Natural gas accumulation processes of tight sandstone reservoirs in deep formations of Songliao Basin, NE China

Abstract

The deep formations (underlying strata of the 3rd Member of Quantou Formation, Upper Cretaceous) in Songliao Basin possess a large natural gas potential, with a proven reserve of more than 200 × 109 m3 (706.3bcf). However, due to deep burial depth, multiple tectonic evolutions, and sets of source rocks, complex gas accumulations and different gas reservoir types developed, which may seriously restrict the natural gas exploration and target selection. This study focuses on the hydrocarbon generation, porosity evolution, and hydrocarbon accumulation processes of the tight sandstone gas (TSG) reservoir in the Cretaceous Shahezi Formation. 1) The timing and cumulative amounts of hydrocarbon generation were ascertained, with the Huoshiling being 143–10 Ma and 108.00 mg/g TOC and the Shahezi being 122–44 Ma and 30.00 mg/g TOC, respectively. 2) The mean original porosity of the Shahezi sandstone was 23.11%, and experienced the porosity decreasing of compaction (avg. = 12.57%) and cementation (avg. = 7.07%), and porosity increasing of dissolution (avg. = 3.25%), and the average present porosity is 6.72%. The sandstone densification (porosity <10%) occurred in the late period of Early Cretaceous (~115–105 Ma). 3) Four main stages of hydrocarbon accumulation in the K1sh occurred: I) Early oil accumulation stage (128–114 Ma)，corresponding to the 1st episode of charging; II) Middle oil and gas accumulation stage (111–90 Ma), corresponding to the 2nd episode of charging; III) Late gas accumulation stage (77–67 Ma), corresponding to the 3rd episode of charging; and IV) Adjustment stage (63 Ma–present), corresponding to the uplift stage. The 2nd and 3rd stages were the main gas accumulation stages. 4) Two types of tight sandstone gas reservoirs developed in the K1sh: I) tight conventional gas, and II) tight deep gas. The tight deep gas reservoirs, with favorable potentials, are the main exploration targets in the Shahezi Formation of Songliao Basin.