Comparative Research On Tectonical Thermal History Of Sedimentary Basins In The North China

The northern part of China is a matching body of different plates and ter-rains. The sedimentary basins in the north China superimpose on the differentstructural units and have different structural forms and different structuralevolution history. Since Mesozoic the basins have been reformed and destroyedby many structural movements. The intersuperimposing of early and laterbasins makes the thermal history very complex. Resuming basins thermal his-tory not only can provide scientific basis for exploration and exploitation of oiland gas in it and directly serve production, but also can provide basis for re-search of intracontinental or intraplate continental dynamics and basin dynam-ics. This paper, directed by "movement theory" and "stage theory" of geologyevolution, synthesizing regional geology surroundings and basins evolutionhistory, using many paleogeothermal research methods, resumes thermal his-tory of the basins in the north China, analyzes and compares the differentstages and different units thermal gradients of basins in the north China,makes a systematic study of the relationship between thermal history and oiland gas of different basins and ascertains favorable units and further explo-ration field of oil and gas exploration.Songliao basin and Erlian basin, located in northeast of the north China,all of which are typical rift basin., have similiar structural evolution history.The present geothermal gradient of Songliao basin is 3.7℃/100m, mean heatflow value is 68.65mW/m~2. The present geothermal field of Songliao basin ishigh in centre, low in border and has characteristic of ring shape distribution.Research in various methods indicates that paleotemperature of Songliao basinis higher than present, paleogeothermal gradient in later Cretaceous is 5.00—6.00℃/100m, which is far more than present geothermal gradient.The study on thermal history of Erlian basin is very little. The presentgeothermal gradient of Erlian basin is 3.10—4.22℃/100m, the mean value ofwhich is 3.50℃/100m, the estimated heat flow value is 56.21—81.59mW/m~2, the mean value is 69mW/m~2. Erlian basin has many depressions and everydepression is different in deposition. Present geothermal gradient and thethreshold of oil generation is distinctly different in every depression and thethreshold of oil generation is controlled by paleotemperature. In Cretaceous the thermal gradient is 3.50—5.50℃/100m, mean of -which is 4.66℃/100m.The thermal gradient of Middle Jurassic is higher.Ordos, Qinshui and Bohaiwan basins in North China are all located inNorth China craton. Deposition and structure of North China basin is stableand the thermal gradient is low in palaeozoic, in later Mesozoic North Chinabasin underwent a severe structural movement, which make the east of NorthChina basin rise and the west of it subsid. The temperature in the east is onthe decrease and the west is on the increase, in later Mesozoic the geothermalgradient of Ordos basin is 3. 30—4.80℃/100m, main value of which is 4.00℃/100m. The geothermal gradient of Qinshui basin is 5.56℃/100m. In theeastern uplifted area of North China geothermal gradient is 5. 25℃/100m.Since Cenozoic Ordos and Qinshui basin is wholelly rising and on the decreaseof temperature, but Bohaiwan basin stretched and fault-subsided and became arift basin and the geotemperature gradually increase. The present geothermalgradients of Ordos and Qinshui Basin are 2.2—3.1℃/100m and 2.60—3.00℃/100m. The heat flow values are 63.4mW/m~2 and 61mW/m~2 separately.The geothermal gradient and heat flow value of Bohaiwan basin are 3. 58℃/100m and 61.53mW/m~2.Jiuquan basin groups in the Hexi corridor is formed By the stacking oftwo-stage different property and different generation bains, tt was rift basinsfrom later Jurassic to early Cretaceous and squeezed sag basin since Cenozoic.The present geothermal gradient and heat flow of Jiudong, Huahai and Jiuxibasins are all very low, of which the present geothermal gradients are 2.51—3.00℃/100m and the heat flow values are 50—57mW/m~2. in later Mesozoic,the paleothermal gradients of Huahai, Jiuxi and Jiudong basin, which are 3.75—4.50℃/100m, are all higher than present thermal gradients. In HuahaiBasin paleotemperature is higer than present and the maturity of thermal evo-lution of source rock is controlled by paleotemperature. In Jiuxi and Juidongbasin the maturity of thermal evolution of source rock is mainly controlled bypresent temperature.Tulufan-Hami Basin is one of the three big basins in Xinjang autonomousregion. The present geothermal gradient of it is 2. 