| Xihu sag is the most important hydrocarbon-rich sag in the East China Sea Shelf Basin,which is not prospected enough of oil and gas resources.And the understanding of hydrocarbon generation of source rocks,origin and enrichment of natural gas in this area is still unclear.Therefore,it is particularly important to systematic research the distribution,organic geochemical and gas generation intensity characteristics of coal-bearing source rocks,and the geochemical,origin and the distribution characteristics of natural gas.Furthermore,the natural gas enrichment rules in Xihu Sag were summarized.According to the study of the above contents,the results can be summarized as follows:(1)It is a basic and critical study to delineating the distribution of source rocks in the lower exploration area,and it has always been one of the difficulties in oil and gas exploration for areas without wells or with few wells.Through the comprehensive analysis of core-logging-seismic data,the lateral distribution and variation characteristics of the 6layers source rocks were identified,including the upper and lower sections of the Huagang Formation and the upper,middle and lower sections of the Pinghu Formation and the Paleocene source rocks.The coal-bearing source rocks in the Xihu sag are widely developed,and the distribution of source rocks in each interval is significantly different.The lower Pinghu Formation and the Paleocene source rocks are the thickest,and the areas with the largest thickness are located in the the Jiaxing structural belt and the Huangyan structural belt in the central reversal tectonic belt.(2)There are three types of source rocks in Xihu Depression:dark mudstone,carbonaceous mudstone and coal.Besides coal seam is good source rock,dark mudstone and carbonaceous mudstone are also important source rocks.The geochemical parameters of the source rocks in the Xihu sag are obviously different in different interval.The organic matter abundance,hydrogen index and hydrocarbon generation potential of the Pinghu Formation source rocks are higher than other intervals,and the Pinghu Formation is the most developed source rock with high hydrocarbon potential.The organic matter types of source rocks are mainly type II?III.The source rocks of the lower Huagang Formation have entered the mature stage,while the source rocks of the Pinghu Formation are at the peak of hydrocarbon generation.Coal and carbonaceous mudstonestones are mainly oil-prone,and mudstonestones are mainly gas-prone.The Xihu Sag is dominated by gas generation.(3)Basin simulation of Huagang Formation,Pinghu Formation and Paleocene source rocks in the Xihu Sag shows that the hydrocarbon generation history of these three sets of source rocks are distinctly different.Generally,characteristics of hydrocarbon generation are late and continuous.The hydrocarbon generation process of Paleocene source rocks is relatively early,it begined to enter hydrocarbon generation period in the early deposition stage of Pinghu Formation(44Ma),and reached hydrocarbon generation peak at the early stage of sedimentation in the middle section of the Pinghu Formation(40.5 Ma).The hydrocarbon generation of Pinghu Formation begined to enter hydrocarbon generation period in the early deposition stage of Huagang Formation(35.4Ma),and reached hydrocarbon generation peak at the stage of sedimentation in Yuquan Formation-Liulang Formation(16.2-7.2 Ma).The hydrocarbon generation of Huagang Formation begined to enter hydrocarbon generation period in the early deposition stage of Yuquan Formation(16Ma),and reached the first hydrocarbon generation peak at the early stage of sedimentation in Liulang Formation(11.6Ma),and it is still generating a large number of hydrocarbons.Xihu sag belongs to generation and expulsion a large number of of hydrocarbon in the late stage,which provides favorable conditions for hydrocarbon accumulation.There are two hydrocarbon generation centers of the gas intensity in the Xihu Sag,which are located in the north of the central reversal tectonic belt and the central part of the western subcontinent.The highest gas intensity can reach 45×10~8m~3/km~2.It can be seen that the source rock in the sag can provide a favorable material basis for the formation of natural gas.(4)The natural gas of the Xihu Sag is dominated by methane,the content is generally ranging from 75%to 94%;the content of heavy hydrocarbon components is less.The composition of natural gas in each structure is obviously different.The average value of methane content of natural gas in Ningbo structure and Tiantai slope is the highest,which reaches 88%.The methane content of western sub-concave natural gas is the lowest,with average value of 74%.The carbon isotopic composition of methane is ranging from-42.5‰to-30.3‰,which is relatively higher;the carbon isotopic composition of ethane is ranging from-30.9‰to-22.7‰,which is also relatively higher;the carbon isotopic values of the alkane series generally increase with the increasing,which shows the positive carbon sequence.The carbon isotopic composition of natural gas in the western slope belt is light,while the central reversal structural belt and the western sub-concave is heavy.At the same time,the carbon isotopic values of natural gas in the western sub-concave is between the first two structural belts.The natural gas is characterized by coal-derived gas and generated from type III kerogen,which origins from the Pinghu Formation and the underlying Paleocene source rocks.The distribution characteristics of C7 series compounds of natural gas in Xihu Sag are as follows:Most of the methylcyclohexane content is more than 50%,most of them are distributed in 47%?62%,indicating that the natural gas in Xihu Sag is mainly coal-type gas.(5)The natural gas enrichment characteristics are as follows:the hydrocarbon generation center controls the distribution of the oil and gas fields in the Xihu sag,and the discovered oil and gas fields are all distributed in or near to the hydrocarbon generation center;the fault distribution and the fault-through strata control the distribution of the reservoir.The fault in the western slope belt is broken to the Huagang Formation,and the natural gas is enriched in the Huagang Formation.The northern part of the central reversal zone is weaker than the southern part,and the early generated natural gas in the south-central part is preserved.The overpressure controls the vertical distribution of oil and gas in the Xihu Sag.At the end of the Miocene,the overpressure formed in the Pinghu Formation has played a good role in sealing and preserving oil and gas in the Pinghu Formation.The vast majority of oil and gas reservoirs are distributed below overpressure zone or overpressure zone. |
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