Pyroclastic Rock Of Yingcheng Formation In Songliao Basin:Fades·Architecture·Application

With the successful practices in volcanic oil and gas reservoir exploration of Yingcheng formationin Songliao basin, the volcanic reservoir rock (including pyroclastic rock) has drown more attention.Pyroclastic rock is a transitional rock developed between lava and sedimentary rocks, and the faciesand archateture of pyroclastic rock is an important part in the reservoir study. There are a large numberof volcanic outcrops in the south-eastern margin of Songliao basin, providing profiles for studying thevolcanic clastic rock facies and archateture.Volcaniclastic rock facies models are built up basedon the study of the outcrops, and these models provide a platform for analysis the volcanicclastic rockreservoir.Based on geochronology study of those main studying objects, which are pyroclasticrocks and lava rocks in the south eastern margin of songliao basin, the chronostratigraphic frameworkof Yingcheng formation with a 1-10Ma scale is built up.Using of geochemical analysis correlates ofthe two pyroclastic rocks outcroped sections,and provides proofs for dividing of volcanic eruptioncycles and stages. The changing laws of the major elements as represented by K2O, is given in a＂base surge-pyroclastic flow-airfall＂cooling unit. The tectonic setting of the basin in Yingcheng stagewas a rifting-extension basin related to post-orogenic geological processes discussed in isotopicgeochronology and geochemistry. Two models of the crater developing process are given, byreconstructing the lithologic sequences and the development process of the craters, which are①＂vapour explosion-vapour magma explosion＂crater model of acidic magmation, and②＂explosion-effusion＂-＂explosion-effluence＂-＂magma extrusion＂crater model ofintermediate-basic magmation.The original erupting and re-worked sediments are studies through the outcrop studycombining with grian size analysis, sieve analysi and detrital component analysis, and the pyroclastic deposits models of facies and architecture (including base surge, pyroclastics flow and airfallsediments) and re-worked sediments models of facies and architecture (including base surge,pyroclastics flow and airfall on fluvial plain and delta-lacustrine, lahar and peperite sediments). Therelationship between lithology-facies and lava-volcaniclastic rock reservoir is discused in Xujiaweizifault depression after those facies and architecture models mentioned above. The conclusion is obtainedthat the reservoir performance of pyroclastic flow is better than base surge and airfall, and pyroclasticsediments on the crater and nearby volcano slope are better than those on the volcano slope -transitional environment. There are 7 main achievements as fellows:1.Determined the volcanic cycle geochronology of Yingcheng formation, and built upchronostratigraphic framework of Yingcheng formationThe sedimengtary geochronology of Yingcheng formation is 110.7-134Ma. Elevenvolcanic/sedimentary cycles in three members of Yingcheng formation. Ages are as following:120-134Ma (Member 1),115-120Ma (Member 2) and 110.7-115(Member 3). Based on thesegeochronology, the chronostratigraphic framework in 1-10Ma scale of Yingcheng formation isestablished.2.Attained pyroclastic rock strata correlation by element geochemistry,determined thetectonic setting in the stage of Yingcheng formation as rifting-extension related to post-orogenicgeological processesThe content curves of Na2O, K2O, Al2O3and SiO2of Yingcheng pyroclastic rocks are received,and strata correlation of pyroclastic rocks is attained. A regularity of K2O content of pyroclastic rocksin the cooling unit with＂base surge- pyroclastic flow-airfall＂is taken. The tectonic setting in the stageof Yingcheng formation was rifting-extension related to post-orogenic geological processes.3.Established the crater model of facies and architectureThere are two types of crater facie and architecture models.①is＂vapour explosion-vapourmagma explosion＂crater model caused by the acidic magmation, and accumulation of lithologyincluding volcanic agglomerate and breccia( production of vapour explosion), agglomerate lava andtuff interbeded with lava (production of vapour lava explosion), and Pearlite and breccia withcryptoexplosive structure (production of intrusion and extrusion);②is＂explosion-effusion＂-＂explosion-effluence＂-＂magma extrusion＂crater model caused by the intermediate-basic magmation,and accumulation of lithology is of alternate layers of agglomerate/ breccia and lava.4.Established the volcano slopes model of facies and architectureEstablished the cooling unit model of archetacture with " base surge-pyroclastic flow -airfall".Base surge model has a two- facies archetacture, the lower is composed of explosion breccia , themiddle-upper is composed of vitric,crystal and debris tuffs. Pyroclastic flow has a three- facies archetactur, the lower is composed of coarse fragment deposition of breccia/ breccia-bearing, themiddle is composed of ignimbrite.and the upper is composed of vitric tuffs in which pumice is enriched.Airfall has a two-facies archetacture. In the bottom there are catapult airfall deposits composed ofbreccia, breccia lava, breccia-bearing tuff. In the top there are ejection airfall deposits ofbreccia-bearing tuff, crystal,debris, vitric tuff.5.Established re-worked pyroclastics model of facies and architectureThere are two types of re-worked architecture models.①is pyroclastic flow and base surge onfluvial plain and delta–lake, and lithofacies sequence is similar to that on volcanic slopes, however thepyroclastics are re-worked in a certain part.②is lahar deposit including the genetic model of collapsedaccumulation at the same time with the eruption and the genetic model of erosion- accumulation afterthe eruption.6.Determined the pyroclastic rock facies of the favorable reservoirWith in a cooling unit,reservoir propertiy of the pyroclastic flow is better than base surge, airfalland mixture of base surge and airfall in Xujiaweizi fault depression. But reservoir performance hasbeen reduced with addtion of fine airfall material.7.Determined the pyroclastic Lithology of the favorable reservoirThe pyroclastics rocks of the favorable reservoirs includs rhyolitic lithic crystal tuff in pyroclasticflow, lithic crystal tuff with breccias in base surge sediments and rhyolitic breccia tuff in airfallsediments in Xujiaweizi fault depression.