The Gravitational And Magnetic Field Research Of Carboniferous Volcanic Rocks In Western-central Junggar Basin

Junggar Basin is a large superimposed composite basin that has undergone complex tectonic evolution.In the Late Carboniferous-Early Permian Period,the Paleo-Asian ocean was completely closed and northern Xinjiang entered the post-collisional period.The mantle-derived magma activities with multi-cycle and multi-stage attributions occurred in the Junggar area under the extension background,and consequently formed the widespread Carboniferous volcanic rocks in the Junggar basin.Volcanic rocks locating in structural high usually are the favorable reservoirs,as they have developed rich secondary pores and fractures after undergoing long-term weathering,leaching,erosion and other reworking in later tectonic activities.Carboniferous volcanic rocks have become a major new field as the hydrocarbon exploration expands to deep part of the basin.Since 2005,great progress has been made in volcanic hydrocarbon exploration in the Junggar basin.The newly discovered gas reserve in Kelameili gas field reachs to 100 billion cubic meters of gas),the newly discovered and increased hydrocarbon reserves in Hong-Che fault zone and Kebai fault zone have all exceeded 100 million tons.Bothhydrocarbon reservoirs and oil and gas shows have almost been found in Carboniferous volcanic rocks in all areas of the basin except for its southern margin,this fact demonstrates huge potential for Carboniferous volcanic hydrocarbon exploration.Therefore,delineating the distribution of Carboniferous volcanic rocks and predicting the favorable areas for volcanic hydrocarbon exploration are of great significance for hydrocarbon exploration and development of the basin.This paper chooses two areas of distinct tectonic setting,Mosuowan-Luxi areas in the center of the basin and Kebei fault zone in the northwest margin of the basin,as the study objects,on the basis of gravity and magnetic data,combines seismic,drilling and magnetotelluric sounding datato delineate the distribution of volcanic rocks and their lithologies in the study area,locate the fault structures and characterizes the basement form.Taking into account the above conclusions and hydrocarbon geological conditions,the Predictions of the favorable hydrocarbon areas are made in the end.This study has the following six major findings.1.Delineating the distribution,attribute and function of fault in the study area.fault structure is a channel for magmatic activity and a bridge linking source rock with volcanic reservoir.18 fracture faults are delineated in Mosuowan-Luxi areas using the horizontal directional derivative,and 5 regional basement faults are determined according to the extension scale.The faults are then compared with the results of seismic fault delineation,it is clear that the NE-trending F1 fault is a new discovery in this study.In Kebei fault zone,two sets of fault systems in the deep and superficial levels are determined by the results of total horizontal gradient of gravity anomalies,aeromagnetic anomalies,and magnetotelluric soundinganomalies.The deep fault systems include Dalabut deep fault,Zhayier Mountain deep fault and Mahuxi deep fault,among which Zhayier Mountain deep fault is the new discovery of this research as the deep channel of magmatic activity in the upper plate of the Kebei fault zone.The main faults in the shallow fault system are Baijiantan south fault(F4),Baijiantan fault(F3),middle Baibai fault(F2)and west Baibai fault(F1).F4 is the frontal fault of the nappe belt and F1 is the boundary fault of the basin and the mountain,the range of NW-SE of Kebai Fault Zone is defined by them.2.In Mosuowan-Luxi Area,the main type of volcanic hydrocarbon reservoirs are buried-hill paleo-weathering crust reservoir,and whether or not the volcanic rocks are in structural high is an important factor in evaluating the favorableness of hydrocarbon accumulation of volcanic edifice.The Euler inversion and tangent inversion results of the magnetic anomaly are used to map the topography of the Carboniferous top interface of the Mosuowan-luxi areas.The volcanic edifices delineated by gravity and magnetic anomaly are superimposed on the topography of the Carboniferous top interface,The degree of overlap between volcanic edifices and structural high is used as an criteria for the prediction of favorable hydrocarbon areas.3.Delineating 6 volcanic activity belts in Mosuowan-Luxi Area based on the distribution of residual gravity and magnetic anomalies and the occurrence of fault structures.The 6 volcanic activity belts are:the Mahu east and Sangequan-Shi'nan ring-shaped volcanic activity belts;the Mosuowan uplift,Mobei uplift and Shixi uplift NE-trending volcanic activity belts;the Dongdaohaizi NW-trending volcanic activity belt.In Kebei fault zone,the F1 fault volcanic activity belt,F4 fault volcanic activity belt and F7 fault volcanic activity belt are demarcated.Depending on residual gravity and magnetic anomalies,several volcanic edifices are identified within each volcanic activity zone.The density of the acid volcanic rocks in Mosuowan-Luxi areas is not different from the sedimentary rocks below the Cretaceous system,rocks in the front of the Kebei fault zone are highly fractured,and the volcanic rocks do not exhibit high density as they should.Therefore,magnetic anomaly is the major criteria and gravity anomaly the supplementary criteria in determinating volcanic edifices.4.Inverting apparent susceptibility and apparent density based on residual gravity and magnetic anomalies and carrying out lithology identification of volcanic rocks according to apparent susceptibility and apparent density values,gravitational and magnetic anomaly intensity and the disclosure of volcanic rock lithology in boreholes.The identification results show that Mosuowan area is dominated by intermediate-acidic volcanic rocks,while Luxi area is dominated by intermediate-basic volcanic rocks.The covered area of Kebei fault zone can be divided into lithological unit ?,?and ? from NW to SE.Unit ? consists mainly of ophiolitic melange and tuff;Unit ? mainly comprises of tuff and volcanic breccia and a small amount of basalt;Unit III is composed of basalt.During lithologic identification,the gravity,magnetic and seismic joint forward modeling was performed using seismic data as a constraint for typical sections.It was found that in Mosuowan-Luxi areas,the gravity and magnetic anomalies caused by volcanic rocks are only a small part of the measured anomalies,it indicated that there are also some strong magnetism,high-density basic intrusion below the volcanic rocks.5.Comprehensive considerations are given to factors of volcanic hydrocarbon accumulation,including the reliability of volcanic edifices,the overlap with structural high,the development of fractures around volcanic edifices,and the proximity of oil-bearing formations to oil and gas reservoirs found in the overlying Mesozoic strata.10 favorable I-level targets and 8 II-level favorable targets are predicted in Mosuowan-Luxi areas.In Kebai Fault Zone,volcanic rocks near the main fault are favorable areas for the accumulation of hydrocarbons.Therefore,Baizhong 7 well block,Baizhong 8 well wlock and F7 fault volcanic activity zone will be the key area for future research.6.Through the analysis of petrographic lithology of the volcanic section of the Gu-29a well in Kebei fault zone,multi-period characteristics of volcanic activity have been further defined.During each interval of volcanic activity,sandstone or sandy conglomerate is formed on the outer edge of the volcanic rock(roughly equivalent to the far crater zone).It is formed under similar tectonic environment as volcanic rocks.Due to long-term weathering,leaching and later structural damage,it gradually becomes the favorable reservoir for oil and gas.A considerable amount of hydrocarbons have been found in these layers in the boreholes(example for Baijiantan 801 well).It adds a new reservoir type relevant to volcanic reservoirs and expands the space of hydrocarbon accumulation about the volcanic rocks.