The samples of 4 wells were obtained to clarify the characteristic of source rocks and sandstone reservoir (Table 1). The source rock samples were tested for geochemical characteristics. And the sandstones were for physical properties of reservoir.

The source rocks of Es4 developed in the initial rifting stage of the rift valley, mainly in the western slope of the northern and southern parts of the depression, with a thickness of 0–700 m, generally about 300 m. The dark mudstone and sandy mudstone account for the majority. The contour map of source rocks (Figure 3A) indicates that the source rocks in Niuxintuo Sag in the northeast of Gaosheng area are the most developed, with a thickness of up to 700 m. According to the evaluation criteria of source rocks, the TOC of source rocks in Es4 Member ranges from 0.14% to 8.02% (average 2.35%), and the organic matter is mainly II₁ and II₂ types. The Ro ranged from 0.3% to 1.26% (average 0.76%).

The source rock of Es3 member is dark mudstone rich in organic matter, which is widely distributed and produced in the deep depression period. The isocontour map of source rock thickness (Figure 3B) indicates that the source rocks of Niuxintuo Sag, Chenjia-Panshan Sag and Qingshui Sag are relatively developed, and the most developed is Qingshui Sag with the maximum thickness of 1,500 m, followed by Chenjia Sag and Panshan Sag with the thickness of source rock up to 1,200 m, and the thickness of Niuxintuo Sag source rock is only 700 m. Generally speaking, this set of source rocks is the best source rock with the widest distribution and the largest thickness in the Western Depression. The TOC of the source rocks in Es4 ranges from 0.03% to 1.19% (average 0.19%). The organic matter is dominated by types II₁ and II₂, and the Ro ranges from 0.28% to 1.29% (average 0.68%).

According to the structural development history, this area is mainly composed of three major structural layers: Paleogene pre-structural layer, Paleogene structural layer and Neogene structural layer. In the Mesozoic to early and middle Paleogene, fault activity was intense and frequent, showing obvious inheritance and neogenesis (Figure 4). In the middle and late Paleogene, fault activity gradually weakened, and there was basically no new fault development in the Neogene, showing stages of tectonic evolution. The reservoir formation model of the established Gaosheng reservoir is shown in Figure 5, which is conducive to revealing the formation mechanism of the reservoir.

The reservoirs include siliceous quartz sandstone, quartzite sandstone, glauconitic quartz sandstone, generally greyish-gray and dark gray, widely distributed in this area. The debris consists of fine particles, coarse and medium particles, sorting is generally good, roundness is mainly subroundness. The clastic composition is mainly monocrystalline quartz and feldspar and siltstone debris are occasionally seen. The content of quartz is generally 95%–99% of the detritus (Figure 6).

According to the porosity experiment, the porosity of quartz sandstone ranges from 0.4% to 22.1%, the porosity of less than 10% accounts for about 50%, the permeability ranges from 0.03 to 10.2 mD, and the permeability of less than 0.1 mD accounts for about 32% (Figure 7). It indicates that the quartz sandstone in Pt formation has a certain degree of density, but the heterogeneity is obvious.

Based on the data of core observation and cast thin sections, the pore types of Proterozoic reservoir rocks in this area are mainly secondary pores and fractures. Among them, the pore types mainly include: residual intergranular pores, intergranular dissolved pores, and intragranular dissolved pores. Most of the fractures are open fractures, and the fracture surface extends longer, and most of them cut the whole core. Multiple groups of fractures develop and are distributed in a macroscopic network, so the core is often broken into small pieces. The presence of a lot of fractures in the core indicates that microfractures in the quartz sandstone are well developed (Figure 8).

This map is based on data collected from drilling Wells. Since Cenozoic, the subduction of the Pacific plate to the Eurasian plate led to the influence of Tanlu fault activity in the early stage of Liaohe Depression. During the Paleocene period, under the action of regional tensile stress, the fault activity was extremely strong, represented by the deep fault activity. The Tai’ an major fault was strongly active and broke off the upper mantle. Then mantle materials poured out along the fracture and erupted to the surface, forming a thick basalt blanket in the Gaosheng—Leijia area (Figure 9).

The Fangshenpao Formation and the Mesozoic basalts are strongly controlled by the main fault and have a large thickness near the fault. It is analyzed that the eruption center is formed in the Gaosheng area, the thickness is larger in the main part of the buried hill, and it is thinner toward the low part of the buried hill (Figure 9). The basalt of this group has the characteristics of large single-layer thickness, multiple layers and large cumulative thickness. And the lithology is dense, which is a good cap layer. For example, the cumulative thickness of basalt in GG 2 well is 632 m, the cumulative thickness of basalt in GG 1 well is 1199 m, and the cumulative thickness of basalt in GG 1 well is 1189 m (Wu, 2012).

