As the main driving force of oil and gas migration, abnormal formation pressure controls the direction of oil and gas migration in the basin. Therefore, the study on the genetic mechanism of abnormal pressure has always been an important content in the analysis of petroliferous basins and the study of oil and gas accumulation. The distribution of pressure field and its genetic mechanism in the eastern basin of China are similar to some extent, and Dongying Depression is taken as an example to illustrate it.
The formation pressure field in Dongying sag is generally characterized by multi-layer structure, which can be roughly divided into three sections in the pressure depth profile: the normal pressure section with buried depth less than 2200 ~ 2500 m; The buried depth of the pressure transition section is 2500 ~ 3000 m, and some boreholes begin to have abnormal pressure, but the pressure coefficient is still low, generally 0.92 ~1.27; The buried depth of the abnormally high pressure section is over 3,000 m, and the pressure of most boreholes is extremely high, with a very high pressure coefficient, which can reach above 1.80 (Figure 2-36). The overpressure system in Dongying sag mainly has the following characteristics: ① the pressure distribution in the plane is annular, and the pressure peak area is generally in the sub-member, sub-member and high-quality hydrocarbon source layer at the top of the sub-member; ② The top sealing layer of overpressure system is roughly bounded by the top of mudstone in the upper member of Shahejie Formation, and the bottom boundary is unclear, so it is presumed to be the salt-gypsum layer in the lower member of Shahejie Formation, and the secondary basin-controlling fault between the slope zone and the depression zone is often used as the lateral boundary of overpressure system; (3) Near the top boundary of the overpressure system, the residual pressure increases rapidly, especially in the depression zone, and its pressure growth rate can sometimes approach or exceed the rock static pressure gradient; ④ The pressure distribution of the source rock is similar to that of its adjacent reservoir, indicating that the source rock is the pressure source and the pressure is transferred from the source rock to the reservoir.
Figure 2-36 Variation of formation pressure, clay minerals and remaining oil saturation with depth in Dongying fault basin
According to the variation trend of residual oil saturation with depth (Figure 2-36), there is a good correlation between the hydrocarbon generation evolution of source rocks in Dongying Depression and the formation of overpressure. Comprehensive analysis shows that hydrocarbon generation by organic matter is the main mechanism of abnormal overpressure, and its main basis is as follows: ① kerogen generates active fluid (mainly including liquid, gaseous hydrocarbon and CO2) in the original solid kerogen, and the fluid volume can be increased by 25% during hydrocarbon generation. The simulation study shows that the CO2 generated by kerogen alone can reach 200m3/t in the low maturity-maturity stage of source rocks in Jiyang Depression. (2) A large amount of CO2 associated with hydrocarbon generation will cause dissolution, migration and precipitation of carbonate minerals, form an effective sealing layer, further hinder the discharge of fluid, and produce abnormal high pressure. (3) With the formation of liquid and gaseous hydrocarbons, the hydrocarbon saturation of source rocks gradually increases, and the fluid system gradually changes from single-phase flow system to multiphase flow system, thus activating the capillary sealing layer. Capillary caprock has high sealing efficiency for hydrocarbon-bearing fluids, which can be approximately regarded as complete sealing ability, so the pressure rises rapidly. According to the characteristics of formation pressure system and its sealing layer, Dongying sag can be divided into three reservoir-forming dynamic systems, namely, shallow atmospheric pressure open system, deep overpressure closed system and the pressure closed system between them. Due to the differences in oil and gas sources and source-reservoir-cap assemblage, each system has its own characteristics in oil and gas migration mechanism, oil and gas reservoir types and reservoir-forming models.
(2) Characteristics of microfractures in source rocks
A large number of studies show that micro-fractures play an important role in the process of hydrocarbon expulsion from source rocks. In recent years, with the discovery of mudstone fractured reservoirs and the need of hydrocarbon expulsion from source rocks, the research on mudstone fractures and microfractures has also made some progress. Based on core observation, combined with fluorescence microscope analysis technology and logging data, the characteristics, morphology, occurrence, fillings and vertical distribution of micro-fracture parent rocks are systematically analyzed, and the relationship between them and hydrocarbon expulsion from source rocks is discussed.
