Formation And Preservation Mechanism Of The High-quality Dolostone Reservoir In The Feixianguan Formation,Xuanhan-Daxian Area

Since the Permian-Triassic dolostones and dolostone reservoirs are widespread in the northeast of Sichuan Basin,where large-scale gas fields such as Puguang,Yuanba and Luojiazhai are discovered,this area becomes an important target for natural gas exploration and exploitation in carbonate successions.In the Xuanhan-Daxian section,thick dolostone intervals are developed in the Feixianguan Formation,and the types of rock texture and pore vary extensively in dolostones.Additionally,dolostones experienced a long diagenetic history on the condition of high-temperature and-pressure deep burial environments,and thus complex pore modifications.So the dolostone reservoir in the Feixianguan Formation is an ideal natural laboratory for the study of development and preservation mechanism of high-quality dolostone reservors.Petrographic observation of drill cores and thin sections,geochemical analyses in C,O,S isotopic values and trace element concentrations,and petrophysical tests are applied to depict the rock texture and pore types,to figure out the distribution of dolostone reservoirs in the stratigraphic framework,to interpret the origins of various diagenetic fabrics,to decipher the developing process of pores,and to make sense of the alteration and preservation of pores.(1)Two third-order sequences are present in the Feixianguan Formation.The high-quality dolomite reservoir is developed in the upper and middle part of the first third-order sequence on the platform rim,the microfacies types are oolite shoal in the middle part and back-shoal tidal flat in the upper part.Dolostones consist of five types of dolomite: micritic and microcrystalline dolomite,finely to medium crystalline dolomite,recrystallized dolomite,dolomite overgrowth and dolomite cement.The types of rock texture include fabric-retentive dolostones,ghost-texture dolostones and fabric-destructive crystalline dolostones.(2)Petrographic observation and high-pressure mercury injection show that three scales of pores types exist in this reservoir.The large-scale types include intergranular pores,intragranular dissolved pores and intercrystalline pores between euhedral crystals,the medium-scale type is sub-intercrystalline pores among dolomite fragments,and the small-scale types include intercrystalline pores between anhedral crystals andintracrystalline pores within euhedral crystals.Intergranular pores,intragranular dissolved pores and intercrystalline pores between anhedral crystals occur in the fabric-retentive and ghost texture of dolostones,while intercrystalline pores,sub-intercrystalline pores and intracrystalline pores occur in the fabric-destructive crystalline dolostones.(3)The rock texture and pore texture of the present reservoir result from the early mineral stablization and dolomitization.The meteoric diagenesis drive the process of mineral stabilization by dissolution and reprecipitation in primary aragonite sediments.High-frequency stratigraphic analyses suggest that early mineral stablization took place during the short-lived subaerial exposure by the high-frequency eustatic regression,consequently pore-type cycles developed in the oolite shoal and back-shoal tidal flat microfacies.Petrographic and geochemical evidence indicates that the five types of dolomite are formed during the regression period of the first third-order sequence.The evaporated seawater which was formed under arid climate conditions reflux from the platform interior,to the platform rim and eventually the Kaijiang-Liangping trough,and the oolite shoal and back-shoal tidal flat were dolomitized by the reflux seawater.The distribution of rock texture and geochemical signal on the reflux pathway is completely consistent with the published reflux dolomitization model.(4)The reservoir is mainly modified by the filling of sulfur-enriched solid bitumen and calcite cement.Crude oil cracking produced bitumen which was rich of sulfur caused by TSR.Calcite cement precipitated from the pressure dissolution fluids and TSR fluid.Using the Photoshop software to count the relative contents of various diagenetic fabric in thin sections,we find that the bitumen causes 0~35% reduction of paleo-porosity,and the bitumen proportion in mediumly porous(6%-14%)dolostones is more abundant than less porous(<6%)dolostones and more porous(>14%)dolostones.Calcite cement occurs almost in the back-shoal tidal flat,while the porous dolostones in the oolite shoal preserve as the absence of calcite cement.