Karst Characteristics And Controlling Factors Of The Longwangmiao Formation In The Early Cambrian,Sichuan Basin And Its Periphery

Karst reservoirs in carbonate are important petroleum exploration targets in the world.The present study found that the karst reservoir of pore-cellular hole was the main production layer of Longwangmiao Formation in the early Cambrian of central Sichuan Basin.However,Longwangmiao Formation in the early Cambrian of the Sichuan Basin and its periphery experienced a multi-period tectonic evolution and multi-period fluid transformation,the type,distribution,evolutionary process,formation mechanisms,and conservation characteristics of the karst lacked of systematic research.These severely restrict petroleum exploration targets of the Longwangmiao Formation in the early Cambrian of Sichuan Basin and its periphery.This work utilized the theories of carbonate sedimentology,sequence stratigraphy,karstology,geochemistry and so on,based on field outcrops,drilling(logging)well and various analytical testing data,combined with geology and geochemistry,identified macro and microscopic features of karstification,classified different types of karst and characteristics of its plane and spatial distribution.Based on the comprehensive tectonic evolution,burial history and sequence of diagenetic evolution,we discussed the development and evolution of karstification,and established the differential development and evolution mode of paleo karst in the study area.The formation mechanism and main controlling factors of typical karstification were discussed from the aspects of eustatic sea level change,rock property,paleoclimate and tectonic evolution.The fluid geochemistry characteristics of fillings and differential preservation mechanism of karst reservoirs in the late burial process were analyzed.Finally,the distribution of karst reservoirs was predicted.Several conclusions were documented as follows:(1)The Sichuan Basin and its periphery is divided into four karstification areas,including northern Sichuan area,eastern Sichuan and Hunan-Hubei area,central Sichuan area and southwest Sichuan area.We identified the five types of karstification,including syngenetic karst,penecontemporaneous karst,buried karst,Caledonian-Hercynian bedding-parallel karst,and bedding-parallel karst formed by structural transformation since the period of Yanshanian,and established corresponding identification marks of karstification.There were significant differences in the types of major karst and superposition in different areas.The northern Sichuan mainly developed syngenetic karst,superimposed by karst formed by structural transformation since the period of Yanshanian.Eastern Sichuan and western Hunan-Hubei area are dominated by syngenetic karst,penecontemporaneous karst,and superimposed by karst formed by structural transformation since the period of Yanshanian.In central of Sichuan area,the Caledonian-Hercynian bedding-parallel karst superimposed on the basis of karstification during the syngenetic and penecontemporaneous stages.The southwestern Sichuan area mainly developed the syngenetic and burial karst,superimposed by Caledonian-Hercynian bedding-parallel karst and karst formed by structural transformation since the period of Yanshanian in local area.The distribution of syngenetic and penecontemporaneous karst is extensive,and the influence of other types of karstification is relatively limited.(2)Laws of karst development in different research areas were dissected.Also,the syngenetic karst evolution model of granular rocks represented grain-beach microfacies in northern Sichuan was established and the densification evolution process model that dolomite infilled dissolution holes from shallow burial period to Caledonian-Hercynian was also outlined.The karst development model during syngenetic and penecontemporaneous period in eastern Sichuan and western Hunan-Hubei was established,and the penecontemporaneous karst development model of the"island type"in the inner part of platform and the"coastal type"in the margin of platform was proposed.Caledonian-Hercynian bedding-parallel karst model superimposed the syngenetic and penecontemporaneous period in central of Sichuan was established,with the thinking that the karstification in the area far away from paleo-uplift is weak,and that the extent of karst development in the east is limited in the west of Huayingshan fault zone.The palaeo-karst evolution model of microcrystalline rocks represented tidal flat microfacies in southwestern Sichuan was established.The evolution processes of the densification solution pores,including penecontemporaneous dolomite filling solution pores to shallow burial dolomitic filling solution pores and to late Caledonian-Hercynian calcite filling,was outlined.(3)Summary of typical control factors of karstification believes that syngenetic karst was mainly controlled by a brief drop in secondary sea levels.In the process of level 4 or even 5 sea level falling,microfacies of grain-beach and dolomitic flat exposed,occurring syngenetic karstification.The penecontemporaneous karst distribution and scale are controlled by the exposure due to the falling of sea level in the end of Longwangmiao Formation,rock property and paleoclimatic condition.The regression of the later Longwangmiao Formation was short and the exposed lithology was the dolomite with weak karst differentiation,dry and hot paleoclimate restricted the development intensity and scale of karstification.Compared with the Dazhou-Kaijiang palaeo-uplift,it was believed that the"exposed"palaeo-uplift in central Sichuan was the foundation of Caledonian-Hercynian karstification.The dissolution holes formed in the early stage were favorable channels of karst water for the later superposition reconstruction.At the same time,Caledonian-Hercynian extensional fault was favorable for the development of Caledonian-Hercynian bedding-parallel karst.The differential uplift was the main controlling factor for the reconstruction of karst since the period of Yanshanian.(4)The filling fluid properties of syngenetic karst in northern Sichuan and penecontemporaneous karst in southeastern Sichuan and the differential preservation mechanism of karst reservoir space was discussed.It was considered that the pores formed by syngenetic karst in northern Sichuan were filled and compacted by two phases of dolomite before the migration of petroleum,two phases of dolomite fluid were affected by meteoric freshwater.In the shallow burial stage,the early fluid rich in Ca Mg(CO₃)₂was presented in relatively closed and reductive environment,and the late fluid rich in Ca Mg(CO₃)₂was presented in relatively open system affected by Caledonian-Hercynian fracture.The karst layer of Longwangmiao Formation in eastern Sichuan and western Hunan-Hubei province shows a relatively weak filling state,which was related to acid fluid entered the penecontemporaneous karst at the deep burial stage.(5)In eastern Sichuan and western Hunan-Hubei area,favorable reservoir development areas were located in microfacies of grain-beach of Songtao-Rongxi,Sangzhi-Cili at platform edge,Xishui-Jinsha area at southeast Sichuan in the inner part of platform and Fengdu-Zhongxian area in the inner part of Sichuan Basin;in northern of Sichuan,the favorable reservoir development areas were located in the microfacies of grain-beach in Xichong-Bazhong areas,the microfacies of grain-beach controlled by Dazhou-kaijiang paleo-uplift and the microfacies with high energy in Chengkou-Wuxi area in the northern margin;in central Sichuan,favorable reservoir development area was located in axis part of the paleo-uplift and the karst area along the slope;in southwestern Sichuan,the favorable reservoir development areas were located in the area from well Laolong-1 to well Gongshen-1.