The rock types of the Long 4 + 5 formation group in the Baoziwan-Majiashan area of the Ji Plateau Oilfield are all dominated by feldspathic sandstones and clastic feldspathic sandstones (Table 1), and the average contents of quartz, feldspar, and clastic are 41.70%, 39.40%, and 19.08%, respectively, and the maturity of the mineral compositions are all low (Figure 3). The sandstone grain

size is fine, dominated by fine sand, rounding degree is dominated by sub-angular shape, sorting is medium-good, the contact relationship between particles is mainly line-point and point-line contact mode, a small amount of point-contact and concave-convex-line contact mode. The cementation type is mainly pore-type cementation, with occasional small amount of enlarged-porosity and pore-film type cementation. The high content of rock fills reflects the lower mineral maturity and medium structural maturity of the study area. The highest content of metamorphic clasts in the sandstone clasts composition, and the higher content of mica reflect the lower compaction resistance of the rocks (Table 2).

The filler contents of the Long 4 + 5 formation group in the Baoziwan-Majiashan area of the Ji Plateau Oilfield are generally high, mainly kaolinite, mica, chlorite, iron calcite, iron dolomite, siliceous and feldspathic (Table 3), of which kaolinite, mica, iron calcite, and siliceous have the highest contents, accounting for 92% of the overall filler contents, whereas chlorite, iron dolomite, and feldspathic have lower contents. Iron calcite cementation and feldspathic kaoliniteization have resulted in the filling of residual primary intergranular pores in the sandstones and the reduction of pore permeability, especially iron calcite cementation, which has a great impact on the physical properties of the whole study area.

Scanning electron microscope observation shows that the quartz grains in the sandstone of the Long 4 + 5 oil formation group in the Baoziwan-Majiashan area of the Ji Plateau Oilfield have clear shell-like fractures, and are obviously compacted and tightly cemented (Figure 4(a)); locally, a large amount of booklet-like kaolinite is found cemented between grains, and is mixed and cemented with the clay minerals (Figure 4(b)); chlorite is in the form of rose petals and is adherent to the surface of the grains (Figure 4(c)); and in addition to that, a large amount of needle-filament-like kaolinization can be observed between grains. Intergranular needle filamentous illite with mixed clay-crystalline calcite collodion can be observed (Figure 4(d)), which also affects the reservoir porosity to some extent.

Through the observation of the cast thin section of the long 4 + 5 section in the study area (Figure 5), the reservoir develops intergranular pores, followed by dissolution porosity, with an overall low facies porosity (Figure 6), and the main types of pore assemblages are intergranular pore-dissolution pore type, intergranular pore-microporous, dissolution pore-intergranular pore type, and dissolution pore-microporous type.The central mechanism for the formation of dissolution pore erosion in rock cores is related to two aspects: the dissolution process of the rock itself and the erosive effect of the fluid. Throughout geological evolution, rocks have been subjected to continuous erosion by dissolved groundwater, especially in areas where the groundwater is rich in dissolved components. In carbonate areas, for example, groundwater is often saturated with carbonic acid, which effectively dissolves the carbonate content of the rock, thus contributing to dissolve pore erosion.

1) Intergranular pore-solution pore type

There are tiny pores between residual particles inside the reservoir, and their surfaces are often covered by chlorite to form a thin film, which results in an extremely heterogeneous pore distribution. In addition, the dissolution pores are also one of the main storage spaces in the reservoir, and they are very unevenly distributed and have good connectivity. These intergranular pores-solution

pores are one of the main types of pore assemblages in the reservoirs of the target section (Figure 7(a)).

2) Intergranular pore-microporous type

The residual intergranular pores in the reservoir are more developed, and the residual intergranular pores are mostly filled with authigenic clay minerals. The sandstone mainly develops micropores within the filler and authigenic mineral intergranular micropores, and chlorite intergranular pores and yi/montmorillonite mixed-layer mineral intergranular pores are common in the study area (Figure 7(b)).

3) Dissolution pore-intergranular pore type

Pores formed between particles as a result of dissolution, as well as dissolution pores within feldspars and clasts, are visible in the study area. At the same time, the distribution of these pores shows significant heterogeneity and poor connectivity between the pores. In this study area, this type of pore assemblage is more widely developed in sandstone reservoirs with poor physical properties (Figure 7(c)).

4) Soluble pore and microporous type

Inter- and intra-solution grain pores are commonly found in the sandstone, and the distribution of pores is relatively uniform and the pores are well connected with each other. In addition, intergranular microporosity exists in the sandstone reservoirs in the study area, and this form of pore assemblage is one of the main types of sandstone reservoirs (Figure 7(d)).

According to the observation of the gross pipe pressure curve of the sandstone reservoir in the long 4 + 5 section of the study area, the discharge and driving pressure of the sandstone reservoir in the long 4 + 5 section is generally high, and it is classified into three types of pore structure types with reference to the regional standard (Figure 8):

Class I (low discharge and driving pressure-fine throat, microfine throat type): when the discharge and driving pressure (Pd) is less than 1MPa, and the median mercury saturation pressure (P50) is less than 4MPa, the maximum radius of the pore and throat is smaller, the curve shows obvious platform area, and the mercury saturation is higher. The pore and throat sorting effect is better, indicating that this is a high-quality reservoir.

Class II (medium-discharge driving pressure-microfine throat type): Pd is 1 - 2 MPa, and under this condition when P50 is about 4 - 8 MPa, the curve shows a clear plateau, and the mercury feed saturation is high. The grading of the pore throat is at a medium level, indicating that this is a better reservoir.

