CN115903034B - Hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution - Google Patents
Hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution Download PDFInfo
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Abstract
The invention discloses a hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution, which introduces a concept of probability density distribution on the basis of analysis of an elastic parameter intersection graph of a logging interpretation lithofacies, and creates a lithofacies probability density function; and introducing the lithofacies probability density function into an interpretation process of elastic parameters obtained by pre-stack inversion, obtaining the probability of each lithofacies of each spatial data point, quantitatively estimating a hydrate and free gas coexistence layer, and quantitatively analyzing risks of the hydrate layer, the high-saturation free gas layer, the low-saturation free gas layer and the hydrate and free gas coexistence layer. According to the method, the lithofacies probability estimation is carried out by utilizing the relation between lithofacies and elastic parameters, the hydrate and free gas coexistence layer identification is carried out, the spatial distribution of the hydrate and the free gas reservoir under complex geological conditions can be objectively and reasonably explained, and a basis is provided for the subsequent exploration and development of the hydrate reservoir.
Description
Technical Field
The invention belongs to the field of petroleum and natural gas hydrate exploration, and particularly relates to a hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution.
Background
The hydrate is a compound similar to ice solid state, and different gas components, water and salinity can form hydrate with different structures of type I, type II and type H under the conditions of low temperature and high pressure, and the type I hydrate is also called methane hydrate. The hydrate stability band floor generally corresponds to the BSR on the seismic section, but it does not fully coincide with the BSR. Type I and heavy hydrocarbon-containing type II hydrates can develop below or above BSR, and the type II hydrate stability zone bottom boundary is deeper than the type I hydrate stability zone bottom boundary, and can form double-like submarine reflexes, i.e., double BSR or II-BSR, on seismic sections. In recent years, drilling has confirmed that there are coexistence states of hydrate and free gas in hydrate stability zones, lower parts of stability zones, and the like, such as in a plurality of sea areas such as the blackish sea table in north america, the edge of the calicadia continental, the new zealand ku langerhans dive zone, the north-south-sea fox sea area, and the north-west coast in south-sea salon, and that geological conditions and coexistence modes in which different basin hydrates coexist with free gas are different.
The hydrate and the free gas coexist under different geological conditions, and the coexistence control factors and the abnormal characteristics of the hydrate are also different, and are mainly divided into four different types of Type1, type2, type3 and Type 4:
type1 is that hydrate and free gas coexist near the bottom boundary of the hydrate stability zone, such as a Blacksea table, mainly related to biogenic gas, and the sediment lithology is a argillaceous reservoir layer due to the coexistence of local hydrate and free gas caused by methane circulation near the bottom boundary of the hydrate stability zone; type2 is a hydrate stable zone in cold spring development area, local area is where hydrate and free gas coexist, such as Cascade continental edge hydrate ridge, related to biogenic gas or thermogenic gas, high flux fluid moves upwards along fault, sand body and the like, and can reach the sea floor to form cold spring system. Because a large amount of free gas moves upwards in a gas phase mode, hydrate is rapidly formed, free water is absent in local stratum, the free gas is difficult to form hydrate, coexistence is caused, the coexistence phenomenon is common in cold spring development areas, and sediment lithology is a argillaceous reservoir; type3 is the coexistence of Type II hydrate below BSR and free gas, such as the open basin of North Zhujiang in south China and the offshore in northwest Veronica in south China, and is related to thermogenic gas, because deep thermogenic gas moves upwards with high flux fluid along the fault, chimney and other paths to form Type II hydrate, II-BSR can be formed on the seismic section, and sediment lithology is a argillaceous and argillaceous silt reservoir; type4 is a hydrate caused by rapid deposition and coexists with free gas, such as new zealand kularan dive zone, hydrate stability zone bottom boundary becomes shallow due to rapid deposition, and hydrate originally formed in sandy reservoir is decomposed, but hydrate decomposition-formation takes time due to high deposition rate, hydrate coexisting with free gas can occur in local sandy reservoir, and sediment lithology is a muddy or sandy reservoir.
