Diagenetic Field Analyses And Causes Of The Anomalously High Porosity Zones In The Deep-Shallow Strata Of Xujiaweizi Fault Depression

On the basis of existing petrographic and mineralogic analyses, certain new tests are ran for deeper understanding of the diagenetic mechanism, porosity evolution characteristics, regional diagenetic rule and its influencing factors of deep horizon in Xujiaweizi faulted area. The study of diagenetic environment is consist of the analysis of Geothermal field, Ancient geothermal, pressure field and fluid field. Production tests of the static temperature data of twenty-six exploratory wells show that, the change range of geothermal gradient is a small one between 3.55℃/100m～3.99℃/100m with an average value of 3.83℃/100m.The study of diagenesis includes petrographic and mineralogic analyses like vitrinite reflectance Ro, sporopollen color index TAI, homogenization temperature of fluid inclusion, X-ray diffraction, combined gas chromatography mass spectrometry, pyrolysis analysis, organic acids analysis, microscopic identification of casting and non-casting thin-sections, scanning electron microscope and electron probe. These data reveal the clastic diagenetic and porosity evolution features of Songliao Basin. During the study, a revision of the current diagenesis division standard was made. The late diagenesis period is divided into two substages. Hence, the diagenetic stages of the clastic rocks in Xujiaweizi faulted area are A and B substages of early diagenetic stage, A1,A2,B1 and B2 substages of middle diagenetic stage, A and B substages of late diagenetic stage, with the corresponding bottom depth of 700m,1050m,1400m,2400m,3000m,3300m,4500m and 4850m. As the results show, there are three anomalously high porosity zones in 600m～2350m,2500m～3500m, 3600m-4300m respectively in the northern Songliao Basin. The first anomalously high porosity zones are distributed in the middle-shallow horizons, mainly formed by organic acids generated from kerogenⅠandⅡdissolving sandstone reservoirs. The second and the third ones are distributed in the deep horizon, formed by organic acid generated from kerogenⅢdissolving volcanic reservoirs and glutenite reservoirs, and meteoric water leaching is important, too. According to the characteristic of the diagenesis evolution, Xujiaweizi Fault Depression can be divided into four diagenesis zones. It shows geothermal gradient's characteristics of "strong in uplifted area and weak in depression area".