A method of identifying a hydrate and free gas coexistence in a mass-transport deposit development zone

Abstract

This invention discloses a method for identifying the coexistence of hydrates and free gas in block transport sedimentary development zones. By analyzing the impact of new sedimentation on the thickness of the hydrate stability zone, it identifies various contact relationships between hydrates and free gas in sandy strata within block transport sedimentary development zones. Through research including well logging anomaly identification, geological and sedimentary condition analysis, seismic impedance inversion, hydrate stability zone thickness adjustment calculation, BSR identification, and stratigraphic lithology identification, it achieves rapid identification of hydrate-free gas coexistence layers in sandy reservoirs within block transport sedimentary development zones. This provides an understanding of the dynamic hydrocarbon accumulation system of hydrates in rapidly sedimentary development zones, with biogenic gas as the primary gas source. It innovatively explains the coexistence of hydrates and free gas in block transport sedimentary zones, and can reasonably explain the spatial distribution of hydrates and free gas in rapidly sedimentary areas, providing a basis for subsequent exploration and development of hydrate reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas hydrate exploration, specifically relating to a rapid method for identifying the coexistence of hydrates and free gas in sandy reservoirs in block transport sedimentary development zones. Background Technology

[0002] Hydrates are ice-like solid compounds that exist under low-temperature and high-pressure environments. Influenced by factors such as gas composition and salinity, they are mainly classified into Type I, Type II, and Type H hydrates in nature. Under static conditions, the depth of hydrate occurrence is affected by temperature and pressure, forming seafloor reflections (BSRs) on seismic profiles. However, multiple or even double BSRs exist in many areas. Currently, double or multiple BSRs have been discovered in various international sea areas. Their formation is due to various factors, such as rapid seafloor sedimentation, erosion by submarine canyons or landslides, sea-level rise and fall and changes in seafloor temperature, and tectonic activity. Different factors can cause BSRs to shift upwards or downwards. The gas source for hydrate formation may be biogenic or thermogenic gas, and the reservoir conditions may be fine-grained muddy sediments or sandy reservoirs. Because BSR adjustment takes time, during the adjustment process before a new equilibrium is reached, natural gas hydrates and free gas may coexist in the adjustment zones above and below the stable zone.

[0003] Massive transported sediments are widely developed in deep-water basins, mainly comprising three structural units: the head extensional zone, the body slip zone, and the toe compression zone. These different structural units exhibit varying seismic characteristics and well logging anomalies. Due to the rapid accumulation of mass-transported sediments, the deposition rate is much higher than that of normal sedimentary strata, leading to rapid changes in seafloor temperature and pressure conditions and affecting the thickness of hydrate stability zones. Therefore, studies suggest that the formation and decomposition of hydrates are closely related to mass-transported sediments or submarine landslides. However, due to limitations in drilling data, direct evidence regarding this relationship is relatively scarce. In strata where mass-transported sediments are developed, seismic profiles show anomalies such as amplitude blanks and weak amplitude reflections. Hydrates are mostly developed in the lower turbidites or normal sedimentary strata of mass-transported sediments, while hydrates are not developed within the mass-transported sediment layers themselves. Previous studies have focused primarily on the formation mechanism and distribution of mass-transported sediments and their impact on hydrate identification and accumulation.

[0004] Seismic response characteristics of transported block sedimentary layers show discontinuous, blank-chaotic, low-amplitude, or transparent reflection features on seismic profiles. The physical properties of the sediments within these layers do not change significantly, but their density and acoustic velocity are higher than those of the surrounding strata, resulting in a higher relative acoustic impedance than the surrounding rocks. They exhibit distinct top and bottom interfaces. The top interface shows a strong positive-polarity reflection, consistent with the seafloor polarity, as acoustic waves propagate from low-impedance to high-impedance strata. The bottom interface shows a negative-polarity reflection, opposite to the seafloor polarity, as acoustic waves propagate from high-impedance to low-impedance strata. Furthermore, the bottom interface often exhibits erosive properties, with slickensides identifiable on the planar surface. Due to dehydration and compression during transport, thrust faults and reduced permeability may form in localized areas of transported block sedimentary layers. Therefore, compared to normal sedimentary strata, hydrate formation in transported block sedimentary development zones is more complex, exhibiting complexity and heterogeneity in hydrate formation and spatial distribution.