From the logging response of hydrate and free gas coexistence, the coexistence layer logging anomalies are similar to the formation gas, and can be easily mistaken for formation gas, for example, in the north-south-ocean-based Zhujiang-kou basin, logging data analysis finds that the formation below the BSR has high resistivity anomalies, high-low-high anomaly changes in longitudinal wave velocity, and slightly increased transverse wave velocity, which indicate that the formation below the BSR does not completely contain free gas, but that the formation may locally contain hydrate and free gas. The related hydrate test production work also confirms the coexistence layer of hydrate and free gas under BSR. If the formation is mistaken for a mixed layer of hydrate and free gas containing free gas, the amount of hydrate and free gas resources will be underestimated.
At present, three-dimensional space study on reservoir physical properties of a reservoir containing hydrate, free gas or coexisting layers is mainly realized by reservoir engraving through utilizing cut-off values of seismic inversion results, namely hydrate, free gas and coexisting layers of the hydrate, the free gas and the coexisting layers are explained as lithofacies based on well logging, then corresponding longitudinal wave speed and longitudinal and transverse wave speed ratio elastic parameter fixed cut-off values are obtained through histogram and intersection graph analysis, finally engraving inversion results are explained through utilizing fixed cut-off values of different lithofacies, and further distribution of the hydrate, the free gas and the lithofacies of the coexisting layers is obtained. From scattered points on the intersection graph, different lithofacies points are concentrated in some positions, some positions are scattered, the data scattered points are mutually overlapped, the distances between the overlapped points in the data point sets are different, and probability is obvious. It is generally considered that the elastic parameters (longitudinal wave impedance, longitudinal wave velocity, longitudinal and transverse wave velocity ratio, etc.) of the hydrate layer, the free gas layer and the coexisting layers of the hydrate layer, the free gas layer and the free gas layer have specific distribution ranges, but because of the differences between physical properties such as clay content, porosity and the like in the lithofacies stratum, the lithofacies are mutually overlapped in the distribution ranges of the elastic parameters, the same elastic parameters represent different lithofacies, the credibility of each lithofacies in the overlapping range of the elastic parameters cannot be estimated, the wrong lithofacies classification can be caused when the elastic parameters are applied to pre-stack inversion elastic parameter interpretation, the lithofacies identification is not mutually performed, and the hydrate and free gas coexisting layers cannot be reasonably recognized and identified, so that the risk of exploration and development of a reservoir cannot be quantified.
Therefore, how to objectively and effectively identify the hydrate and free gas coexisting layers and evaluate the uncertainty in the interpretation of the hydrate and free gas layers becomes a problem to be solved.
Disclosure of Invention
The invention provides a hydrate and free gas coexisting layer identification method based on elastic parameter probability distribution, which aims to solve the problems that in the prior art, rock phase identification is out of phase, misjudgment or partial rock phase is lost, so that hydrate and free gas coexisting layers cannot be reasonably recognized and identified, and the like, so that the spatial distribution of porous natural hydrate, free gas layer and coexisting layers of the porous natural hydrate, free gas layer and the free gas coexisting layers can be reasonably evaluated.
The invention is realized by adopting the following technical scheme: a hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution comprises the following steps:
step A, establishing a lithofacies probability density function: introducing a concept of probability density distribution on the basis of analysis of an elastic parameter intersection graph of a logging interpretation lithofacies, determining prior information of each lithofacies, and creating a lithofacies probability density function;
step B, pre-stack seismic inversion is carried out to obtain an elastic parameter body of longitudinal wave speed and longitudinal wave speed ratio;
step C, rock phase probability conversion: and C, introducing the lithofacies probability density function in the step A into the interpretation process of the elastic parameter body obtained by pre-stack inversion in the step B, realizing the identification of the hydrate and free gas coexistence layer distribution, and giving risk assessment.
Further, the step a specifically includes the following steps:
step A1, developing rock facies interpretation of a hydrate and free gas coexisting layer based on logging data, and dividing a target layer into a plurality of rock facies, wherein the plurality of rock facies comprise mudstone, a hydrate layer, a high-saturation free gas layer, a low-saturation free gas layer and the hydrate and free gas coexisting layer, and the rock facies serve as subsequent elastic parameter intersection analysis basis;
step A2, carrying out intersection graph analysis based on the longitudinal wave speed and the longitudinal and transverse wave speed ratio of logging data and the lithofacies interpretation result of the step A1, and determining the distribution characteristics of different lithofacies on the intersection graph;
and A3, determining prior information of each lithofacies according to the distribution characteristics of different lithofacies on an intersection chart determined in the step A2, aiming at a target layer where a researched hydrate and free gas coexisting layer is located, counting the lithofacies explained through logging to determine the proportion of logging sampling points of each lithofacies to all logging sampling points of the target layer, and fitting a lithofacies probability density function.
