Overpressure systems in the Qingshankou Formation of the Gulong Sag have a significant impact on unconventional shale oil accumulation, but their distribution and genesis are unknown. This study uses a comparative analysis of three primary pressure prediction methods—the equivalent depth method, the Eaton method, and the Bowers method—to investigate the genetic mechanisms of overpressure and their controlling factors. The study clarifies the link between overpressure and hydrocarbon distribution. The key findings are as follows. (1) The Eaton method is identified as the best approach for estimating current formation pore pressure. The Qingshankou Formation exhibits mild overpressure development, with a maximum pressure coefficient of 1.44. (2) Hydrocarbon-generating overpressure, driven by source rock maturation, is confirmed as the dominant mechanism through integrated acoustic velocity–density cross plots and logging analysis. (3) Tectonic-sedimentary factors, such as burial depth, source rock thickness, sand-mud ratio, and faults, collectively control the spatial variability of overpressure. (4) The distribution of the Gulong shale oil and the Fuyu tight oil is influenced by overpressure, with the northwestern part of the sag and the adjacent sand bodies being the respectively favorable areas. These results lay the groundwork for accurately reconstructing paleopressure and better understanding the hydrocarbon accumulation potential of shale oil and Fuyu tight oil. They also provide guidance on the exploration and development of unconventional resources.

To better characterize the pore development and evolution characteristics in the fourth member of the Quantou Formation (Q4), an accurate evaluation of the reservoir properties and oiliness for different types of reservoirs was conducted in this study. Polarization microscope and scanning electron microscope experiments were used for lithology, mineral composition, pore types, and morphology observation. High-pressure mercury intrusion experiments were used to characterize the porosity, permeability, and pore size distribution, and the Beard and Weyl model was used to calculate the porosity evolution characteristics. The results reveal the following: (i) siltstone and pelitic siltstone dominate the Q4 tight reservoir, while feldspar and quartz are the main rock-forming minerals. (ii) Intergranular pores, intergranular dissolution pores, intragranular dissolution pores, intercrystalline pores, and microfractures are the main pore types, with a porosity of 4% to 12% and a permeability of 0.001 to 1 mD. (iii) After the Q4 reservoir was divided into Grade I, II, and II reservoirs from good to poor, their current average porosity values were found to be 14.82%, 8.98%, and 3.57%, respectively. The results from this work are of great significance for accumulation space and oiliness evaluation, and they provide theoretical support for the exploitation of Q4 tight reservoirs in the study area.

Huoduomoer structural zone is one of the most favorable oil and gas accumulation zones in Beier depression of Hailaer basin. However, the exploration in this zone has experienced a long and tortuous process, and the exploration is almost stagnant until the natural production capacity of well x1 has obtained daily output of 472m³, which has achieved a major breakthrough in the exploration of tectono-stratigraphic and stratigraphic unconformity reservoirs in this structural zone. Based on the comprehensive study of the subdivided layers of the structural belt, this paper holds that the Nantun Formation suffered from long-term denudation after deposition, the Nantun Formation at the high part of the uplift belt was denuded completely, and the remaining Nantun Formation at the periclinal area was the main target layer for exploration. Based on the analysis of the main controlling factors of hydrocarbon accumulation, it is considered that feldspar lithic sandstone and lithic sandstone mainly developed in the subaqueous distributary channel deposition of braided river delta front are easy to be leached and eroded, forming high-quality reservoir with secondary porosity near the unconformity surface, and the multi-direction oil and gas migration system is the key to the oil and gas accumulation; the stable and thick mudstone deposition in the large section provides a good sealing condition for traps.