Characterization And Development Model Of Archean Metamorphic Buried Hill Reservoirs In The Bohai Sea Area

In recent years,a number of large-scale oil and gas fields have been discovered in the Bohai Sea area,showing the great potential of the buried hill area,and the buried hill oil and gas reservoir has gradually become an important target of oil exploration in the Bohai Sea.The number of buried oil and gas fields with metamorphic rocks as reservoirs in the sea area is small,and ultra-deep metamorphic rock buried hill reservoirs above 4500 meters are even rarer.Compared with other rock types and shallow reservoirs,this type of reservoir is relatively poorly studied,and due to the early formation time,large burial depth,complex diagenesis and diverse tectonic events,the ultra-deep metamorphic rock-latent mountain reservoir has a high degree of heterogeneity and is difficult to explore.The scale of metamorphic latent mountain reservoir is a key factor restricting metamorphic rock exploration,and the implementation of the formation conditions and development law of large-scale submerged mountain reservoir has important reference value and guiding significance for the further exploration and development of deep-ultra-deep oil and gas reservoirs.This study focuses on the buried hill reservoirs of Archean metamorphic rocks in the BZ19-6,BZ13-2,QHD30-2,QHD35-1,and QHD36-1 tectonic areas of the Bohai Sea area.Through core and wall observation,casting thin sections,scanning electron microscopy identification,and imaging logging data,the petrological characteristics,physical properties,and reservoir spatial characteristics of the reservoirs in the study area were analyzed;Based on the production capacity test data and physical property test results of multiple drilling encounters with the Archaean metamorphic rock buried hill in the Bohai Sea area,the Archaean metamorphic rock buried hill reservoirs in the Bohai Sea area are divided into 3 major categories and 6 subtypes using mathematical function fitting;Based on the results of high-pressure mercury injection,nitrogen adsorption,nuclear magnetic resonance,and rock CT testing,the reservoir space composition and pore structure characteristics of six types of reservoirs were analyzed;Analyzing the formation mechanism of reservoirs in the study area by comparing the characteristics of six types of reservoirs;Based on the differences in imaging logging fracture characteristics and dissolution reservoir space characteristics,combined with conventional logging response characteristics,the division and identification of weathered crust and buried hill interior structural units in the study area were achieved;Based on the vertical development characteristics of single well reservoirs in 27 wells,six types of reservoir structural unit combinations in the study area were summarized,and the vertical distribution patterns of high-quality reservoirs in three types of buried hills were determined;Based on the above research,an analysis was conducted on the control effects of lithology,structure,fluid,and paleogeomorphology on reservoirs,establishing three high-quality reservoir development models,and conducting planar prediction of reservoirs in the study area.Insights are as follows:1.Basic characteristics of metamorphic rock submerged mountain reservoirs in the Bohai Sea area.There are two types of rocks in the Archean metamorphic rock submerged mountains in the Bohai Sea area,namely metamorphic rocks and intrusive rocks.The development of mixed granite,mixed gneiss and gneiss laid a solid foundation for the formation of metamorphic latent mountain reservoirs in the sea area,and the extensive development of intrusive rocks proved that large-scale magmatic activity existed in the study area.The average porosity of the reservoir in the study area was 4.23%,and the average permeability was 0.99×10^-3μm²,and the overall performance was medium and low porosity-low permeability.The reservoir space has the characteristics of dual medium,with intergranular dissolution pores in pores,dissolution pores in primary mineral particles,dissolution pores in secondary mineral granules,residual pores and micropores.Fractures develop tectonic fractures,dissolution fractures,intragranular microfractures and intergranular microfractures.2.Classification and qualitative characterization of metamorphic buried hill reservoirs in the Bohai Sea areaThe reservoirs of Archean metamorphic rocks in the Bohai Sea can be divided into3 categories and 6 subcategories,with porosity greater than 8%or permeability greater than 1×10^-3μm² as type I reservoirs,and type II reservoirs with porosity of 5%-8%or permeability of（0.1-1）×10^-3μm².Porosity 2%-5%or permeability between（0.01-0.1）×10^-3μm² are type III reservoirs,each type of reservoir can be further divided into fracture-pore type and fracture type according to the relative size of porosity and permeability contribution to the reservoir two subcategories.From type I.to type III.,the dissolution and fracture development degree gradually weakened,and the average values of dissolution