Prediction Of Xujiahe Formation Fracture-porosity Reservoir Xinchang Gas Field

The xujiahe formation of west Sichuan depression possesses the condition of forming big gasfields. According to the statistic of 2003,the discovery degree of resources is only 0.45%.X851 obtained breakthrough in Xinchang structure in 2000,that reveal the prospect of xujiahe exploration. But the gas reservoir of xujiahe formation was buried depth,the sand body is compact and the heterogeneities are strong. It is very hard to predict and drill. The gas reservoir's feature and distribution are not found. For many years,it is commom to deploy the well according to the fault development and structural high. For the better of exploration,reveals the distribution of gas of xujiahe,lauch the 3D3C seismic exploration. This research select principal producing formation of X851(TX24) of 3D3C(about 530km~2) and have been made a breakthrough in predicting reservoir,fracture and gas-bearing properties.The analysis on reservoir character show that the pore of high-quality reservoir comes from denudation and developes in mouth bar. The degree of ripeness of mouth bar is relatively low,therefore,it contains more feldspar. The mouth bar contacted the frontal matting-like sand,and acidic water entered into mouth bar easily. The feldspar in sandstone of mouth bar was dissolved and formed secondary pore easily. Oil and gas enrichment model is built: The epoch of hydrocarbon-generation,with the fault as passageway and thick delta front sandbody as dredging-layer,acidic water entered into mouth bar. Denudation arose in arcose. Oil and gas entered into mouth bar in early times and formed oil and gas abundance zone. Prediction of mouth bar favorable depositional fades is the key point in exploration of xujiahe formation gas reservoir. By analyzing X851 bulit the prediction model of mouth bar and oil and gas abundance zone: Mouth bar presents typical sigmoid reflection configuration. It displays conchiform reflection in cross section and distributes horseshoe in plane. The article predicts high-quality reservoir and distribution of oil and gas abundance zone. The prediction is in good agreement with drilling.Because the reservoir of xujiahe formation suffered destructive diagenesis ,the physical properties were poor. The reservoir of xujiahe formation-2 is characterized by low matrix porosity about 2-4% and the rock permeability is under 0.1×10-3μm2; They belong in the field of tight-ultratight sandstones.The gas forms scale with the help of improving the penetrability with fracture. Major principal stress is nearly east-westward in this area,that means east-westward fracture is efficacy. S-wave splitting is important value of multi-component. When the S-wave passes by the anisotropic media,it will be splitted fast s-wave and slow s-wave. It can be determined the fracture strike by means of detecting the direction of fast s-wave. And the fracture density can also be obtained by the time difference between fast s-wave and slow s-wave. Combined with predominant frequency band coherency algorithm,the curvature,dip and azimuth means,realize the fracture predicition of single reservoir (TX24) of xujiahe formation-2.Identification of gas-bearing bed in hypercompact sandstone reservoir is very difficult all over the wrold.We use the advantage of 3D3C of the prediction of p-wave and s-wave to predict the gas-bearing of single reservoir (TX24) . The velocity is influenced by formation lithology chance and fluid property.P-wave propagates through rock matrix and pore fluid, but s-wave through rock matrix not pore fluid.The velocity propagated by gas or liquid is less than propagated by solid.The velocity will be difference when Sand is occoured, especially the hydrocarbon is contained in the rocks.That results in abnormality of the ratio by p-wave and s-wave and poisson ratio. Oil-bearing Reservoirs has relation to poisson ratio.We definite the limit of tight sand,Oil-bearing sand,fractured Oil-bearing sand and shale according to convergence. At last,the result is put in seismic section to obtain the thickness map.Base on the conclusions above,we predict the TX24 reservoir of xujiahe formation-2 and devide the enrichment zone,favourable zone and less favourable zone.