[0005] Well-while-drilling data from different stations in Shikurangi, New Zealand, show that at the bottom of the block transported sedimentary formations at different well locations, the P-wave velocity and resistivity increase slightly, but the density also increases. Pressure coring analysis shows that there are no hydrates in the block transported sedimentary formations, but hydrates are developed on the lower BSR. In the formation of the U1519 well, which has multiple phases of block transported sedimentary formations, there are no hydrates near the seafloor, but two BSRs are identified from the seismic profile. The upper BSR has strong reflections, while the lower BSR has weak reflections. There are multiple updip sandstone reservoirs between the two BSRs. Chloride ion anomalies in the cores indicate that the formations above the BSRs and between the two BSRs contain hydrates.

[0006] Therefore, how to effectively identify the occurrence and distribution of hydrates and free gas in block sedimentary development areas using well logging and seismic data, and how to obtain information on how rapid deposition affects changes in the hydrate system and creates hydrate and free gas coexistence layers are the practical needs currently facing hydrate exploration and development. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by proposing a method for identifying the coexistence of hydrates and free gas in block transport and depositional development zones. By combining well logging and seismic data with changes in deposition rate, the method analyzes the impact of rapid deposition in biogenic gas development zones on hydrate accumulation systems.

[0008] This invention is achieved using the following technical solution: a method for identifying the coexistence of hydrates and free gas in a bulk transport and deposition development zone, comprising the following steps:

[0009] Step A: Identify hydrates and free gas layers:

[0010] Step A1: Calculate the bottom boundary of the hydrate stability zone based on well logging data, and calculate the saturated water P-wave velocity and saturated water resistivity to identify the hydrate layer and free gas layer;

[0011] Step A2: Based on well logging data and seismic data, synthesize seismic records, establish the time-depth relationship between well logging and seismic data, and perform constrained sparse pulse wave impedance inversion to obtain wave impedance data. By comparing with well logging data, use wave impedance data to delineate hydrate and free gas layers. Perform cross-validation with the hydrate and free gas layers identified in Step A1 to identify the spatial distribution of hydrates and free gas layers.

[0012] Step B: Analyze the changes in BSR affected by rapid deposition:

[0013] Step B1: Based on the seismic data and geological understanding of the target area, identify the spatial distribution range of the block transport sedimentary layers, and determine the thickness of the block transport sedimentary layers by combining well logging data; collect the corresponding core dating data and calculate the sedimentation rate.

[0014] Step B2: Using seismic data and geological knowledge of the target area, determine the BSR standard and identify all BSR reflections within the entire study area; analyze the changes in seismic amplitude between multiple BSR reflections, and use the time-depth relationship of the synthetic seismic record to convert the time between BSRs into thickness.

[0015] Step B3: Based on the geological data of the core area, analyze the thickness of the newly deposited strata on the seabed during the corresponding depositional time.

[0016] Step B4: Based on steps B1 and B2, and combined with step B3, interactively verify and identify the changes in the thickness of the BSR or hydrate stability zone caused by bulk transport and deposition.

[0017] Step C: Combining the hydrate and free gas layers identified in Step A and the BSR changes affected by rapid deposition analyzed in Step B, analyze the contact relationship between BSR and hydrates and free gas in the block transport sedimentation development zone, and realize the identification of the hydrate and free gas coexistence zone in the block transport sedimentation development zone.

[0018] Furthermore, in step A2, the measured resistivity and P-wave velocity are compared with the saturated water resistivity and saturated water P-wave velocity calculated in step A1. Combined with the calculated bottom boundary of the hydrate stability zone, if the measured result is greater than the calculated background trend above the bottom boundary of the stability zone, the formation is considered to contain hydrates; otherwise, it is considered to contain free gas.

[0019] Furthermore, in step B3, after the formation undergoes new deposition, the formation temperature change is a function of depth z and time t, expressed as:

[0020]

[0021] In the formula, G is the geothermal gradient, v is the erosion rate or deposition rate of the strata, κ is the thermal conductivity of the strata, Δz = z - z0, T0 and z0 are the initial seafloor temperature and depth, respectively, and the thermal conductivity of the strata κ is obtained from core testing; assuming that the seafloor temperature and geothermal gradient remain constant, the thickness change of the bottom boundary of the hydrate stability zone due to deposition is calculated by combining the hydrate phase equilibrium curve with the above formula.

