Gas-Bearing System Classification and Gas Reservoir Formation Characteristics for Unconventional Natural Gas of Upper Paleozoic in Ordos Basin

Coal seams, organic shale, and sandstones undergo vertical interbedded development in coal-bearing strata of Upper Paleozoic in Ordos Basin. Based on the sedimentary background analysis of research areas, and in combination with comparison of gas isotope, gas measurement and hydrology, the paper comprehensively analyzed the gas-bearing system classification for unconventional natural gas of Upper Paleozoic in Ordos Basin. Through an in-depth analysis of gas-bearing property and reservoir physical property of various gas-bearing systems, the paper also expounded gas reservoir formation characteristics for unconventional natural gas of Upper Paleozoic in the research areas. Results showed that the gas-bearing systems of Upper Paleozoic in Ordos Basin were divided into the shale gas-bearing system in Member 1 of Taiyuan Formation and the mixed-source gas-bearing system in Member 2 of Taiyuan Formation ~ Member 1 of Xiashihezi Formation. The shale was the type of gas source rock of the shale gas-bearing system in Member 1 of Taiyuan Formation, with an average organic matter content of 3.3% and an average gas amount of 5 m³/t. There was a high content of brittle minerals in the reservoir and a relatively developed micro-fracture. It was characterized by “low porosity, low permeability, and low gas saturation” on the whole. Coal seams and shale were the main types of gas source rock of the gas-bearing system in Member 2 of Taiyuan Formation∼Member 1 of Xiashihezi Formation, of which the coal seams had an organic matter content of 22.32%∼81.51% and a gas amount of 17 m³/t, and the shale had an organic matter content of 0.04%∼2.73% and an average gas amount of 3 m³/t. The main reservoir contained coal seams, dark shale, and tight sandstones, and was characterized by “low porosity and low permeability” on the whole. The shale in Member 2 of Xiashihezi Formation and in Shangshihezi Formation presented itself as regional cap rock that had a continuous planar distribution and a good sealing ability, and whose thickness was from 50 m to 200 m. The unconventional natural gas reservoirs in the research areas were mainly divided into the in-source combination gas reservoir in Member 1 of Taiyuan Formation and the mixedsource combination gas reservoir in Member 2 of Taiyuan Formation∼Member 1 of Xiashihezi Formation.