A method for determining the lower limit of sealing thickness in complex lithologic layers based on temperature field simulation

Abstract

The present invention discloses a method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation, comprising: determining the upper and lower superposition relationship and the total thickness of the siltstone layer and the mud shale layer according to the lithologic vertical distribution characteristics of the overlying strata within a set range; obtaining the formula for preparing siltstone and mud shale respectively through experimental fitting; preparing a top plate rock sample by integral molding; arranging a heating device and a temperature sensor on the bottom surface of the rock sample, and arranging multiple temperature sensors on the side of the rock sample at set intervals; heating the rock sample by the heating device until the bottom temperature is a first set temperature, and stabilizing at this temperature until the measured values ​​of each temperature sensor are stable, and determining the lowest position where the rock sample temperature is no higher than a second set temperature; determining the lower limit of the top plate sealing thickness according to the superposition relationship and this position; and similarly determining the lower limit of the bottom plate sealing thickness. This method can reasonably quantify the lower limit of the top and bottom plate sealing thickness of a shale reservoir by simplifying the lithologic and temperature field simulation methods.

Description

Technical Field

[0001] The present invention relates to the fields of petroleum geology and shale reservoir mining technology, and in particular to a method for determining the lower limit of the sealing thickness of a complex lithology layer based on temperature field simulation. Background Art

[0002] my country's medium- to low-maturity oil shale reserves hold enormous potential, and in-situ mining technology, as an effective approach to industrializing its production, is gaining increasing attention. In-situ mining involves directly heating the shale reservoir underground, causing it to crack underground. The resulting oil and gas is then extracted through production wells.

[0003] Within the same geological environment, the sealing strength of the caprock is highly correlated with its lithology. Various geological processes form caprocks of varying lithologies. Based on the caprocks that have been explored to date, gypsum, argillaceous rock, and dense carbonate are the predominant lithologies, with few examples of other lithologies being explored as caprocks.

[0004] Whether the overburden can effectively seal shale reservoirs during in-situ mining is one of the key factors in improving oil recovery. However, there is no effective means to evaluate the roof sealing performance during in-situ mining of shale reservoirs. Summary of the Invention

[0005] In order to at least partially solve the above-mentioned technical problems existing in the prior art, the inventors have made the present invention. Through a specific implementation method, a method for determining the lower limit of the sealing thickness of complex lithologic layers based on temperature field simulation is provided. The method can reasonably quantify the lower limit of the sealing thickness of the shale reservoir roof by simplifying the lithologic and temperature field simulation methods.

[0006] An embodiment of the present invention provides a method for determining a lower limit of a plugging thickness of a complex lithologic layer based on temperature field simulation, wherein the complex lithologic layer is an overlying stratum of a shale reservoir, and the complex lithologic layer is an interbedded layer of siltstone and mud shale. The method includes:

[0007] Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the overlying strata within a set range;

[0008] Based on the measured properties of multiple siltstone samples, a first formula for siltstone is prepared by experimental fitting; based on the measured properties of multiple shale samples, a second formula for shale is prepared by experimental fitting;

[0009] According to the upper and lower overlapping relationship, the total thickness of the siltstone layer and the total thickness of the shale layer, the first formula and the second formula, a roof rock sample is prepared by an integrated molding method;

[0010] A first heating device and a first temperature sensor are arranged on the bottom surface of the roof rock sample, and multiple first temperature sensors are arranged on the side of the roof rock sample at first set intervals. The first heating device is used to heat the roof rock sample until the measured temperature of the first temperature sensor at the bottom is the first set temperature, and stabilize it at this temperature until the change amplitude of the measured value of each first temperature sensor is less than the set threshold value. The lowest position where the temperature of the roof rock sample is not higher than the second set temperature is determined based on the measured value of the first temperature sensor, and the lower limit of the roof sealing thickness is determined based on the upper and lower overlapping relationship and the lowest position.

[0011] In some embodiments, preparing a first formula of siltstone by experimental fitting based on measured properties of a plurality of siltstone samples comprises:

[0012] Measuring the particle size distribution, mineral composition and porosity of rock samples of each siltstone layer within the set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness-weighted averaging;

[0013] Using the average values ​​of the particle size distribution and the mineral composition of the siltstone layer as the current formula, preparing a siltstone sample, and adjusting the current formula until the error between the porosity of the currently prepared siltstone sample and the corresponding average values ​​meets an error threshold, thereby obtaining a first formula for preparing siltstone;

[0014] Accordingly, the second formula for preparing shale by experimental fitting based on the measured properties of the plurality of shale samples includes:

[0015] Measuring the particle size distribution, mineral composition, TOC, porosity, and kerogen initial hydrocarbon generation temperature of rock samples of each shale layer within the set range, and obtaining the average value of the corresponding parameters by thickness-weighted averaging;

[0016] A shale sample is prepared using the average values ​​of the particle size distribution and the average values ​​of the mineral composition of the shale layer as the current formula. The current formula is adjusted until the errors between the TOC, porosity, and initial kerogen hydrocarbon production temperature of the currently prepared shale sample and the corresponding average values ​​meet the error threshold, thereby obtaining a second formula for preparing shale.

[0017] In some embodiments, the first set temperature is the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches a set conversion rate threshold.

[0018] In some embodiments, the first set temperature is the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches 90%.

[0019] In some embodiments, the second set temperature is the average of the initial hydrocarbon production temperatures of the kerogen of each shale sample within the set range of the overlying formation.

[0020] In some embodiments, the first heating device is a one-way heating device;

[0021] The first heating device is located at the center of the bottom surface of the roof rock sample;

[0022] The area of ​​the bottom surface of the roof rock sample directly heated by the first heating device does not exceed 1 / 4 of the bottom surface area.

