Geochemical Study Of Hydrocarbon Accumulations With Multiple Sources And Multiple Generation Stages In The Middle Yangzi Area

Based on the study of the prototypes and tectonic evolution of the marine Mesozoic basins in the Middle Yangzi area,the deformation of the Jianghan Basin was studied, which underwent bi-directional compression of the DaNe orogenic belt and the Jiangnan - Xuefeng basement detachment during the Indosinian - Early Yanshanian movement.It indicated that the Paleozoic and Mesozoic basins in the Yangzi area had undergone the Late Sinian - Ordovician inner cratonic depression - peripheral cratonic basin,the Silurian foreland basin,the Devonian - Middle Triassic inner cratonic depression,the Late Triassic - Jurassic foreland basin and the CretaceousTertiary extensional strike-slip basin since the Late Proterozoic Xuefeng movement (800Ma),resulted in recent superimposed basin.According to the principle of " basin as the pattern and modification as the boundary",the Jianghan basin was divided into "structural units of the IndosinianEarly Yanshanian compressional basin" and "structural elements of the Himalayan sqeezing and strike slip basin".The Paleozoic and Mesozoic Jianghan basin was divided into 5 classâ…¡structural units,10 classâ…¢structural units,and 16 depressions and uplifts.On basis of evolution and division of structural units of the Jianghan Basin,the burial history of the basin was divided into shallow buried Paleozoic - deep buried Mesozoic in Danyang and Jinzhou - Gaiyang area,shallow buried Paleozoic and long erosion in the Mesozoic and Cenozoic in Lexianguan - Tianmen area,shallow buried Paleozoic to Cenozoic in Xianning - Daye area,and shallow buried Paleozoic and deep buried Mesozoic and Cenozoic in Yichang slope area.Geothermal gradient in every stages of basinal evolution was calculated with ratio of mixed layer clay minerals and crastallinity of illite.Geothermal gradient was 3.7℃/100m in Himalayan period(K-E),2.7 - 2.2℃/100m in IndosinianYanshanian period(T3-J3),3.1℃/100m in Hercynian period(D-P-T2)and 3.1℃/100m in Caledonian period(Z-S).In order to study of thermal - structural history,11 samples were carried out apatite fission - track analysis.Fission-track age of apatite was 24.8 - 28.3 Ma in the southern part of the Jianghan basin,indicating 2500m of uplift during the Himalayan movement in the Oligocene and Miocene.In Danyang area,the geo-temperature showed rising - falling - rising during the Paleogene and Late Cretaceous,i.e.it was in temperature rising annealing zone during sedimentation of the CretaceousPaleogene sequences;temperature was fall during uplift(erosion level less than 500m) due to Jianghan movement between the Oligocene and the Miocene,but not retreated from the annealing zone;then the temperature was slightly rose as a result of superimposition of the Miocene sequence.The denudation amount of the marine Paleozoic and Mesozoic strata was calculated by TSM basin modeling method,and the burial history and thermal history of the basin was restored.Result of hydrocarbon generation simulation of 3 sets of Paleozoic source rocks(C1,O3-S1,P1)in the Middle Yangzi area showed that different hydrocarbon generation of different sequences resulted in the difference of hydrocarbon generating amount in different periods in the Jianghan Basin.The total hydrocarbon generating amount of the Paleozoic source rocks in the DevonianMiddle Triassic was 24.797^*10¹²m³,accounting for 31.31%;39.939^*10¹²m³ in the Late Triassic-Jurassic,accounting for 50.43%;12.567^* 10¹²m³ in the CretaceousPaleogene, accounting for 15.87%;with small amount since the Eocene.Till recent, total hydrocarbon generating amount of the Paleozoic source rocks in the Jianghan Basin was 78.239^* 10¹²m³.In terms of its distribution,the Gaiyang and Jiangling depressions accounted for the biggest hydrocarbon generating amount,with 31.82^*10¹²m³ and 22.208^* 10¹²m³ respectively,and the other areas were less than 10^*10¹²m³.The geochemical sections of the marine Paleozoic and Mesozoic sequences in northern margin,western margin and wells in the Jianghan Basin were analyzed. Especially,the geochemical feature and thermal evolution of 3 sets of source rocks of C1,O3-S1 and P1 age were studied in detail.By correlation of the bitumen/source rocks in Nanshanping ancient oil reservoir and correlation of gas/source rocks in Jiannan gas field,study of Miaoshi,Beicamen and Yanmenkou Permian oil show,and correlation of oil/source in well 631 Cretaceous oil reservoir,a multiple source rocks and multiple hydrocarbon accumulation was indicated.Based on study of basin prototype,structural evolution,burial and thermal history of the Jianghan Basin of the Middle Yangzi area,and analysis of geochemical characteristics of the marine Paleozoic and Mesozoic source rocks,combining with modeling of hydrocarbon generating history of the source rocks by TSM basin modeling method in the Jianghan Basin,the hydrocarbon generating intensity and hydrocarbon amount was calculated in different evolutional stages.The results provided some basis for assessment of marine exploration targets in the Jianghan Basin.The evaluation of traps was in the line of "near source rocks,good preservation, original block,weak deformation and in footwall of thrust" for the marine Paleozoic and Mesozoic traps of the Jianghan Basin of the Middle Yangzi area.Typeâ… area:The marine Paleozoic and Mesozoic source rocks were well preserved. Favorable area for late(secondary)hydrocarbon generation is in Gaiyang thrustfolds belt(Gaiyang depression,Xiaoban depression,Tonghaikou low uplift and Yekou low uplift)and Jiangling depression.Typeâ…¡area:The marine Paleozoic and Mesozoic source rocks were generally preserved with part denudation.It showed medium intensity of late(secondary) hydrocarbon generation,such as in Jingzhou thrust - folds belt(Qianjiang depression, Yaxin low uplift),Yichang slope area(Zhijiang depression)and Danyang synclinorium(Herong depression).Typeâ…¢area:The marine Paleozoic and Mesozoic source rocks were denudated and were poorly preserved with small late(secondary)hydrocarbon generation.For examples,Daye thrust - folds belt,Tianmen - Lexiangguan syclinorium(Jingmen depression,Haishui depression,and Yonglonghe and Lexiangguan uplift),HonghuTongshan thrust belt and Bahong thrust belt.In accordance with analysis of the Paleozoic and Mesozoic trap conditions in the Middle Yangzi area,comprehensive assessment of exploration areas and optimization of exploration targets was carried out.And future exploration decision was proposed.