Abstract
The vague understanding of the coupling relationship among natural gas charging, reservoir densification, and pressure evolution restricted the tight gas exploration in the Lower Shihezi Formation of the Hangjinqi area, north Ordos Basin. In this study, the quantitative porosity evolution model, the pressure evolution process, and the natural gas charging history of tight sandstone reservoirs were constructed by integrated investigation of the reservoir property, the thin section, SEM and cathode luminescence observations, the fluid inclusion analysis and the 1D basin modeling. The results show that the compaction and cementation reduced the primary porosity by 21.79% and 12.41%, respectively. The densification of the reservoir occurred at circa 230 Ma, which was before the natural gas charging time from 192 to 132 Ma. The paleo-overpressure within the tight reservoirs occurred since the Middle Jurassic with the pressure coefficients between 1.1 and 1.55. The continuous uplifting since the Late Cretaceous resulted in the under- and normal-pressure of the Lower Shihezi Formation with the pressure coefficients ranging from 0.67 to 1.05. The results indicate that the densification of the reservoirs was conducive to the formation of paleo-pressure produced by gas generating. The gas predominantly migrated vertically, driven by gas expansion force rather than buoyance and displaced the pore water in the reservoirs near source rocks.
Highlights
The tight sandstone gas is an important unconventional hydrocarbon resource with widespread distribution and considerable potential over the world [1–5]
As more attentions were paid to margins of the basin, a large quantity of tight gas with geological reserves more than 700 × 109 m3 was found in the Upper Paleozoic coal-bearing successions in the Hangjinqi area [10]
It has been widely believed that the tight sandstone reservoirs in the Ordos Basin have been subjected to deep burial and complex diagenetic modifications, resulting in tiny pores and throats, strong heterogeneity, and poor physical properties with porosity and air permeability generally less than 10% and 1 mD [11–14]
Summary
The tight sandstone gas is an important unconventional hydrocarbon resource with widespread distribution and considerable potential over the world [1–5]. The continuous or quasi-continuous gas accumulations tended to be formed when the reservoir densification occurred before gas charging, which were generally characterized by extensive distribution without distinct boundaries, complicated gas-water contacts and abnormal original reservoir pressure [2,11,16]. It is very difficult for the gas to migrate over long distance due to the great capillary resistance and strong heterogeneity in this type of tight reservoir [17]. Some researchers have studied the reservoir quality and its controlling factors [18,19], the porosity evolution and densification time of the Permian tight sandstone reservoirs are still unclear in the Hangjinqi area. The hydrocarbon charging time and accumulation process have been demonstrated by previous works [20,21], but their relationship with reservoir densification was not properly constrained in the research area
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