Characteristics And Mechanism Of Cretaceous Volcanic Reservoirs In Huanghua Depression

Volcanic rocks are one of the important reservoir types of Mesozoic in Huanghua Depression.Volcanic rocks are also an important horizon for increasing oil and gas reserve and production in Mesozoic in Dagang prospect area.However,the study of Mesozoic volcanic reservoirs in the exploration area is relatively weak.Based on logging,drilling,seismic profile,core and cuttings,oil testing and production testing data,The petrological characteristics and development regularity,reservoir characteristics and reservoir controlling factors of Cretaceous volcanic rocks in Huanghua Depression are studied by means of analytical testing methods which includes core observation,casting thin section identification,conventional porosity and permeability test,mercury injection analysis,whole rock geochemical analysis.On this basis,the types of volcanic reservoirs are divided,the diagenetic evolution process and reservoir formation mechanism of different types of volcanic reservoirs are analyzed,and the reservoir formation models of different types of volcanic reservoirs are established.Differential analysis of the formation of different types of volcanic reservoirs can effectively guide the exploration of Mesozoic volcanic rocks buried hill reservoirs in Huanghua Depression.The results show that:(1)The Mesozoic volcanic rocks in Huanghua Depression are concentrated in the Early Cretaceous and limited in the Late Jurassic.The lithology of Early Cretaceous volcanic rocks is complex,which can be divided into 3 major types and 10 sub-types.The lithofacies is diverse and can be divided into 4 lithofacies and 7 sub-facies.The lithology is dominated by volcanic lava type,andesite and basalt are the most common.The lithofacies is dominated by the slab lava flow subfacies of overflow facies.Volcanic activity can be divided into two cycles and four stages.(2)Volcanic reservoir space types can be divided into three categories: primary pore,secondary pore and fracture,and can be further subdivided into 13 sub-categories.Among them,almond inner pore,secondary dissolution pore,structural fracture and weathering fracture are the most common.Generally speaking,the reservoir belongs to medium porosity and low permeability reservoir with strong heterogeneity.The capillary pressure curves of reservoirs are divided into three types: type?belongs to platform type,type?belongs to long slope type,type ? belongs to short slope type.Reservoir diagenesis is complex,including 12 types.The diagenesis stage of reservoir can be divided into four stages in turn: the cooling and consolidation diagenesis stage,the magmatic hydrothermal stage,the supergene diagenesis stage and the burial diagenesis stage.Among them,tectonism and weathering-leaching of supergene period,filling and dissolution of burial period have great influence on the physical properties of reservoir.(3)The core and MFW geochemical weathering indices show that the volcanic rocks are weakly weathered on the whole.CIA,CIW,WIP and other weathering indices in the study area change regularly vertically.Volcanic reservoirs of different lithological and lithofacies types have different oil-bearing properties and reservoir-forming probability.The Cretaceous andesite in Huanghua Depression has the greatest reservoir-forming probability and is easiest to form reservoirs.(4)Lithology,lithofacies type,weathering-leaching and tectonism are the main factors affecting the reservoir performance of Cretaceous volcanic reservoirs in Huanghua Depression.The dominant lithology of volcanic reservoir development in the study area is andesite,and the dominant lithofacies is plate lava flow subfacies of overflow facies.Volcanic reservoirs near unconformities and large faults have the best reservoir performance.(5)Cretaceous volcanic reservoirs in Huanghua Depression are divided into three types:primary pore type,weathering-leaching type and buried dissolution type.The diagenetic evolution process and reservoir-forming model of each type of reservoir are different.The primary pore type reservoir is mainly controlled by the primary diagenesis,which retains a large number of primary pore.This kind of reservoir is characterized by undeveloped faults and stable stratigraphic distribution.Weathering-leaching type reservoirs are close to unconformities.The reservoir are exposed to the surface after being uplifted by tectonism.Then it is subjected to weathering and leaching,resulting in the formation of structural fractures,weathering fractures and leaching and dissolution pores.Buried dissolution type reservoirs are close to large faults,which are directly buried after uplifting by tectonism.During the burial period,organic acid fluids of Paleogene entered the interior of volcanic rocks along bedding planes and large faults.It reacted with soluble minerals in volcanic rocks to form a large number of dissolution pore.