Characteristics Of Tuff And Its Relationship With Oil And Gas In The Lucaogou Formation In Jimusar Sag,Permian,Zhunggar Basin

Jimusar Sag is located in the east of Junggar Basin,Xinjiang.The Middle Permian Lucaogou Formation in the basin which is widely distributed and has a huge thickness is a favorable horizon for tight oil exploration and exploitation.The Lucaogou Formation was formed in reduced-weakly oxidized salt lake with a background of rift-fault depression,forming a set of volcanic eruption and hydrothermal activity Related to special rock and mineral combinations with the affection of volcanic activity and alkaline magmatic-hydrothermal activity.Considering the regional tectonic setting and sedimentary environment of the study area,it is considered that the oil-bearing zones of Lucaogou Formation are mainly composed by tuff based on the observation of a large number of cores and slice.According to the different debris of the volcano,tuff can be divided into carbonated tuff and andesitic tuff.Tuffs in the study area mainly composed of quartz,feldspar,calcite,dolomite and other minerals,but little cuttings.The electron probe data of feldspar and the discovery of aegirine indicating that the alkaline-parlkaline magma are their source.Scattered calcite in carbonate tuffs are surrounded by the layers.Organic matter near the calcite has a higher maturity than other parts in the rock.The fluid inclusion in the calcite are zonal pattern.Combined with fluorescence and cathodoluminescence characteristics,we judge that this type of calcite is debris of carbonated magma,the heat brought by which accelerated the organic matter's mature surrounded.The fluid inclusion is captured when calcite was increasing affected by the deuteric hydrothermal.Geochemical characteristics show that the source of the andesitic tuff in the Lucaogou Formation is mainly andesitic volcanic rocks.The similar rare earth element allocation patterns with andesitic tuff and Permian volcanic rocks in the southern margin of Bogda shows they have the same source region.SEM and NMR experimental show that both the primary and secondary pores are developed in the tuff,and the dissolution pores and remaining intergranular pores are most common.The content of the tuff material has a positive correlation with the free fluid saturation.In summary,it is considered that tuff is not only a kind of high quality source rock but also reservoir rock,which plays an important role in the enrichment and preservation of organic matter in the study area.The uneven distribution of tuffaceous rocks leads to an important reason for the heterogeneous distribution of tight oil in the study area.