Diagenesis And Genetic Mechanism Of Tight Oil Reservoir Of The Permian Lucaogou Formation,Jimusar Sag,China

Tight oil,the most realistic and potential unconventional hydrocarbon resources,has acted as one of the most important target for enhancing oil and gas reserve and production.Tight oil is characterized by para-genetic source and reservoir,oil accumulation within or adjacent to the source and unconstrained by trap.As a consequence,looking for reservoir is the soul of tight oil exploration.Tight oil reservoir is featured by various rock components,diversified diagenesis,complicated pore-throat structures and strong oil heterogeneity,which increase the difficulty of tight oil exploration.Diagenesis and formation mechanism of reservoir are the key factors resulting in the strong reservoir heterogeneity and are regarded as the core but difficult subject in tight oil research.Taking the tight oil of the Permian Lucaogou Formation(P₂l)in the Jimusar Sag as research object,based on sedimentary features and reservoir properties characterizing,reservoir space formation,evolution and its relation with diagenetic alteration were analyzed combing with study of diagenesis characteristics and mechanism in different lithofacies in this dissertation.Finally,the genetic mechanism of tight oil reservoir was clarified,and formation model of the P₂l tight oil reservoir was established.The studied tight oil reservoir has complex rock composition.The mainly developed lithofacies are dolomicrite,silty dolomite,dolomitic siltstone,silty tuffite,tuffaceous siltstone and tuffaceous dolomite.Comparative analysis of porosity,permeability and oil-bearing in different lithofacies shows that the reservoirs are mainly developed in dolomitic siltstone/sandstone,tuffaceous siltstone and tuffaceous dolomite.Physical properties vary largely in these different reservoirs.For tuffaceous dolomite,reservoir space is dominated by pyroclastic dissolving pores and pores generated from volcanic material devitrification.This reservoir is characterized by small pore and throat radius,large pore-throat radius ratio and relatively poor pore-throat connectivity.The reservoir space of tuffaceous siltstone is mainly composed of strong dissolved pores of feldspar,medium-basic volcanic debris and pyroclastics.The pore radius in tuffaceous siltstone are various greatly,with small pore radius,medium throat radius,small pore-throat radius ratio and medium pore-throat connectivity.The reservoir space in dolomitic siltstone/sandstone is dominated by dissolved pores of feldspar and andesite debris,followed by residual intergranular primary pores.The pore throat radius distribution shows relatively large pore and throat radius,small pore-throat radius ratio and relatively good pore-throat connectivity.The diagenesis and their mechanism show great differences between different reservoirs with complicated multicomponent.Tuffaceous dolomite reservoir is characterized by strong dissolution and devitrification of medium-basic volcanic material and precipitation of authigenic quartz,accompanied by albite and smectite precipitation and smectite gradually transformation to chlorite.The authigenic quartz is sourced from tuff devitrification and dissolution,bio-siliceous and clay mineral transformation.Tuffaceous siltstone is featured by strong dissolution of K-feldspar and pyroclastic,affected by earlier formation water and organic acids and CO₂ from organic matter evolution from the adjacent layer.Minor authigenic albite and quartz were precipitated under relatively open diagenetic system.Na HCO₃ formation water,smectite illitization and chloritization all accelerate K-feldspar dissolution and albite precipitation.Dolomitic siltstone is characterized by moderate to strong dissolving of feldspar and debris,accompanied with quartz overgrowth and carbonate cement.Quartz overgrowth has a wide range temperature from 65?to 115?,indicating a continuous precipitation process.The silicon was mainly sourced from feldspar dissolution and pressure dissolution of quartz grains,secondly from volcanic material dissolution and smectite transformation.There are various carbonate cements,among which Ca of the early carbonate was from the original sedimentary formation water and C was derived from microbial fermentation,while the later ferro-carbonate with negative?¹³C was affected by decarboxylation of organic matter,and abundant Ca²⁺?Mg²⁺?Fe²⁺?Mn²⁺from volcanic material dissolution.Tight oil reservoirs can be divided into source-reservoir integrated and source-reservoir interbedded tight oil according to source and reservoir configuration and assemblage.The source-reservoir integrated tight oil is mainly found in fine-grained ash-rich rocks,and the typical lithofacies are muddy tuffite and tuffaceous dolomicrite.In this source-reservoir integrated tight oil,reservoir is formed within the source rock,and the organic acids,CO₂ and hydrocarbon generation from organic matter were accelerated by catalysis of volcanic material,advancing the oil generation window.Volcanic material dissolving pores supplied the major reservoir space,and was charged by low maturated oil,with crude oil presenting absorbed state within nano-pores.Bedding surface and stylolite are developed,which could also act as reservoir space and migration pathway.Source-reservoir interbedded tight oil is mainly developed in relatively coarse grained ash-poor rocks,reservoirs formed adjacent to the source rocks,represented by dolomitic siltstone/sandstone and tuffaceous siltstone.The unstable feldspar and tuff were dissolved by acid fluids from adjacent organic-rich mudstone and tuffaceous rocks,forming the major reservoir space.Devitrification also has obvious enhancement on porosity.Crude oil distributes in the pore throat with radius greater than 0.05-0.1?m,presenting as free state in pores or migrating along favorable layers and stylolite.The P₂l source rocks began to enter the hydrocarbon generation threshold from the Late Triassic,and then generated oil in large quantities from the Middle Jurassic,after then continuing hydrocarbon producing and charging.Under a certain source and reservoir assemblage,the time and space matching of early hydrocarbon generation and continuous charging,reservoir space formation process by tuff and feldspar dissolution,and pore preservation by overpressure and earlier oil charge,corporately controls the formation mechanism of tight oil reservoir.