Proximity to the hydrocarbon generating center is conducive to oil migration and accumulation, which is the basic guarantee of reservoir formation (Zhao et al., 2019). The isocontour thickness maps of Es3 and Es4 in the Western Depression indicate the source rocks are thick (Figure 3). According to Section 4.1 of the results, the Es3 and Es4 source rocks have high abundance, good types, and their Ro are at the oil generation window, indicating that the source rocks have the potential to provide a large amount of crude oil and thus form large reservoirs. High hydrocarbon generation intensity is conducive to reservoir formation and the value of large reservoirs is generally greater than 350 × 10⁴ t/km² (Hu et al., 2017; Feng et al., 2021; Song et al., 2021). The hydrocarbon generating intensity of Es4 is generally 5 × 10⁶–10 × 10⁶ t/km², and that of Es3 is generally greater than 5 × 10⁶–15 × 10⁶ t/km², which also indicates that the hydrocarbon generating potential of source rocks is good (Wu, 2012). It should be pointed out that proximity to the hydrocarbon generating depression is not a sufficient condition for hydrocarbon accumulation. The possibility of accumulation increases only in the dominant direction of oil and gas migration (Guo et al., 2017) and Gaosheng Oilfield meets this requirement (Figure 5).

Palaeouplift area is the location of oil and gas accumulation, which is beneficial for the formation of various reservoir traps, such as structure, lithology-stratigraphy. The systematic analysis of reservoir development characteristics in palaeouplift area is of great significance for oil and gas exploration. If the palaeouplift is in the dominant direction of oil and gas migration, it is conducive to accumulation (Jiang et al., 2023).

The archetype of Gaosheng buried-hill palaeouplift was formed at the end of Mesozoic, and it was basically formed in the middle of Eocene (late sedimentation of Es3 Member) (Wu, 2012).

Uplift is a positive tectonic unit (Jin, 2012). In the oil-gas accumulation unit, uplift is located in the low potential area of hydrocarbon fluid migration, which is a favorable place for oil-gas accumulation. At the same time, the traps formed during the formation and evolution of uplift are mainly anticlinal traps. The migration of oil and gas in such traps is more affected by buoyancy and tends to converge to the apex. Therefore, this type of reservoir is formed by the movement of oil from the deep to the shallow layer through faults and unconformities under the action of buoyancy. The oil migrated to the direction of regional uplift and accumulated in the suitable trap (Figure 5) (Liu et al., 2015).

Reservoir physical property is an important factor affecting the formation of oil and gas reservoirs, especially for tight sandstone reservoirs. The development degree of reservoir porosity, permeability and fracture have an influence on the formation of hydrocarbon reservoirs (Liu et al., 2017; Liu and Xiong, 2021).

The main reservoir in the study area is the sandstone of Pt formation. The pore types mainly include residual intergranular pores, intergranular dissolved pores, intra-granular dissolved pores and fractures (Figure 8), which provide sufficient space for the accumulation of crude oil.

Fracture can improve the permeability and the physical properties of the reservoir, and also seriously influence on the distribution of the reservoir (Jiang et al., 2015; Zhao et al., 2022)

From the analysis of failed wells in and around this area, it can be concluded that undeveloped reservoir or poor reservoir physical properties are the main factors leading to failure, accounting for the largest proportion. In the favorable sand body distribution area, good exploration results have been obtained in recent years. This is also evidenced by the high oil flow rates of wells GG14, SG169, and SG158. On the contrary, good result was not achieved due to poor reservoir conditions in the wells GG1 and GG15.

The distribution of sand body and the physical properties of reservoir restrict the enrichment and high production of oil and gas, so the comprehensive study on the distribution of reservoir and sand body should be strengthened.

The formation of a large number of reservoirs requires an overlying layer as a good preservation condition (Guo et al., 2017; Liu et al., 2017; Jiang et al., 2022). The thickness of cap layer is positively correlated with oil and gas reserves (Liu et al., 2017). The basalt cap layer with large thickness is developed in the Gaosheng area, and the largest area can reach 2100 m. For example, the cumulative thickness of basalt in GG 2 is 1903 m, and the cumulative thickness of basalt in GG 1 is 1189 m. The cap layer has large thickness and stable distribution, so it is a regional cap layer to preserve the reservoir. In addition, the reservoir and overlying basalt constitute a good reservoir-cap combination.

On the basis of comprehensive analysis of main controlling factors of accumulation in Gaosheng area, favorable exploration zones are pointed out. According to the evaluation ideas and indicators, the reservoir forming factors such as structural conditions, source rock conditions, reservoir development, preservation conditions and trap development in Gaosheng area are evaluated. Combined with previous studies (You et al., 2006; Yu, 2016), it is considered that the formation conditions of the Gaosheng area are superior and the Proterozoic and Mesozoic oil-gas accumulation associations are the most promising in the Gaosheng area. Due to the small number of drilled wells, the low degree of exploration and the poor quality of seismic data, the development of fault system and the connection between reservoir and source rock are not fully evaluated. The Proterozoic and Mesozoic oil and gas accumulation associations in the middle-buried hill belt are the direction of oil and gas migration, and are potential exploration fields with great potential, which must be paid sufficient attention to.

In addition, the formation high pressure caused by hydrocarbon generation may also be one of the main controlling factors of oil and gas accumulation, but there is a lack of related research, so this should be one of the future research focuses.