Further, the step B is implemented by the following manner:
step B1, converting the superposition speed obtained by seismic processing into a low-frequency model by utilizing rock physical relation analysis results of longitudinal wave speed and longitudinal wave impedance;
step B2, performing well shock calibration based on pre-stack seismic data and logging data, and extracting seismic wavelets of near-medium-far incidence angle partial superposition data;
and B3, carrying out pre-stack seismic inversion based on the pre-stack seismic data, the low-frequency model obtained in the step B1 and the seismic wavelets extracted in the step B2 to obtain elastic parameter bodies such as space longitudinal wave speed, longitudinal-transverse wave speed ratio and the like.
In the step C, further, rock phase probability conversion is performed on the longitudinal wave velocity and the longitudinal wave velocity ratio elastic parameter body obtained by the pre-stack seismic inversion of the step B based on the rock phase probability density function determined in the step a, so as to obtain five rock phase probability bodies of mudstone, a hydrate layer, a high saturated free gas layer, a low saturated free gas layer and a hydrate and free gas coexistence layer; the rock facies with the maximum probability in the five rock facies probability bodies are the rock facies with the maximum likelihood;
and further, the risk assessment is carried out on the inversion result by obtaining the spatial three-dimensional lithofacies probability and the spatial distribution of the hydrate layer and the free gas layer of the maximum likelihood lithofacies interpretation area and adopting a probability mode, wherein the high probability indicates that the lithofacies probability is high, otherwise, the probability sum of any point on the elastic parameter body is 1.
Further, in the step A3, the lithofacies probability density functions all adopt a gaussian model.
Compared with the prior art, the invention has the advantages and positive effects that:
according to the scheme, a concept of probability density distribution is introduced on the basis of analysis of an elastic parameter intersection graph of a logging interpretation lithology, and a lithology probability density function is created by giving prior information of each lithology; then introducing the lithofacies probability density function into an interpretation process of elastic parameters obtained by pre-stack inversion, obtaining the probability of each lithofacies of each spatial data point, quantitatively estimating a hydrate and free gas coexistence layer, and quantitatively analyzing risks of the hydrate layer, the high-saturation free gas layer, the low-saturation free gas layer and the hydrate and free gas coexistence layer;
the method effectively avoids the defect that the lithology cannot be correctly distinguished for the local stratum when the longitudinal wave velocity and longitudinal and transverse wave velocity ratio elastic parameter body of the seismic inversion are interpreted by adopting the fixed cut-off value, solves the problem of elastic parameter superposition by adopting a lithology probability mode, more objectively interprets the seismic inversion result, improves the capability of resolving hydrate and free gas reservoirs by seismic inversion in an innovative manner, predicts the risk of reservoir interpretation, can reasonably interpret the spatial distribution of the hydrate and the free gas reservoirs under complex geological conditions, and provides basis for the subsequent exploration and development of the hydrate reservoirs.
Drawings
FIG. 1 is a schematic flow chart of a method for identifying a hydrate and free gas coexisting layer according to an embodiment of the present invention;
FIG. 2 is a diagram of pre-stack seismic data, with the upper diagram being near angle of incidence superimposed data, the middle diagram being medium angle of incidence superimposed data, and the lower diagram being far angle of incidence superimposed data, according to an embodiment of the present invention;
FIG. 3 is a lithofacies interpretation result diagram of an embodiment of the invention;
FIG. 4 is a graph showing probability density distribution of different lithofacies fluids, with scattered points being measured longitudinal wave velocity and longitudinal and transverse wave velocity ratio data points, and with ellipsoids being probability density function distribution fitted by the scattered points, as explained in the well logging of the embodiment of the present invention;
FIG. 5 is a schematic diagram of a longitudinal wave velocity and a longitudinal-transverse wave velocity ratio profile of simultaneous inversion before stack according to an embodiment of the present invention, wherein the upper diagram is a longitudinal wave velocity data volume, and the lower diagram is a longitudinal-transverse wave velocity ratio data volume;
FIG. 6 is a schematic diagram of a maximum likelihood lithofacies volume profile and probability volume profiles of different lithofacies in accordance with an embodiment of the present invention; the upper part is the section of the maximum likelihood lithofacies body, and the lower part is the section of the probability body of different lithofacies.
