| The Cambrian system in Sichuan Basin consists of marine facies carbonatite sediments with a large stratigraphic thickness.Exploration lasted for more than 60years and no major breakthroughs have been achieved.Until 2012,when a major breakthrough was achieved in Moxi area in Central Sichuan.By the end of 2017,the number of wells with a production capacity of one million cubic meters in Longwangmiao Formation in the area exceeded 30,and the proven geological reserves amounted to 440 billion cubic meters,making it an extremely large-scale marine facies carbonate rock gas field.After the breakthrough of the Longwangmiao Formation in the Moxi area was achieved,a lot of drilling and core data revealed that the Longwangmiao Formation in the area is a set of thick layer dolomite deposits with20-70 m thick granular beach deposits and a large exploration potential.Based on the previous research results,and guided by the theory of sedimentology,stratigraphic paleontology,geochemistry,geophysics,petroleum geology,etc.,and based on the fine description of the 653m cores taken from 12 wells,combined with observation on103 pieces of thin sections,systematic studies have been carried out as introduced in this paper on the precipitation facies characteristics,lithofacies palaeogeographic evolution,reservoir types and main controlling factors of development of Cambrian Longwangmiao Formation in central Sichuan Basin using 27,000 km 2D seismic data and 6,000 km~2 3D seismic data,and the following four research progresses have been obtained:Based on the drilling and core data,the lithofacies palaeogeographic characteristics of the Longwangmiao Formation in central Sichuan Basin are studied.It is believed that the Longwangmiao Formation of the Cambrian system in Sichuan Basin consists of carbonate-striped platform deposits;restricted platform sediments including three subfacies,namely grain beach,dolomite tidal flat and interband sea subfacies.To the westward are the mixed shelf deposits and to the east are the lagoon deposits.The lithofacies palaeogeography of the Longwangmiao Formation in central Sichuan Basin is closely related to the palaeogeographic pattern formed by the tectonic movement in the Tongwan period.Due to the extension action in the Tongwan period,a large-scale intracratonic rift was developed in Mianzhu-Changning area,and in the eastern part of the rift,a large-scale underwater paleo-uplift was formed in the central Sichuan area.The paleo-uplift had a relatively high landform and strong sedimentary hydrodynamics,making Longwangmiao Formation deposited with large-area distributed thick layer of granular beach facies with a thickness of20-70m.Combined with substantive seismic data,the characteristics and periods of fault distribution in central Sichuan Basin are described.The control effects of micro-geomorphology formed by faults of different periods on the distribution of beach bodies are analyzed.It is believed that multi-periods of synsedimentary faults are developed in central Sichuan Basin,controlling the sedimentation and distribution of granular beach facies in the Longwangmiao Formation.Faults are extremely developed in the central Sichuan region,mainly with approximately the north-south,NEE,NWW direction distribution.Combined with seismic section analysis,it is found that three stages of faults are main developed in the Longwangmiao Formation.The first stage and second stage faults are controlled sedimentary faults,and the third stage faults are related to oil and gas enrichment.The first stage North-South direction basal fault controlled the structural pattern of rifts and sags in the central and western Sichuan Basin as well as the formation of the underwater palaeo-uplifts in the central Sichuan Basin.The paleo-geomorphology was relatively high,which laid the foundation for the large area sedimentation of granular beach facies in central Sichuan.The active period was Sinian-early Cambrain;the second stage faults with an approximately east-west direction controlled the differential sedimentation of beach bodies in the Longwangmiao Formation in the palaeo-uplift.The faults of this stage formed multiple local ancient landform highs in central Sichuan Basin,with an ancient landform relatively high in both sides of the faults but low between the faults.The water was relatively deep and the hydrodynamic force is weak in this sedimentary period,forming interbank sea sediments,and the reservoirs are dense,splitting central Sichuan Basin into four large granular beach facies sedimentation zones,namely Gaoshiti,Moxi,Shehong-Nanchong,Yanting-Xichong zones.The fault active period was Early Cambrian-Early Permian;the third stage of faults controlled the enrichment of oil and gas.They are mainly approximately east-west direction faults accompanied by the northwest direction faults,which divided Gaoshiti-Moxi area into multiple small ancient landform highs.Oil&gas can be easily enriched in these ancient landform high positions.The fault active period was Permian.The basic characteristics of the Longwangmiao Formation reservoirs in central Sichuan Basin are analyzed,and the reservoir types are classified.The high-quality reservoir rocks in the Longwangmiao Formation are dominated by sand dolomite and fine-coarse grained dolomite.The reservoir space is solution cavity,intergranular dissolved pores,crystalline dissolution pores and intercrystalline pores.Among them,a large number of small and medium-sized pores can be seen,and they are characterized by medium-low porosity and medium-high permeability.According to reservoir space difference combined with analysis on the relationships between porosity and replacement pressure,medium value pressure,maximum throat radius and medium value throat radius,the reservoirs in the Longwangmiao Formation can be classified into three types,namely dissolution cavern type,dissolution pore type and matrix pore type.Dissolution cavern type:macroscopically,it is dominated by solution cavity.On the electrographic image,dark black patches are found,and vertical fractures are developed.Dissolution pore type:macroscopically,pinholes are developed,with solution cavities occasionally found.On the electrographic image,dark black pitting can be found,with horizontal fractures and vertical fractures developed.Matrix pore type:macroscopically,no pores can be found,and logging interpretation results are effective reservoirs with horizontal fractures and diagenetic fractures developed;the four types of fractures,namely the high angle structural joints,horizontal fractures,map fractures,vertical diagenetic fractures and suture lines,and the three types of reservoirs formed an integrally connected fracture and cavity system.Based on the comprehensive study on seismic,core,thin section and massive carbon&oxygen isotopic data,a four-element control mechanism is proposed.It is believed that the formation of the dolomite reservoirs between beaches in the Longwangmiao Formation in central Sichuan Basin was mainly controlled by four factors:namely,the paleo-uplift,paleo-sea level,paleo-environment and multi-stage karstification in central Sichuan Basin.The paleo-uplifts in central Sichuan Basin laid a foundation for the sedimentation of granular beach facies,making the large area distributed granular beach facies developed in central Sichuan Basin.The Longwangmiao Formation experienced multiple periods of sea level rises and falls.The frequent vibration of seawater provided sufficient nutrients for the growth of the organism,which was conducive to the development and exposure of the beach bodies,and the development of large-scale quasi-syngenetic karstification.The Longwangmiao Formation of the Cambrian in central Sichuan Basin was under a sea water temperature of around 18°C to 28°C,and a salinity value of around 118 to 125,with an average of 123.At the same time,it is adjacent to the sedimentary environment of the Gypsum rock lacustrine facies,which is beneficial to the dolomitization of the beach bodies.In the late period of the granular beach facies of Longwangmiao Formation,it experienced weathering crust karstification and burial dissolution to form high-quality reservoirs with dissolved pores developed.The dolarenite experienced three stage of denudation namely,quasi-syngenetic,epigenetic and burial periods.The needle-like dissolution pores are closely relate to the first stage syngenetic~quasi-syngenetic denudation.The large denudation caverns are related to the second stage weathering crust karstification,and the late period expanded dissolution fractures and pores are related to the third stage burial dissolution;The medium-fine grained dolarenite granular beach facies formed three reservoir types:dissolution cavern type,dissolution pore type and matrix pore type,to form high quality reservoirs with fractures and cavities developed;the three types of reservoirs are laterally stacked and connected with a large area distribution,extending5-20km,with sound interwell connectivity. |
PDF Full Text Request