Yulin Upper Paleozoic gas field is located in Yulin area in the north-central Ordos Basin. The structure is located in the northeast of Yishan slope and inclined to the west. The regional structure is extremely gentle, the local structure is undeveloped, and only the big nose is deflected. The provenance in the northern part of the basin controlled the development of sedimentary systems in different stages of Upper Paleozoic (Figure 5- 10).
Figure 5- 10 Geological Comprehensive Map of Yulin Gas Reservoir
5.3.2 Reservoir forming conditions
5.3.2. 1 hydrocarbon source conditions
A set of Benxi-Shanxi coal measure source rocks is widely distributed in Ordos basin, which has the characteristics of extensive gas generation. Yulin area is located on the flank of the main gas generating center in the east of the basin, with the gas generating intensity of (20 ~ 40) × 108m3/km2, and is rich in natural gas sources. This area has been in the direction of natural gas migration since the early Yanshan period, which is favorable for natural gas accumulation (Figure 5- 1 1).
Figure 5- 1 1 Isothickness Map of Sandstone of Shanxi Formation of Lower Permian
Reservoir characteristics in 5.3.2.2.
(1) sand body distribution
Influenced by the Hercynian tectonic movement, the regression at the end of Carboniferous caused great changes in the northern sedimentary environment, that is, the Carboniferous continental basin evolved into the Permian continental lake basin. A large number of terrigenous debris flowed into the basin, forming a widely distributed lake-delta sedimentary system. Because the paleotopography is high in the north and low in the south, the Shanxi Formation in the northern area is mainly fluvial facies and delta facies deposits. Shanxi Formation in Yulin area is located in the delta plain and delta front environment, and the reservoir sand body of Shan 2 member is controlled by delta distributary channel subfacies. The main sand body is distributed in the north-south direction, with a width of about 10km from east to west and a thickness of about15 m. The general trend is that it is thick in the north and thin in the south. At the junction of distributary channel, the thickness of sedimentary sand body locally increases obviously. The sand bodies are vertically superimposed on each other and belong to delta reticulated river plain deposits.
There are flood plain deposits on both sides of the compound sand body of reticulated rivers, and there are some small inter-branch depressions and swamps between rivers. The sand body has three development centers from north to south, and the maximum thickness of the sand body is 20 ~ 30m (Figure 5- 1 1).
(2) Diagenesis
From north to south, the mineral maturity and structural maturity of reservoir sandstone in Shan 2 member in Yulin area gradually increase. In the south, seasonal sandstone accounts for more than 70% ~ 85% of the total thickness. The sorting degree of sandstone is mainly medium-good, mainly sub-circular, and the grain size of sandstone tends to become finer to the south.
The above sandstone reservoirs have experienced strong diagenesis in geological history. (1) Early diagenetic stage: compaction and pressure solution, in which compaction makes particles contact each other, concave-convex inlay and suture line develop, and timely particles break into cracks, and the accompanying pressure solution leads to timely secondary growth and development, and strong siliceous cementation makes sandstone porosity lose 5% ~13%; ② Late diagenetic stage A: Clay minerals such as kaolinite were precipitated from the diagenetic medium filling intergranular pores, which further reduced the reservoir pores; ③ Late diagenetic stage B: cementation and metasomatism of carbonate rocks (ankerite and siderite) lead to pore loss of sandstone reservoir in Shan 2 member. Due to the above diagenesis, most intergranular pores in sandstone reservoirs of Shanxi Formation are lost and become tight sandstone reservoirs. Different lithology shows different reservoir characteristics;
1) Seasonal sandstone has dense lithology, mainly siliceous cementation, and the cementation forms are enlarged gap type and pressed embedding type. The swelling phenomenon generally develops in time, and the compaction and pressure solution are remarkable. Due to the high compressive strength of quartz sand particles, some intergranular pores remain in the reservoir, and the kaolinite filled in the pores is well crystallized, and intergranular pores develop, forming a combination of micropores-intergranular pores and composite pores; The average porosity and permeability of this kind of reservoir in Shan 2 1 1 block are 6.6% and 3.5× 10-3μm2 respectively.
2) Clastic sandstone is embedded with dense structure and siliceous cementation. The intergranular pores of kaolinite are the main reservoir space, and intergranular pores are developed locally. The average porosity and average permeability of the reservoir in Shaan 2 1 1 well area are 5.8% and 1.8× 10-3μm2.
(3) Pore types
There are intergranular pores, intergranular pores and dissolved pores (including matrix dissolved pores, cuttings dissolved pores and carbonate dissolved pores). ) and microfractures, and the reservoir space is the sandstone reservoir of the second member of Shanxi Formation in this area. A large number of micro-cracks formed in the late diagenesis play an important role in pore communication. According to the observation results of thin-section microscope and reservoir structure characteristics, the sandstone in Shan 2 member in this area is divided into four reservoir types.