[0023] In some embodiments, if the upper and lower overlapping relationship is that the siltstone layer is on top, determining the lower limit of the roof plugging thickness based on the upper and lower overlapping relationship and the lowest position includes:

[0024] If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness;

[0025] If the lowest position is located in the siltstone layer of the roof rock sample, the method is ineffective.

[0026] In some embodiments, if the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lower limit of the roof plugging thickness based on the upper and lower overlapping relationship and the lowest position includes:

[0027] If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness;

[0028] If the lowest position is located in the siltstone layer of the roof rock sample, the total thickness of the siltstone layer is determined as the lower limit of the roof sealing thickness.

[0029] In some embodiments, if the underlying rock formation of the shale reservoir is a mud shale formation, the method further includes performing the following step of determining the lower limit of the bottom plate plugging thickness:

[0030] A bottom plate rock sample is prepared using a coring section of an underlying rock formation of a shale reservoir, and a side surface of the bottom plate rock sample is wrapped with a heat-insulating material that does not contain pressure. A second heating device and a second temperature sensor are arranged on the top surface of the bottom plate rock sample, and a plurality of second temperature sensors are arranged on the side surface of the bottom plate rock sample at second set intervals.

[0031] The bottom plate rock sample is heated by the second heating device until the temperature measured by the second temperature sensor on the top reaches the first set temperature, and is stabilized at this temperature until the amplitude of the temperature change measured by each second temperature sensor is less than a set threshold, and the highest position at which the bottom plate rock sample temperature is not higher than the third set temperature is determined based on the measured temperatures of each second temperature sensor;

[0032] The lower limit of the bottom plate plugging thickness of the shale reservoir is determined based on the distance between the position and the top surface.

[0033] In some embodiments, the third set temperature is the initial hydrocarbon production temperature of kerogen in the underlying rock formation.

[0034] In some embodiments, if the underlying rock layer of the shale reservoir is an interbedded layer of siltstone and mud shale, the method further includes performing the following step of determining the lower limit of the bottom plate plugging thickness:

[0035] Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the underlying rock layer within a set range;

[0036] preparing a third formulation for siltstone by experimental fitting based on measured properties of multiple siltstone samples within a set range of the underlying rock formation, and preparing a fourth formulation for shale by experimental fitting based on measured properties of multiple shale samples within a set range of the underlying rock formation;

[0037] According to the upper and lower overlapping relationship of the siltstone layer and the mud shale layer of the underlying rock layer, the total thickness of the siltstone layer and the total thickness of the mud shale layer, and the third and fourth formulas, a bottom plate rock sample is prepared by an integrated molding method;

[0038] A second heating device and a second temperature sensor are arranged on the top surface of the bottom plate rock sample, and multiple second temperature sensors are arranged on the side of the bottom plate rock sample at second set intervals. The second heating device is used to heat the bottom plate rock sample until the measured temperature of the second temperature sensor on the top is the first set temperature, and stabilize it at this temperature until the change amplitude of the measured value of each second temperature sensor is less than the set threshold value. The highest position where the temperature of the bottom plate rock sample is not higher than the fourth set temperature is determined based on the measured value of the second temperature sensor. Based on the overlapping relationship between the siltstone layer and the mud shale layer of the underlying rock stratum, the lower limit of the low plate plugging thickness is determined according to the highest position.

[0039] In some embodiments, the fourth set temperature is an average value of the initial hydrocarbon production temperatures of the kerogen of each shale sample within the set range of the underlying rock formation.

[0040] The beneficial effects of the above technical solutions provided by the embodiments of the present invention include at least:

[0041] (1) The method provided by the embodiment of the present invention for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation is to prepare a roof rock sample by integral molding, and to heat the roof rock sample by using a first heating device so that the measured temperature of the first temperature sensor at the bottom is stabilized at the first set temperature until the measured temperature of each first temperature sensor is stabilized. The lowest position where the temperature of the roof rock sample is not higher than the second set temperature is determined based on the measured temperature of each first temperature sensor; and the lower limit of the roof sealing thickness is determined based on the upper and lower overlapping relationship of the siltstone layer and the mudstone layer and the lowest position. The lower limit of the roof sealing thickness is determined based on the temperature field simulation by the experimental method, and the roof sealing conditions in the in-situ heating production of the shale reservoir are specifically quantified, which can effectively guide production and improve the economic benefits of in-situ heating production of shale oil and gas.

[0042] (2) The method provided in the embodiment of the present invention for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation determines the vertical superposition relationship of the siltstone layer and the mudstone layer, as well as the total thickness of the siltstone layer and the total thickness of the mudstone layer, based on the lithologic vertical distribution characteristics of the overlying strata within a set range; obtains the formula for preparing siltstone and mudstone respectively through experimental fitting; and prepares the roof rock sample by integral molding. The prepared synthetic sample takes into account both the properties of mudstone and siltstone and the vertical superposition relationship between the two, rationally simplifies the sample, and thus realizes the determination of the lower limit of the sealing thickness of the complex lithologic roof with interbedded mudstone and siltstone.

[0043] (3) The method provided in the embodiment of the present invention for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation is such that during the experiment, the temperature at the bottom of the roof is kept constant at the lowest temperature at which the kerogen conversion rate in the reservoir reaches 90%. At this temperature, the reservoir almost completes the conversion to hydrocarbons, achieving maximum oil and gas production. That is, this temperature is the highest temperature at which the reservoir may be heated during the in-situ heating and exploitation of an actual shale reservoir. Therefore, the setting of this temperature ensures the safety and rationality of the determined lower limit of the roof sealing thickness.

