinal variations. During deposition of the Shahezi Formation, the climate was mid-subtropical. Vegetation consisted of coniferous forest and herbage. During deposition of the Ying- cheng Formation, the climate was south Asian tropical. Vegetation consisted mainly of coniferous forest and herbal shrub. In addition, fresh and saline non-marine water dominated the lacustrine setting during deposition of these formations. Deposition of the Denglouku Formation, however, occurred under a hot and dry tropical climate. The vegetation was mostly coniferous forest and lake waters became saline. Palaeoclimate variation is correlated by the lake level change and the development of sedimentary facies. Palaeoclimate contribute to the formation of hydrocarbon source rocks and reservoir.

Accumulation and exploration of continental shale gas resources of Cretaceous Shahezi Formation in Lishu fault depression,Songliao Basin,NE China

Distribution characteristics,organic matter development characteristics,gas-bearing characteristics,reservoir characteristics,and preservation conditions of the Shahezi Formation shale of Lower Cretaceous in the Lishu fault depression,Songliao Basin,NE China,are analyzed using organic geochemical,whole rock,and SEM analysis data,and CO_(2)and N_(2) adsorption and high-pressure mercury injection experiment data in combination with the tectonic and sedimentation evolution of the region to reveal the geological conditions for enrichment and resource potential of continental shale gas.The organic-rich shale in the Lower Cretaceous of the Lishu fault depression is mainly developed in the lower submember of the second member of the Shahezi Formation(K_(1)sh_(2)^(1) Fm.)and is thick and stable in distribution.The shale has high TOC,mainly types II_(1) and II_(2) organic matter,and is in the mature to the over-mature stage.The volcanic activity,salinization,and reduction of the water environment are conducive to the formation of the organic-rich shale.The shale reservoirs have mainly clay mineral intergranular pores,organic matter pores,carbonate mineral dissolution pores,and foliated microfractures as storage space.The pores are in the mesopore range of 10–50 nm,and the microfractures are mostly 5–10μm wide.Massive argillaceous rocks of lowland and highstand domains are deposited above and below the gas-bearing shale separately in the lower submember of the K_(1)sh_(2)^(1) Fm.,act as the natural roof and floor in the process of shale gas accumulation and preservation,and control the shale gas enrichment.Based on the above understandings,the first shale gas exploration well in Shahezi Formation was drilled in the Lishu fault depression of Songliao Basin.After fracturing,the well tested a daily gas production of 7.6×10^(4) m^(3),marking a breakthrough in continental shale gas exploration in the Shahezi Formation(K1 sh Fm.)of the Lishu fault depression in Songliao Basin.The exploration practice has reference significance for the exploration of continental shale gas in the Lower Cretaceous of Songliao Basin and its periphery.