1) composite pore type: it is the best reservoir type in this area. It is distributed in the middle and lower part of the main sand body of Shan 2 member, and the lithology is gray medium-coarse grained timely sandstone and dark gray quartz sand conglomerate. The average porosity is 7.8% and the average permeability is 9.98× 10-3μm2.
2) Micropores-intergranular pores are the most important reservoir types in this area, and they are distributed in composite types. The lithology is gray siliceous seasonal coarse sandstone and glutenite, and secondary enlargement is common in the season, forming a granular mosaic structure. The common residual intergranular pores are connected by curved flaky throat, and the pores are filled with kaolinite, which has good crystallization and well-developed intergranular pores. The average porosity of this kind of reservoir is 6. 14% and the average permeability is 2.29× 10-3μm2. The pores are mainly intergranular pores and intergranular pores, with a few carbonate dissolved pores, microcracks and micropores, with an average pore size of 67.438±0 μm m.
3) The micropore type is the secondary reservoir type in the second member of Shan in this area, which is distributed in the upper part of the gas-bearing sand body in the second member of Shan. The lithology is dark gray muddy medium-coarse clastic sandstone and glutenite, and some are timely sandstone with dense lithology. The intergranular pores of kaolinite are the main reservoir space. The reservoir permeability is worse than the first two types of reservoirs, with an average porosity of 5.58% and an average permeability of 0.9 1× 10-3μm2. This kind of reservoir accounts for 25% of the total reservoir thickness.
The types of tight sandstone are found in the tight interlayer in massive sandstone and the tight sandstone in the upper part of Shan 2 member. The lithology is coarse clastic sandstone and mudstone clastic glutenite in gray carbonate rocks. The average porosity of tight sandstone is 2.8%, and the average permeability is 0.26× 10-3μm2. Tight sandstone accounts for 15% of the total reservoir thickness. The low porosity and high permeability in Yulin area are mainly caused by the development of micro-fractures. The rational allocation of the above reservoirs and caps constitutes a good natural gas accumulation combination, which is one of the key factors for the formation of large gas reservoirs in the second member of Yulin Mountain.
5.3.2.3 trap condition
The formation conditions of upper Paleozoic lithologic traps in caprock include the combination mode of caprock and reservoir, sealing ability and distribution area of caprock. The fluvial composite sand body in the lower part of Shan 2 member in Yulin area is the main gas layer in this area, and black mudstone and carbonaceous mudstone with coal seam (the thickness of single mudstone is 8 m) are the direct caprocks of gas-bearing sand bodies. According to the breakthrough pressure test of four samples in this section, and compared with the evaluation standards of argillaceous rock sealing ability at home and abroad, it is considered that its sealing ability should be the best level. Its uphill 1 profile is 47m thick, and mudstone accounts for 74% of the direct caprock. Upper Shihezi Formation is 182m thick, of which mudstone is 142m thick, accounting for 85%. It is an indirect regional caprock with wide distribution of upper Paleozoic gas reservoirs and strong sealing ability.
Complex traps controlled by both hydrodynamic and lithologic factors are described by fine gas reservoirs. The temporal and spatial distribution of reticulated river sand bodies in Shan 2 member is more clear. The main sand belt is SN-oriented in the plane, and the reticulated rivers are divided and compounded, and they are superimposed with multiple rhythms in the longitudinal direction, forming a huge complex sand body, which is blocked by hydrodynamic and lithology in the northern direction. The argillaceous rocks deposited in the flood plain on the east side of river sand bodies constitute the lateral rock shielding of gas reservoirs. These conditions created the petroleum system of the Lower Permian.
5.3.3 Reservoir-forming assemblage
5.3.3. 1 natural gas accumulation combination
The Upper Paleozoic in Yulin is divided into Shan 2 member and Tai 1 member.
The source-reservoir assemblage is self-generating and self-storing, which is conducive to the formation of primary gas reservoirs dominated by early accumulation, and thus to the preservation of a considerable number of primary pores, characterized by the accumulation of primary intergranular pores (Shan 2) and sandstone primary pores-limestone fractures (platform 1). The upper Shan 1 mudstone of Shan 2 is its direct caprock, which mainly generates hydrocarbon-bearing CO2 gas. The limestone of Taiyuan Formation in the east also entered the hydrocarbon generation threshold and began to generate liquid hydrocarbons.
