--> Origin, Pore Structure and Recognition of a New Kind of Low Permeability-Tight Sandstone Reservoirs: High GR Sandstone

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Origin, Pore Structure and Recognition of a New Kind of Low Permeability-Tight Sandstone Reservoirs: High GR Sandstone

Abstract

There developed low permeability-tight sandstone in Cretaceous, Hailar-Tamtsag Basin, China. Different from normal sandstones that have low GR in well logs, a special kind of sandstone with high GR developed in Tongbomiao Formation, Cretaceous. It was usually interpreted as shale in well logs. This study is focus on the high GR reservoir. Through drilling core observing, petrology analysis of thin slice, well logging analysis and petro-physical test, the sandstone is found to have high volcanic debris content. Sedimentology study indicates that volcano activities were very active in Tongbomiao stage, Cretaceous in the basin. In the area far from volcano craters, tephros deposited with terrigenous clastic. So the sandstone has high volcanic debris content and this causes high GR in well logging. Tuffaceous component plays an important role in reservoir digenesis and pore structure evolution. High tuffaceous content intensified compaction and destroyed the major inter-particle pores in the early digenesis stage. Pore structures were also influenced during this process. In the late digenesis stage, tuffaceous component was dissolved to produce secondary pores and reservoir porosity and permeability are improved. Sweet spots in tight sandstone are developed during this process. Pore structures are classified as 4 types with microscale image analysis and capillary pressure test of rocks. Macro-physical properties of rocks, such as porosity and permeability, have good positive correlation with pore structures. The second origin of high GR reservoir is clay mineral types and contents. In the analysis of relationship between GR value of reservoirs and clay contents, content of illite and I/S is found to have a positive correlation with reservoirs' GR value. The illite and I/S content in high GR reservoir is 6 times of that in normal sandstone reservoirs. In recognition of high GR reservoir originated by high tuffaceous content, photoelectric absorption index(Pe), which cannot be influenced by rock's radioactivity, is chosen. Pe from natural gamma ray spectrum logging is used to build a Pe-GR method to interpret high GR sandstone from shale. For high GR reservoir originated by high illite and I/S contents, a well log interpretation method based on volume model is built.