[0044] (4) The method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation provided by an embodiment of the present invention determines the lower limit of the roof sealing thickness based on the fact that the temperature at the bottom of the roof is constant at the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches a set conversion rate threshold, and the lower limit of the roof sealing thickness is determined according to the lowest position where the temperature of the roof rock sample is not higher than the initial hydrocarbon production temperature of the kerogen in the overburden. Once the overburden begins to produce hydrocarbons, the kerogen material changes, which in turn causes changes in the rock structure, destroying the original sealing properties. Therefore, only roofs below this temperature have sealing properties. Therefore, the setting of this temperature ensures that the lower limit of the determined roof sealing thickness is reasonable and has strong production guidance significance.

[0045] (5) In the method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation provided by an embodiment of the present invention, the first heating device is a unidirectional heating device, which saves resources; the heating position is located at the center, and the direct heating area of ​​the bottom surface of the top plate rock sample does not exceed 1 / 4 of the bottom surface area, so that the heat source can be spread upward as much as possible.

[0046] (6) When studying the sealing properties of shale reservoirs, existing technologies often focus only on the sealing properties of the roof, while ignoring the sealing function of the floor. However, for in-situ heating and production of shale reservoirs, poor floor sealing performance will still lead to oil and gas loss, affecting the recovery rate. The method for determining the lower limit of the sealing thickness of complex lithologic layers based on temperature field simulation provided by the embodiment of the present invention not only considers the sealing properties of the roof, but also fully considers the sealing properties of the floor, thus ensuring the reliability of the determination of the sealing performance and further providing a guarantee for improving the recovery rate.

[0047] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings.

[0048] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0050] Figure 1 This is a flow chart of a method for determining the lower limit of the plugging thickness of a complex lithologic layer based on temperature field simulation in Example 1 of the present invention;

[0051] Figure 2 This is a flow chart of a method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation in Example 2 of the present invention;

[0052] Figure 3 This is a flow chart of a method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation in Example 3 of the present invention;

[0053] Figure 4 This is a schematic diagram of the axial cross-section of the roof rock sample in Example 4 of the present invention;

[0054] Figure 5 This is a temperature field simulation diagram in Example 4 of the present invention;

[0055] Figure 6 for Figure 5Oil saturation variation diagram at the middle temperature line of 200℃ and 220℃. DETAILED DESCRIPTION

[0056] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0057] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0058] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the invention belongs. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail.

[0059] In the description of the present invention, it should be noted that the terms “include”, “including”, “have”, “contain”, etc. are all open terms, which mean including but not limited to.

[0060] The inventors found in their work that the existing technology has the problem of difficulty in effectively evaluating the roof sealing performance during the heating and mining process of shale reservoirs. After further research and development, the inventors found that the difference in heat conduction between the roof and bottom plates of the shale reservoir during the in-situ heating and mining process of the shale reservoir determines the difference in its sealing thickness. Therefore, the sealing thickness of the roof and bottom plates can be reasonably determined through temperature field simulation experiments. In addition, it is generally believed that only mud shale can produce a sealing effect during the in-situ mining of shale reservoirs, but the inventors found that the complex lithology of interbedded mud shale and siltstone may also produce a sealing effect during the in-situ mining of shale reservoirs. The embodiment of the present invention provides a method for determining the lower limit of the sealing thickness of complex lithology layers based on temperature field simulation, which can reasonably quantify the lower limit of the sealing thickness of the shale reservoir roof by simplifying the lithology and temperature field simulation methods.

[0061] Example 1

[0062] The first embodiment of the present invention provides a method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation, referring to Figure 1 As shown, the following steps are included:

[0063] Step S11: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer according to the vertical distribution characteristics of the lithology of the overlying strata within a set range.

[0064] The complex lithologic layer is the overlying stratum of the shale reservoir, which is composed of interbedded siltstone and mud shale.

[0065] Empirical data indicates that the roof seal thickness during in-situ heating of shale reservoirs is typically within 300-400 centimeters. Therefore, the length (thickness) of the roof rock sample should be no less than 500 cm. Any thicker increases experimental difficulty and cost; any thinner will likely fall below the actual lower limit of the roof seal thickness, leading to experimental failure (unable to measure the required roof seal thickness for in-situ heating of shale). This means that the overlying stratum should be within a 500 cm thickness range, preferably within 500 cm from the location closest to the top of the shale reservoir upward.

[0066] The set range is divided into an upper and a lower part by an averaging method. If the cumulative thickness of the lower shale layer exceeds the first set ratio (for example, more than 60%), the superposition relationship is determined to be the shale layer at the bottom and the siltstone layer at the top; if the cumulative thickness of the lower shale layer is lower than the second set ratio (for example, lower than 40%), the superposition relationship is determined to be the shale layer at the top and the siltstone layer at the bottom; if the cumulative thickness of the lower shale layer is within the set ratio range (for example, 40% to 60%), that is, the ratios of the two are equivalent, then the lower part within the set range is further divided into an upper and a lower part by an averaging method, and the above method is repeated to determine the upper and lower superposition relationship of the siltstone layer and the shale layer.

[0067] Step S12: Based on the measured properties of multiple siltstone samples, a first formula of siltstone is prepared by experimental fitting; based on the measured properties of multiple shale samples, a second formula of shale is prepared by experimental fitting.

[0068] In some embodiments, fitting the first formula for preparing siltstone may include: measuring the particle size distribution, mineral composition and porosity of rock samples of each siltstone layer within a set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness-weighted averaging; preparing the siltstone sample using the average value of the particle size distribution and the average value of the mineral composition of the siltstone layer as the current formula, and adjusting the current formula until the error between the porosity of the currently prepared siltstone sample and the corresponding average value meets the error threshold, thereby obtaining the first formula for preparing siltstone.