[0022] Furthermore, in step B4, the depth of upward adjustment of the bottom boundary of the stabilization zone calculated in step B3 is used to cross-validate and explain the changes in the BSR caused by rapid block transport and deposition. As a result, the hydrate stabilization zone is adjusted upward, which will form a new BSR in a local location. In the strata where the bottom boundary of the stabilization zone is adjusted, hydrates and free gas coexist.

[0023] Furthermore, in step C, the new BSR is the interface between the hydrate and the hydrate-free gas coexistence layer, the original BSR is the interface between the hydrate-free gas coexistence layer and the free gas layer, and the new BSR and the original BSR belong to the hydrate-free gas coexistence interval.

[0024] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0025] This scheme utilizes the temperature variation with deposition rate and calculates the deposition rate based on the time of the newly deposited strata thickness. It considers the influence of deposition on temperature and quantitatively calculates the adjustment thickness variation of the bottom boundary of the stabilization zone. This provides a better explanation for why hydrates and free gas develop in the hydrate stabilization zone or below the identified BSR, avoiding incorrect misjudgments that assume poor logging data quality or erroneous inversion results, and effectively reducing interpretation risks.

[0026] Furthermore, it quantitatively explains the upward adjustment of the bottom boundary of the hydrate stability zone due to rapid deposition. This upward adjustment is accompanied by the decomposition of hydrates in the original stability zone and the formation of new hydrates in the new hydrate stability zone, resulting in the appearance of hydrate layers, free gas layers, and hydrate and free gas coexisting layers in the strata. The adjustment of the bottom boundary of the stability zone provides a theoretical basis for understanding the distribution of hydrates and free gas.

[0027] In addition, considering the influence of deposition rate and the time lag in temperature conduction, the adjustment of hydrate stability zone cannot be completed instantaneously, and the adjustment of stability zone also requires time. This effectively explains the coexistence of hydrate and free gas and the double BSR phenomenon observed in well logging and seismic observation.

[0028] This innovative method explains the formation of double BSRs and the enrichment of hydrates and free gas in the biogenic gas-controlled zone of the block transport sedimentation area. It is mainly due to the change in the thickness of the hydrate stability zone caused by rapid deposition, which leads to dynamic adjustments in the hydrate system. This method overcomes the geological phenomena that traditional static hydrate systems cannot explain and can reasonably explain the spatial distribution of hydrates and free gas in the rapid deposition area, providing a basis for the subsequent exploration and development of hydrate reservoirs. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the overall process of the identification method described in the embodiment of the present invention;

[0030] Figure 2 This is a schematic diagram comparing the resistivity and P-wave velocity of the saturated water formation calculated from the logging curves of well U1519 on the West Kurangi continental margin in New Zealand with the measured resistivity and P-wave velocity, as per an embodiment of the present invention.

[0031] Figure 3 The diagram shows the wave impedance profile (top) obtained by constrained sparse pulse inversion after stacking through well U1519 and the seismic profile (bottom).

[0032] Figure 4 This diagram illustrates the region where the bottom boundary of the hydrate stability zone is adjusted upwards due to new sedimentation, resulting in partial hydrate decomposition, coexistence of hydrates and free gas, and the formation of new BSRs. Detailed Implementation

[0033] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other. Many specific details are set forth in the following description to provide a thorough understanding of the present invention; however, the present invention may also be implemented in other ways than those described herein, and therefore, the present invention is not limited to the specific embodiments disclosed below.

[0034] Previous studies have found that bulk transported sediments are widely distributed in deep-water basins along continental margins, which hinders the identification of hydrate layers. Bulk transported sedimentary formations (BSRs) are generally difficult to identify on seismic profiles in areas with well-developed BSRs, mainly because these sediments are typically parallel to the strata and seafloor, and drilling is limited. Although hydrates and bulk transported sediments are widely developed, the relationship between the two is not clear, and existing studies are primarily qualitative. Currently, dual BSRs and the coexistence of hydrates and free gas have been discovered in several international sea areas. This is mainly due to thermogenic gas controlling the development of type II hydrates and tectonic activity, which is of great significance for the accurate evaluation and development of hydrate resources. However, in areas with widespread bulk transported sediments, research on hydrates and free gas is relatively limited. Current research mainly utilizes the lower boundary of the hydrate stability zone, combined with well logging and seismic anomalies, to identify and study the distribution of hydrate layers. Whether more types of hydrates exist in these rapidly depositing areas remains unclear.

