1) Geological characteristics of A-P units

A-P units are distributed in all lithologies and lithofacies; however, reservoir spaces in different lithologies and lithofacies are highly variable and of different petrogenetic origins, leading to varying degrees of reservoir space development and distribution characteristics of volcanic A-P layers (Table 10.1).

1. Effusive facies A-P unit. Vesicles are the primary reservoir space. A-P layers are developed predominantly in the top, upper, and lower subfacies, with vesicular lava being the main lithology. Barriers and interbeds are developed mainly in the middle subfacies, lithologically represented by tight volcanic lava.

2. Explosive facies A-P unit. Intergranular pores are the main reservoir space. A-P layers are mainly developed in the hot clastic flow, fallout, and the splash subfacies, represented by welded tuff and breccia, crystalline tuff, and volcanic breccia. Barriers and interbeds are mainly developed in the hot base surge subfacies, lithologically dominated by crystalline tuff.

3. Volcanic conduit facies A-P unit. Vesicles and fractures are the main reservoir space in volcanic rocks of the conduit facies. A-P layers are developed mainly in the volcanic neck and cryptoexplosive breccia subfacies, and the representative lithology is vesicular lava and autoclastic brecciated lava. Barriers and interbeds are developed mainly in the volcanic neck subfacies, lithologically typical of tight volcanic lava.

4. Subvolcanic facies A-P unit. Interstitial pores and dissolution pores are the primary reservoir space in volcanic rocks of subvolcanic facies. A-P layers are developed mainly in the inner-zone and mesozone subfacies, and the lithology is typically weathered and dissolution-affected orthophyre and granite porphyry. Barriers and interbeds are developed mainly in the inner-zone subfacies, and the representative lithology is tight orthophyre and granitic porphyry.

5. Volcanic sedimentary A-P unit. Intergranular pores are the primary reservoir space in volcanic sedimentary facies. A-P layers are developed mainly in the xenoclast-bearing subfacies, represented by sedimentary volcanic breccia and sedimentary tuff. Barriers and interbeds are developed mainly in the coal-bearing tuff and transported subfacies, dominated lithologically by tuffaceous siltstone, tuffaceous mudstone, and coal.

2) Identification markers of accumulation capacity

Accumulation capacity is used to assess the size of the effective reservoir space of an A-P unit and is related to the scale and petrophysical properties of the unit. In general, it is evaluated on the basis of effective porosity that reflects reservoir properties in wells. As a result, accumulation capacity is identified with the three-porosity curves of nuclear magnetic logging and conventional logging (Table 10.2).

Identification markers in nuclear magnetic logging

In terms of its occurrences, fluid can be divided into mobile and irreducible types by the T₂ spectrum of nuclear magnetic logging, defined by the T₂ cutoff threshold (red line in Figure 10.2). The total area of mobile fluid spectrum reflects the accumulation capacity of the A-P unit. The larger the total area, the higher the effective porosity and effective accumulation capacity of the volcanic rock (see Figure 10.2). The T₂ spectrum characteristics of nuclear magnetic logging vary with A-P layers, barriers, and interbeds (see Table 10.2). Volcanic rocks have a relatively small total area of irreducible fluid spectrum, volcanic A-P layers have a relatively large total area of mobile fluid spectrum, and volcanic barriers and interbeds have a relatively small total area of mobile fluid spectrum. Sedimentary volcanic rocks have a relatively large total area of irreducible fluid, sedimentary volcanic A-P layers have a moderate total area of mobile fluid, and sedimentary volcanic barriers and interbeds have a relatively small total area of mobile fluid spectrum.

Identification markers in conventional logging

The three-porosity curves can be used to reflect the accumulation capacity of volcanic rocks. Lithological separation results in various response characteristics of the three-porosity curves that reflect volcanic rocks and sedimentary volcanic rocks (see Table 10.2):

1. A-P layers. The amplitude values of density, interval transit time, and neutron porosity curves are moderate, and their curves commonly have a smooth to microdentoid box shape and a moderate to good accumulation capacity (Figure 10.3).