Bedding or low-angle fractures: these fractures are mainly distributed along bedding and rarely run through the parent rock; Most of the fractures are filled with secondary bright calcite, and its crystals are arranged in a comb shape, and the long axis direction is generally perpendicular to the stripe plane, which is in steep contact with the surrounding parent rock stripe layer, indicating the secondary cause (Figure 2-37). In the interval where fractures develop, the abundance of fractures is very high. It is estimated that the total thickness of bright calcite in some intervals can account for more than 40% of the total core thickness. According to the observation of core samples, this kind of microcracks mainly exist in high-quality source rocks of the lower member of Shahejie Formation and the upper member of Shahejie Formation, among which the anoxic phase is the most developed, and the probability of appearing in intermittent oxidation phase is quite different, but it is rare in anoxic phase and oxidation phase. The organic carbon content in the fractured zone is relatively high, while the organic carbon content in the fractured zone is relatively low, and the organic carbon boundary between them is roughly between 2.0% and 4.0%. Therefore, the development of fractures is directly or indirectly related to high organic matter abundance. The development characteristics of the above-mentioned bedding microcracks are very similar to those of Posidonia shale rich in organic matter (Littke et al., 1988), indicating that the development of bedding microcracks in organic matter-rich laminated rock may be universal.
Figure 2-37 Microscopic characteristics of bedding microcracks
Atto142,3292, upper submember of the fourth member of Shahejie Formation; Well B-He 130, 3245.53m, lower member of the third member of Shahejie Formation, yellow fluorescence, ×200.
In addition, bedding micro-fractures are widely developed in the deep depressions of Jiyang Depression, and the top boundary depth is generally 2900～3000m m. Combined with the analysis of hydrocarbon generation evolution of source rocks, the initial development depth of fractures is basically consistent with the period when high-quality source rocks reach maturity and rapid hydrocarbon generation in saline environment. Vernik( 1994) also thinks that such bedding fractures are often more closely related to the thermal evolution, hydrocarbon generation process and primary migration of source rocks than vertical fractures. Under the fluorescence microscope, sometimes a large number of oil inclusions can be seen at the edge and inside of comb calcite crystals, which are bright yellow fluorescence, indicating that fractures were once the channels for oil and gas migration.
According to the analysis of the relationship between the development of this kind of microcracks and the lithology, organic matter abundance and maturity of source rocks, organic matter hydrocarbon generation is the main reason for the formation of bedding microcracks. After the buried depth of high-quality source rocks reaches 2900 ~ 3000 m, it enters the mature stage and begins to generate a large number of hydrocarbons. In the process of hydrocarbon generation, the pressure in the pores of source rocks rises rapidly, which exceeds the fracture strength of rocks, and thus micro-cracks are formed. The hydrocarbon expulsion process after the formation of micro-cracks in source rocks makes the fluid pressure in the cracks decrease and carbonate gradually supersaturate, resulting in the precipitation of secondary calcite. In addition, from the structure and structural characteristics of microcracks, there is neither bifurcation nor mutual cutting phenomenon in microcracks, and the possibility of structural cracks can be basically ruled out.
Vertical or high-angle fractures: The occurrence of these fractures is mostly vertical to the bedding plane or at a large angle to the bedding plane, and often bifurcate and intersect, some of them are feathered, most of them are tensile fractures, and sometimes there are dissolution phenomena. This is consistent with the extensional environment of Paleogene strata in Jiyang Depression. Ci Xinghua et al. (2006) systematically described the high-angle tensile fracture in Sikou sag of Zhanhua sag, with an inclination of 50 ~ 80, an opening of 0.2 ~ 3.0 cm and a length of 70cm. The fracture surface is uneven, and most of it has been completely or partially filled. For example, this kind of fracture was found in Luo 67 well at 330 1m and 3448m m. Fracture fillings are complex, and the minerals are mainly secondary minerals such as granular calcite, gypsum and salt rock, which are often impregnated with liquid hydrocarbons or contain solid asphalt, as shown in Figure 2-38.