[0035] This scheme proposes a rapid identification method for the coexistence of hydrates and free gas in block transport depositional zones based on BSR adjustment and updip sand layer anomalies. The basic principle is as follows: First, based on regional geological data such as seismic data, well logging data, and core samples, and the thickness of the hydrate stability zone, the method analyzes anomalies to identify BSR anomalies on seismic profiles. Gamma-ray logging is then used to identify the corresponding stratigraphic lithology and analyze the stratigraphic lithology in the BSR anomaly area. Next, by combining seismic and well logging data, constrained sparse pulse inversion impedance is performed to calculate the velocity and resistivity curves of the water-saturated formation. These curves are compared with the measured velocity and resistivity to identify areas deviating from the background anomaly. Combined with the calculated hydrate stability zone floor boundary, well logging-indicated hydrate and free gas layers are identified. A synthetic seismic record is then created, and the inverted impedance is compared with the well logging anomaly layers. Combining stratigraphic lithology characteristics and the deposition rate of block transport deposition, the adjusted thickness of the stability zone floor due to deposition is calculated, and the coexistence of sandy hydrates and free gas in the lower strata of the BSR is analyzed.

[0036] Specifically, this embodiment proposes a rapid identification method for the coexistence of hydrates and free gas in sandy reservoirs within a block transport sedimentary development zone. By analyzing the impact of new sedimentation on the thickness of the hydrate stability zone, it identifies various contact relationships between hydrates and free gas in sandy strata within the block transport sedimentary development zone. Through research including well logging anomaly identification, geological and sedimentary condition analysis, seismic impedance inversion, hydrate stability zone thickness adjustment calculation, BSR identification, and stratigraphic lithology identification, it achieves rapid identification of hydrate-free gas coexistence layers in sandy reservoirs within a block transport sedimentary development zone. This provides an understanding of the dynamic hydrocarbon accumulation system in rapidly sedimentary development zones where biogenic gas is the primary gas source. This embodiment combines well logging and seismic data from the U1519 well drilled during the IODP 372 expedition to the West Kurangi continental margin of New Zealand to analyze the practical application effect. Figure 1 Specifically, it includes the following steps:

[0037] Step 1: Calculate the bottom boundary of the hydrate stability zone based on well logging data, and calculate the P-wave velocity and resistivity of saturated water to identify the hydrate layer and free gas layer;

[0038] Specifically, existing traditional methods were used to calculate the lower boundary of the hydrate stability zone. Then, a simplified three-phase medium velocity model and the Archie equation were used to calculate the saturated water P-wave velocity and resistivity as background trends. When calculating the saturated water P-wave velocity, sediment lithology was obtained from core samples, with average contents including 44.6% clay, 25.9% quartz, 15.7% feldspar, and 13.8% calcium. When calculating the saturated water resistivity using the Archie equation, the Archie constants a and m were obtained using stratigraphic factors and density-porosity cross-plots, with a = 4.8 and m = 0.89. The measured resistivity and P-wave velocity were compared with those calculated using the above methods. Combined with the calculated lower boundary of the hydrate stability zone, if the measured results were greater than the calculated background trend above the lower boundary, the formation was considered to contain hydrates; otherwise, it was considered to contain a free gas layer. Figure 2 The image shows the logging curves of well U1519 on the West Kurangi continental margin in New Zealand. The calculated resistivity and P-wave velocity of the saturated water formation are compared with the measured resistivity and P-wave velocity. High anomalies indicate the presence of hydrates in the formation, while low anomalies indicate the presence of free gas. Gas layers and hydrate layers can be identified.

[0039] Step 2: Based on well logging data and seismic data, synthesize seismic records, establish the time-depth relationship between well logging and seismic data, and perform constrained sparse pulse wave impedance inversion to obtain wave impedance data. By comparing with well logging data, use wave impedance data to delineate hydrate and free gas layers. Perform cross-validation with the hydrate and free gas layers identified in Step 1 to identify the spatial distribution of hydrate and free gas layers.

