Geological conditions

Geological conditions for lateral sealing of active faults and relevant research methods

Oil and gas exploration practices indicate that there are a large number of faults developed in petroliferous basins or sags. These faults play a pivotal role in the accumulation and distribution of hydrocarbons, acting as the pathways for the migration of oil and gas and providing lateral blocking conditions for oil and gas to accumulate and distribute near them [1], [2], [3], [4], [5], [6]. However, as a lateral blocker of hydrocarbon accumulation, a fault manifests it sealing capacity not only in its inactive stage, but also in its active stage. Is the geological condition for lateral blocking in the inactive stage consistent with that in the active stage? Are their study methods identical to each other? The above-mentioned questions are one of the key to oil and gas exploration in fault developmental zones of the sag. There have been a lot of research and discussion on the geological conditions and research methods for the lateral blocking of faults. And it is considered that the geological conditions for lateral blocking of faults in the stationary stage is that fault rocks contain a displacement pressure greater than or equal to that of reservoir rocks in the oil and gas migration wall. Once the displacement pressure of fault rocks and that of the reservoir rocks in the migration wall are confirmed, the lateral sealing capacity of fault rocks can be quantitatively studied [7], [8], [9], [10], [11], [12], [13]. But these studies were mainly conducted by analyzing the composition of the fault rocks without considering the influence of the internal structural characteristics on the lateral sealing capacity of fault. It was considered that faults in the active stage show no lateral sealing capacity, causing the rare mention of the lateral sealing of faults in the active stage. Therefore, there is a lack of research methods aiming at the lateral sealing of faults in the active stage.

In fact, not all the faults and positions in the same fault are open laterally. Due to the influence of compositions and structures of the filling materials in the fault belt, the lateral oil and gas sealing capacity is not as strong as that of fault rocks experiencing diagenesis of compaction, but there is still some sealing capacity for a certain amount of oil and gas, thus affecting the accumulation and distribution law of oil and gas [14], [15], [16], [17], [18]. Whether the problem can be correctly understood is the key to oil and gas exploration in the fault developmental zone of the sag. Therefore, to conduct study on the geological conditions and their research methods for lateral sealing of faults in the active stage is of great significance to correctly understanding the distribution law of oil and gas and guiding oil and gas exploration in the fault developmental zone.

Geological conditions for lateral sealing of faults in the active stage

During the activity of faults, the opening is formed by associated fractures and induced fractures. And the vertical opening of faults is the migration channel for the vertical migration of oil and gas and the infeasibility of vertical sealing has been an uncontroversial fact. However, whether an active fault plays a role in sealing the migration of oil and gas mainly depends on the compositions of the filling materials. Only with the fault zone filling materials with shaly compositions being the lateral migration blockers, the displacement pressure will be greater than or equal to that of the reservoir rock in the migration wall, thus there is fault lateral sealing, otherwise there is none (Fig. 1). However, the condition for this kind of sealing is that the fault should be in a reverse direction and the faulted strata are dominated by mudstone. Due to the dominance of mudstone in faulted strata, the filling materials in the fault zone after fracturing can also be dominated by shaly compositions; otherwise it will be dominated by argillaceous composition. Only when there was no development of induced fracture zone in the footwall of reverse faults [19], the fault zone filling materials with shaly compositions will act as the direct blocking materials, forming lateral sealing; otherwise the induced fracture zone will act as the blocking materials and there is no formation of lateral sealing.

Fig. 1. Conditions for lateral sealing of faults in the active stage.

Research methods for the lateral sealing of faults in the active period

As can be seen from the above, once the displacement pressure of fault rock and that of the reservoir rock in the oil and gas migration wall are confirmed, the lateral sealing of faults in the active stage can be studied based on the above two kinds of pressure.