Figure 2-38 Filling Characteristics of High Angle Fractures in Jiyang Depression
A—— Filling asphalt and granular calcite; B- filled with salt rock particles; C— Filled with gypsum, asphalt, calcite and pyrite.
Different from the development of bedding microcracks, the parent rocks of vertical fractures are mainly high-quality source rocks, but not limited to high-quality source rocks. At present, it has been revealed in various rocks such as oil shale, shale, massive mudstone and marl. According to the observation of continuous coring, it is found that the probability of occurrence in brittle rocks is higher than that in plastic rocks. Generally speaking, the source rocks rich in calcareous carbonate are the most developed in the slope zone. In addition, many source rocks in the vertical fracture zone are accompanied by a large number of low-angle fractures, which are considered to be caused by tectonic stress acting on source rocks with strong anisotropy. The fractures are mostly located in the transition zone between the depression slope and the gentle slope bottom, the end of faults and the intersection between faults, and such fractures are often abundant, such as Li 89 well, Fengshen 1 well, Luo 1 1 well, Da 93 well, Fan 4 1 well and so on. The above characteristics show that tensile fractures are closely related to structural activities, and the development degree of fractures is closely related to structural positions. In addition, such fractures are often concentrated in the inter-salt strata at the top of the lower member of Shahejie Formation, such as Fengshen 1 Well and Liang 107 Well. Liu Hongwei (2002) considered that the arching action also played an important role in controlling this kind of fractures according to the characteristics that the fractures in the inter-salt mudstone in the central uplift zone of Dongpu Depression are very developed.
From the buried depth, it generally appears below 2500m and above 3000m, and the abundance gradually increases. When the buried depth is close to more than 4000 meters, such high-abundance fractures can be seen even without faults, such as the 65,438+0 well in Xinlishen. According to the analysis of the generated products of source rocks, 4000 meters roughly corresponds to the rapid increase stage of gas production of source rocks, indicating that the hydrocarbon generation process of source rocks also plays an important role in controlling such fractures, and the strata where such fractures develop often correspond to very high abnormal pore pressure.
According to the distribution characteristics of vertical fractures in Jiyang Depression, the formation of fractures is controlled by fracture development, fluid overpressure and lithofacies (Figure 2-39). With the increase of buried depth and the process of hydrocarbon generation and clay transformation, the mechanical properties of source rocks gradually change from plasticity to brittleness, and at the same time, the abnormal fluid pressure increases, which can greatly reduce the effective stress acting on rock particles, thus reducing the fracture resistance of rocks. Under the action of regional and local stress fields, brittle rocks will fracture at the stress concentration, and oil and gas will accumulate to form fractured oil and gas reservoirs.
Figure 2-39 Diagenetic evolution sequence and diagenetic stage division of salt-generated source rocks in Jiyang Depression
(3) Hydrocarbon expulsion mechanism and mode of high-quality source rocks
According to the hydrocarbon generation process, evolution of hydrocarbon expulsion phase, characteristics of pressure field and vertical evolution of micro-fracture development of source rocks, a dynamic model of hydrocarbon expulsion from salinized source rocks is proposed, and the whole hydrocarbon expulsion process is divided into free hydrocarbon expulsion stage, oil and gas accumulation stage and micro-fracture hydrocarbon expulsion stage (Figure 2-40). Here's an example to illustrate.