[0040] Specifically, conventional well-seismic calibration is performed using P-wave velocity and density logging data and seismic data to establish the time-depth relationship between logging and seismic data. Then, constrained sparse pulse inversion is performed to obtain acoustic impedance data. This data is then compared with logging data to delineate hydrate and free gas layers. Combined with the hydrate stability zone bottom boundary identified in step 1 from the logging data, acoustic impedance data near the hydrate stability zone greater than 4.5 × 10⁻⁶ is considered. 6 kg / (m 2 The region with the s) value is considered to possibly contain hydrates, while the wave impedance data below the bottom boundary of the stable band is less than 3.2 × 10. 6 kg / (m 2 The region with the s) value is considered to potentially contain free gas, and is cross-validated with the hydrate layer and free gas layer identified in step 1 to identify the spatial distribution of hydrates and free gas layers, such as... Figure 3 As shown, the acoustic impedance profile obtained by confined sparse pulse inversion after passing through well U1519 (top figure) and the seismic profile obtained through the well (bottom figure) show that the acoustic impedance increases after the formation contains hydrates, and decreases significantly after the formation contains free gas. BSR1 and BSR2 are seismic profiles that identify two BSRs, and there is an alternating high-low acoustic impedance between them.

[0041] Step 3: Using seismic data and geological knowledge (regional geological data) of the target area, identify the spatial distribution range of block transport sedimentary layers using existing technical methods, and determine the thickness of the block transport sedimentary layers by combining well logging data; collect the corresponding core dating data, and calculate the deposition rate by dividing the thickness by time.

[0042] Step 4: Using seismic data and geological knowledge of the target area (regional geological data), traditional methods are used to determine BSR criteria (such as polarity opposite to the seabed, amplitude increase, approximately parallel to the seabed, etc.). All BSR reflections within the entire study area are identified. By comparing the amplitudes with the seabed and adjacent strata, the changes in seismic amplitude between multiple BSR reflections are analyzed. Using the time-depth relationship of the synthetic seismic record, the time between BSRs is converted into thickness.

[0043] Step 5: Based on regional geological data such as core samples, analyze the thickness of newly deposited strata on the seabed during the corresponding depositional time. After the occurrence of new deposition, the temperature change of the strata is a function of depth (z) and time (t), expressed by the following formula:

[0044]

[0045] In the formula, G represents the geothermal gradient, v represents the erosion rate (or deposition rate), κ represents the thermal conductivity of the formation, Δz = z - z0, T0 and z0 are the initial seafloor temperature (5.6℃) and depth (1019m) respectively, the geothermal gradient G (0.0212℃ / m), the erosion or deposition rate v (0.9mm / year), and the thermal conductivity κ is obtained from core testing and is approximately 1.3W / m². -1 K -1 .

[0046] Assuming constant seafloor temperature and geothermal gradient, the thickness change of the hydrate stability zone floor boundary due to sedimentation can be calculated by combining the hydrate phase equilibrium curve with the above equation. Calculations show that 45m of new seafloor sedimentation occurred, and the hydrate stability zone floor boundary was adjusted upwards. Figure 4 The display has shifted upwards by 65m; Figure 4In the study, the lower boundary of the hydrate stability zone adjusts upward with new sedimentary processes. Under initial conditions, the thickness of the hydrate stability zone is determined by the intersection of the geothermal gradient and the phase equilibrium curve. With new sedimentary processes, the formation temperature changes over time due to the influence of sediment rate, causing the lower boundary of the hydrate stability zone to adjust upward. The adjustment of the lower boundary of the stability zone takes time. During the upward adjustment of the stability zone, hydrates in the original stability zone decompose, but not necessarily completely. During the adjustment process, hydrates and free gas coexist. Furthermore, due to the presence of inclined sand layers in the study area, and the high permeability of sandy reservoirs, fluids migrate upward along the sand layers, resulting in the appearance of free gas in local stability zones.

