The Hydrocarbon Accumulation Characteristics Of Mesozoic Volcanic Rocks In Fulin Sag

Volcanic rocks are exposed extensively on the top of Mesozoic in Fulin Sag,and their reservoir property has been improved to a certain degree by weathering.The volcanic reservoirs and the oil-generating Shahejie Formation are well contacted and have great potential for exploration.However,the discovered reservoirs are small in size and far from the expected breakthrough.The hydrocarbon accumulation characteristics and controlling factors of Mesozoic volcanic reservoirs in Fulin Sag are urgent to be researched.Under the guidance of the theory of petroleum geology,this paper analyzes the fault characteristics and tectonic evolution,volcanic rock characteristics and distribution,volcanic reservoir characteristics and distribution,hydrocarbon accumulation characteristics,etc.comprehensively using the data of core,thin slice,seismic,well logging and organic geochemistry and so on.The results show that:The Mesozoic tectonic evolution of Fulin Sag was developed on the basis of the evolution of the entire Zhanhua and even Jiyang Depression.During the Indosinian period,the development of Chengnan-Guxi thrusting tectonic belt uplifted the area of Fulin Sag.In Early Jurassic,the study area was a stable shallow lake,and Fangzi Formation characterized by shallow lake-river swamp facies were deposited,with few volcanic rocks.In Lake Jurassic,the lake became smaller,and the purple-red clastic rocks of Santai Formation were deposited,with limited distribution of volcanic rocks.In Late Jurassic-Early Cretaceous,the Indosinian NW thrusting tectonic belt negativly reversed and the volcanic eruption was strong,forming thick volcanic rocks and pyroclastic rocks in Mengyin and Xiwa Formations.At the end of the Mesozoic,the Late Yanshan Movement caused an overall uplift of the Bohai Bay Basin,and the Mesozoic volcanic rocks of Fulin Sag was weathered and leached,causing the improvement of the property of volcanic rock reservoirs.The present-day tectonic outline of Fulin Sag was mainly formed in Cenozoic.The Mesozoic volcanic rock fault-block was furtherly complicated.The source rocks of the fourth and third members of Shahejia Formation were deposited,and the Mesozoic volcanic rock reservoir experienced a hydrocarbon accumulation process during the deposition of Minghuazhen Formation.The Mengyin and Xiwa Formations of Lower Cretaceous in Fulin Sag mainly consists of andesites,andesitic pyroclastic rocks and some subvolcanic rocks.The volcanic facies can be divided into overflow,eruptive and volcano-sedimentary facies.The combination of volcanic facies can be divided into overflow?eruption facies,overflow?volcano-sedimentary facies,explosive? volcano-Sedimentary facies,overflow?eruption? volcano-sedimentary facies.The Mesozoic unconformity is characterized by regular lithology changes: primary volcanic rock?weathering volcanic rock?basal conglomerate with volcanic rock gravels?fluvial facies glutenite from the bottom up.The corresponding logging response characteristics are obvious.It is speculated that two volcanic craters existed near the present area of Well Fu 111 and Fugu 4 in the Guxi Fault Zone.The thickness of volcanic and pyroclastic rocks on both sides gradually decreases,and the thickness of sedimentary strata increases.Various fractures such as high-angle fractures,network fractures,grain-edged fractures and few intergranular pores are developed in the tuff.Various pores such as intergranular dissolved pores,intragranular dissolved pores,phenocryst dissolved pores and few network fractures and grain-edged fractures are developed in volcanic breccia.Various fractures and pores such as high-angle fractures,microfratures and intergranular dissolved micro-pores,phenocryst dissolved pores are developed in andesite.Reservoir properties are generally poor.The permeability and porosity of tuff are relatively good,and those of andesite are poorer.The Mesozoic volcanic reservoirs experienced syngenetic,supergene,shallow burial,uplifting exposed and deep burial evolution stages.The effective reservoirs are basically distributed within 50 m below the top of the Mesozoic unconformity,and are mainly distributed near the faults laterally.The seismic phase-controlled nonlinear random inversion method can predict good reservoirs about 5 m thick and predict its distribution range,space distribution and occurrence.The probability index and the comprehensive index calculated from the value of the well logging curve can predict the fracture development sections in tuff.The both methods can accurately predict the reservoir sections in Well Fu 117 and Fu 121.Hydrocarbon accumulation characteristics of the Mesozoic volcanic rocks in Fulin Sag are near-source,late-accumulation,and “young source in old reservoir”.The source rocks have a low degree of thermal evolution,and hydrocarbon generation begins during the deposition of Minghuazhen Formation.Except for the eastern and middle sections of the Kenli Fault and a few secondary faults internal the sag,which maintain a certain degree of activity during the hydrocarbon generation period,other faults basically have no transporting capacity.The main trap types are buried hill-fault block traps,buried hill-inner reservoir traps,unconformity-cap traps and structural-unconformity traps.These traps are formed mainly during Cenozoic evolution period and finalized during the deposition of the Dongying Formation,which is late than the hydrocarbon generation period,favoring hydrocarbon accumulation.According to the different hydrocarbon accumulation characteristics in the different structure position,the oil hydrocarbon accumulation patterns are divided into three types: steep-slope belt pattern,central-sag belt pattern,and gradual-slope belt pattern.According to the different hydrocarbon accumulation characteristics,this paper decide to deploy the exploration of the inner reservoir in the area of Well 93-94 in the steep-slope zone,and to deploy the exploration of expanding the hydrocarbon-bearing range and the confirming the oil-pool types in the area of Well 117-29 in the central sag.