Petrological,Chronological And Geochemical Features Of The Middle And Upper Jurassic,Hailaer Basin

As a peripheral exploration area of Daqing oilfield,Hailaer Basin has huge potential for oil and gas exploration because of the existence of Jurassic volcanic strata in deep layer.However,the understanding of the sequence,chronology and distribution characteristics of the Jurassic strata in the basin is insufficient.In the early exploration practice,the Jurassic strata were uniformly named "Tamulangou Formation" or "Xinganling Group",and the eruption period was ambiguous,which restricted the exploration and deployment of volcanic reservoirs in the basin.In this paper,the volcanic rocks of Tamulangou Formation of Middle Jurassic and Manketouebo Formation of Upper Jurassic developed on the periphery of Hailaer Basin are taken as the research object.Combined with previous research results of Jurassic and upper and lower stratigraphic sequences in Greater Khingan Mountains area,the formation age and tectonic setting of Middle and upper Jurassic in Hailaer Basin are determined by field geological survey,petrology,chronology and geochemistry.Establish stratigraphic sequence around Hailaer Basin.On this basis,combined with the study of Jurassic strata in Wuerxun Sag,the correlation between the Jurassic strata in Hailaer Basin and its margin was carried out,the corresponding relationship between the Jurassic strata inside and outside the basin was summarized,and the stages of the Jurassic Tamulangou Formation were detailed,which provided the basis for the exploration of the key volcanic rock reservoirs in the basin.Zircon U-Pb dating of Middle and Upper Jurassic volcanic rock samples collected from the peripheral section of Hailaer Basin and Wuersun Sag shows that the Tamulangou Formation was formed in the Middle Jurassic(167~158.1Ma),and the lower part mainly developed middle basic lava and a few pyroclastic rocks and acidic lava.The upper part mainly developed the middle basic lava and acid pyroclastic rocks.The Manketouebo Formation was formed in the Late Jurassic(156.1~152.7Ma),and the lithology is mainly acid lava.The first member of the Tamulangou Formation was formed in the Late Jurassic(158~157.5Ma).The lithology is composed of basic volcanic rocks,acidic volcanic rocks and sedimentary rocks,corresponding to the upper part of the Tamulangou Formation in the basin margin.The second member of the Tamulangou Formation was formed in the Late Jurassic(156~152.4Ma),and the lithology is mainly acid lava,corresponding to the Manketouebo Formation on the rim of the basin.Acidic volcanic rocks of Tamulangou Formation and Manketouebo Formation on the periphery of Hailaer Basin are dominated by rhyolitic lava and pyroclastic rocks,with loss of Sr,Ba,Eu,Ti and P.It has obvious europium negative anomaly and has obvious geochemical affinity with A-type granite.The geochemical characteristics of high aluminum(12.96%~15.18%)indicate that the crust magma plays a decisive role in the formation of rocks,and the volcanic rocks are mostly high potassium calc-alkaline series and potassium basalt series,which have the characteristics of major elements of post-orogenic granite.The Mongolian-Okhotsk Ocean was formed in the early Paleozoic,the continental crust was thickened after the collision orogeny in the Middle Jurassic,and the delisting occurred in the Late Jurassic,resulting in an extensional environment,similar to the formation time of volcanic rocks in the study area.The similar geochemical characteristics of the two sets of acid volcanic rocks of Tamulangou Formation and Manketouebo Formation indicate that the study area was always under the influence of the Mongolian-Okhotsk Ocean tectonic domain during the late Middle Jurassic to Late Jurassic.Therefore,it is considered that the volcanic rocks of Tamulangou Formation and Manketouebo Formation of Middle and Upper Jurassic in Hailaer Basin are related to the lithosphere extension environment after the closure of Mongolic-Okhotsk Ocean.Based on the stratigraphic correlation between the inner basin and the outer basin,combined with the previous research results,the inner basin Tamulangou Formation can be divided into two stages: the first stage,the first member of the inner Basin Tamulangou Formation(volcanic rocks with the age of 166~157Ma revealed by drilling)corresponds to the Middle Jurassic Tamulangou Formation(168~157Ma)on the edge of the basin,and the assemblage of basic-acid volcanic rocks and sedimentary rocks is developed.In the second stage,the second member of the Tamulangou Formation in the basin(volcanic rocks of 157~148Ma revealed by drilling)corresponded to the Late Jurassic Manketouebo Formation(157~144Ma)on the basin margin,with acidic volcanic rocks and a small amount of mesobaric rocks.In the course of field geological investigation,it is found that pore structure is developed in the rhyolite of Manketouebo Formation,and there are a lot of residual bitumen,which proves that Jurassic volcanic rocks in Hailaer Basin have a certain capacity of reservoir formation.The acidic volcanic rocks in the upper member of the Tamulangou Formation can be used as the key exploration horizon for the development of volcanic rocks in the subsequent basin oil and gas exploration.