Determination of the displacement pressure of filling materials in fault zones

As a fault zone in the active stage was in its open state, its filling materials were not compacted in diagenesis, they are equivalent to sediments. Their lateral sealing capacity to oil and gas migration is significantly weaker than that of the compacted fault rocks which experienced diagenesis. Because the magnitude of their displacement pressure is not only mainly affected by the shaly content like the displacement pressure of fault rocks, although they can be compacted by the weight load of overlying sediments, they were not diagenetic yet. They resulted in a displacement pressure significantly lower than that of fault rocks [20], [21], [22]. According to the perspective [23] that the displacement pressure of fault rocks is proportional to the diagenetic depth of compaction and the shaly content, the empirical relationship between the displacement pressure of filling materials in the fault zone and shaly content and depth (Equation (1)) can be derived as follows.

where pdf indicates the displacement pressure of filling materials in the fault zone, MPa; c, d indicates the constants varying with areas; Rf represents the shaly content of filling materials in the fault zone; and Zf is the burial depth of filling materials, m.

As can be seen from Equation (1), the depth of filling materials in the fault zone can be obtained from the direct reading of drilling wells and seismic sections, and the shaly content of filling materials in the fault zone can be calculated from the faulted distance, thickness of faulted strata, and shaly content in Equation (2) [24].

where n is the number of faulted strata; Hi is the thickness of strata i in the faulted strata, m; Ri is the shaly content of strata i in the faulted strata; L represents faulted distance, m.

Once the empirical relationship between the displacement pressure of filling materials in the fault zone and their burial depths and shaly content are determined (Equation (1)), the displacement pressure of filling materials in the fault zone can be derived. Due to the restriction of drilling and coring, the displacement pressure of filling materials in the fault zone can’t be tested directly; however, the empirical formula of Equation (1) can only be indirectly obtained by means of physical simulation experiment.

Clay and siltstone were mixed respectively in accordance with the ratios of 100:0, 80:20, 60:40, 40:60, 20:80 and 0:100 to simulate the fault zone filling materials with different shaly content. Then they were mixed in the stirrer, and the mixture obtained was wetted with an atomizer before it was poured into the template. Finally a manual pressure pump was used to exert respectively a pressure of 1 MPa, 5 MPa, 10 MPa, and 15 MPa in order to simulate the fault zone filling materials under a burial of 100 m, 500 m, 1000 m and 1500 m. The compacted 20 samples with a diameter of 2.5 cm were obtained and were kept in the incubator with a constant temperature of 40 °C. After the samples were dried, test samples of fault zone filling materials with different shaly content and different depths were thus obtained. These samples were subjected to vacuum and saturated kerosene for a few days and were put in the displacement pressure test device [25]. The 20 fault zone filling material samples obtained from simulation were tested in terms of their displacement pressure under normal temperature conditions and the results are shown in Table 1. It can be seen from Fig. 2 that the displacement pressure of fault zone filling materials is proportional to the burial depth and shaly content respectively, shown in Equation (3), namely, as the depth and mud content increases, the displacement pressure increases; and vice versa. This agrees with the actual relationship between the displacement pressure of fault filling zone and its burial depth, shaly content, indicating that the experimental data is credible, and their relationship is shown in Equation (3). According to the burial depth of faults in the active stage and shaly content of fault zone filling materials (Let us assume that the shaly content of fault zone filling materials in different geological periods is different), the displacement pressure of fault zone filling materials in the active stage can be calculated from Equation (3).

Table 1. Relationship between displacement pressures and shaly contents & depths of fault zone filling materials from physical simulation experiments.
Fig. 2. Relationship between displacement pressures and shaly contents & depths of fault zone filling materials from physical simulation experiments (pdf vs. Rf Zf/100).

Determination of reservoir displacement pressure of oil and gas migration wall in the active stage

Displacement pressure of oil and gas migration wall can be used to restore the palaeo-depth of oil and gas migration wall in the active stage by the palaeo-thickness recovery method [26]. Assume a condition that the shaly content in the strata is constant and by using the shaly content prediction method in Ref. [27], the natural gamma logging data was used to predict the shaly content of reservoir in oil and gas migration wall in the active stage. Then take the two into the experimental equation between displacement pressure and compaction of diagenesis, shaly content, thus the reservoir displacement pressure in oil and gas migration wall in the active stage can be obtained.

