The origin of the overpressure in the northern Qaidam Basin has not been clearly understood, which has caused some difficulties in hydrocarbon exploration. Using a compaction study, we applied a modified acoustic-velocity and effective-stress diagram to identify the overpressure transfer in the study area. This phenomenon has not been discussed in previous studies. For the present study, we approximately calculated the magnitude of the transfer overpressure and analyzed the cause of the overpressured aquifer at the crest of the anticline in the study area. Our study indicates that the effect of overpressure transfer is very distinct, and the largest contribution to the total overpressure is 57%. The main media of overpressure transfer include vertical faults and lateral conducting layers. The vertical faults can connect deep overpressured strata, and the lateral conducting layers can connect overpressured strata at the top and wing of the anticline. During anticline formation, the crest fractures, and then the overpressured water in the anticline wing flows into the fractured crest and forms the overpressure compartment that prevents the charging of deeper natural gas.

Lowly permeable sandstone reservoirs play an important role in the exploration and exploitation of natural gas and petroleum in China. The reservoirs are major lowly permeable sandstone reservoirs in Chang 2 division, Yanchang Formation, Upper Triassic in Zhang-Han oilfield, which located in the northern Shaanxi slope of Ordos Basin. According to the distribution and composition of sand beds, integrated measured physical properties, micro-pore structure analysis, cast thin section observation, scanning electron microscopy, the impacts of deposition and diagenesis on porosity evolution are analyzed. The essential diagenesis causing the porosity loss is evaluated quantitatively, and finally the origin mechanisms of low permeability reservoir in Zhang-Han oilfield are discussed. The results show: (1) Fine particle and low compositional maturity arkose are the material foundation of the formation of poor physical property sandstone; (2) The main pore space of reservoir is secondary pores. There are two types of combined pores that including dissolve-residual pores and dissolve-micropores. The porosity values display an approximately normal distribution, and permeability values are asymmetric distribution of the logarithm in lowly permeable sandstones. Their correlation coefficient becomes more and more worse with the decrease of permeability; (3) There are four diagenetic facies, in which three diagenetic facies belong to extra-lowly permeable and ultra-lowly permeable reservoir sandstones and widely distributed, and they are diagenetic lithofacie background of lowly permeability sandstone; (4) In low compositional maturity arkose, its initial porosity is 1/4 lower than conventional reservoir, the secondary and dissolved pores are main pore types of lowly permeable reservoir rocks. It is also a key factor of effective oil-bearing of lowly permeability sandstone.

Palaeogeomorphology plays a crucial role in the development of sedimentary facies and the reservoir distribution in the late period of basin evolution, and to a certain extent, controls the assemblage of the late hydrocarbon reservoirs and caprock formations. Therefore, it has a function in controlling the enrichment of oil and gas. Caledonian orogeny uplifted the Ordovician Majiagou Formation in the Ordos basin and thus led to the formation of the weathering crust reservoirs in the process of weathering, erosion and leaching for over 130 Ma. Such palaeogeomorphology represents the background of the palaeokarst, which has an essential influence on the formation of reservoir rocks. Based on Pre-carboniferous paleogeologic map plotted by 305 wells, and the thickness of bauxite in 272 wells, methods of residual thickness and moulage are applied to reconstruct the palaeogeomorphology in the studied area. In combination with the thickness of horizon to the weathering crust, the criteria for the second order palaeogeomorphologic units were proposed. They are karst highland, karst slope and karst basin, which can be further subdivided into the third order palaeogeomorphologic units of remnant hill, depression, gentle hill, shallow depression and erosional grooves. According to the criteria and refined identify results of ancient grooves using geological data of 272 wells, palaeogeomorphology of the Ordovician weathering crust in the northern Jingbian gas field has been restored accurately. The results show that karst highland developed in the northwest part of the studied area, with remnant hill and depression occurring locally. Karst slope was present in the central and east, predominatly in the form of gentle hills and shallow depressions, with 5 nearly EW-trending ancient erosional grooves developed locally. Finally, integrated data of reservoir property with gas production test implied that the remnant hill in the karst highland, the gentle hill in the karst slope and the two sides of the erosional grooves were favorable positions for gas accumulation.