Detailed Description
In order that the above objects, features and advantages of the invention will be more readily understood, a further description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced otherwise than as described herein, and therefore the present invention is not limited to the specific embodiments disclosed below.
The embodiment provides a method for identifying a hydrate and free gas coexisting layer based on elastic parameter probability distribution, which identifies the hydrate and free gas coexisting layer based on the mode of elastic parameter probability distribution, as shown in fig. 1, and comprises the following steps:
step A, establishing a probability density function of the ratio of the longitudinal wave speed to the transverse wave speed: developing hydrate lithology interpretation based on the logging data; and according to the lithofacies interpretation result, carrying out cross map analysis by combining the longitudinal wave speed and the longitudinal wave speed ratio of the logging data; introducing a concept of probability density distribution, and fitting a lithofacies probability density function;
step B, pre-stack seismic inversion is carried out to obtain an elastic parameter body of longitudinal wave speed and longitudinal wave speed ratio;
step C, rock phase probability conversion: the lithofacies probability density function in the step A is introduced into the interpretation process of the elastic parameters obtained by pre-stack inversion in the step B, so that reasonable interpretation of hydrate and free gas coexistence layer distribution is realized, and risk assessment is given; the probability of each lithofacies for each spatial data point is obtained, and quantitative estimates of the hydrate and free gas coexistence layer are made.
In the embodiment, the hydrate drilling of a pore-type hydrate development area and the acquisition of pre-stack three-dimensional seismic data of the area are utilized, the application effect analysis is carried out on a pre-stack seismic inversion longitudinal wave velocity and longitudinal wave velocity ratio data body (namely the longitudinal wave velocity and longitudinal wave velocity ratio elastic parameter body) through rock phase probability conversion, the probability analysis of a hydrate and free gas coexistence layer is shown, the research is carried out on the basis of the elastic parameter probability analysis of the rock phase, the research is carried out by combining the longitudinal wave velocity and longitudinal wave velocity ratio data body of the pre-stack seismic inversion, the reasonable explanation on the distribution of the hydrate and free gas coexistence layer is realized, and the risk assessment is given, so that the scheme of the invention is more clearly understood, and the scheme of the invention is described in detail below:
1. in the step a, when the lithofacies probability density function is established (the probability density function of the ratio of the longitudinal wave velocity to the longitudinal transverse wave velocity is shown in fig. 1), the determination of the lithofacies division and the elastic parameter probability distribution is mainly considered, and specifically includes:
step A1, carrying out rock phase interpretation of a hydrate and free gas coexistence layer based on logging data, wherein a target layer is divided into five rock phases of a mudstone, a hydrate layer, a high-saturation free gas layer, a low-saturation free gas layer and the hydrate and free gas coexistence layer, and the five rock phases are used as a subsequent elastic parameter intersection analysis basis; the logging curves include a velocity curve, a neutron curve, a resistivity curve, a borehole diameter curve, a lithology composition curve, and the like.
As shown in FIG. 3, the lithofacies interpretation result diagram of the present embodiment is shown, and the left 1 st trace is a natural gamma curve; lane 2 is the measurement depth; lane 3 is a resistivity curve; lane 4 is the density and density porosity curve; lane 5 illustrates the results of the volume model for ELAN logging; lane 6 is the total porosity and water saturation curve; the 7 th to 8 th channels are longitudinal wave speed and longitudinal wave speed ratio in turn; the right-most curvelet lithofacies explain the outcome.
And A2, carrying out intersection graph analysis based on the longitudinal wave speed and the longitudinal and transverse wave speed ratio of logging data and the lithofacies interpretation result of the step A1, setting an X axis as the longitudinal wave speed, setting a Y axis as the longitudinal and transverse wave speed ratio, and enabling colors of intersection data points to be represented by lithofacies, so as to further determine distribution characteristics of different lithofacies on the intersection graph.
