Mechanism And Pattern Of The Classification And Distribution Of Natural Gas Resources In Xihu Sag,east China Sea Shelf Basin

The Xihu Sag,which is located in the East China Sea Shelf Basin,is the largest Cenozoic petroliferous sag in offshore China.After years of exploration and development and multiple rounds of resource evaluation,it has been proved to be a gas-rich sag with great exploration potential.In recent years,with the deepening of exploration,major breakthroughs have been made in the fields of the middle-deep layers and the unconventional resources.It is revealed that the Paleogene reservoirs contain abundant natural gas resources and present the characteristics of coexistence of multiple grades as convention,low permeability and tight.Limited by the history,extent and data conditions of exploration,the natural gas resources located in the middle-deep layers have not been reasonably classified and effectively evaluated.The accumulation mechanism of natural gas resources at all grades has not been clarified,the allocation and the law and mode of distribution of natural gas resources have not been clear either.It is difficult to meet the needs of the accumulation research and exploration of natural gas in Xihu Sag at the present stage.Based on drilling,seismic,logging,analytical testing,productivity testing and other data,this thesis makes a comprehensive evaluation of the source rock conditions in the Xihu Sag.Then,the oil and gas resources were quantitatively predicted by the comprehensive utilization of organic carbon restoration method,Trinity basin simulation method and resource abundance analogy method.The mechanism of the classification and distribution of effective natural gas resources is elucidated through the classification evaluation and accumulation anatomy of high-exploration traps.Combined with the classification prediction results of resources in low-exploration traps,the effective natural gas resources in Xihu Sag are comprehensively evaluated according to different areas,layers and grades.Finally,the rule and pattern of the classification and distribution of natural gas resources can be summarized.The main achievements and understanding of the thesis are as follows.1.The source rocks of Eocene Baoshi Formation,Eocene Pinghu Formation and Oligocene Huagang Formation are the three main sets of coal-bearing source rocks in Xihu Sag,and the lithology is dark mudstone and coal.Dark mudstone source rocks are widely distributed in the transverse and vertical direction,with large thickness and strong continuity.They are mainly poor to medium source rocks,and some are good source rocks.The type of organic matter of dark mudstone mainly belongs to typeⅢ.The process of hydrocarbon generation and expulsion of dark mudstone can be divided into five stages as follows,1)Ro=0.5%-0.7%,when oil was generated slowly without expulsion;2)Ro=0.7%-1.0%,when oil was generated and expelled rapidly;3)Ro=1.0%-1.5%,when oil began to be cracked into hydrocarbon gas;4)Ro=1.5%-2.3%,when gas generation predominated;and 5)Ro>2.3%,when only dry gas was generated.Coal source rocks are mainly distributed in the Eocene Pinghu Formation in the vertical direction and in the intertidal zone on slope break in the horizontal direction,with thin thickness of single coal bed and poor continuity.They are characterized by high abundance of organic matter with mainly typeⅡ₂.The process of hydrocarbon generation and expulsion of coal can be divided into four stages as follows,1)Ro=0.5%-0.7%,when oil was generated slowly without expulsion;2)Ro=0.7%-1.0%,when oil was generated and expelled rapidly;3)Ro=1.0%-3.5%,when oil began to be cracked into hydrocarbon gas and the generation of gas were predominated;and 4)Ro>3.5%,when only dry gas was generated.2.Xihu Sag is rich in oil and gas resources,with total effective resources exceeding 30×10⁸of oil equivalent.Among them,natural gas resources account for more than 80 percent of the total,which is the main resource of the sag.The spatial distribution characteristic of effective natural gas resources is that more in the middle areas and the middle-deep layers of Xihu Sag,while less in the surrounding areas and deep layers.3.According to reservoir physical properties,the effective natural gas resources in Xihu Sag can be divided into four grades as follows,1)conventional middle-high porosity gas resources,when the porosity of the gas reservoir is greater than 15 percent;2)conventional low porosity gas resources,when the porosity of the gas reservoir is between 12 percent and 15 percent;3)unconventional low porosity gas resources,when the porosity of the gas reservoir is between 10 percent and 12 percent;4)tight gas resources,when the porosity of the gas reservoir is between 7 percent and 10 percent.Effective natural gas resources at all grades are accumulated and enriched under the joint control of hydrocarbon source conditions,reservoir conditions and charging dynamics conditions.The accumulation and enrichment of conventional middle-high porosity and low porosity gas resources are mainly controlled by hydrocarbon source conditions,while the accumulation and enrichment of unconventional low porosity and tight gas resources are mainly controlled by hydrocarbon source conditions and charging dynamics conditions.Under the constraints of controlling factors of gas accumulation,the multi parameter prediction model of reservoir average gas saturation based on gas expulsion intensity of source rock,reservoir thickness,reservoir average porosity and residual pressure is established.The model can be effectively applied to the classification and evaluation of natural gas resources in low exploration degree traps.4.The total amount of effective natural gas resources in traps in Xihu Sag exceeds20×10⁸of oil equivalent,of which tight resources are the main resources,accounting for56 percent.The natural gas resources which are favorable objects for natural gas exploration in Xihu Sag are mainly distributed in Huagang Formation of Ningbo inversion zone in the middle of the inversion zone of central subsag and Upper Huagang Formation of Jiaxing inversion zone in the north of the inversion zone of central subsag.And the preferred target is concentrated in Upper Huagang Formation of Ningbo inversion zone and Jiaxing inversion zone.5.In the vertical direction,the grade of natural gas resources in Xihu Sag generally tends to be tight with the increasing of the buried depth of gas reservoirs.The accumulation of natural gas resources at all grades is mainly controlled by hydrocarbon source conditions.On the one hand,the sufficient natural gas expelled from the source rocks can be filled into the reservoir to form gas reservoirs.On the other hand,the overpressure formed by hydrocarbon generation and expulsion of source rocks can provide sufficient power for natural gas filling to adjacent reservoirs,and can also form efficient plugging for underlying gas reservoirs.The charging dynamics conditions is also a necessary condition for the accumulation and enrichment of natural gas,and also controls the value of gas saturation to a certain extent.The pattern of the classification and distribution of natural gas resources can be summarized as follows,the buried depth of reservoirs controls their physical properties,thus controlling the grade of natural gas resources;hydrocarbon source conditions control the accumulation of gas;and the charging power affects the amount of natural gas resources.