2. Volcanic barriers and interbeds. These components are characterized by high-amplitude density logging curves, low-amplitude interval transit time and neutron logging curves, and a microserrated to dentoid box shape, indicating poor accumulation capacity (see Figure 10.3).

3. Sedimentary volcanic A-P layers. These layers typically have moderate- to low-amplitude density logging curves, moderate- to high-amplitude interval transit time and neutron logging curves, and dentoid bell and funnel shapes, indicating moderate accumulation capacity (Figure 10.4).

4. Sedimentary volcanic barriers and interbeds. These components have the characteristics of low-amplitude density logging curves, moderate- to high-amplitude interval transit time and neutron logging curves, and dentoid to serrated digitate bell and funnel shapes, denoting poor accumulation capacity (see Figure 10.4).

3) Identification markers for permeation capacity

Permeation capacity is used to assess the capacity of fluid flow of an A-P unit, which is related to the degree of pore development, throat radius, and permeability. Generally, permeation capacity is evaluated on the basis of permeability parameters. Single-well permeation capacity is identified mainly through nuclear magnetic logging, three-porosity curves of conventional logging, and resistivity logging curves (Table 10.3).

Identification markers of nuclear magnetic logging

The permeation capacity of volcanic rocks is related to pore throat size, porosity, and fractures. The porous zones with various pore throats are distinguishable on the T₂ spectrum in nuclear magnetic logging curves. A higher T₂ peak value of the mobile fluid and larger area of the spectral peak indicate large-radius pores and therefore higher permeation capacity (Figure 10.5). As a result, the permeation capacity of volcanic rocks can be assessed by the T₂ time distribution and the area of mobile fluid peak in the T₂ spectrogram (see Table 10.3).

Volcanic formations have an extensive mobile fluid peak, and overall its T2 average value is higher than that of sedimentary volcanic rocks. The trap area of the mobile fluid peak is large for volcanic A-P layers and small for barriers and interbeds (see Table 10.3).

The T2 value of the mobile fluid peak of sedimentary volcanic rocks is generally smaller than that of volcanic rocks. The trap area of the mobile fluid peak is moderate for sedimentary volcanic A-P layers and small for barriers and interbeds (see Table 10.3).

Response characteristics of conventional logging

The electrical conductivity of rocks has a good correlation with its permeability (Figure 10.6). For conventional logging, beside the three-porosity logging curves, the resistivity curve is often used to reflect the permeation capacity of volcanic rocks. Similarly, the conventional logging response characteristics reflecting the permeation capacity of volcanic rocks and sedimentary volcanic rocks are highly variable due to the variety of lithologies (see Table 10.3):

1. Volcanic A-P layer. Moderate-amplitude density, acoustic travel time, and resistivity logging curves and relatively large positive separation between deep and shallow laterolog resistivity curves (Figure 10.7)

2. Volcanic barrier and interbed: High-amplitude density logging curve, low-amplitude acoustic logging curve, and high-amplitude resistivity logging curve, as well as small separation between deep and shallow laterolog resistivity curves (see Figure 10.7)

3. Volcanic A-P layer. Moderate to low amplitude density logging curve, moderate to high amplitude acoustic travel time logging curve, and low-amplitude resistivity logging curve, as well as small separation between deep and shallow laterolog resistivity curves

4. Sedimentary volcanic barrier and interbed. Low-amplitude density logging curve, high-amplitude acoustic travel time logging curve, and low-amplitude resistivity logging curve, as well as small separation in the deep-shallow laterolog resistivity curve

1) Assessing A-P capacity and identifying A-P layers

The A-P capacity of a target reservoir can be qualitatively assessed on the basis of the identification markers of volcanic A-P layers, in combination with the identification of A-P units by lithology and lithofacies characteristics. This further facilitates the quantitative evaluation of the A-P capacity of a target reservoir through reservoir parameter interpretation and gas tests. And the identification results are verified by the cutoff criteria of effective reservoirs.

The left of Figure 10.8 is an example of the division and identification of A-P layers in Well XX9. The strata are classified, in descending order, into seven intervals.