Lateral sealing of faults in the active stage

If the displacement pressure of the fracture zone filling materials is greater than or equal to that of the rocks in the oil and gas migration wall, there is lateral sealing in the active stage; otherwise there is no sealing.

Application

In this paper, three faults of F1, F2, and F2 in the Nanpu No. 1 Structure of the Nanpu Sag were selected. The above-mentioned method was used to study the lateral sealing of the late activity of the deposition of Neogene Minghuazhen Fm to natural gas in the first member (F1, F3) of Paleogene Dongying Fm and the Neogene Guantao Fm (F2). Comparison between analytical result and the distribution of natural gas currently discovered was made to verify the feasibility of lateral sealing study of faults in the active stage.

Nanpu No. 1 Structure is located on the southwest slope of southern depression. In general, the structural morphology is a complicated drape structure developed on the background of a buried hill. The structural trend is NE and is intersected and complicated by the faults trending NE and nearly NS (Fig. 3). Upwardly, it developed the strata of Paleogene Shahejie Fm, Dongying Fm, Neogene Guantao Fm, Minghuazhen Fm, and the Quaternary in the structure. The Dong 1 Member and the Guantao Fm are the major pay horizons of natural gas in the Nanpu No. 1 Structure. And industrial gas flows have been obtained in many wells (Fig. 3).

Fig. 3. Schematic section of natural gas reservoirs in the Nanpu No. 1 Structure.

Comparison of gas source shows that the natural gas in this area mainly comes from the underlying source rock of Sha 3 Member or Sha 1 Member and the caprock is the volcanic rock in the Guan 3 Member and the mudstone in the Ming 1 Member.

The type of natural gas reservoir is the mainly reverse fault block, however, the three faults of F1, F2, F3 blocking the natural gas were active faults in the accumulation period of natural gas – the late deposition period of the Minghuazhen Fm [8]. They are the migration faults for the migration of natural gas generated in the underlying Sha 3 Member or Sha 1 Member to the overlying Dong 1 Member or Guantao Fm. Due to the blocking of volcanic rocks of the Guan 3 Member and the mudstone of the Lower Minghuazhen Fm, the natural gas migrated distributively to the reservoirs in the Dong 1 Member and the Guantao Fm. Whether the three faults in the active stage can seal the distributively migrated natural gas is key to whether the natural gas can accumulate in the Dong 1 Member and the Guantao Fm of the Nanpu No. 1 Structure.

The current burial depths (2449 m, 2153 m and 2446 m, respectively) of F1, F2, and F3 respectively in the Dong 1 Member, Guantao Fm, and the Dong 1 Fm were statistically obtained from the wells of Np101, Np1, and the Np1-2 in Fig. 3, from which the burial depths (587 m, 596 m and 619 m, respectively) of the three faults in the natural gas accumulation stage (the late deposition period of the Minghuazhen Fm) were substracted, and the palaeo-burial depths of the three faults in the late deposition period of the Minghuazhen Fm are respectively 1862 m, 1557 m and 1827 m. According to the fault distance, faulted strata thickness, and shaly content of F1, F2, F3 in the Dong 1 Member, Guantao Fm, and the Dong 1 Member, and the fault zone filling materials in the Dong 1 Member, Guantao Fm, and the Dong 1 Member were respectively calculated according to Equation (2). And the results are 63%, 61%, and 60%, respectively, indicating that the faulted strata are mudstone-dominated and can be blocks for lateral sealing. And then the palaeo-depth and the shaly content of F1, F2, and F3 in the late deposition period of the Minghuazhen Fm were taken into Equation (3), and the displacement pressures of the fault zone filling materials are respectively 0.97 MPa, 0.65 MPa, and 0.84 MPa.

