Mechanism Of Reservoir Formation And Hydrocarbon Accumulation Of Sedimentary Organic-matter-bearing Tuffaceous Tight Oil In The Malang Sag,Northwest China

The discovery of the Permian Tiaohu Formation tuff reservoir in the Santanghu Basin in Northwest China has offered an excellent opportunity to further enhance our knowledge of the hydrocarbon accumulation mechanism of the tuffaceous tight oil.This reservoir has the peculiar property of being sedimentary organic-matter-bearing tuff formed by airfall volcanic ash in a lake.Based on the organic geochemistry,reservoir petrology characteristics,reservoir petrophysical property,pore structures and rock wettability,combined with comprehensive analysis of the geological conditions,distribution characteristics of the Tiaohu Formation tuff,reservoir formation mechanism,oil charging mechanism and hydrocarbon accumulation model as well as controlling factors of the tuffaceous tight oil reservoir are studied in this paper.Vitric tuff,crystal-vitric tuff,politic tuff and silicified tuff constitute the tuff reservoir.Vitric tuff is usually distributed far away from the volcanic activity belt and vertically within the central-lower part of the tuff.Crystal-vitric tuff,interbedded with vitric tuff,is usually distributed near the volcanic activity belt and also vertically within the central-lower part of the tuff.Politic tuff is generally distributed far away from the volcanic activity belt and vertically within the upper part of the tuff.Silicified tuff is generally distributed within the bottom of the tuff,where it has direct contact with the lower basalt.Types of tuffs vary due to the different distances from the volcanic activity belt,and the thickness is controlled by both volcanic activity belt and sedimentary palaeotopography.Thus,depositional depressions on both sides of volcanic activity belt are the favorable locations for the development of tuffs.The sedimentary organic-matter-bearing tuff in the Tiaohu Formation is characterized by medium-high porosity and low permeability.O f the four,vitric tuff has the properties best suited to a reservoir.The devitrification of volcanic glass in the tuff is the main cause of micropores.Organic acids that were produced during kerogen maturation facilitated the process of devitrification and the formation of reservoir micropores.Tuff porosity evolution is controlled mainly by the original composition and burial depth.The porosity of vitric tuff is higher than crystal-vitric tuff at the same depth,but both their porosities are characterized by an initial decrease and then an increase with increasing depth.For depths >3000 m,vitric tuff retains the trend of high porosity,but the porosity of crystal-vitric tuff scarcely changes or even decreases.The porosity evolution of the Tiaohu tuff can be divided into three stages,and they are the stage of decreasing porosity caused by normal compaction,the stage of increasing porosity caused by devitrification and the stage of normal evolution after increasing porosity,respectively.The three stages occurred before the early Cretaceous,the Cretaceous and before oil charging,and after the late Cretaceous,respectively.The crude oil in the tuffs mainly originates from underlying source rocks in the second member of the Lucaogou Formation,the oil is not indigenous,and it is far from the source rock.Adsorption of polar components of the oil generated from original organic matter leads to tuff wettability of partia l lipophilicity.Wettability of partial lipophilicity and small pore throat ratio are the main causes of the low starting pressure gradient of oil displacing water,and they also contribute efficiently to hydrocarbon charging and accumulation.Hydrocarbon accumulation in the tuff is controlled by reservoir quality,rock wettability,and good hydrocarbon expulsion conditions of the second member of the Lucaogou Formation and migrating system of fault-fracture.Therefore,the main hydrocarbon accumulation model that "wetting by the hydrocarbon produced by itself,accumulating by the non-indigenous oil,migrating through faults and fractures,charging from many filling points,and oil enrichment in effective tuffaceous reservoir with large area" is proposed.