Reservoir-forming Characteristics Of Carboniferous-Permian Coal-formed Gas In Chezhen Sag

In recent years,Chezhen Sag has been made great breakthroughs in oil and gas exploration and has been changed the previous understanding of "unfavorable oil and gas source conditions" in this area.However,there are still some basic geological understanding is not clear,which is urgent to further study.This dissertation mainly focused on the division and correlation of Carboniferous-Permian in Chezhen Sag,the development and evolution of sedimentary facies,the characteristics of coal measures source rocks(coal and dark mudstone)development and hydrocarbon generation,the physical properties of sandstone reservoirs and their genetic mechanism,and the characteristics of Source-reservoir-cap combination.Under the guidance of sedimentology,petrology,coal geology,oil-gas geology and other theories,combined the theories and practices,though the means of field investigation,systematic sampling and experimental analysis,the following understandings were obtained:1)The Carboniferous-Permian standard stratigraphic section had been established.In Chezhen Sag,the Carboniferous-Permian(remnant)was mainly distributed in the northwest-north-northeast,followed by the south,and the denudation in the central-southwest-southeast was severe or even exhausted.2)The tidal flat facies,barrier-lagoon facies,river-controlled shallow water delta facies,fluvial facies and lacustrine facies were identified,the development characteristics of various sedimentary facies were analyzed,and the distribution pattern of those sedimentary facies in different periods was reconstructed.3)The development and distribution law of Carboniferous-Permian coal measures source rocks were found out,their hydrocarbon generation potential was analyzed,which were determined to be medium-better gas source rocks.The characteristics of primary and secondary hydrocarbon generation from coal measures source rocks were studied.The secondary hydrocarbon generation was the main gas generation process of coal-formed gas reservoir forming,and a typical hydrocarbon generation model was established.4)The development(remnant)and distribution law of Carboniferous-Permian sandstones had been found out.The Carboniferous-Permian high-quality reservoirs were mainly Taiyuan Formation sandstone and Upper Shihezi Formation quartz sandstone.The porosity and permeability of these reservoir varied greatly,with medium-low porosity(0?15%)and low to ultra-low permeability(0.1?10×10-3 m2)and strong heterogeneity.The reservoirs were deeply buried,the porosity had a negative correlation with the buried depth and a positive correlation index with the permeability.The median particle size had a linear positive correlation with porosity and a positive correlation index with permeability.5)Sandstone reservoirs were mostly in the middle diagenesis stage A,and some belong to the late diagenesis stage B;The pore type of sandstone reservoir was mainly secondary intergranular dissolved pores,followed by primary intergranular pores.Compaction,cementation and dissolution had the greatest influence on reservoir physical properties,and dissolution plays a major role in increasing porosity of deep reservoirs.6)There were three types of source-reservoir-cap combinations for coal-formed gas accumulation:self-generating and self-storing,upper-generating and lower-storing,lower-generating and upper-storing.From the Upper Carboniferous Benxi Formation to the Upper Permian Upper Shihezi Formation,five sets of source-reservoir-cap combinations could be divided,and got better from bottom to top.The Permian coal-formed gas source-reservoir-cap combination was better-good in the study area.