[0069] Small-sized samples can be synthesized first, the porosity of the synthesized samples can be measured, and the results can be calibrated with the measurement results of natural samples. Then, the current particle size distribution and mineral composition can be adjusted and optimized, and new artificial samples can be synthesized. Finally, the error between the measurement results of the synthetic samples and the natural samples does not exceed the error threshold (which can be set to 15%).

[0070] In some embodiments, fitting the second formula for preparing mud shale may include: measuring the particle size distribution, mineral composition, TOC, porosity and kerogen initial hydrocarbon production temperature of rock samples of each mud shale layer within a set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness weighting; preparing mud shale samples using the average value of the particle size distribution and the average value of the mineral composition of the mud shale layer as the current formula, and adjusting the current formula until the errors between the TOC, porosity and kerogen initial hydrocarbon production temperature of the currently prepared mud shale sample and the corresponding average values ​​meet the error threshold, thereby obtaining the second formula for preparing mud shale.

[0071] Optionally, in the process of fitting the first and second formulas, the parameters used for calibration may include not only the above parameters, but also the fracture pressure, breakthrough pressure and critical damage strain value of fracture, etc. Correspondingly, the measurement results of natural samples of these parameters are obtained by thickness-weighted averaging the measurement results of the corresponding rock samples.

[0072] Step S13: preparing a roof rock sample by integral molding according to the upper and lower overlapping relationship, the total thickness of the siltstone layer and the total thickness of the shale layer, the first formula and the second formula.

[0073] Roof rock samples of the same cross-sectional area (eg cylindrical with a diameter of 5 cm) can be prepared.

[0074] Taking the above setting range of 500 cm thickness as an example, the thickness of the roof rock sample is also 500 cm.

[0075] Furthermore, the sides of the roof rock sample are wrapped with a heat-insulating material that does not contain pressure. The heat insulation can prevent heat from not being dissipated to the surroundings during the heating process; if there is no pressure, the pressure can be released outwards.

[0076] Step S14: Arrange a first heating device and a first temperature sensor on the bottom surface of the roof rock sample, arrange multiple first temperature sensors on the side of the roof rock sample at a first set interval, use the first heating device to heat the roof rock sample until the measured temperature of the first temperature sensor at the bottom is the first set temperature, and stabilize at this temperature until the measurement value change amplitude of each first temperature sensor is less than the set threshold value, determine the lowest position where the roof rock sample temperature is not higher than the second set temperature based on the measurement value of the first temperature sensor, and determine the lower limit of the roof sealing thickness based on the upper and lower overlapping relationship and the lowest position.

[0077] In the embodiment of the present application, from bottom to top corresponds to from bottom to top.

[0078] In some embodiments, the first heating device is a one-way heating device to save resources.

[0079] The first heating device is located at the center of the bottom surface of the top plate rock sample. Further, the first heating device can directly heat an area of ​​the bottom surface of the top plate rock sample that does not exceed 1 / 4 of the bottom surface area, so that the heat source can be spread upward as much as possible.

[0080] The first temperature sensors are arranged axially on the side of the roof rock sample, and the first set interval of the arrangement is set according to the measurement accuracy requirement, usually at an interval of 10 cm.

[0081] In some embodiments, the first set temperature is the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches a set conversion rate threshold.

[0082] The first set temperature can be determined by using a source rock thermal simulation experiment or a kerogen activation energy experiment.

[0083] For example, multiple samples of shale reservoirs are taken, and source rock thermal simulation experiments are conducted on each sample to measure the lowest temperature at which the kerogen conversion rate reaches a set conversion rate threshold, and the minimum value of the multiple measured minimum temperatures is determined as the first set temperature.

[0084] Furthermore, the first set temperature is the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches 90%, which is usually 290-330°C.

[0085] The kerogen conversion rate reaches 90%. At this temperature, the reservoir is almost completely converted to hydrocarbons, achieving maximum oil and gas production. That is, this temperature is the highest temperature at which the reservoir can be heated during the in-situ heating and production process of actual shale reservoirs. Therefore, the setting of this temperature ensures the safety and rationality of the determined lower limit of the roof sealing thickness.

[0086] In some embodiments, the second set temperature is the average value of the initial hydrocarbon production temperature of the kerogen of each shale sample within the set range of the overlying formation, which is usually 200-250°C.

[0087] The initial hydrocarbon generation temperature of kerogen in rock samples can be measured using source rock thermal simulation experiments.

[0088] The lower limit of the roof plugging thickness is determined based on the minimum temperature at the bottom of the roof being constant, at which the kerogen conversion rate in the shale reservoir reaches the set conversion threshold. The lower limit of the roof plugging thickness is determined based on the lowest location where the roof rock sample temperature does not exceed the initial hydrocarbon production temperature of the kerogen in the overburden. Once hydrocarbon production begins in the overburden, the kerogen material changes, leading to changes in the rock structure and destroying the original plugging properties. Therefore, only roofs below this temperature have effective plugging properties. Therefore, this temperature setting ensures that the lower limit of the roof plugging thickness is reasonable and has strong production guidance significance.

[0089] The first set temperature, ie, the lowest temperature T0 at which the kerogen conversion rate in the shale reservoir reaches 90%, is determined in the above manner, and the initial hydrocarbon production temperature Tx of the kerogen in the overlying rock layer is determined.

[0090] Use the first heating device (heat source) to slowly heat (not exceeding 20°C / day), with the maximum temperature of the heat source between 400 and 650°C. Adjust the heat source power to make the bottom temperature of the roof rock sample constant at T0 until the measured temperature change amplitude of each first temperature sensor is less than the set threshold, that is, after the temperature stabilizes, collect the measured temperature of each first temperature sensor. Since the temperature of the roof rock sample gradually decreases from bottom to top, the measured temperatures of the first temperature sensors can be collected in sequence from bottom to top until the currently collected temperature is Tx or not greater than Tx. Determine the lowest position where the roof rock sample temperature is not higher than the second set temperature based on the position of the corresponding first temperature sensor.

