Formation Mechanism Of High Quality Reservoir Of The Upper Paleozoic Tight Sandstones In Daniudi Gas Field, Ordos Basin

Large-scale typical tight-gas reservoirs of the Upper Paleozoic developed in Daniudi gas field,Ordos Basin.Based on analycial datum including physical data and tests including observations of casting thin sections and scanning electronic microscope,and stable C&O isotope test and temperature measurement of fluid inclusions,the paper clears the diagenetic evolution and formation mechanism of reservoirs,taking the First Member of Permian Xiashihezi Formation?P₁x¹?and the Second Member of Carboniferous Taiyuan Formation?C₃t²?for examples.By the method of inversion and back stripping which based on pore structures,the paper reveals formation processes of porosity of different types of reservoirs.And thepaper also clears formation mechanism of high quality reservoirs,combining the analysis of control factors on reservoirs.By the inversion of well logging and batch processing of modeling software,the paper also reveals spatial distribution features of high quality reservoirs.By the recovery and correction of calcite and kaolinite replacements,the content of detrital feldspar in primary rock composition of P₁x¹ and C₃t² members can be as high as 20%.Their primary rock types are litharenite and feldspathic litharenite?P₁x¹?,and lithiarenite,subquartzose and quartzose?C₃t²?.Enough smecitite sources such as some rock fragments and synsedimentary volcanic glasses in P₁x¹ member and early pore fluid containing more K⁺impacted by sea water in C₃t² member make different reaction mechanisms between K-feldspar,kaolinite and illite.Illitization of smectite from late eodiagenesis to the middle and late mesodiagenesis?50¹20oC?strongly influences the dissolution of K-feldspars in P₁x¹ member and kaolinite,chlorite and calcite are the main by-products.But in C₃t² member,most of K-feldspars preserved because of enough K⁺in pore fluid and absence of smectite in middle mesodiagenesis.Then K-feldpars experienced profund dissolution drived by illitization of kaolinite?120¹40oC?.And quartz and illite cements produced as a cost of consumption of kaolinite and K-feldspar.Tight reservoirs can be divided to three diagenetic facies in the light of their diagenetic characteristics:dissolution diagenetic facies,compaction-pressolution-siliceous-cementation diagenetic facies and calcareous-replacement-cementation diagenetic facies.Pseudomatrix formed in egodiagenesis made the main porosity loss for compaction-pressolution-siliceous-cementation diagenetic facies and calcareous-replacement-cementation diagenetic facies developing in fine-grained lithofacies.And quartz cements also play an important role to the former one.Schisitc and curved lamellar throats among the intergranular spaces of grains which are in close contact and microthroats among pseudomatrix are the main contribution to the relative low permeability.Due to relatively less porosity loss by compaction because of high content of quartz grains and porosity increase by feldspar dissolution in mesodiagenesis,reservoirs of dissolution diagenetic facies and calcareous-replacement-cementation diagenetic facies developing in coarse-grained lithofacies are the main types for high quality reservoirs.Intragranular porosity and microporosity?areal porosity>2%?are the dominant in high quality reservoir of P₁x¹ member and they are intergranular and intragranular porosities?areal porosity>2%?in C₃t² member.Pore-throat structure consisting of micropores,thin throats and micro-thin throats which is comprised by spaces among kaolinite and other clay minerals is the main contribution to the permeability of high quality reservoirs.Under the background of enough acid fluid sources supplied by thermal evolution of thick coals,reservoirs which developed in coarse-grained lithofacies and possessed high content of quartz grains and low content of rock fragments experienced relatively less porosity loss in egodiagenesis and relatively higher porosity increase in mesodiagenesis.That is the formation mechanism of high quality reservoir in Daniudi gas field.Relative coarse lithofacies and preponderant diagenetic facies controls the distribution of high quality reservoirs.It shows patterns of“lithofacies controlling the districts of high quality reservoirs”and“diagenetic facies controlling the redistribution of high quality reservoirs”.Reservoirs of dissolution diagenetic facies and calcareous-replacement-cementation diagenetic facies developing in coarse-grained lithofacies controls the distribution of high quality reservoirs.The remarkable symbols are the centres of accumulative thickness of those two diagenetic facies.They both distribute in the areas controlled by the accumulative thickness centres of medium-grained lithofacies?>15m,in P₁x¹ member;>12m,in C₃t² member?.The former distributes in the core of accumulative thickness centres of medium-grained lithofacies and the latter distributes in their edges.Whether controlled by fine-grained lithofacies at the edges of the area is the key to judge if they are high quality reservoirs.