An Approach To Seismic Prediction For Sweet Spot Of Shale Gas

The shale gas has a variety of storage forms,complex rock composition in the strata,large regional changes in organic matter types and abundance,stronger heterogeneity,and diverse pore spaces.These characteristics determine that the exploration and development of shale gas is quite different from conventional sandstone or carbonate rock reservoirs.And higher requirements are put forward for its evaluation ideas and methods.At present,the research and evaluation of shale gas is mainly aimed at the accumulation model and conditions,reservoir geology and resource potential evaluation.The technology is becoming mature.And shale gas geophysical modeling,seismic simulation and prediction have also made breakthroughs.However,there is still a huge room for research work in this area.The shale gas reservoir of the Longmaxi Formation-Wufeng Formation in Nanchuan-Pengshui district is a typical set of deep sea shelf sedimentary strata and has undergone multiple stages of tectonic movement transformation.The main gas-bearing reservoir in the research area is the first section of the Longmaxi Formation-Wufeng Formation,and the high-quality shale is 1-5 layer starting from the bottom of the Wufeng Formation.And it forms the distinctive reservoir characteristics of Nanchuan overpressure reservoir and Pengshui atmospheric pressure reservoir.In order to analyze the laws of high-quality shale reservoirs and predict the " Sweet spot " of shale gas in the research area,and to form a " Sweet spot" seismic prediction system for atmospheric shale gas,based on the geological conditions in Pengshui and its surrounding areas,this thesis had study on shale gas well-seismic simulation and shale gas " Sweet spot" seismic prediction.Firstly,based on the logging data,the petrophysical analysis of the target layer of the study area had been carried out.And the sensitive parameters and sensitive parameters combination of the high quality shale had been found.Then,through the work of rock physics modeling,a physical volume model of rock is constructed for the anisotropy of shale strata.On this basis,the petrophysical simulation of key geological parameters was carried out,and the petrophysical characteristics of the change of geological parameters such as carbon concentration and porosity of the switchboard were analyzed.On the basis of calibrating downhole seismic,AVO forward simulation of actual wells was carried out in atmospheric pressure area and overpressure area.The AVO response characteristics of each well are analyzed and looking for the laws of seismic response in normal and overpressure areas.At the same time,AVO forward modeling of varying formation thickness,TOC and porosity had been carried out.Then,by analyzing the main controlling factors of “Sweet spot” in the study area,the post-stack inversion of each factor was carried out,and the formation thickness,porosity,TOC,formation pressure,crack and brittleness index were predicted.On the basis of the AVO forward simulation,the pre-stack AVO attribute inversion is performed to realize the direct prediction of the “Sweet spot” distribution.Finally,based on the actual situation of the study area,establish a "Sweet spot" evaluation standard to achieve a comprehensive forecast of " Sweet spot".Conclusions as follow:(1)Rock physics analysis suggests that the first section of Longmaxi formation can be divided into three sub-sections: upper middle and lower.Logging curves can distinguish different lithology well.p-wave velocity and s-wave velocity can distinguish between siliceous shale and rich clay siliceous shale at high velocity.By analyzing the sensitive parameters of TOC,porosity and Poisson ratio,high quality shale corresponds to low density,low p-wave and s-wave velocity ratio,and low Lambda impedance and shear impedance ratio.The change rate of physical parameters of rock is closely related to the content of silica and organic matter in different geological parameters.As porosity increases,the p-wave velocity,s-wave velocity and density decreases,but the longitudinal wave velocity decreases less in silicon shale.(2)Taken Nanchuan-Wulong-Pengshui overpressure Zone to the atmospheric pressure Zone,the SY1 well,the JY194-3 well,the LY1 well,and the PY1 well as the representative well,AVO forward modeling simulation of the high-quality shale formation in the lower part of Longmaxi formation researched.The well SY1 is the fourth type of AVO response characteristics in the overpressure zone.And well JY194-3 is the third type of AVO response characteristics.In the atmospheric pressure zone the well LY1 is characterized by the third type of AVO response,and the well PY1 is the fourth type of response.(3)The AVO response of atmospheric and overpressure wells indicates that the AVO response characteristics at different pressures are very small or the difference is not due to pressure differences.Formation pressure has no or little effect on the AVO response of shale reservoirs.Influential is also caused indirectly by the change of gas content caused by pressure changes.(4)The “Sweet spot” elements such as high-quality shale thickness,total organic carbon,porosity,formation pressure,crack and brittleness index were predicted by poststack inversion methods.The thickness of the high-quality shale layer in the Longxi Formation Section-Wufeng Formation in the research area is 25m~40m.The average TOC is greater than 2.5 %.And the overall porosity is greater than 2 %.The pressure coefficient in overpressure area such as Nanchuan is generally greater than 1.1.The pressure coefficient from west to east decreases gradually,the pressure coefficient in Wulong area is about 0.9-1.2,and the pressure coefficient in Pengshui area is less than 1.0 on average.The first part of Longmaxi formation has high brittleness in the study area.The average brittleness index of high quality shale layer reaches more than 60 %.(5)Through AVO attribute extraction and well-seismic comparison analysis,it is determined that the difference p-wave velocity(DVP),the variation rate of p-wave(DRS)and the difference density(DRHO)are hydrocarbon sensitive parameters.The average negative anomaly can effectively carry out hydrocarbon indication.It can provide an important reference for the comprehensive prediction of " Sweet spot".(6)Evaluation criteria for favourable zones had been built in research areas.For high-quality shale,the thickness requirement is greater than 25 m,the TOC is greater than 2%,the porosity is greater than 2%,the crack is more developed,the formation pressure coefficient in the normal pressure zone is greater than 0.9,and the brittleness index is greater than 50%.A plurality of factors are superimposed to initially divide the favorable area,and then the AVO inversion result is used as a reference and an indication to optimize the favorable target area.