The first interval (3520 to 3578 m) consists of sedimentary volcanic rock and volcanic rock characterized by low resistivity, low density, high interval transit time, and high neutron value. The logging curves are serrated-digitate in shape, with large amplitude of irreducible fluid peak and underdeveloped mobile fluid peak in a nuclear magnetic logging curve showing poor A-P capacity. As a result, the interval is assessed systematically as a non-accumulation-permeation (A-P) sedimentary volcanic layer. Its effective porosity is less than 1% and permeability is less than 0.1 mD, thus interpreted as a dry layer by logging characteristics.

The lithology of the second, fourth, and sixth intervals (green color) comprises rhyolite with moderate-amplitude resistivity, density, interval transit time, and neutron logging curves in microserrated box and bell shapes. The same development of irreducible fluid peak and mobile fluid peak in a nuclear magnetic logging curve suggests some A-P capacity for the intervals. As a result, the intervals are assessed systematically as effusive facies A-P layers. In logging interpretation, their porosity is 6%, 5.6%, and 5.8%, and permeability is 1.2 mD, 0.14 mD, and 0.33 mD, respectively. These values are higher than the cutoff of effective reservoirs in the XX gas field, strongly indicating that these intervals as gas layers.

The lithology of the third, fifth, and seventh intervals is rhyolite for the third and fifth intervals and dacite for the seventh interval; this is characterized by high resistivity, high density, low interval transit time, and low neutron value. The logging curves have a smooth box shape, locally serrated-digitate. The inferior development of irreducible fluid peak and underdeveloped mobile fluid peak in the nuclear magnetic log indicate poor A-P capacity for the intervals. As a result, systematic evaluation ranks them as effusive facies non-A-P layers. In logging interpretation, their porosity is 0.9%, 2.7%, and 2.3%, and permeability is 0.05 mD, 0.02 mD, and 0.01 mD, respectively. All of these values are below the cutoff for effective reservoirs, thus rendering them as dry layers.

The second and sixth intervals are perforated for gas tests. Natural production is low (0.0134 × 10⁴ m³/d), but the production reaches 20.9 × 10⁴ m^3/d after hydraulic fracturing. As a result, these intervals are commercial gas layers, which verified their initial interpretation as A-P layers.

2) Tracing extension and distinguishing barriers and interbeds

Both barriers and interbeds are integral parts of A-P layers. Barriers usually have a large thickness and lateral extension, located between A-P units, and can obstruct fluid seepage. Interbeds have a small thickness and limited lateral extension, located within A-P layers, and can decrease fluid seeping. Barriers and interbeds are distinguishable based on their thickness features and the identification of A-P layers, through well-seismic calibration to track the horizontal extension of non-A-P layers in seismic profiles. This provides the basis for the division of A-P units.

In the case study of Well XX9 (see Figure 10.8), the lithology of non-A-P layers between the second, fourth, and sixth intervals is rhyolite. Due to the small thickness (about 12 m) and limited lateral extension, they cannot be identified and tracked in seismic profiles and are thus determined as interbeds with poor petrophysical properties. The first interval consists of sedimentary volcanic rock and volcanic-sedimentary rock with large thickness, which has unconformable contact with its underlying volcanic rock and is widespread in the XX gas field. As a result, the interval is determined as the cap rock of lower volcanic A-P units. The lithology of the seventh interval is dacite with a thickness of some 150 m, which appears with moderate-amplitude, low-frequency, and good-continuity reflection characteristics and a laminar-like reflection shape (on the right in Figure 10.8) and can be tracked in Well Block XX9. As a result, the interval is interpreted as a petrophysical property barrier.

3) Analyzing the relationship among cap rocks, reservoirs, and basal bed assemblage to divide A-P units

The classifications of A-P units delineation of the vertical distribution patterns of A-P units are conducted with reference to the classification scheme for carbonate reservoir units [14,15], using various static and dynamic data and taking into consideration the geological features of volcanic gas reservoirs.

Using dynamic data to verify division results and determine vertical distribution modes of A-P units in wells

Fluid flow characteristics vary with A-P units. Based on the separation in produced fluid properties, the consistency of the water-gas relationship, and the relationship between the pressure system and production performance, the division results can be verified and the vertical distribution patterns of A-P units in single wells can be modified and improved. Comprehensive analyses of Well DD1001 show that the volcanic A-P unit is the similar to a carbonate reservoir unit (Figure 10.9a), characterized by the rock assemblage of an upper cap rock, middle reservoir, and lower basal bed.