[0091] If the currently collected temperature is Tx, the position of the corresponding first temperature sensor can be directly determined as the position where the roof rock sample temperature begins to drop to Tx; if the currently collected temperature begins to be less than Tx (the first temperature less than Tx), based on the currently collected temperature and the previous collected temperature and the position of the current first temperature sensor and the position of the previous first temperature sensor, the lowest position where the roof rock sample temperature is not higher than Tx is determined by the proportional method.

[0092] In some embodiments, based on the vertical superposition relationship between the siltstone layer and the mudstone layer, the lower limit of the roof plugging thickness is determined according to the lowest position where the roof rock sample temperature is not higher than the second set temperature, which may include the following two cases:

[0093] 1. The superposition relationship is that the siltstone layer is on top.

[0094] (1) If the lowest position is located in the mudstone layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof sealing thickness.

[0095] (2) If the lowest position is located in the siltstone layer of the roof rock sample, the temperature field simulation method is invalid, and it is impossible to determine whether the overlying rock layer has sealing properties, and it is even more impossible to quantify the lower limit of the roof sealing thickness.

[0096] 2. The superposition relationship is that the siltstone layer is at the bottom.

[0097] (1) If the lowest position is located in the mudstone layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof sealing thickness.

[0098] (2) If the lowest position is in the siltstone layer of the roof rock sample, because the mud shale layer is located above, the temperature in the mud shale layer is definitely lower than the second set temperature. As long as the thickness is greater than the total thickness of the siltstone layer, the mud shale layer above can produce a plugging effect. Therefore, the thickness of the siltstone layer of the roof rock sample is determined as the lower limit of the roof plugging thickness.

[0099] In some embodiments, other factors affecting the plugging performance may be further considered to determine a safety correction factor (greater than 1), and the safety correction factor may be used to correct the lower limit of the roof plugging thickness determined above.

[0100] The method provided in the first embodiment of the present invention for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation comprises: preparing a roof rock sample by integral molding; heating the roof rock sample by a first heating device so that the measured temperature of the first temperature sensor at the bottom stabilizes at a first set temperature; and determining the lowest position where the temperature of the roof rock sample is not higher than the second set temperature based on the measured temperature of each first temperature sensor; and determining the lower limit of the roof sealing thickness based on the upper and lower overlapping relationship of the siltstone layer and the mudstone layer and the lowest position. The lower limit of the roof sealing thickness is determined based on the temperature field simulation by an experimental method, and the roof sealing conditions during the in-situ heating exploitation of the shale reservoir are specifically quantified, which can effectively guide production and improve the economic benefits of in-situ heating exploitation of shale oil and gas.

[0101] The method provided in Example 1 of the present invention for determining the lower limit of the sealing thickness of complex lithologic layers based on temperature field simulation determines the vertical superposition relationship of siltstone and mudstone layers, as well as the total thickness of the siltstone and mudstone layers, based on the vertical lithologic distribution characteristics of the overlying strata within a set range. Formulas for preparing siltstone and mudstone are obtained through experimental fitting, respectively. A roof rock sample is then prepared by integral molding. The prepared synthetic sample takes into account both the properties of mudstone and siltstone, as well as their vertical superposition relationship, rationally simplifying the sample and thereby achieving the determination of the lower limit of the sealing thickness of a complex lithologic roof with interbedded mudstone and siltstone.

[0102] Example 2

[0103] The second embodiment of the present invention provides another method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation, specifically a method for determining the lower limit of the bottom plate sealing thickness of a shale reservoir, which is applicable to the case where the underlying rock layer is a mud shale formation. Specifically, refer to Figure 2 As shown, the following steps are included:

[0104] Step S21: Prepare a bottom plate rock sample using the coring section of the underlying rock formation of the shale reservoir, wrap the side of the bottom plate rock sample with a non-pressure-holding insulating material, arrange a second heating device and a second temperature sensor on the top surface of the bottom plate rock sample, and arrange multiple second temperature sensors on the side of the bottom plate rock sample at a second set interval.

[0105] The second temperature sensor may be the same temperature sensor as the first temperature sensor. The "first" and "second" here are only used to distinguish whether the temperature sensor is set on the top plate rock sample or the bottom plate rock sample.

[0106] A bottom rock sample of the same cross-sectional area (e.g., cylindrical with a diameter of 5 cm) is prepared from a cored section of the underlying stratum of the shale reservoir. The length (thickness) of the bottom rock sample is not less than 500 cm.

[0107] The second heating device can be the same heating device as the first heating device. The "first" and "second" here are just to distinguish whether the heating device is set on the bottom surface of the top plate rock sample or the top surface of the bottom plate rock sample.

[0108] The second heating device is located at the center of the top surface of the bottom plate rock sample. Further, the second heating device can directly heat the top surface of the bottom plate rock sample to an area not exceeding 1 / 4 of the top surface area, so that the heat source can be spread downward as much as possible.

[0109] Specifically, the top surface of the bottom plate rock sample refers to the side closest to the shale reservoir at the corresponding original underground position.

[0110] The second temperature sensors are arranged axially on the side of the bottom plate rock sample. The second set interval is set according to the measurement accuracy requirement and can be consistent with the first set interval, usually 10 cm.

[0111] Step S22: Use the second heating device to heat the bottom plate rock sample until the measured temperature of the second temperature sensor on the top is the first set temperature, and stabilize at this temperature until the measured temperature change amplitude of each second temperature sensor is less than the set threshold value, and determine the highest position where the bottom plate rock sample temperature is not higher than the third set temperature based on the measured temperature of each second temperature sensor.

[0112] In some embodiments, the third set temperature is the initial hydrocarbon production temperature of kerogen in the underlying rock formation, and its determination method can refer to the determination of the second set temperature.