And A3, introducing a concept of probability density distribution on the basis of analysis of an elastic parameter intersection graph of a logging interpretation lithofacies according to the intersection point distribution characteristics of the longitudinal wave speed and the longitudinal and transverse wave speed ratio in the step A2, determining prior information of each lithofacies, and fitting a lithofacies probability density function, wherein the function has probabilities of different lithofacies at any intersection point of the longitudinal wave speed and the longitudinal and transverse wave speed ratio, the intersection point dense area is simulated to have high probability density, otherwise, the probability density is low, and the probability density function of each lithofacies adopts a Gaussian model.
In this embodiment, the prior information of each lithofacies refers to a proportion of logging sampling points of each lithofacies, which are determined by counting lithofacies explained by logging, to all logging sampling points of a target layer, with respect to the target layer where the hydrate and free gas coexistence layer under study is located; for example, if all the well logging sampling points of the target layer are set to be 100, the mudstone phase well logging sampling point is set to be 32, the hydrate layer rock phase well logging sampling point is set to be 24, the high-saturation free gas layer rock phase well logging sampling point is set to be 18, the low-saturation free gas layer rock phase well logging sampling point is set to be 14, and the hydrate and free gas coexisting layer rock phase well logging sampling point is set to be 12, the corresponding determination according to the research shows that the proportion of the mudstone phase is 0.32, the proportion of the hydrate layer rock phase is 0.24, the proportion of the high-saturation free gas layer rock phase is 0.18, the proportion of the low-saturation free gas layer rock phase is 0.14, the proportion of the hydrate and free gas coexisting layer rock phase is set to be 0.12, and then the probability density function of each rock phase is fitted. Wherein the probability density function distribution of the mud lithology is set as: vp is mean=1768, std dev=108, and Vp/Vs is mean=3.57, std dev=0.42; the probability density function distribution of the hydrate layer lithofacies is set as: vp is mean=2113, std dev=142, and Vp/Vs is mean=2.83, std dev=0.26; the probability density function distribution of the high saturation free gas layer lithofacies is set as: vp is mean=1794, std dev=93, and Vp/Vs is mean=2.38, std dev=0.11; the probability density function distribution of the low saturation free gas formation facies is set as: vp is mean=1713, std dev=89, and Vp/Vs is mean=2.98, std dev=0.19; the probability density function distribution of the hydrate and free gas coexistence layer lithofacies is set as follows: vp is mean=1837, std dev=154, and Vp/Vs is mean=2.69, std dev=0.12, mean represents the expectation, std dev represents the standard deviation.
As shown in fig. 4, probability density profiles (scattered points are measured longitudinal and transverse wave velocity ratio data points, and ellipsoids are probability density function distributions fitted by the scattered points) for different lithofacies fluids interpreted for logging.
2. In the step B, longitudinal wave speed and longitudinal wave speed ratio space data of pre-stack seismic inversion are key to rock phase probability conversion, and are directly related to identification of a hydrate and free gas coexisting layer, and the step B is specifically realized by the following modes:
step B1, utilizing rock physical relation analysis results of regional longitudinal wave velocity and longitudinal wave impedance to convert the superposition velocity obtained by seismic processing into a low-frequency model: establishing a longitudinal wave speed and transverse wave speed and longitudinal wave speed and density relation by analyzing the petrophysical relation of the fitting longitudinal wave speed, transverse wave speed and density of intersection points of the regional logging curve; and then converting the superposition velocity obtained by seismic processing into a layer velocity through a Dix formula to serve as a longitudinal wave velocity low-frequency model, and obtaining a transverse wave velocity low-frequency model and a density low-frequency model by utilizing the conversion of a relational expression.
The relational expression of the low-frequency model of the converted transverse wave speed is as follows: vs= -853+0.75vp, where Vs is transverse wave velocity and Vp is longitudinal wave velocity; the relation of the conversion density low-frequency model is as follows; density=1218+0.32Vp, where Density is Density and Vp is longitudinal wave velocity.