The buried depth in free drainage stage is less than 2200m, and the source rocks are in immature stage and stable compaction stage. The oil saturation of source rocks is low, and the discharged fluid is mainly water, and the driving force of fluid discharge is mainly the instantaneous formation pressure generated during compaction. In the stage of oil and gas and energy accumulation, the buried depth is 2200 ~ 3000 m, and the source rocks are in the stage of low maturity and sudden compaction. The source rocks began to generate hydrocarbons slowly, and the oil saturation gradually increased, but generally it was still below 20% ~ 30%, which could not reach the critical saturation of free hydrocarbon phase. Due to the existence of water and hydrocarbon multiphase fluid and the sealing of capillary, abnormal pressure began to appear. The ability to generate immature oil in the lower member of Shahejie Formation is poor, and the discharged fluid is still mainly water, which may contain a small amount of low molecular weight hydrocarbons. The buried depth of microfracture displacement stage is more than 3000 m, and the source rocks are in mature stage and compact stage. Due to the peak period of hydrocarbon generation, the oil saturation of source rocks increases rapidly, reaching the critical saturation of free hydrocarbon phase for oil displacement. With the rapid increase of formation fluid pressure and the formation of a large number of micro-fractures, large-scale hydrocarbon expulsion occurs in the form of miscible inflow. This stage is in the late stage of compaction, and there is less water in the source rock, and the discharged fluid is mainly petroleum hydrocarbons.
The hydrocarbon expulsion process of the fourth member of Shahejie Formation in salinization environment is basically the same as that of the third member of Shahejie Formation. The main difference is that the fourth member of Shahejie Formation is a salt lake deposit, and the hydrocarbon generation process is earlier than that of the lower member of Shahejie Formation. Therefore, the main difference of hydrocarbon expulsion is that after the buried depth of the fourth member of Shahejie Formation is more than 2500 m, that is, the late stage of oil and gas accumulation and energy accumulation, the immature oil generated in source rocks can be expelled from the free phase after accumulation, and in some cases, industrial accumulation can be formed.
From the above analysis, it can be seen that hydrocarbon expulsion is the inevitable result of abnormal pressure accumulation after a series of transformations of organic matter, inorganic minerals and fluids in source rocks under the background of continuous burial, heating and pressure. The evolution of hydrocarbon generation, compaction diagenesis and the formation of abnormal pressure of source rocks are closely related to the primary migration and subsequent accumulation of source rocks.
Figure 2-40 Dynamic model of hydrocarbon generation and expulsion of high-quality source rocks in salt lake facies in Jiyang Depression
The pressure sealed box and its evolution in the basin will also have a great impact on the hydrocarbon expulsion process. Pressure chamber is a relatively stable system, which is in metastable state for most of its evolution, and the rupture of its sealing layer, especially the thick muddy sealing layer, is very difficult in the structural stability period. There are two situations in the process of sealing layer opening and hydrocarbon release: first, during the relatively stable period of the structure, the sealing layer is broken and hydrocarbons are discharged due to the rising pressure in the sealing box; Second, faults and faults occurred during tectonic activity, which led to cap rock rupture and large-scale hydrocarbon expulsion.
Structural stability period: from the analysis of hydrocarbon generation process of source rocks, on the same profile, the lowest source rocks generally reach the threshold and peak of hydrocarbon generation first because of their high maturity. When the sedimentary characteristics and hydrocarbon generation conditions are similar, the lowest source rock will first form an abnormal high pressure zone and lead to micro-fractures. For different intervals, the order of fracture formation is from bottom to top. For the source rocks in the same interval, the lower source rocks are also broken first. However, due to the heterogeneity of source rock development and the difference of source rock quality, fracture development does not completely follow this law. For example, in Nanlin sag and Chexi sag of Huimin sag, the source rocks of the upper fourth member of Shahejie Formation are poor, and there may be no fractures.