4) Determining the interfaces of A-P units by comprehensive analyses

Based on the identification of A-P layers, barriers, and interbeds as well as the division results of A-P units in a single well, the type of interface between volcanic A-P units is determined through petrogenetic analysis, according to the established identification markers in single wells. This provides well point constraints for profile tracing.

The interfaces of volcanic A-P units are classified into lithological interface, petrophysical property interface, and sealing fault barrier in terms of their petrogenesis. Interface identification in single wells generally involves the former two types.

Identification of lithological interfaces

A lithological interface results from changes in petrophysical properties due to lithological variation, and can be subdivided into the following three varieties.

Abrupt interface between sedimentary and volcanic rocks

This type of interface is developed mainly in the basal rock units formed at the beginning of a volcanic eruption as well as the top rock units formed at the end of a volcanic eruption. In logging curves, the interface appears as a sudden change from sedimentary rocks with high interval transit time and neutron value and low density and resistivity to volcanic rocks with low interval transit time and neutron value and high density and resistivity (Figure 10.10).

Lithofacies interfaces

This results from changes in petrophysical property due to lithofacies or environmental variation. In logging curves, the interface appears as a trending change. Figure 10.11 is a typical example of Well DD14, where the lower lithology is a brecciated tuff barrier and the upper lithology is a tuffaceous volcanic breccia A-P layer. The logging curves from the bottom up are characterized by intensified radioactivity, reduced density, increased interval transit time, and increased neutron value and resistivity.

Lithological interfaces

This results from poorer petrophysical properties due to variations in the magma source and petrogenesis. For example, Well DD1824 penetrated mainly orthophyre and occasionally andesitic welded breccia and sedimentary tuff; the lithological interface is formed due to relatively poor petrophysical properties. Figure 10.12 shows that the lower part is an andesitic welded breccia barrier and the upper part is an orthophyre A-P layer. The logging curves show an upward increase in radioactivity, reduced density, increased interval transit time and neutron value, and decreased resistivity.

Identification of petrophysical property interfaces

This refers to an A-P interface between the same volcanic lithologies due to relatively poor petrophysical properties of the middle subfacies of effusive facies, which clearly defines the interface between the upper and middle subfacies and between the middle and lower subfacies of the effusive facies. Also, due to the relatively poor petrophysical properties of the hot base surge subfacies, this interface is easily delineated by the borders between the hot base surge subfacies of explosive facies and other subfacies. Similar to the poor petrophysical properties of the inner-zone subfacies of subvolcanic rocks, this interface is also easily defined between the mesozone and inner-zone subfacies of subvolcanic rocks.

Relatively speaking, volcanic A-P layers with good petrophysical properties have the characteristics of low density, high neutron value, high interval transit time, and moderate-low resistivity, and non- A-P layers with poor physical properties have the characteristics of high density, low neutron value and interval transit time, and moderate resistivity in logging curves. Based on the lithology identification, logging response characteristics are analyzed to identify the petrophysical property interface of A-P units. In Figure 10.13, the map with logging curves of Well DD1804 demonstrates that orthophyre has good petrophysical properties in the upper part and poor petrophysical properties in the lower part, with a petrophysical property interface located near depth 3645 m.

1) Interwell combination principles

Volcanic A-P units have complex distribution patterns and multiple interwell combination modes, controlled by various mechanisms, such as modes of eruption, structures, and diagenetic processes. The principles and approaches for interwell combination and the correlation of A-P units are determined by taking into consideration the rock architecture, lithology and lithofacies, fault distribution, reservoir space type, and petrophysical characteristics of volcanic rocks:

1. Establishing the framework model of rock architectural units as well as framework constraints for the identification of A-P units based on the architectural dissection of volcanic rocks

2. Assessing fault sealing properties to provide a fault control basis for interwell combination of A-P units

3. Establishing a standard profile, dividing single-well profile, and dividing single-well A-P units by plotting the classified single-well A-P units on standard profile in the same scale