[0113] Use the second heating device (heat source) to slowly heat (not exceeding 20°C / day), with the maximum temperature of the heat source between 400 and 650°C. Adjust the heat source power to make the top temperature of the bottom plate rock sample constant T0 until the measured temperature change amplitude of each second temperature sensor is less than the set threshold, that is, after the temperature stabilizes, collect the measured temperature of each second temperature sensor. Since the temperature of the bottom plate rock sample gradually decreases from top to bottom, the measured temperatures of the second temperature sensors can be collected in sequence from top to bottom until the currently collected temperature is the third set temperature or starts to be less than the third set temperature. According to the position of the corresponding second temperature sensor, determine the highest position where the bottom plate rock sample temperature is not higher than the third set temperature.

[0114] Taking the third set temperature as Tx as an example, if the currently collected temperature is Tx, the position of the corresponding second temperature sensor can be directly determined as the position where the bottom plate rock sample temperature begins to drop to Tx; if the currently collected temperature begins to be less than Tx (the first temperature less than Tx), based on the currently collected temperature and the previous collected temperature and the position of the current second temperature sensor and the position of the previous second temperature sensor, the highest position where the bottom plate rock sample temperature is not higher than Tx is determined by the proportional method.

[0115] Step S23: determining the lower limit of the bottom plate plugging thickness of the shale reservoir according to the distance from the position to the top surface.

[0116] The distance from this position to the top surface can be directly determined as the lower limit of the bottom plate plugging thickness of the shale reservoir; other factors affecting the plugging performance can also be further considered to determine a safety correction factor (greater than 1), and the product of the safety correction factor and the determined distance can be used as the lower limit of the bottom plate plugging thickness of the shale reservoir.

[0117] Example 3

[0118] Embodiment 3 of the present invention provides another method for determining the lower limit of the sealing thickness of complex lithologic layers based on temperature field simulation, specifically a method for determining the lower limit of the bottom plate sealing thickness of a shale reservoir, which is applicable to the case where the underlying rock layer of the shale reservoir is an interlayer of siltstone and mudstone. The method for determining the lower limit of the top plate sealing thickness of the shale reservoir in the above embodiment 1 is still applicable here.

[0119] Specifically, refer to Figure 3 As shown, the following steps are included:

[0120] Step S31: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the underlying rock layer within a set range.

[0121] The setting range of the underlying rock layer can be within a thickness range of 500 cm, preferably, within a thickness range of 500 cm downward from a position closest to the top surface of the shale reservoir.

[0122] Step S32: Based on the measured properties of multiple siltstone samples within the set range of the underlying rock formation, a third formula for siltstone is prepared by experimental fitting, and based on the measured properties of multiple shale samples within the set range of the underlying rock formation, a fourth formula for shale is prepared by experimental fitting.

[0123] Step S33: preparing a bottom plate rock sample by integral molding according to the upper and lower overlapping relationship of the siltstone layer and the mud shale layer of the underlying rock strata, the total thickness of the siltstone layer and the total thickness of the mud shale layer, the third formula and the fourth formula.

[0124] Step S34: Arrange a second heating device and a second temperature sensor on the top surface of the bottom plate rock sample, and arrange multiple second temperature sensors on the side of the bottom plate rock sample at a second set interval. Use the second heating device to heat the bottom plate rock sample until the measured temperature of the second temperature sensor on the top is the first set temperature, and stabilize at this temperature until the change amplitude of the measured value of each second temperature sensor is less than the set threshold. According to the measured value of the second temperature sensor, determine the highest position where the bottom plate rock sample temperature is not higher than the fourth set temperature. Based on the upper and lower overlapping relationship, determine the lower limit of the low plate sealing thickness according to the highest position.

[0125] The fourth set temperature is the average value of the initial hydrocarbon production temperature of the kerogen of the shale samples within the set range of the underlying rock formation.

[0126] Existing technologies often focus only on the sealing properties of the roof when studying the sealing properties of shale reservoirs, while ignoring the sealing function of the floor. However, for in-situ heating and extraction of shale reservoirs, poor floor sealing performance can still lead to oil and gas loss, affecting recovery rates. The methods for determining the sealing thickness of complex lithologic adjacent layers in shale reservoirs based on temperature simulation, provided in Examples 2 and 3 of the present invention, fully consider the sealing properties of the floor, ensuring the reliability of the sealing performance determination and further guaranteeing an increase in recovery rates.

[0127] The above three embodiments can be applied individually; embodiment one and embodiment two can also be applied in combination, or embodiment one and embodiment three can be applied in combination, to determine both the lower limit of the top plate plugging thickness of the shale reservoir and the lower limit of the bottom plate plugging thickness of the shale reservoir. Only when the top and bottom plate thicknesses of the shale reservoir both meet the corresponding lower limits of the plugging thickness can it be determined that the plugging requirements are met.

[0128] Example 4

[0129] A fourth embodiment of the present invention provides a specific application of a method for determining a lower limit of the roof plugging thickness of a shale reservoir, and verifies its accuracy through numerical simulation, including the following steps:

[0130] 1) Count the total thickness of the siltstone layer and the total thickness of the shale layer in the overlying strata within the set range.

[0131] 2) The particle size distribution, mineral composition, initial kerogen hydrocarbon generation temperature Tx, TOC, porosity Φ, fracture pressure Px, and critical damage strain value εx of different shale layer samples were measured respectively, and the average values ​​of the above parameters of the shale layer were calculated using the thickness-weighted average method.

[0132] 3) The particle size distribution, mineral composition, porosity Φ, fracture pressure Px, and critical damage strain value εx of different siltstone layer samples were measured respectively, and the average values ​​of the above parameters of the siltstone layer were calculated using the thickness weighted average method.