[0047] Traditionally, hydrates are not typically found in the lower part of a BSR (Biogenic Sedimentary Stream) in hydrate development zones dominated by biogenic gas, primarily due to the lack of gas source conditions for the formation of Type II hydrates (which require thermogenic gas). This study utilizes the effect of temperature variations with deposition rate on the thickness of the hydrate stabilization zone. The adjustment of the stabilization zone thickness is not simply a matter of adjusting upwards by the number of meters of strata deposited, but rather influenced by the deposition rate. Based on the time required for a new 45m stratum thickness to be deposited, and considering that the hydrate stabilization zone thickness also requires time to reach a new equilibrium state, during which time the hydrates may not completely decompose, this method effectively explains why a thicker hydrate and free gas coexistence layer develops in the hydrate stabilization zone or in the lower part of the BSR. This avoids incorrect misjudgments based on poor logging data quality or erroneous inversion results, effectively reducing interpretation risks.

[0048] Step 6: Based on Step 3 and Step 4, and combined with Step 5, verify and identify the changes in the thickness of the BSR or hydrate stability zone caused by bulk transport and deposition.

[0049] Using the upward adjustment depth of the hydrate stabilization zone floor calculated in step 5, we cross-validate and explain the changes in BSR caused by rapid block transport and deposition. This upward adjustment of the hydrate stabilization zone leads to the formation of new BSRs in localized areas. Because the original BSRs in the lower strata are not completely decomposed, they still exist in some localized strata, resulting in discontinuities or weakening of the original BSRs (e.g., ...). Figure 3 BSR2), while newly formed BSRs are relatively stronger (e.g., BSR2). Figure 3 BSR1 is the newly formed BSR, and BSR2 is the original BSR. Figure 4 As shown, the hydrates in the upper part of the original BSR will decompose. The decomposition of hydrates takes time. In the 65m stratum at the bottom boundary of this stable zone, hydrates and free gas will coexist. There are inclined sand bodies in this area, and the gas may migrate upward along the high-permeability sand bodies.

[0050] Step 7: Combining the identified hydrate and free gas layers (Step 2) and the changes in BSR due to rapid deposition (Step 6), analyze the relationship between BSR and the contact between hydrates and free gas in the block transport sedimentation development zone. The new BSR represents the interface between hydrates and the hydrate-free gas coexistence layer. Figure 3 BSR1); the original BSR is the interface between the hydrate and free gas coexistence layer and the free gas layer ( Figure 3 (BSR2). The new BSR and the original BSR belong to the coexistence zone of hydrates and free gas. The coexistence zone contains both hydrate layers with high wave impedance and free gas layers with low wave impedance (wave impedance parameter cutoff value in step 2), thereby determining the distribution range of the hydrate and free gas coexistence layer.

[0051] The above analysis shows that the method in this embodiment combines seismic inversion results with the influence of sedimentation on the thickness of the stabilization zone to identify hydrate reservoirs. It utilizes the principle that when the rapid sedimentation rate of block transport sedimentation affects the adjustment of the hydrate stabilization zone and the formation is rich in sandy reservoirs, hydrates and free gas coexist, thus forming an identification technique. This effectively avoids the shortcomings of traditional methods in understanding hydrate accumulation in block transport sedimentation areas, especially in biogenic gas development areas. It adopts the principle that high sedimentation rates lead to an upward adjustment of the seafloor stabilization zone thickness, and that this adjustment requires a certain amount of time. This approach proposes a new understanding of hydrate and free gas enrichment in block transport sedimentation development areas, providing a basis for accurately understanding the hydrate and free gas resources in these areas and offering a reasonable and accurate explanation for the coexistence of hydrates and free gas in block transport sedimentation areas.

[0052] In summary, this scheme, combined with the wave impedance data obtained from post-stack constrained sparse pulse inversion, discovered alternating layers of high and low wave impedance in the lower strata of the BSR identified on the seismic profile. By comparing this with the wave impedance data of the water-saturated strata, it was interpreted as the formation containing hydrates and free gas, which is in good agreement with the results identified on the well logging.

