Study On The Characteristics Of The Tight Tuff Reservoir In The Kalagang Formation In The Malang Sag

This thesis is based on the scientific and technological research project of the Tuha Oilfield Company and China University of Petroleum(East China).Malang Sag is the major oil-producing area of the Santanghu Basin and has rich oil and gas resources.The volcanic rocks in the basin are generally appeared,and the types of volcanic rock reservoirs are diverse.They have strong heterogeneity.Since the discovery of tuff reservoirs in the Permian Tiaohu formation in 2013,they have had a preliminary understanding of the tuff reservoirs in the sag.However,there is a lack of systematic understanding of its macroscopic and microscopic features,especially the understanding of the tuff reservoirs discovered in 2015 in the Carboniferous Kalagang formation.This article starts with the macro-basic geological studies of reservoir lithology,lithofacies and mineral composition,and characterization of the reservoir microscopic reservoir space,and combines the characteristics of diagenesis to analyze the reservoir control factors.The tuffs with high debris content in the study area were obtained.The debris was presented in four forms: rock debris,crystal debris,glass shavings and volcanic ash.Three types of lithofacies were developed: crystal-vitric tuff phase,vitric tuff phase,and argillaceous tuff phase.The overall pores are universal developed,including dissolved pores,structural fractures,dissolved fractures,interlayer fractures,intergranular/devitrified pores,and devitrified pores.Among them,dissolved pores and structural fractures and dissolved fractures are better developed,followed by devitrification.At the same time,reservoir interconnected pore space is divided into four large-scale pores: macropores(>1000 nm),mesopores(100–1000 nm),pores(20–100 nm),and micropores(<20 nm).Combined with the above basic understanding,studies have shown that the vitric tuff phase is the best lithoface in the tuff reservoir,with a porosity of 7.61% to 22.31%,an average of 12.919%,and a permeability of 0.082 mD to 1.24 mD,with an average of 0.622 mD.The pore size distribution of tight tuff reservoirs in the study area is wide,ranging from 0.85 nm to 100?m.The pore size is mainly distributed below 0.1?m.The proportion of pores larger than 1?m is smaller,and micropores and small pores are mainly developed.Nano-sized pores dominate.These features are formed by a series of sedimentary diagenesis.Because of the distance between the craters and other factors,the mineral content and species in the rocks are different.Both devitrification and dissolution increase a certain amount of reservoir space,and the faults and cracks are more connected.It is in favor of the formation of tight tuff reservoirs in the Kalagang Formation.These understandings provide some guidance for the next exploration and development of tuff reservoirs.