4. Connecting boundary lines of volcanic edifices, eruption cycle and period, lithology and lithofacies, and A-P units, following the approach of level-by-level control from large to small, with reference to the combination relationships between faults and associated A-P units

5. Analyzing the connectivity of A-P layers, and verifying and amending the rationality of the interwell combination mode of A-P units based on produced fluid properties, the pressure system, and the production performance of the pay zone

2) Multiwell profile identification

Based on the principles of interwell combination, the integrated multiwell profile identification methods using geological, logging, and production performance data are developed in combination with various static and dynamic data, and the interwell distribution mode of A-P units is also determined. The process is illustrated using Well DD10 as an example.

Dissecting volcanic rock architecture and establishing a constraint framework for the interwell combination of A-P units

The volcanic edifice in Well Block DD10 is cone shaped as a result of three stages of eruption from a single volcanic crater, and volcanic strata of the second and third stages are absent from the topographic highs (Well DD10) due to denudation.

The constraint framework of the interwell combination of A-P units is established (Figure 10.14) according to the results of architectural dissection and sequence division: the black lines on the top and at the bottom are the upper and lower boundaries of volcanic eruption cycles, and red lines in the middle denote the boundaries between various eruption stages of the cycles. There is no fault developed between wells.

3) Verification by dynamic data

Well DD10 is about 1000 m away from Well DD1001, and their major gas layers have obviously different rock compositions and lithology. In Well DD10, the lithology of two gas-tested intervals is rhyolitic welded tuff and dacitic welded breccia, respectively, and in Well DD1001, the lithology of gas-tested intervals is andesitic volcanic breccia.

Pressure test results show that the three gas-tested intervals belong to different pressure systems (Figure 10.15). During the early production stage, gas production accounts for 76.2% in Well DD1001, but 23.8% in Well DD10, of total gas production in this area. Compared to the relatively steady production of Well DD1001, Well DD10 has unsteady output after beginning production. The wellhead tubing pressure of Well DD10 declined from 18.5 MPa to 12.2 MPa, and daily gas production decreased from 6 × 10⁴ m³/d to 2.2 × 10⁴ m³/d within two months. Considering their quite different production performances, the three intervals are primarily interpreted as disconnected.

Furthermore, an interference well test is conducted between Wells DD10 and DD1001 to determine the connectivity of A-P units. Based on the pressure–rate history curves (Figure 10.16), no active signal of Well DD1001 is received from observation Well DD10 during the test, which indicates that the producing intervals in the wells connected poorly or disconnected. This verifies that there are multiple discrete A-P units in Well Block DD10.

1) Identification model

The A-P units of volcanic gas reservoirs are classified into four types in terms of the lithofacies types and characteristics. Each type of A-P unit has its own seismic reflection characteristics and configurations.

Volcanic conduit facies A-P units

The volcanic conduit facies occurs in the lower part of a volcanic edifice, in which interclast pores and internal primary pores, dissolution pores, blast fractures, contraction fractures, and columnar joint fractures are primary reservoir spaces, with good petrophysical properties. In the seismic profile (Figure 10.17), this type of A-P unit appears as umbrella- or cloud-shaped reflections with weak amplitude, high frequency, and poor continuity. Better-developed A-P units are correlated with weaker amplitude, higher frequency, and poorer continuity of the seismic reflection wave.

Explosive facies A-P units

The explosive facies mostly forms at the early stage of a volcanic eruption, in which intergranular pores, intragranular primary pores, dissolution pores, interbedded fractures, intergravel fractures, and structural fractures are primary reservoir spaces, with moderate petrophysical properties. In the seismic profile (Figure 10.18), this type of A-P unit mostly appears as a mound and shows chaotic reflections. Better-developed A-P units are represented by weaker amplitude of the seismic reflection wave. In general, discontinuous, traceable, and relatively strong reflections represent the interfaces between an A-P layer and an external flow resistance zone.