50℃/100m and the heatflow value is 44.48mW/m~2. The geothermal gradient of it is high in cast andlow in west. The resumed paieotemperature indicates that the geothermal gra-dient of Tulufan-Hami basin from later Jurassic to early Creataceous is higher,2.31—3.61℃/100m, the mean of which is 3.00℃/100m, in early and middleJurassic the geothermal gradient is lower than 3.00℃/100m. The structuraland thermal evolution of the Hami depression, Taibei depression and Tuokex-un depressions is very different. In Hami deprssiion and parts of Tuokexun de-pression the paleotemperature is higher than the present and the stage of oilgeneration is early. Although from later Jurassic Taibei depression underwent a short time rise and denudation but the temperatrue have been rising as awhole, which is beneficial to oil generation and-preservation. Badaowan for-mation and Xisanyao formation, the present main source rocks, is at the highstage of oil generntioin, Taibei depression is the most favorable explorationarea. The oil and gas pools in Tuloufan-Hami basin is formed in many times,which can be divided into three stages : Later Jurassic-early Cretaceous, LaterCretaceous and Oligcene-Miocene.North China basin and Talimu basin in the northern part of China in Pale-ozoic are all located in stable craton, whose area is big and in which the deposi-tion and stTucture is stable and the sedimentary layer is thin, whose geother-mal gradient and paleotemperature is low. Tuloufan-Hami basin and Zhungeerbasin located in Hasakesitan plate underwent extension-rift, the activity ofwhich is severe and the geothermal gradient of which is higher than Talimuand North China basins.In later Mesozoic the northern part of China is effected by surround plateand its activity is intense. According to activity, it can be divided into four u-nits: (1). tn Northeast unit Erlian and Songliao basins, in which volcanism issevere and crust is chasmiced, formed a Jurassic and Cretaceous faulted basinand the geothermal gradient is high. (2)In North China unit Ordos basin, Qin-shui basin, Bohaiwan region, in later Mesozoic geothermal gradient of whichwas evidently rising. At east Bohaiwan region was uplifting and cooling. Atwest Qinshui and Ordos basin was depressing and the geotemperature was ris-ing. (3)On the east of Aerjin fault in the Alashan and Hexi corridor area theJiuquan gain groups and Yingen basin extended from Jurassic to early Creta-ceous and formed many faulted basins. The structural pattern of the faultedbasins is smiliar to Erlian basin in northeast china, but the volcanism is fewerand geothermal gradient is high. (4)On the west of Aerjin fault Tuloufan-Ha-mi, Talimu ,Zhungeer basins are all depression basins, the geothermal gradi-ents in lower, Tuloufan-Hami basins activity is severer than Talimu basin andZhungeer basin, the geothermal gradient is higher than the two basins.In later Mesozoic, On the east of Aerjin fault the structural activity is se-vere. The severe structural movement results in volcanism and ptutonic ther-mal activity, which is a severe regional structural-thermal event and has regionality. Because of the confinement and adjustment by boundary faults, struc-tural pattern and movement activity of different tectonic terrains is different.It resulted in geothermal abnormality in volcanic area. The structural-thermalevent controlled the generation, migration and play-forming of oil and gas inthe basins on the east of Aerjin fault.Stretching and graben faulting on Bohaiwan area in the east of North Chi-na form Bohaiwan rift basin since Cenozoic, geothermal gradient and heat floware all high in bohaiwan basin, the Tertiary formation is very thick and the geotemperature is on the increase in the basin. Ordos, Qinshui basins in thewest of North China uplift and was denuded, the geothermal gradients arelowering and the geotemperture is on the decrease. Erlian, Songliao basins inthe east of North China is crust thinning area, the geothermal gradient is high-er, the values are 3.50℃/100m and 3.70℃/100m separately. Because of thenorthward movement of India plate, the crust of the west of the north China isfurther shortening and thickening the geothermal gradients and heat flows ofTuloufan-Hami, Talimu, Zhungeer basins and Jiuquan basin groups are alllower.The types of thermal history of the main basins in the northern part ofChina is very complex, which can be divided into cooling type, alternativechange type and thermal event type etc. The thermal history type of basin iscontrolled by its dynamic background of the earth and structural evolution.The basins having different structure and thermal history are very different inthermal maturation of source rock, threshold of oil generation and main stageof oil and gas generation. In high thermal gradient type of basins, the thresh-old of oil generation is shallow, which is advantageous to quick generation ofoil and gas. In low thermal gradient type of basins, the threshold of oil gener-ation is deep and the effective exploration depth scope and exploration depthare all increased.According to the comprehensive analysis of basin structural and thermalhistory, the mature evolution of source rock and preservative condition, It isthought that Songliao basin, Bohaiwan basin, Ordos basin, Tuloufan-Hamibasin is the favorable exploration-area and is worth for detail exploration. TheCarboniferous and Permian in Bohaiwan basin and Qinshui basin, the deep lay-er of Permian and Triassic in Taibei depression of Tuloufan-Hami basin andthe lower Cretaceous in Jiudong basin is the further exploration fields.