Sedimentary And Reservoir Characteristics Of The Cambrian Longwangmiao Formation In The Yibin Area,Southern Sichuan Basin

Based on field investigation,core observation and testing methods,this study integrates a variety of data including thin sections,scanning electronic microscope,cathodoluminescence,seismic survey,mercury intrusion and physical property data.Eventually this study conducts a systematic research on stratigraphy distribution characteristics,sedimentary characteristics and facies distribution,reservoir petrological features,physical properties and pore structure.Based on reservoir distribution pattern,the study further analyzes the reservoir controlling factors,establishes classification and evaluation standards and predicts the distribution of high-quality reservoirs.The study results indicate that: 1.Longwangmiao Formation in the Yibin area can be divided into two members.The lower member mainly consists of crystal-power dolomite,the upper member develops gypsum dolomite,grainstone and doloarenite with thin layers of carbonate and siliciclastic mixed deposits.The stratum thickness ranges between 65 m?130m with an average value of 112 m,the upper formation is around 58 m thick and the lower formation thickness is nearby 61 m.Overall this formation tends to be thick in the southeast and thin in the northwest in the Yibin area.2.During the deposit period,the Longwangmiao Fm.is dominated by mixed-deposits restricted rimmed-platforms depositional system.The environment has weak wave-reworking and poor circulation.Based on lithology and other characteristics,there are five types of sedimentary subfacies:peritidal,shoal,lagoon,bank and sabkha 3.Doloarenite and oolitic dolomite are the primary reservoirs,followed by fine-middle crystalline dolomite and dissolved crystalpower dolomite.There are various types of storage space in reservoirs,for example,intercrystallite pores and dissolved intercrystallite pores as main storage space,with some intergranular pores,dissolved intergranular pores and dissolved gypsum pores.Cracks caused by tectonic movements are relatively less developed.Combined with physical property data,the reservoir has overall low porosity and permeability,regional high porosity and permeability.4.Reservoir development and distribution are mainly controlled by three factors: sedimentary environment,karstification,and geomorphology as the major control factor.Longwangmiao formation gently slope towards southeast,and can be further divided into four types of paleogeomorphic units: uplift areas,slope areas,depression areas and underwater uplifts.Influenced by the geomorphology features,grain beach with primary pores are mainly developed in slope areas.In addition,extensive secondary intergranular dissolved pores and intercrystalline dissolved pores are formed by bedding karstifications,which makes the paleomorphic slope area a favorable area for carbonate reservoirs to develop.Overall,the distribution of reservoirs has significant features: horizontally,reservoirs develop in the karst slope area in north region of the study area where shoals are concentrated;vertically,reservoirs are more developed in the upper section of Longwangmiao formation.