Study On Lithological Characteristics And Forming Environment Of The Volcanic Rocks In Xujiaweizi Region, Daqing Oil Field

Recently, people attached importance to study prospection and exploitation of volcanic reservoirs more and more along with that people demand gas resource increasingly and the volcanic reservoirs which have industry meaning were discovered in nation and overseas. But the study of volcanic rocks which can supply basic theory sustain for the upon is not in-depth and systemic. In order to solve this problem, the author have studied the Lithological characteristics, forming environment,volcanic facies and reservoir quality of volcanic rocks in Xujiaweizi region, and try to supply essential basic theory sustain for prospection and exploitation of volcanic reservoirs in this region.Xujiaweizi fault depression lies in east of the Daqing changyuan, north part of the Songliao basin, extending from south to north, with 80 km long, 40 km wide, and covered 5350 km2.It is the largest Mesozoic continental volcanic-sedimentary basin occurred in the Songliao basin. The Xujiaweizi fault depression develops volcanic rocks widely in Yingcheng Formation of the Lower Cretaceous and Huoshiling formation of the Upper Jurassic, and Yingcheng formation is richer in volcanic rocks.There are mainly two kinds of volcanic rocks in this area: lavas and pyroclastic rocks. The lavas mainly include basalt, dolerite, fumarolic, amygdaloidal andesite, trachy-andesite and as well as rhyolite. The pyroclastic rocks mainly include nomal pyroclastic rocks, pyroclastic rocks which transit to lava, pyroclastic rocks which transit to sedimentary rocks. The normal pyroclastic rocks mainly include andesitic, dacitic and rhyolitic volcanic breccia. They also include andesitic welded agglomelate and rhyolitic welded tuff. The pyroclastic rocks which transit to lava mainly have breccia lava and tuff lava etc. The pyroclastic rocks which transit to nomal sedimentary rocks are mainly sedimentary volcanic brecciaites and stuff gritstones etc.The volcanic rocks are mainly neutral and acid volcanic rocks, the basic volcanic rocks take the second place in research area. They mainly develop rhyolite, dacite and trachy-andesite, but few basalt. The volcanic rocks mainly belong to middle or strongcalc-alcaline and alkalescence series, with some alkali and strong alkali series. The volcanic rocks are mainly middle and high-k calc-alcaline series. The stages of volcanic eruption are at least three and not consanguinity in this area. The volcanic magmas of this field mainly come from the earth crust and curtain melted mixture or only from the earth crust. The forming environment of the volcanic rocks in this area should be island arc or active continental margin.There are three volcanic facies in the research area: eruption facies (fallout facies, pyroclasitc flow facies), effusion facies and volcanic-sedimentary facies.The volcanic clinothem is optimistic reservoir belt in the eruption facies;the volcanic rocks of effusion facies are mainly gas reservoir in this area;the ash tuff of volcanic-sedimentary facies is better part blanket. The primary pores and late stage structure fractures are mainly reservoir space of volcanic rocks in this region. The composite relation between this two factors often determines the vocanic reservoir quality. Based on comprehensive analysis of volcanic facies and vocanic reservoir quality, the author considers andesite and rhyolite reservoirs are the best volcanic reservoirs in the Xujiaweizi region.