In the early stage of crack development, the crack development range is limited, the thickness is small, and the pressure and crack sealing box are small. Because many source rocks have no cracks, large thickness and poor porosity and permeability, they form a very good seal, which is difficult to be broken only by the accumulation of internal pressure. However, because the pressure sealing box is close to the bottom of the source rock mass and the bottom sealing layer is thin, it may break through the sealing layer because the abnormal pressure exceeds the displacement pressure of the source rock or causes vertical or high-angle cracks. Once the sealing layer is broken, the oil and gas in the sealing box can easily enter the beach bar sand body in the lower part (generally confined to the upper part) of the high-quality source rock in the upper fourth member of Shahejie Formation, and migrate laterally along the bottom surface of the source rock mass. Although the thickness of these beach bar sand bodies is not large, they extend widely. Although they are poor as reservoirs, they can be used as good diversion layers. In recent years, the exploration of deep subtle oil and gas reservoirs shows that the beach bar sands of the upper fourth member of Shahejie Formation and the fourth member of Kongdian Shahejie Formation in Dongying Depression have good oil-bearing properties. Typical ones, such as Liangjialou-Chunhua Slope Belt, Boxing Depression and Niuzhuang Depression, often contain oil in a large area. All these are favorable evidences for downward hydrocarbon expulsion from source rocks.
With the discharge of hydrocarbon-bearing fluid, the fluid pressure decreases, the fracture will heal again, and a new round of hydrocarbon generation, energy and pressure accumulation, sealing box rupture and hydrocarbon expulsion will begin. With the development of sedimentation in the basin, the buried depth of strata is increasing, the horizon of mature source rocks is gradually getting new, the sealing layer of abnormal high pressure zone is intermittently opened and closed by fracturing, and the positions of sealing layer and fracture top boundary will be continuously adjusted upward until hydrocarbon generation of source rocks is completed.
Tectonic active period: In fact, argillaceous rocks have strong plasticity, so a closed and independent high-pressure fluid tank is sometimes difficult to be broken only by the accumulation of internal pressure. Therefore, many petroleum geologists believe that the fracture system dominated by vertical fractures is often the product of tectonic activities.
In the period of tectonic activity, according to the stress judgment, vertical or high-angle faults are easy to occur in the process of extrusion and tension, accompanied by a large number of vertical or high-angle cracks and micro-cracks. If the pressure seal box has been formed during the fracture period, the structural movement can lead to the rupture of the cover layer of the seal box, and realize the communication between the pressure seal box and the external reservoir. Once this communication is realized, hydrocarbons and other fluids gathered in the pressure sealing box will rapidly complete hydrocarbon expulsion and accumulation of source rocks in the form of mixed inflow under the driving of abnormal high pressure. After the tectonic active period, with the discharge of fluid and the decrease of pressure, the fracture will gradually be cemented and closed, and a new process of energy accumulation, pressure release and hydrocarbon expulsion will begin. From this point of view, the hydrocarbon expulsion process is more or less episodic in both structural stability and tectonic activity, but obviously the scale of hydrocarbon expulsion in each episode is much larger in tectonic activity.
The hydrocarbon expulsion ability of a fault, that is, the permeability or sealing ability of the fault, is mainly affected by many factors, such as the nature, scale, drop size, cross-section shape, contact relationship between two reservoirs of the fault, and whether the cross-section can form a low permeability zone with high displacement pressure.
Compared with compressional faults, extensional faults are more conducive to hydrocarbon expulsion. A typical example is the central uplift belt of Dongying sag, where extensional faults lead to a large number of upward hydrocarbon expulsion from source rocks. The larger the fault scale, the greater the fault distance, the more fractured horizons, the larger the oil supply range and the more thorough hydrocarbon expulsion. For large-scale primary and secondary faults, due to the wide fault zone and fracture zone, the upper or top sealing layer of the source rock complex is easy to be destroyed, and the overpressure in the source rock will be released quickly, and hydrocarbon expulsion should mainly start from upward drainage.
For faults or small faults with inconspicuous tensile characteristics, the vertical dredging ability may be limited, and the hydrocarbon expulsion process is different. In the center of the depression, the caprock on the upper part of several sets of source rocks is very thick, and the mudstone on both sides of the fault contacts with mudstone, so the permeability is limited. Perhaps only a part of the hydrocarbons generated in the upper part can break through the closed layer and be discharged upward, while the other part will migrate laterally. In the absence of lateral shunt system, oil and gas can be discharged downward and then migrated laterally. Therefore, the upward migration of oil and gas generally has the oil source characteristics of the upper source rock, while the downward migration of oil and gas has the characteristics of the lower source rock.