Research On Reservoir Evaluation And Composite Prediction For Xu-2 Gas Pool In Xinchang Gas Field

Aiming at geologic features of Xu-2 (the second member of Xujiahe formation) in Xinchang gas field, based mainly on reservoir forming geology, combined multidiscipline of geology, seismic, logging and gas reservoir engineering, intergration research of interpenetration was made. Proceeded with developed gas reservoir for Xujiahe formation in Western Sichuan Depression, each data of completion well and pilot production well was analyzed deeply, and then research on single well, profile, plane and space were organically combined. The structural characteristics, sedimentary facies, sedimentary microfacies, division of reservoir sandbodies, the reservoir characteristics and heterogeneity were described thoroughly by the conventional and specific reservoir description techniques; then combining production behavior and the geophysical features of response, gas-bearing identification was analyzed; and combining seismic, prediction of deep gas-bearing sand body and new prediction techniques of effective fractures, especially applications of 3D3C outcomes, the favorable enrichment areas were comprehensively predicted and evaluated; reserves in gas enrichment areas were calculated and it laid a foundation for formulating development scheme of oil and gas.Through research in detail, understanding of geologic features for Xu-2 in Xinchang gas field has obtained:1. Xinchang-Xiaoquan structure totally exhibites NEE-trending, developing multiple variable trending and highs.These highs formed a large composite anticline structure with trap area of 164.75 km2 and closure amplitude of 151m. Minor fractures develops in this area, mainly distributed in the middle and eastern part, exihibiting near NNE and NW-trending; its extending length is approximately 2-4 km, and fault throw is small, approximately 20-45m.2. Sedimentary facies for Xu-2 in Xinchang gas field is dominated by deltaic front sandbar, partly developed deltaic plain distributary fluvial channal. Thickness of sand body is big, with extensive range and stable thickness, and its burial depth is 4500-5200 m. Sedimentary microfacies for main gas layers is dominated by channal mouth bar and distributary fluvial sand bar. Xu-2 is divided into 9 sand groups from the bottom to top, in which TX22,TX24,TX25,TX27 are the main gas-bearing sand groups, and sand body exihibits layered distribution, with large thickness, and its average thickness is 45.9-86.7m.3. Xu-2 in Xinchang gas field is dominated by litharenite, debris quartzitic sandstone and quartz lithic sandstone. Rocks are very tight with high content of feldspar in middle-sub member, which includes clay mineral. Xu-2 is super low porous, tight and super tight with low water bearing reservoir. Affected by diagenesis and epigenesis, the reservoir porosity takes induced porosity as priority, primary porosity secondly, and some microfissure added. The reservoir is dominated by the combination of micropore-microthroat and pinhole-microthroat. The reservoir type to get the stable and high production should be fracture-pore type.4. The differences of sedimentary microfacies, sand-body distribution, lithology, diagenetic epigenesis and fracture development led to the characteristics of strong heterogeneity in interlamination, intralamination and plane, and it also increases the difficulty of usage of gas reservoir reserve and reasonable and effective development.5. Comprehensive prediction of fracture is based on seismic cube and seismic stratification plane curvature, inclination+P wave azimuthal anisotropy+shear wave splitting of reservoir surface, and comprehensive prediction of fracture development zone is evaluated taking reservoir as unit. It is divided into developd area and relatively developed area in plane. Agreement of fracture prediction has been proved by new completed wells.6. Gas-bearing properties of Xu-2 is predicted by the following four types: absorption and attenuation proceeded by combining wavelet decomposition technique, AVO composite index inversion, composite gas-bearing index(AiEi27/Si)and seismic data structural body. In general, gas-bearing properties is predicted accuratedly through absorption coefficient law and AVO pridiction law.7. Combined with static and dynamic characteristics, Xu-2 gas pool can be defined as layered dry gas and edge water drive gas reservoir which is characterized by tightness and abnormally-high pressure and controlled by structural trap. Negotiability is poor among interlayers, but it is good in layers.8. Combined with reservoir, fractures and the outcomes of gas-bearing detection, comprehensively prediction techniques- seismic " dark spot" techniques for high-permeable area has been preliminaryly developed. "dark spot" is divided into 2 types by this techniques; one is PP wave and PS wave ( "dark spot"), with high production and stable production in this kind of gas well; another is PP wave dark spot, PS wave continuity, with low production in this kind of gas well. On the basis of dark spot prediction, gas-bearing area and its reserves for high-peameable area were calculated as 103.6km2 and 843.8×108m3 respectively. It laid a foundation for formulating scheme of rolling exploration and development.