The current diagenesis burial depths (2449 m, 2153 m and 2446 m, respectively) of F1, F2, and F3 respectively in the Dong 1 Member, Guantao Fm, and the Dong 1 Fm were statistically obtained from the wells of Np101, Np1, and the Np1-2, from which the compaction diagenesis burial depths (587 m, 596 m and 619 m, respectively) of the three faults in the natural gas accumulation stage (the late deposition period of the Minghuazhen Fm) were substracted, and the palaeo compaction diagenesis burial depths of the three faults in the late deposition period of the Minghuazhen Fm are respectively 1862 m, 1557 m and 1827 m. The shaly content of F1, F2, and F3 in the Dong 1 Member, Guantao Fm, and the Dong 1 Member can be calculated from the natural gamma logging data of the three wells by using the shaly content prediction method in Ref. [27] under the assumption that the shaly content is different. The results are respectively 37%, 35% and 36%. From the reorganization of the measured displacement pressure of reservoir rocks, the relationship between the displacement pressure of reservoir rocks and the shaly content, diagenetic burial depth can be obtained (Fig. 4, Eq. (4)). And it is calculated from this that the palaeo-displacement pressure of the reservoir rocks of the Dong 1 Member, Guantao Fm, and the Dong 1 Fm in the accumulation stage are respectively 0.16 MPa, 0.15 MPa and 0.18 MPa.

where pds represents the displacement pressure of reservoir rock, MPa; Rs represents the shaly content of reservoir rock; Zs represents depth of reservoir rock, m.

Fig. 4. Relationship between displacement pressures and shaly contents & depths of reservoir rocks in the Nangpu Depression.

It can be seen that the displacement pressure of fault zone filling materials of the three active faults of F1, F2, and F3 in the Dong 1 Member, Guantao Fm, and the Dong 1 Fm are greater than the palaeo-displacement pressure of reservoirs, indicating that the three active faults show lateral sealing to the natural gas in the reservoirs of the Dong 1 Member, Guantao Fm, and the Dong 1 Fm in the late deposition of the Minghuazhen Fm, favorable for the accumulation of natural gas. Industrial gas flow was uniformly obtained in the wells of Np101 (Dong 1 Member), Np1 (Guantao Fm) and Np1-2 (Dong 1 Member) drilled in the Nanpu No. 1 Structure, which can further confirm this conclusion.

Conclusion

1)Geological conditions for the lateral sealing of faults in the active stage is that the fault are in reverse direction and the faulted strata are dominated by mudstone, thereby making their displacement pressure greater than or equal to that in the reservoir rocks of the oil and gas migration well. And the lateral sealing is formed.

2)By comparing the displacement pressure of fault activity period with that of reservoir rocks in oil and gas migration wall, a set research method for the lateral sealing of faults in the active stage has been established and it was applied in the study of the sealing of the three active faults F1, F2, and F3 to the lateral distributive migration natural gas in the natural gas accumulation period–late deposition period of the Minghuazhen Fm in the Nanpu No. 1 Structure of the Nanpu Sag. The results show that: the three active faults of F1, F2, and F3 contain fault zone filling materials dominated by shaly content. The displacement pressure in the late deposition period of the Minghuazhen Fm was uniformly greater than that of the reservoirs in the Dong 1 member, the Guantao Fm and the Dong 1 member. And they are laterally sealed, which is in agreement with the fact that the current wells Np101, Np1-2, and Np1 obtained industrial gas flow in the Dong 1 Member and the Guantao Fm. The results show that the application of the method to the study of active faults is feasible.

3)The method has many shortcomings, such as the use of physical simulation to establish the empirical relationship between the displacement pressure of fault zone filling materials and their burial depths, shaly content. Restricted by the experimental conditions, this relationship obtained in a physical simulation experiment may not necessarily represent the relationship between the displacement pressure of fault zone filling materials and their burial depths. There are certain errors between the calculated displacement pressure of fault zone filling materials and the actual displacement pressure of fault zone filling materials, bringing risks to the lateral sealing in the active stage. It can be seen that the method is not perfect, and it needs to be improved in the future.

Source: Geological conditions for lateral sealing of active faults and relevant research methods

Authors: Guang Fu, Mingwang Zhan

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