Influence Mechanisms Of Diabase Intrusion On Reservoir Quality Of Clastic Wall-rocks

Magmatism influences or modifies wall-rocks,thereby forming magmatic and wall-rock hydrocarbon reservoirs.Clastic rock reservoirs influenced by magmatism are new exploration target in hydrocarbon sucessions in eastern China.Therefore,it is of great significane to investigate the controlling mechanisms of magmatic activities on their wall-rocks.This dissertation takes the case studies from the Gaoyou sag and an outcrop from southern Songliao Basin,aiming to explore magmatic influences on reservoir quality of wall-rocks.Based on petrologic,seismic,and well-log analyses,the reservoir spaces and properties of intrusive diabase and contact metamorphic rocks are investigated in the Funing Formation?T₁f?of northern slope in the Gaoyou sag,and also the diagenesis of intrusive-metamorphic reservoir complex is discussed.For the outcrop of Tuanshanzi quarry located in southern Songliao Basin,the diagenetic and metamorphic features of wall-rocks influenced by diabase emplacement are investigated,and subsequently,their porosity and permeability characteristics as hydrocarbon reservoirs are studied on the basis of dense and consecutive sampling.On the basis of previous investigations,the controlling mechanisms of the reservoir development is discussed.As a result,comprehensive reservoir models of contact metamorphic rocks are proposed based on previous analyses.The study on core samples from T₁f of northern slope in the Gaoyou sag reveals that the impermeable mudstones are altered into permeable metamudstones,including intermediate-metamorphosed hornfelses and low degree-metamorphosed slates after contact metamorphism,with numerous fractures and microfissures being main factors of reservoir quality enhancement.Similarly,the contact sandstones are modified into metasandstones,whose porosity types are similar to those in normal sandstones.The reservoir porosities mainly include primary intergranular pores,dissolved intergranular pores,dissolved intragranular pores,moldic pores,ultralarge pores,and dissolved micropores.The diagenesis of intrusive-metamorphic reservoir complex can be divided into four stages according to different controlling factors,including solidification,hydrothermal breaking,intensvie erosion and burial diagenesis stages,and the corresponding controlling factors include crystallization and metamorphic crystallization,filling and metasomatic erosion,acidic dissolution,and burial compaction.The tectonic fracturing is persistent throughout all diagenetic stages.The hand specimens from the Tuanshanzi quarry indicates that contact hornfelses influenced by diabase emplacement have characteristics in several aspects:1)The sericite content is decreasing with increasing distances from diabase intrusion;2)Due to heat influence caused by diabase intrusion,the clay matrices are characterized by development of illite and chlorite,which commonly represent later-stage diagenesis;3)Vesicle-amygdule structures,which are filled with laumontite,develop exclusively in hornfels;4)Microfissures are developed with carbonate veins.Diagenetic and metamorphic features of overlying metasandstones influenced by diabase intrusion are characterized by 1)characteristic distribution of authigenic quartz,2)uneven distribution of clay minerals,and 3)forming characteristics structures reflecting mechanical pressure.The formation of reservoir porosities in contact metamudstones is mainly controlled by diabase emplacement,with controlling mechanisms including physical fracturing,hydrothermal activity,cooling contraction,and dissolution.Compared with T₁f contact metamudstones which are potential hydrocarbon reserovoirs,hornfels from Tuanshanzi quarry exhibit no permeability potential,although developing porosity at certain extent.It is mainly because that the occurrence of laumontite and depleted kaolinite reflect alkaline diagenetic environment,which prevents calcite dissolution in the microfissures.By contrast,metamudstones from T₁f of the Gaoyou sag show favorable reservoir potential due to intensive dissolution of carbonate veins induced by hydrocarbon transformation and possible meteoric leaching.The reservoir quality of the contact sandstones is primaryly controlled by lithofacies and diagenesis before intrusion.However,later-stage diabase intrusion exerts considerable influences on the contact sandstone by introduceing physical pressure,dissolution,and cementation.Although dissolution increases porosity moderately,it insufficiently compensates for the porosity reduction caused by physical pressure and cementation.Thus,the reservoir quality of the contact metasandstones is generally decreased by diabase intrusion.Comprehensive reservoir models of contact metamorphic rocks are proposed based on the preceding analyses,which reveal characteristic distribution of petrophysical properties in the contact metamorphic reservoirs:in metamudstones,metamorphic degree,hydrothermal fluid breaking,and physical stressing decrease gradually with increasing distances from the diabase intrusion,thereby causing reservoir quality decrease.In metasandstones,physical pressing and microcrystalline quartz filling cause reservoir quality decrease at location adjacent to diabase intrusion.At certain distances from diabase,reservoir quality is enhanced at certain extent due to hydrothermal dissolution.At locations distal to diabase intrusion,reservoir quality is then decreased by authigenic quartz and/or clay minerals cementation due to slackened hydrothermal flowing.