Description Of The Permian Magmatism And Its Geological Significance In Tazhong Area,Tarim,NW China

Permian magmatic activity is very intensity in Tarim basin,and the mafic rocks which are mainly composed by basalts cover 250,000 km2.In recent years,all the studies like geochronology of Permian basalts(the eruption duration of the Permian basalts was less than 5 Ma),and geochemical studies(characteristic like OIB pattern was found in Permian basalts)refer to the origin of the mantle plume.Because of the cover of the Taklimakan Desert,the research on Permian magmatic rocks of Tazhong area in the center part of Tarim basin is still poor.The 3D seismic and drilling deployed in Tarim abdominal area by oilfield enterprises light up the new way for geophysical methods used in magmatic studies.Our research is based on 1445km2 3D seismic data and 6 wells(w1,w11,w12,w16,w17,w18)in K1 area of Tazhong.Our studies aim at lithology interpretation,lithofacies division,seismic identification and space distribution of igneous rocks.Combined with the basic studies,we built 3D geological modeling for Permian magmatic rocks,resolved eruption pattern,and discussed the effect of magmatic activity on Paleozoic oil and gas reservoirs.The combination of well log curve cross analysis and mud logging debris description to identify volcanic rock lithology:near crater,rocks are mainly basalt,andesite,volcanic breccia and tuff;far from the crater,rocks turn to thick basalt and tuff.Compared logging data with seismic data,five lithofacies were defined:volcanic vent facies,sub-volcanic facies,volcanic eruption facies,overflow facies and volcanogenic sedimentary facies.Four types of reflection patterns for the igneous rocks were found,mushroom-shaped reflections,chimney-shaped reflections,strata-concordant reflections and funnel-shaped reflection.The upper part and surrounding of the volcanic rocks exhibit high amplitude seismic reflection and the internal of the volcanic rocks is chaotic reflection.Seven carters whose area was bigger than 12 km2 are found in our study and the distance between the nearby carters is less than 10 km.The whole working area is covered by Permian volcanic rocks.The thickness of the volcanic rocks decreases from the south and northwest part to the northeastern part The thickness of the volcanic rocks in No.6 and No.7 in south part and No.1 in northwest part are up to 1100 m,the thickness near w11 is 120 m.Based on the 3D geological modeling,the volume of the igneous rocks in the study area is estimated to be 178 km3.Expand to the whole Tarim basin(250,000 km2),the maximum volume is predicted to 0.308 x105 km3.The results of six synthetic seismogram calibration show that no crater is observed in younger formations above T51 or in older layers beneath T52.Thus,the timing of the volcanic eruptions is constrained between the Early and Middle Permian,coincident with the deposition of the Kaipaizileike Formation.In the range of 35 km(EW)x 40 km(SN),all the characteristic of the igneous rocks as muti-large craters,short duration and huge thickness suggest that the eruption center of Tarim large igneous province is Tazhong area.The craters show circle or oval in the plane.No obvious thickening beneath fault 2 and fault w18 are found.On seismic profiles and attribute slices,there is no obvious fault beneath the volcanic cones.We conclude that a central-vent eruption is the main eruption patterns,local area may be fissure eruption.Hydrocarbon charging and the development degree of lower-middle Ordovician carbonate rocks are the main factors for hydrocarbon accumulation in Tazhong area.The w17 drilled for 22 m diabase in Yingshan formation in Ordovician and 14.8 m dyke can be tracked on seismic profiles.The Caledonian fractured-vuggy carbonate reservoirs were filled or jammed by late magmatic activity.The negative effect of Permian magmatic activity on hydrocarbon accumulation is more than the promoting effect.