Step B2, performing well-shock calibration on pre-stack seismic data and logging data, and extracting seismic wavelets of near-medium-far incidence angle partial superposition data; the pre-stack seismic data comprises near-incidence angle superposition seismic data, medium-incidence angle superposition seismic data and far-incidence angle superposition data, as shown in fig. 2, wherein the upper graph is the near-incidence angle superposition data, the medium graph is the medium-incidence angle superposition data, and the lower graph is the far-incidence angle superposition data; the method comprises the steps of carrying out well shock calibration on pre-stack seismic data and logging data, determining a time depth relation, and respectively extracting wavelets of near-incidence angle superposition data, wavelets of medium-incidence angle superposition data and wavelets of far-incidence angle superposition data in a target layer range.
Step B3, carrying out pre-stack seismic inversion based on pre-stack seismic data, the low-frequency model of the step B1 and the seismic wavelet of the step B2 to obtain an elastic parameter body with space longitudinal wave speed and longitudinal wave speed ratio;
3. and C, performing rock phase probability conversion on the elastic parameter body with the longitudinal wave velocity and the longitudinal wave velocity ratio obtained by the pre-stack seismic inversion of the step B based on the rock phase joint probability density distribution function determined in the step A on the basis of the work of the step A and the step B, and obtaining five rock phase probability bodies and a maximum likelihood rock phase body of a mudstone, a hydrate layer, a high saturated free air layer, a low saturated free air layer and a hydrate and free air coexistence layer, wherein the rock phase body corresponding to the maximum probability in the five rock phase probability bodies is the maximum likelihood rock phase body. The distribution of the hydrate layer and the free gas layer of the region is explained, the probability mode is adopted to carry out risk assessment on inversion results to obtain probability bodies of different lithofacies, the high probability indicates that the lithofacies are high in probability, otherwise, the probability is low (the high probability is a relative concept, that is, the probability is higher as the probability is higher), the probability sum of any point on a data body of each lithofacies is 1, and meanwhile, the maximum likelihood lithofacies are obtained through comprehensive analysis according to the probability bodies of each lithofacies.
As shown in fig. 5-6, fig. 5 is a schematic diagram of a longitudinal wave velocity and a longitudinal-transverse wave velocity ratio section of the simultaneous inversion before stack, wherein the upper diagram is a longitudinal wave velocity data body, and the lower diagram is a longitudinal-transverse wave velocity ratio data body. If the velocity of the longitudinal wave is high in the figure, the velocity of the longitudinal wave is considered to be increased after the stratum contains hydrate, and if the velocity of the longitudinal wave is low in the figure, the velocity of the longitudinal wave is considered to be obviously reduced after the stratum contains free gas; both the hydrate and free gas exhibited low aspect ratio, but the non-hydrate and free gas layer exhibited slightly higher aspect ratio.
FIG. 6 is a schematic diagram of a maximum likelihood facies volume profile and probability volume profiles for different facies, i.e., facies probability interpretation results; the upper part is a maximum likelihood lithofacies section, the spatial distribution of different lithofacies is clearly displayed and is matched with the rock facies explained on the well, the spatial explanation of the lithofacies is realized, the lower part is a probability body section of different lithofacies, the dark color represents high probability distribution, the probability of which region the lithofacies is the highest can be visually seen from different probability diagrams, the problem that the reliability of the lithofacies cannot be quantitatively evaluated when the longitudinal wave speed and the longitudinal and transverse wave speed ratio of elastic parameters are overlapped can be effectively solved by the method, and the lithofacies probability analysis is carried out on spatial data points. The maximum likelihood lithofacies body and lithofacies probability body effectively solve the problem that lithofacies elastic parameters are overlapped at intersection, a probability interpretation mode is provided for an overlapping area, misjudgment occurring when inversion results are interpreted by utilizing longitudinal wave speed and longitudinal and transverse wave speed ratio cutoff values of elastic parameters is avoided, and risks are effectively quantified.