Class III (medium-discharge driving pressure-microthroat type): Pd ranges from 1 to 2 MPa, P50 is higher than 8 MPa, the curve shows double porosity characteristics, and the mercury feed saturation is relatively low. The pore-throat sorting effect is poor, indicating that the reservoir belongs to poorer reservoirs.

From the statistics of oil test data, the implementation of I, II type of hole and throat structure is effective, and the proportion of I, II type of long 4 + 5 reservoir is high.

According to the statistics of core analysis data of 618 samples in the study area, in Majiashan-Baoziwan area, the porosity of the long 4 + 5 reservoir has a wide range of variations, with a maximum of 17.22% and a minimum of only 6.06%, and an average value of 11.11%. The porosity is mainly concentrated between 10% and 12% (see Figure 9(a)). Correspondingly, the permeability varies greatly, with the highest being 24.2 × 10⁻³ µm² and the lowest being 0.10 × 10⁻³ µm², and the average value is 1.16 × 10⁻³ µm². The permeability is mainly distributed between 1.1 × 10⁻³ µm² and 2.1 × 10⁻³ µm² (see Figure 9(b)), which is in the category of low-porosity and low-permeability reservoirs.

The sedimentary phase of the Long 4 + 5 formation group in the study area belongs to the delta front subphase. In this subphase, a variety of sedimentary

microphases are developed, mainly including submerged diversion channels, submerged natural dikes, tributary interbays and estuarine dams. The sediments consist mainly of light gray, gray, gray-brown, and gray-green fine sandstones and siltstones. The lithology of the underwater diversion channel is mainly light gray medium sandstone and fine sandstone, with low mud content, and the development of small-scale cross-stratification and parallel stratification, etc.; the underwater natural dyke is mainly interbedded with light gray siltstone and grayish-black mudstone, with fine grain size, which is not very conducive to the development of reservoirs, and the development of horizontal stratification and undulating cross-stratification; the inter-bay of tributaries is mainly dark gray and grayish-black mudstone, which is not very conducive to the development of reservoirs, and the development of horizontal stratification, blocky stratification; the estuary dam is mainly composed of light gray, grayish brown, grayish brown, grayish green fine sandstone and fine sandstone. The estuary dam is mainly powder-fine sandstone, with better sandstone sorting and less mud content, which is also favorable for reservoir development, and develops parallel laminations, undulating laminations and horizontal laminations (Figure 10). From the existing thin-section identification data and physical analysis data of the long 4 + 5 oil formation group in the Baotiwan-Majiashan area (Table 4), it can be seen

that the physical properties of fine sandstones in general are medium-good, and the physical properties of fine sandstones are poor, while sandstones with high mud content are basically poor or non-reservoirs, which indicates that the physical properties of reservoirs are positively correlated with the grain size of sandstones, and that the grain size of sandstones is also controlled by the depositional microfacies, of which the microfacies that are most favorable for the development of reservoirs is the front of the delta The most favorable microfacies for reservoir development are the delta front and the underwater diversion channels and estuarine dams.

The main lithogenic effects affecting the physical properties of the reservoir are compaction, dissolution and cementation [15] - [19] . Through the observation of various diagenetic components, pore types and authigenic mineral assemblages and other characteristics in the thin section of rock casts, it is concluded that the main diagenetic characteristics of the study area are:

Plastic clasts such as mica in the sandstone generally experienced strong deformation, and some of the muddy clasts formed pseudohybrid bases under pressure. In addition, it can be observed that individual rigid grains have fractured during compaction, and the clastic grains are tightly packed together in linear contact (e.g., Figure 11(a) and Figure 11(b)). This indicates that the

studied reservoir sandstones underwent significant changes under compaction. The proportion of primary intergranular pores in sandstones usually does not exceed 50%, and in most cases is in the range of 30% - 45%. In contrast, the proportion of inter- and intra-granular solution pores is usually higher, typically between 40% - 55%. These two types of porosity play a dominant role in the study area and form by very different mechanisms (Table 5).

1) Compaction

In the study area, due to the high clay mineral content in the original sediments, under compaction, clay minerals and heterogeneous bases lead to a significant decrease in the porosity and permeability of the reservoir through the process of migration and adjustment. At the same time, compaction can also lead to the fracture of friable minerals such as feldspar, which in turn generates feldspar fracture fissures and expands through the process of dissolution and erosion, increasing the permeability of the reservoir (Figure 11(c) and Figure 11(d)).

2) Cementation

The long 4 + 5 cementation in the study area is mainly manifested as chlorite ring edge cementation and local carbonate cementation, etc. The cementation can fill both primary and dissolved pore space. The colluvium can fill both primary and dissolved pores, greatly reducing the storage space; however, the colluvium formed at the early stage of diagenesis has a certain inhibitory effect on compaction while filling pores, such as chlorite colluvium at the early stage of diagenesis, which makes a certain amount of primary pores to be preserved, and also lays a material foundation for the later acidic dissolution (Figure 12).

3) Dissolving

Previous studies have shown that the long 4 + 5 compositional rock action in Ji Plateau area has gone through the early orogenic A and B phases, and is currently in the middle orogenic A phase [20] . There are two phases before and after the development of the dissolution of the long 4 + 5 in the study area, each time increasing the secondary dissolution porosity of the reservoir, of which the dissolution of the meso-mesozoic phase A is more intense, and the improvement of the physical conditions of the reservoir is obvious. The second period of dissolution action in the middle orogenic stage B has a relatively weak influence, but overall it also plays a certain role in the improvement of the reservoir (Figure 13).