Diverse effects of intragranular fractures on reservoir properties, diagenesis, and gas migration: Insight from Permian tight sandstone in the Hangjinqi area, north Ordos Basin

Abstract

As petroleum industry tilts towards unconventional petroleum resources, the significance of fractures in exploration and development becomes increasingly prominent. In this study, thin sections and scanning electron microscopic (SEM) observation, porosity and permeability measurement, mercury injection porosimeter (MIP) and computerized tomography (CT) analysis were carried out to investigate the morphology and development characteristics of intragranular fractures, and their effects on physical properties, reservoir diagenesis, and gas migration of the He-1 Member tight sandstones in the Hangjinqi area. Results show that the tight sandstones have intergranular and intragranular dissolved pores, intercrystalline pores and heterogeneous distributed intragranular fractures. The average porosity and permeability of the tight sandstones with intragranular fractures are 12.76% and 4.70 × 10−3 μm2, while those lack of fractures are 6.06% and 0.574 × 10−3 μm2, respectively. The fracture density increases with the enlargement of clastic-grain size and the content of rigid grains. Results indicate that the intragranular fractures greatly enhance permeability, but they have limited direct contribution to total porosity. And the sandstones rich in intragranular fractures have a more homogeneous and connective pore structure with larger radius of pore throat. Intragranular fractures are beneficial to the latter feldspar and debris dissolution caused by organic acids derived from hydrocarbon generation in coal-bearing strata. The dissolved pores directly enhance the porosity of tight sandstones compared to intragranular fractures, and together they form a percolation network to increase permeability. Faults mainly control the lateral petroleum migration from the south to the north, and intragranular fractures affect the local accumulation of gas. This study helps to illustrate the influences of intragranular fractures on reservoir properties, diagenetic difference, and gas migration of the He-1 Member tight sandstones in the Hangjinqi area, and can guide the evaluation and exploration of tight sandstone reservoirs in the Ordos Basin and other basins with similar settings around the world.