[0053] This patent proposes a rapid identification method for hydrates and free gas in block transport sedimentary zones, which quantitatively explains the upward adjustment of the bottom boundary of the hydrate stability zone caused by rapid deposition. This upward adjustment is accompanied by the decomposition of hydrates in the original stability zone and the formation of new hydrates in the new hydrate stability zone. As a result, hydrate layers, free gas layers, and hydrate and free gas coexisting layers appear in the original stability zone strata. The adjustment of the bottom boundary of the stability zone provides a theoretical basis for understanding the distribution of hydrates and free gas. Because the technical method considers the influence of deposition rate and the time lag in temperature transmission, the adjusted thickness of the hydrate stabilization zone is not simply the thickness of the newly deposited strata, but rather greater than the thickness of the newly deposited strata. Furthermore, the adjustment cannot be completed instantaneously; the adjustment of the stabilization zone also requires time. This effectively explains the coexistence of hydrates and free gas observed in well logging and seismic observations, as well as phenomena such as double BSRs appearing on seismic profiles. It innovatively explains that even in the absence of thermogenic gas, in block transport depositional zones, the formation of double BSRs and the accumulation of hydrates and free gas are due to the dynamic adjustment of the hydrate system caused by rapid deposition, overcoming geological phenomena that traditional static hydrate systems cannot explain. This technical method can reasonably explain the spatial distribution of hydrates and free gas in rapidly deposited areas, providing a basis for subsequent exploration and development of hydrate reservoirs.

[0054] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for identifying the coexistence of hydrates and free gas in a bulk transport and depositional development zone, characterized in that, Includes the following steps: Step A: Identify hydrates and free gas layers: Step A1: Calculate the bottom boundary of the hydrate stability zone based on well logging data, and calculate the saturated water P-wave velocity and saturated water resistivity to identify the hydrate layer and free gas layer; Step A2: Based on well logging data and seismic data, synthesize seismic records, establish the time-depth relationship between well logging and seismic data, and perform constrained sparse pulse wave impedance inversion to obtain wave impedance data. By comparing with well logging data, use wave impedance data to delineate hydrate and free gas layers. Perform cross-validation with the hydrate and free gas layers identified in Step A1 to identify the spatial distribution of hydrates and free gas layers. Step B: Analyze the changes in BSR affected by rapid deposition: Step B1: Based on the seismic data and geological understanding of the target area, identify the spatial distribution range of the block transport sedimentary layers, and determine the thickness of the block transport sedimentary layers by combining well logging data; collect the corresponding core dating data and calculate the sedimentation rate. Step B2: Using seismic data and geological knowledge of the target area, determine the BSR standard and identify all BSR reflections within the entire study area; The study also analyzed the changes in seismic amplitude among multiple BSR reflections and used the time-depth relationship of the synthetic seismic record to convert the time between BSRs into thickness. Step B3: Based on the geological data of the core area, analyze the thickness of the newly deposited strata on the seabed during the corresponding depositional time; after the occurrence of new deposition, the change in stratum temperature is a function of depth z and time t, expressed as: , In the formula, G is the geothermal gradient, and v is the erosion rate or deposition rate. For the thermal conductivity of the formation, , and These represent the initial seabed temperature and depth, and the thermal conductivity of the formation, respectively. Obtained from core testing; assuming the seabed temperature and geothermal gradient remain constant, the thickness change of the bottom boundary of the hydrate stability zone due to sedimentation is calculated by combining the hydrate phase equilibrium curve with the above formula. Step B4: Based on Step B1 and Step B2, and combined with the depth of the upward adjustment of the bottom boundary of the stabilization zone calculated in Step B3, cross-validate and explain the changes in the thickness of the BSR or hydrate stabilization zone caused by rapid bulk transport and deposition. Step C: Combining the hydrate and free gas layers identified in Step A and the BSR changes affected by rapid deposition analyzed in Step B, analyze the contact relationship between BSR and hydrates and free gas in the block transport sedimentation development zone, and realize the identification of the hydrate and free gas coexistence zone in the block transport sedimentation development zone.

2. The method for identifying the coexistence of hydrates and free gas in a bulk transport and deposition development zone according to claim 1, characterized in that: In step A2, the measured resistivity and P-wave velocity are compared with the saturated water resistivity and saturated water P-wave velocity calculated in step A1. Combined with the calculated bottom boundary of the hydrate stability zone, if the measured results are greater than the calculated background trend in the upper part of the bottom boundary of the stability zone, the formation is considered to contain hydrates; otherwise, it is considered to contain free gas.

3. The method for identifying the coexistence of hydrates and free gas in a bulk transport and deposition development zone according to claim 1, characterized in that: In step C, the new BSR is the interface between the hydrate and the hydrate-free gas coexistence layer, the original BSR is the interface between the hydrate-free gas coexistence layer and the free gas layer, and the new BSR and the original BSR belong to the hydrate-free gas coexistence interval.

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