In summary, the invention utilizes the elastic parameter interpretation analysis method of seismic inversion to develop the identification method of the complex hydrate and free gas coexisting layer of the high-porosity argillaceous stratum, and establishes the technical method of the complex hydrate and free gas coexisting layer identification suitable for the high-porosity argillaceous stratum based on the research contents of logging lithofacies interpretation of the hydrate and free gas coexisting layer, elastic parameter longitudinal wave impedance and longitudinal wave velocity ratio intersection analysis of the hydrate and free gas coexisting layer, lithofacies probability density function analysis, prestack seismic inversion, lithofacies probability conversion interpretation and the like, thereby obtaining the spatial distribution and probability analysis results of the hydrate layer, the free gas layer and the coexisting layer of the hydrate and free gas.
The present invention is not limited to the above-mentioned embodiments, and any equivalent embodiments which can be changed or modified by the technical content disclosed above can be applied to other fields, but any simple modification, equivalent changes and modification made to the above-mentioned embodiments according to the technical substance of the present invention without departing from the technical content of the present invention still belong to the protection scope of the technical solution of the present invention.
Claims (3)
1. The hydrate and free gas coexistence layer identification method based on the elastic parameter probability distribution is characterized by comprising the following steps of:
step A, establishing a lithofacies probability density function: introducing a concept of probability density distribution on the basis of analysis of an elastic parameter intersection graph of a logging interpretation lithofacies, determining prior information of each lithofacies, and creating a lithofacies probability density function;
step A1, developing rock phase interpretation of a hydrate and free gas coexisting layer based on logging data, and dividing a target layer into a plurality of rock phases, wherein the plurality of rock phases comprise mudstone, a hydrate layer, a high-saturation free gas layer, a low-saturation free gas layer and a hydrate and free gas coexisting layer;
step A2, carrying out intersection graph analysis based on the longitudinal wave speed and the longitudinal and transverse wave speed ratio of logging data and the lithofacies interpretation result of the step A1, and determining the distribution characteristics of different lithofacies on the intersection graph;
step A3, according to the distribution characteristics of different lithofacies on the intersection map determined in the step A2, aiming at a target layer where a hydrate and free gas coexistence layer is located, counting the lithofacies explained through logging to determine the proportion of logging sampling points of each lithofacies to all logging sampling points of the target layer, further determining prior information of each lithofacies, and fitting a lithofacies probability density function;
step B, pre-stack seismic inversion is carried out to obtain an elastic parameter body of longitudinal wave speed and longitudinal wave speed ratio;
step C, rock phase probability conversion: the lithofacies probability density function in the step A is introduced into the interpretation process of the elastic parameter body obtained by the pre-stack inversion in the step B, so that the identification of the hydrate and free gas coexistence layer distribution is realized, and risk assessment is given, and the method specifically comprises the following steps:
performing lithofacies probability conversion on elastic parameter bodies with the longitudinal wave velocity and the longitudinal wave velocity ratio obtained by pre-stack seismic inversion in the step B by using the lithofacies probability density function determined in the step A to obtain five lithofacies probability bodies of mudstone, a hydrate layer, a high-saturation free gas layer, a low-saturation free gas layer and a hydrate and free gas coexistence layer; and carrying out comprehensive comparison analysis according to each lithofacies probability body to obtain a maximum likelihood lithofacies body; and further, the spatial three-dimensional lithofacies probability and the spatial distribution of a hydrate layer and a free gas layer of the maximum likelihood lithofacies interpretation area are obtained, risk assessment is carried out on the inversion result in a probability mode, and the high probability indicates that the lithofacies probability is high, and otherwise, indicates that the lithofacies probability is low.
2. The hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution according to claim 1, wherein: the step B is realized by the following steps:
step B1, converting the superposition speed obtained by seismic processing into a low-frequency model by utilizing rock physical relation analysis results of longitudinal wave speed and longitudinal wave impedance;
step B2, performing well shock calibration based on pre-stack seismic data and logging data, and extracting seismic wavelets of near-medium-far incidence angle partial superposition data;
and B3, carrying out pre-stack seismic inversion based on the pre-stack seismic data, the low-frequency model obtained in the step B1 and the seismic wavelets extracted in the step B2 to obtain the elastic parameter body with the space longitudinal wave velocity and the longitudinal wave velocity ratio.
3. The hydrate and free gas coexistence layer identification method based on elastic parameter probability distribution according to claim 1, wherein: in the step A3, the lithofacies probability density functions all adopt a Gaussian model.
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