[0133] 4) According to the geological conditions, the artificial samples in this experiment were determined to have a siltstone layer at the bottom and a shale layer at the top.

[0134] 5) According to the averaged parameter results, based on the superposition relationship and thickness, a roof rock sample containing a homogeneous siltstone layer and a homogeneous mudstone layer is synthesized at one time (the mudstone layer thickness is equal to 400 cm, the siltstone layer thickness is equal to 100 cm, and the cumulative deviation of the above parameters from the design value does not exceed 15%, and the sample preparation is qualified. The roof rock sample is a cylinder with a diameter of 5 cm, see Figure 4 The figure shows a schematic cross-sectional view of the roof rock sample along the axial direction.

[0135] 6) The T0 of the reservoir rock sample was measured to be 300°C (90% conversion temperature of kerogen).

[0136] 7) The average Tx (initial hydrocarbon generation temperature of kerogen) of rock samples in the mudstone layer was determined to be 200°C.

[0137] 8) Wrap the roof rock sample with rock wool as insulation, without compressing the material. Place the wrapped roof rock sample in the experimental apparatus. Place a unidirectional heat source at the bottom of the roof rock sample. Arrange temperature sensors starting from the bottom and spaced 10 cm apart along the axis of the roof rock sample.

[0138] 9) The heat source is heated at a rate of 20°C / day, and the peak temperature is set to 300°C (T0).

[0139] 10) After the bottom of the roof rock sample is kept at a constant temperature of T0 for 30 minutes (the temperature measured by each temperature sensor reaches stability), continuously collect the roof rock sample temperature sensor data. The distance from the position where the temperature Tx is measured to the bottom of the roof rock sample is 140 cm, which is the lower limit of the roof plugging thickness.

[0140] 11) A numerical model consistent with the samples and experimental conditions was constructed. A shale layer, 400 cm thick, was placed at the top. Its TOC, porosity, Tx, Px, εx, and mineral composition were identical to those of the artificial samples used in the experiment. A siltstone layer, 100 cm thick, was placed at the bottom. Its porosity, Px, εx, and mineral composition were identical to those of the artificial samples used in the experiment. The kerogen activation energy parameters for the shale were set so that the Tx (initial kerogen hydrocarbon generation temperature) of the roof rock sample was 200°C.

[0141] 12) Keep the temperature at the bottom of the roof rock sample constant at T0 (300°C), complete the numerical simulation of the temperature field, and wait for the grid temperature to stabilize.

[0142] 13) See Figure 5 The figure shows the simulated temperature field of the roof rock sample. Figure 5 Oil saturation at the Tx temperature line (200°C) (see Figure 6 There was no change during the entire simulation period, indicating that oil and gas were blocked at this location.

[0143] 14) Observation Figure 5 Oil saturation at 220°C (see Figure 6 During the entire simulation period, there are obvious changes, indicating that the oil and gas are not blocked at this location.

[0144] 15) The thickness of the Tx temperature line is 138 cm. Comparison shows that a roof thickness of 138 cm meets the sealing requirements. The error between the numerical simulation results and the experimental results (140 cm) is 1.4%, thus the numerical simulation verifies the reliability of the experimental results.

[0145] It should be understood that the specific order or hierarchy of steps in the disclosed processes is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process can be rearranged without departing from the scope of the present disclosure. The accompanying method claims present elements of the various steps in an exemplary order and are not intended to be limited to the specific order or hierarchy described.

[0146] In the foregoing detailed description, various features are grouped together in a single embodiment to simplify the disclosure. This method of disclosure should not be interpreted as reflecting an intention that embodiments of the claimed subject matter require more features than are expressly recited in each claim. On the contrary, as reflected in the appended claims, the invention comprises less than all the features of any individual disclosed embodiment. The appended claims are therefore hereby expressly incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment of the invention.

[0147] The above description includes examples of one or more embodiments. Of course, it is not possible to describe all possible combinations of parts or methods for the purpose of describing the above embodiments, but it will be appreciated by those skilled in the art that the various embodiments may be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations that fall within the scope of protection of the appended claims. Furthermore, to the extent that the term "comprising" is used in the specification or claims, the word is intended to be encompassed in a manner similar to the term "including," as explained in terms of "including," used as a transitional word in the claims. Furthermore, any term "or" used in the specification of the claims is intended to mean a "non-exclusive or." The terms "first," "second," etc. are used for descriptive purposes and are not to be understood as indicating or implying relative importance.

Claims

1. A method for determining the lower limit of the sealing thickness of a complex lithologic layer based on temperature field simulation, wherein the complex lithologic layer is the overlying stratum of a shale reservoir, and the complex lithologic layer is an interbedded layer of siltstone and mud shale, characterized in that: The method comprises: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the overlying strata within a set range; Based on the measured properties of multiple siltstone samples, a first formula for siltstone is prepared by experimental fitting; based on the measured properties of multiple shale samples, a second formula for shale is prepared by experimental fitting; According to the upper and lower overlapping relationship, the total thickness of the siltstone layer and the total thickness of the shale layer, the first formula and the second formula, a roof rock sample is prepared by an integrated molding method; A first heating device and a first temperature sensor are arranged on the bottom surface of the roof rock sample, and multiple first temperature sensors are arranged on the side of the roof rock sample at first set intervals. The first heating device is used to heat the roof rock sample until the measured temperature of the first temperature sensor at the bottom is the first set temperature, and stabilize it at this temperature until the change amplitude of the measured value of each first temperature sensor is less than the set threshold value. The lowest position where the temperature of the roof rock sample is not higher than the second set temperature is determined based on the measured value of the first temperature sensor, and the lower limit of the roof sealing thickness is determined based on the upper and lower overlapping relationship and the lowest position.

