Forming Process Of Underpressure And Its Impact On Shale Gas Enrichment, Shan1 Member, Southeastern Ordos Basin

With the continuous discovery of underpressure oil and gas reservoirs worldwide,the origins of underpressures,the dynamic evolution of underpressure systems,the relationship between underpressures and hydrocarbon accumulations have attract petroleum geologists' and oil field engineering experts' great attention.In-depth analysis and quantitative evaluation of the underpressure of oil and gas reservoirs are helpful for the study of the formation and distribution of underpressure reservoirs,enriching modern oil and gas accumulation theory,and providing a scientific basis for the exploration and development of underpressure reservoirs.In recent years,great breakthroughs have been made in the exploration of the Upper Paleozoic shale gas in the southeastern Ordos Basin.Industrial gas flow was drilled in multiple intervals,and a number of underpressure shale gas reservoirs were discovered.However,the origins of underpressure in these gas reservoirs have been controversy,and their influence on shale gas enrichment is unknown.In this thesis,shale gas from Shan1 member of Shanxi Formation in the southeast of Ordos Basin is taken as an example.For the problems of the formation of underpressure and its relationship with shale gas enrichment,the current anomalous pressure system is first divided by determining the water potential surface,then the loading/unloading curve graphical method was used,tectonic uplift and depression model was established,and PetroMod basin simulation,overburden porosity measurement,triaxial stress test and high pressure isothermal adsorption experimental analysis were all used to quantify the formation process of underpressure and analyze its effect on shale gas enrichment.The following results and awareness were achieved :(1)The true abnormal pressure which calculates from the water potential surface shows that pore pressure of Shan1 member is dominated by underpressure,and also exists hydrostatic pressure and small overpressure in Southeastern Ordos Basin.(2)Tectonic uplift and pore rebound are the main factors for the formation of theunderpressure in Shan1 member.(3)The formation process of underpressure mainly experienced three stages of undercompaction,hydrocarbon generation and tectonic uplift.The undercompaction basically occurred in the late Triassic,and the amount of excess pressure formed by undercompaction was being maximum in the end of early Cretaceous,with 8-29.7 MPa;the hydrocarbon generation pressurization began at the end of the Triassic and reached the peak at 0-2MPa at the end of the Early Cretaceous;at the late Cretaceous,Ordos basin began to undergo uplift and strata erosion,which leaded to decrease of pore pressure to hydrostatic,at when undercompaction and the hydrocarbon generation pressurization disappeared,and then due to the pore rebound,the formation pressure continued to decrease about 3.23-18.9MPa,and finally formed the current underpressure.(4)During the formation of the underpressure in Shan1 member of Shanxi formation in the southeastern Ordos Basin,the amount of shale gas adsorption decreased slightly,the greater pore pressure decreased,the greater the amount of gas desorption had;the reduction of fluid pressure has a greater impact on the solubility of shale gas than decrease of temperature,during the forming of underpressure,the solubility of shale gas in water of Shan1 member of Shanxi formation decreased by 2.51-5.2.(5)At the end of the Early Cretaceous,during the maximum hydrocarbon generation period,there were four types of matching relationships between the longitudinal excess pressure and the conductive medium: shale gas leaked partially under mud-mud stacking with high excess pressure difference,shale gas completely leaked at mud-sand stacking with high excess pressure difference,shale gas partially retained with mud-sand stacking with a little excess pressure difference,and shale gas fully retained in shale with mud-sand superimposed negative excess pressure difference.The latter two types are favorable for hydrocarbon retention in Shan1 shale,and these two types mainly distributed in the middle and east of the study area.Accumulation of shale gas in Shan1 member in northwest of study area formed mainly affected by the distribution of both the lower gas potential and multiple transport in shale.(6)The effects of tectonic uplifting during the Late Cretaceous and depressurizing on shale gas accumulation include: the horizontal bedding fractures in shale in Shan1 member developed,which leaded to the horizontal migration of shale gas;the low gas potential area in the middle of the northern margin had become a relatively high-value gas potential area,which reduced the accumulation of shale gas there,the gas potential in the southeast decreased rapidly,which formed a lower gas potential area and promoted shale gas migrating laterally to the southeast;pore pressure of the Shihezi Formation was relatively low longitudinal,which affected the retention of shale gas in the source.