Volcanic Stratigraphy And Tectonic Control Of Yingcheng Formation, In Songliao Basin

Yingcheng formation is one of the mainly reservoir beds in oil/gas explorationand development in Songliao basin. It developed in different fault depressions,including both volcanic strata and clastic strata. The seimic reflectances of both typesof strata have something similarity in seismic frofiles, and the surfaces areuncomformity surfaces. It is a basic geological problem in oil and gas exploration thatthe stratigarphy and archetecture of volcanic srata correlating in the different faultdepressions and in the different parts of the same depression or with the clastic stratadeveloped during the same time. The precision of this research influences theprogresses of the oil and gas exploration and development.This study is aimed at volcanic strata of Yingcheng formation in eastern marginand Xujiaweizi fault depression of Songliao basin combining geology withgeophysics researching dividion,correlation, mechanism of volcanic cycle and statgeof Yingcheng formation using outcrops data, drilling data, logging data and seismicdata. By making balanced cross sections and drawing fault displacement curves, toanalyse the tectonic control of the growth of volcanic strata. Through analysis of thevolcanic sructures and ranks to discuss the development of volcanic basin inXujiaweizi fault depression.The main achivements are followings in this paper:1. The main rock types of Yingcheng formation in Songliao basin are lava andpyroclastic rocks. Lava rocks are with5types including rhyolite, andesite, basalt,perlite, pitchstone and dacite. Pyroclastic rocks are with10types including breccialava, tuff lava, ignimbrite breccia, ignimbrite tuff, agglomerate lava, cryptoexplosion breccia, tuff, breccia, agglomerates, tuffites, tuffaceous sedimentary rocks.2. According to the measured and collected dating data, the time spans for themembers of Yingcheng formation and volcanic cycle are given. It is120-134Ma formember1of Yingcheng formation, it is115-120Ma for member2of Yingchengformation, and it is110.7-115Ma for the member3of Yingcheng formation. It iswithin10Ma for the time spans of volcanic cycles of Yingcheng formation, and thechronostratigraphic framework with1-10Ma scale is established.3. The volcanic facies of Yingcheng formation is classified into5facies and15sub-facies. The facies are volcano vent, explosive, effusive, extrusive and volcanicsedimentary. Volcano vent includes volcanic neck, subvolcanic rocks, hiddenexplosive breccias. Explosive includes airfall, base surge, pyroclastic flow. Effusiveincludes lower, middle and upper effusive sub-facies. Extrusive includes inner,intermediate and extrusive sub-facies. Volcanic sedimentary facies includes thepyroclastic deposit containing debris re-transportation pyroclastic deposit and tuffwith coal intercalation sub-facies.4. The cooling unit or flue/cumulative unit is the basic original unit of volcanicstrata in Yingcheng Formation and this basic unit is using in deviding volcanic strata,developping in considerable scale and scope with a certain geometric featuresreflecting forming mechanisms. There are four types of cooling unit of Yingchengformation in the outcrop areas in the southeast margin Xujiaweizi fault depression ofSongliao basin, including pyroclastic, lava, pyroclastic+lava and lava+pyroclastic.5. Volcanic eruption stage (strata of sub cycle) is stacked by cooling units in thespatial and temporal context. There are7types of volcanic stages of YingchengFormation in Songliao basin, including vapor explosion, lava explosion, volcanicsedimentary, vapor-lava explosion, lava-vapor explosion, vapor-volcanic sedimentaryand vapor-lava-volcanic sedimentary.6. Volcanic cycle(strata of cycle) is stacked by volcanic stages(strata of subcycle)) in the spatial and temporal context. Its time span is within10Ma as the thirdorder sequence. There are3types of volcanic cycles of Yingcheng formation in Songliao basin,including clastic type, lava type, and mixed type of clastic and lava.7. The strata both of volcanic cycles and volcanic stages of Yingcheng formationare confined by unconformities. The volcanic strata of member1is divided into3volcanic cycles and9volcanic stages, and the volcanic strata of member3is dividedinto4volcanic cycles and9volcanic stages.8. The fault displacement and rate curves of Yingcheng Formation areestablished. Based on these curves and the regular devepment pattern of theextensional basin, the forming and development of the volcanic stratagraphy both instages order and cycles order are all controlled by the faulting.The fault activities caused the volcanic eruption, and the tectonic subsidence andthe collapse of craters controlled conservation of the volcanic rocks. The verticaldisplacement determined the tectonic subsidence of the basin, and the horizontalthrow determined basin stretching. Both also determined the range of the basin byfault dip and its changes or the geometry of fault plane. The important causes of theconservation of volcanic strata were tectonic subsidence and the collapse of craters.Only a suitable settlement for subsidence of basin and accumulation of volcanicmaterials could preserve comparatively complete volcanic strata. If volume ofvolcanic materials were larger than the extension space, the volcanic rocks could beeroded severely. On the contrary, that could be well preserved, while the clasticsequence could be formed and the volcanic rocks could exist as systems tracts orinterlayers.9. Volcanic structures can be distinguished into three classes: â…¢ is volcanicedifice, â…¡ is volcanic edifice assembly and â… is volcanic edifice assembly group.The volcanic edifices of Yingcheng formation can be recognised four mainly types,including shield volcano, composite volcano, pyroclastic cone and lava dome. Amongthem, â…¡is controlled by regional fault and depression-uplift structure, and â… iscontrolled by large regional faults and trending fundamental structure. The sizes andfeatures of volcanic structure of â…¡ are same with single fault depression of Songliaobasin, including volcanic uplift and volcanic depression. Xujiaweizi fault depressionis a V type of volcanic depression, in which the Yingcheng formation developed. The volcanic structure of â…  is the same as the group of fault depression in Songliaobasin.10. The relationship between lithology and reservoir features of the volcanicrocks is summarized. The lithology was the main influence factor of the physicalproperties. The airhole rhyolite, rhyolitic tuff and breccia are better in physicalproperties. Changes of physical properties of the volcanic lava areittle in the deep, butchanges of pyroclastic rocks are clear.11. The relationship between facies and reservoir features of the volcanic rocksis summarized. Volcanic facies control the reservoir features of the volcanic rocks,thereservoir features are good in upper effusive sub-facies,inner extrusive sub-facies,and hidden explosive breccias sub-facies. It is also good in volcanic neck sub-faciesand pyroclastic flow sub-facies.12. It is significant to predict reservoir in the volcanic strctural stratigraphicframework. The favorable reservoirs are predicted in the framework.