porosity of fracture-pore reservoir were 4.02%,2.84%and 0.77%,respectively.The average fracture densities of fracture imaging logging were 1.8pieces/m,1.26 lines/m and 0.87 lines/m,respectively.3.Quantitative characterization of pore structure of metamorphic buried hill reservoirs in the Bohai Sea areaThe high-pressure mercury capillary pressure curve has a large morphological distribution range,a large distribution range of pore structure parameters,and a wide distribution of pore throat radius.The adsorption isotherms were all between type II.and type III.,and the hysteresis loop showed the characteristics of H₃ and H₄.The T₂value of NMR samples was mainly distributed between 0.1ms～1000ms.The curve morphology is mainly bimodal,and unimodal and trimodal peaks are also developed,and the peak height is different.These characteristics reflect the complex pore structure of metamorphic rock reservoirs in the study area and strong heterogeneity.The reservoir is from type I.to type III.,the mercury inlet curve of the high-pressure mercury capillary pressure moves from the lower left like the upper right,the discharge pressure gradually increases,the maximum mercury saturation gradually decreases,the radius distribution of the pore throat transitions from a wide single peak to a narrow bimodal,the pore volume gradually decreases,and the proportion of large pore size gradually decreases.The nitrogen adsorption capacity of the nitrogen adsorption isothermal adsorption curve under equilibrium pressure gradually decreased,the area of the lagging loop gradually decreased,and the pore volume corresponding to the same pore size gradually decreased.Under the same relaxation time of NMR,the porosity component gradually decreased,the macroporous content gradually decreased,and the saturation of movable fluid decreased.4.Vertical structural combination style and high-quality reservoir development pattern of metamorphic buried hill reservoirs in the Bohai Sea areaThe metamorphic rock reservoir is divided into weathered clay belt,weathered sand and conglomerate belt,weathered dissolution zone,weathered disintegration zone,fracture zone,fracture zone and fracture dense zone.There are six structural unit combinations of the three types of submerged mountains in the study area,namely"weathered gravel belt+weathered dissolution zone+weathered disintegration zone+inside"type,"weathered sand and conglomerate belt+weathered disintegration zone+inside"type,"weathering dissolution zone+weathering disintegration zone+insider"type,"weathered disintegration zone+insider"type,"weathered clay belt+weathering disintegration zone+insider"type,and"insider"type.The weathered gravel belt（Univariate Hidden Mountain）,fragmentation zone and weathering dissolution zone are the dominant development zones of fracture-pore reservoirs,while weathered disintegration zones and thick fracture dense zones are dominant development zones of fracture-type reservoirs.Univariate Hidden Mountain has developed high-quality reservoirs in both weathered crust and insider,and the contribution rate of single well weathered crust and insider to the reservoir under different geological conditions is different.The high-quality reservoirs of Binary Hidden Mountain are mainly contributed by insiders,and there are many Class II.reservoirs developed in the weathered crust.The ternary submerged mountain reservoir does not develop weathered crusts,and the reservoirs are all contributed by insiders.5.Control factors and development patterns of metamorphic rock latent mountain reservoirs in the Bohai Sea areaDifferent types of structures have different lithological foundations,different tectonic activities,and different uplift and burial times,resulting in different reservoir formation and evolution processes.The control factors of metamorphic rock latent mountain reservoirs in the Bohai Sea include four aspects:lithology,structure,fluidity and paleolandform.The development of mixed granite,mixed gneiss and gneiss has laid a good foundation for the reservoir development of the Bozhong Tectonic Area,and the extensive development of metagranulite in the Qinhuangdao Tectonic Area is one of the important factors restricting the formation of reservoirs.The structure is not only the dynamic condition for the formation of fracture reservoir space,but also the channel for communicating fluid and bedrock water-rock reaction dissolution and pore expansion gap.Atmospheric freshwater leaching develops weathered crust reservoirs for a long time,and the pores and cracks that occur in the early stage of organic acid activity transformation greatly improve the storage and seepage capacity of the reservoir.There are three reservoir development modes in the Archean metamorphic rock submerged mountains in the Bohai Sea:mode 1"atmospheric freshwater+organic acid"mode,mode 2"atmospheric freshwater+structure"mode,and mode 3"structure+organic acid"mode.Mode 1 and mode 2 developed in the BZ19-6 structural area,and mode 3 developed in the BZ13-2 binary submerged mountain and in QHD tectonic Area the ternary submerged mountain.