Study On Reservoir Geology Of Coal-formed Gas In Pre-tertiary In Gubei Deeply Buried Hill

The gas pools existing in shallow-middle depths are important gas exploration targets in Jiyang basin. The found of Coal-formed gas in Gubei Deeply Buried Hill marked a large breakthrough of Coal formed gas exploration in Jiyang depression. Thus,it's important to study the gas reservoir. Based on review of the previous studies, according to core observation data, the sedimentary facies, rule of space distribution of sand body, lithologic characteristics and evolution, the type of storage space, the genetic mechanism and controlling factors of gas reservoir were studied by means of some technical methods such as measured porosity data, thin sections, blue epoxy resin-impregnated thin sections, electronic scanning and grain size analyses, X-ray diffractions and fluid inclusion testing technique and log and well logging data. Through investigation, it was considered that: (1) sandy deposit, which is mainly featured by river facies developed in Upper Shihezi Formation of Permian and deltas faces developed in Lower Shihezi of Permian and Shanxi Formation of Permian become the main reservoir of coal gas. The faces can be divied into four sub facies, nine micro-facies the channel fill formed by channel detention and marginal bank, water channels sub-water channels and debouch bar were the favorable reservoir facies. (2) In this area sandstone diagenesis of Permian system is very srong, Sandstone diagrnetic evolution has meched the A-B matured degree of mesodiagenesis. Based on diagenetic analysis, the reservoir can be divided into four types of diagenetic facies such as strong compaction pressolution facies, strong dissolution facies, strong carbonate cementation facies and fracture facies, and pick out the diagenetic evolution model of reservoir. (3) Reservoir rocks are poor in physical properties whieh are characterized by low porosity san low permeability.The Secondary dissolution porosity is the major reservoir space of the sandstone and it's about 80% of porosity. Fresh water leaching and Dissolution resulted from the organic acid and CO2 resulting from organic thermal evolution of coal-measure strata are important mechanisms in producing large secondary porosity in the area .Diageneses, Sedimentary facies, tectonic movement and fluid and so on is the important control factors. The Secondary dissolution pores exist in the channel fill formed by channel detention and marginal bank, water channels sub-water channels and debouch bars. Secondary pores.often discovered beneath the unconformities, clastic rocks near the coal and fracture developed areas.