Natural gas accumulation period in 5.3.3.2
During the slow burial period of Jurassic Carboniferous-Permian coal and mudstone, due to the high geothermal gradient, the source rocks were further buried and heated up, reaching the mature-high mature stage. The limestone of Taiyuan Formation reached the peak of liquid hydrocarbon generation at J 1, during which hydrocarbons entered the reservoir, which inhibited the continuous evolution of pores and was beneficial to preserve more primary pores in Yulin gas field, while the Carboniferous-Permian source rocks entered the peak of gas generation at the end of Middle Jurassic. During the rapid burial period of early Cretaceous, the source rocks quickly reached the over-mature stage, and continued a large number of gas generation processes, and the liquid hydrocarbons generated in the early stage were further cracked to form natural gas; At the end of Early Cretaceous, the whole uplift occurred. Due to the temperature drop and stratum uplift, some natural gas was lost in the northern part of Yulin gas reservoir, and the pressure was also reduced. The production of hydrocarbons tends to stop.
Gas reservoir characteristics
Gas reservoir pressure +0
According to the pressure measurement data of three wells in Shaanxi 14 1 well area 13, the pressure in the middle of the gas reservoir and the altitude depth in the middle of the gas reservoir are plotted, and the correlation coefficient is 0.92.
According to the formula, the static pressure gradient of gas layer is 1.89MPa/km. The static pressure gradient of gas reservoirs obtained from the unstable well testing of Shan 9 and Shan 1 18 in this area is 1.75MPa/km, which are close to each other, indicating that the gas reservoirs belong to the same pressure system. The pressure coefficient of this gas reservoir is 0.9592, and the pressure coefficient of each gas reservoir in Yulin gas field is between 0.95 and 0.98, which is normal pressure. Comprehensive drilling, testing and laboratory analysis data show that the fluid properties are stable, indicating that the gas reservoir belongs to the same pressure system. Natural gas produced by main sand body does not contain water.
5.3.4.2 formation temperature
The northern part of the gas field 1 1 well (9 wells in Shaanxi 14 1 well area) is selected for comparative analysis, and the correlation between them is good, with a correlation coefficient of 0.82.
According to the formula, the geothermal gradient of Upper Paleozoic is 3.06℃/ 100m, which is close to that of Changqing gas field (3.09℃/ 100m), indicating that there is a unified geothermal field in Changqing gas field.
5.3.4.3 fluid properties
Natural gas in Shan 2 gas reservoir of Yulin gas field is stable in physical properties and composition, with high CH4 content. C 1 index of most samples is greater than 0.95 (92.62% ~ 95.68%), C2+ is relatively low (ethane content is 2.02%), mainly dry gas with a little moisture.
Gas well productivity in 5.3.4.4
There are drilling wells 10 and testing wells 10 in the gas-bearing area of Shaanxi14/well area. The open flow varies greatly (50 ~ 20) × 104m3/d or (20 ~1) ×104m3/d.
Gas reservoir type
The main controlling factor of the gas reservoir in Yulin gas field is the lithologic trap formed by the lithologic pinch-out of the main sand body deposited by reticular rivers in different periods of Shan 2, which is in the northeast direction and the diagenesis of sandstone is blocked. The upward distribution of gas and water in the northern part of the basin comes from the conventional sandstone water-bearing area in the northern part of Yimeng uplift, the gas-water transition area of low permeability sandstone in the middle and southern part of the uplift and the large area of tight sandstone water-bearing area on the slope of northern Shaanxi. This gas-water inversion phenomenon is very similar to the "water seal" trap characteristics of Alberta deep basin gas. The results show that the gas reservoir belongs to a compound natural gas reservoir controlled by both hydrodynamic and lithologic factors.
5.3.6 Main control factors of natural gas accumulation
The sedimentary sand bodies of NNE reticulated rivers go deep into the hydrocarbon generation center and are rich in natural gas sources. In the low potential area of fluid migration period, the sand bodies in the main channel are vertically superimposed in multiple stages and rhythms, forming a huge reservoir space. The argillaceous rocks deposited in the flood plain on the east side of the channel sand body constitute the lateral rock cover of the gas reservoir The upwarping direction of sand bodies in the northern channel is a composite natural gas reservoir controlled by both hydrodynamic and lithologic factors.
5.3.7 Enriching high-yield factors
Shan-2 gas reservoir in Yulin and the eastern region benefited from the Archean granitic parent rock provenance in the eastern region, forming a timely sandstone reservoir with developed primary pores. At the same time, compared with the early hydrocarbon generation, the eastern source rocks have the ability to generate natural gas and light liquid hydrocarbons, which makes the hydrocarbons charge in the early stage, inhibits the secondary increase in time, and is conducive to the preservation of primary pores, which is the basis of relatively high production of Yulin gas reservoir.