2. The method according to claim 1, wherein The method of preparing a first formula of siltstone by experimental fitting based on measured properties of a plurality of siltstone samples comprises: Measuring the particle size distribution, mineral composition and porosity of rock samples of each siltstone layer within the set range of the overlying stratum, and obtaining the average value of the corresponding parameters by thickness-weighted averaging; Using the average values ​​of the particle size distribution and the mineral composition of the siltstone layer as the current formula, preparing a siltstone sample, and adjusting the current formula until the error between the porosity of the currently prepared siltstone sample and the corresponding average values ​​meets an error threshold, thereby obtaining a first formula for preparing siltstone; Accordingly, the second formula for preparing shale by experimental fitting based on the measured properties of the plurality of shale samples includes: Measuring the particle size distribution, mineral composition, TOC, porosity, and kerogen initial hydrocarbon generation temperature of rock samples of each shale layer within the set range, and obtaining the average value of the corresponding parameters by thickness-weighted averaging; A shale sample is prepared using the average values ​​of the particle size distribution and the average values ​​of the mineral composition of the shale layer as the current formula. The current formula is adjusted until the errors between the TOC, porosity, and initial kerogen hydrocarbon production temperature of the currently prepared shale sample and the corresponding average values ​​meet the error threshold, thereby obtaining a second formula for preparing shale.

3. The method according to claim 2, wherein The first set temperature is the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches a set conversion rate threshold.

4. The method according to claim 3, wherein The first set temperature is the lowest temperature at which the kerogen conversion rate in the shale reservoir reaches 90%.

5. The method according to claim 3, wherein The second set temperature is the average value of the initial hydrocarbon production temperature of the kerogen of each shale sample within the set range of the overlying formation.

6. The method according to claim 1, wherein The first heating device is a one-way heating device; The first heating device is located at the center of the bottom surface of the roof rock sample; The area of ​​the bottom surface of the roof rock sample directly heated by the first heating device does not exceed 1 / 4 of the bottom surface area.

7. The method according to claim 1, wherein If the upper and lower overlapping relationship is that the siltstone layer is on top, determining the lower limit of the roof sealing thickness according to the upper and lower overlapping relationship and the lowest position includes: If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness; If the lowest position is located in the siltstone layer of the roof rock sample, the method is ineffective.

8. The method according to claim 7, wherein If the upper and lower overlapping relationship is that the siltstone layer is at the bottom, determining the lower limit of the roof sealing thickness according to the upper and lower overlapping relationship and the lowest position includes: If the lowest position is located in the mud shale layer of the roof rock sample, the distance from the lowest position to the bottom surface of the roof rock sample is determined as the lower limit of the roof plugging thickness; If the lowest position is located in the siltstone layer of the roof rock sample, the total thickness of the siltstone layer is determined as the lower limit of the roof sealing thickness.

9. The method according to claim 1, wherein If the underlying rock layer of the shale reservoir is a mud shale formation, the method further includes performing the following steps of determining the lower limit of the bottom plate plugging thickness: A bottom plate rock sample is prepared using a coring section of an underlying rock formation of a shale reservoir, and a side surface of the bottom plate rock sample is wrapped with a heat-insulating material that does not contain pressure. A second heating device and a second temperature sensor are arranged on the top surface of the bottom plate rock sample, and a plurality of second temperature sensors are arranged on the side surface of the bottom plate rock sample at second set intervals. The bottom plate rock sample is heated by the second heating device until the temperature measured by the second temperature sensor on the top reaches the first set temperature, and is stabilized at this temperature until the amplitude of the temperature change measured by each second temperature sensor is less than a set threshold, and the highest position at which the bottom plate rock sample temperature is not higher than the third set temperature is determined based on the measured temperatures of each second temperature sensor; The lower limit of the bottom plate plugging thickness of the shale reservoir is determined based on the distance between the position and the top surface.

10. The method according to claim 9, wherein The third set temperature is the initial hydrocarbon production temperature of kerogen in the underlying rock formation.

11. The method according to claim 1, wherein If the underlying rock layer of the shale reservoir is an interbed of siltstone and mud shale, the method further includes performing the following step of determining the lower limit of the bottom plate plugging thickness: Determine the vertical superposition relationship of the siltstone layer and the mud shale layer, and the total thickness of the siltstone layer and the total thickness of the mud shale layer based on the lithologic vertical distribution characteristics of the underlying rock layer within a set range; preparing a third formulation for siltstone by experimental fitting based on measured properties of multiple siltstone samples within a set range of the underlying rock formation, and preparing a fourth formulation for shale by experimental fitting based on measured properties of multiple shale samples within a set range of the underlying rock formation; According to the upper and lower overlapping relationship of the siltstone layer and the mud shale layer of the underlying rock layer, the total thickness of the siltstone layer and the total thickness of the mud shale layer, and the third and fourth formulas, a bottom plate rock sample is prepared by an integrated molding method; A second heating device and a second temperature sensor are arranged on the top surface of the bottom plate rock sample, and multiple second temperature sensors are arranged on the side of the bottom plate rock sample at second set intervals. The second heating device is used to heat the bottom plate rock sample until the measured temperature of the second temperature sensor on the top is the first set temperature, and stabilize it at this temperature until the change amplitude of the measured value of each second temperature sensor is less than the set threshold value. The highest position where the temperature of the bottom plate rock sample is not higher than the fourth set temperature is determined based on the measured value of the second temperature sensor. Based on the overlapping relationship between the siltstone layer and the mud shale layer of the underlying rock stratum, the lower limit of the low plate plugging thickness is determined according to the highest position.

12. The method according to claim 11, wherein The fourth set temperature is the average value of the initial hydrocarbon production temperature of the kerogen of each shale sample within the set range of the underlying rock formation.

Images