Effusive facies A-P units

The effusive facies usually forms during the middle-late stage of a volcanic eruption, in which primary pores, dissolution pores, cooling contraction fractures, and structural fractures are primary reservoir spaces and the upper and lower subfacies have relatively good petrophysical properties. In the seismic profile (Figure 10.19), this type of A-P unit mostly appears as laminoid reflections with good continuity. In the zone where A-P layers are developed, the seismic reflection has a weaker amplitude and lower frequency. If the effusive facies A-P layers are in direct contact with sedimentary rocks, a locally continuous strong reflection may occur.

Subvolcanic facies A-P units

Subvolcanic rocks are usually formed at the late stage of a volcanic eruption; they have the same source as volcanic rocks, but with intrusive occurrence, in which interstitial pores, dissolution pores, cooling contraction fractures, and structural fractures are primary reservoir spaces with relatively poor petrophysical properties. In the seismic profile (Figure 10.20), subvolcanic rocks mostly appear in an irregular massive shape and are marked by chaotic internal reflections. In the zone with A-P layers developed, the seismic reflection has a relatively weak amplitude, low frequency, and slightly better continuity. Discontinuous and traceable strong reflections usually represent the interfaces between an A-P layer and a flow resistance zone.

It is difficult to identify the A-P units directly with seismic profiles. The seismic responses presented here can provide the basis for a qualitative identification of A-P units by integrating seismic profiles with geological models.

2) Interface tracing of A-P units

Based on single-well calibration, several typical interfaces of A-P units can be traced using the identification models as a guide.

Identifiability varies with lithological interfaces. In seismic profiles, an abrupt interface from sedimentary rock to volcanic rock appears as a strong reflection of unconformity surface and is easily recognizable (Figure 10.21a); a lithofacies interface appears as a moderate reflection peak and is locally traceable (Figure 10.21b); a lithological interface resulting from variations in rock composition or grain size appears as a relatively strong reflection peak and is locally discontinuous and traceable (Figure 10.21c).

The traceability of a petrophysical property interface varies with interface types but is generally poor. An effusive facies petrophysical property interface appears as laminar reflections with relatively strong amplitudes, and locally it has a relatively good traceability (Figure 10.21c). An explosive facies petrophysical property interface appears as chaotic reflections with moderate amplitude, and its traceability is relatively poor. A subvolcanic facies petrophysical property interface appears as chaotic reflections with moderate-poor amplitudes, and it may be locally traceable (Figure 10.21d).

1) Sensitive attributes analysis for A-P layers

Both theoretical and experimental studies show that high-frequency components are strongly absorbed when a seismic wave passes through a gas-bearing reservoir. As a result, the seismic wave of effective A-P layers is generally characterized by low energy for high-frequency amplitude and high energy for low-frequency amplitude. The case study in the DD gas reservoir shows that in the frequency acquisition of seismic trace near Well DD18, there is as an obvious trend of change from high frequency to low frequency corresponding to the transition from nonreservoir sedimentary rocks to an orthophyre gas layer (from top to bottom). According to the stratum spectral analysis of a gas-bearing property, the wavelet spectrum is marked by the phenomenon of enhancement of low frequency and attenuation of high frequency. As a result, amplitude, frequency, and attenuation are the most sensitive seismic attributes for identifying A-P units of a volcanic gas reservoir.

2) Seismic attribute extraction

The method of seismic attribute extraction involves time slice, horizon slice, and interlayer attribute extraction. The extraction targets include 2D profile and 3D data volume [18]. Both horizon slice and interlayer attribute extraction can be used to predict planar distribution.

Considering that seismic waves for volcanic A-P units have different energy levels for high-frequency and low-frequency components, the horizon slice method is used to extract high-frequency and low-frequency amplitude attributes (Figure 10.24) based on the frequency division processing of seismic data. This will render effective A-P layers as weak amplitude for high-frequency components (20 Hz) (blue area in Figure 10.24a) and relatively strong amplitude for low-frequency components (12 Hz) (red area in Figure 10.24b).

3) Optimization and combination of seismic attributes

If individual seismic attributes have a poor correlation with reservoirs, the combination and optimization of multiple attributes can be used to reduce the prediction uncertainty of the planar distribution of A-P units.

The stratum absorption coefficient obtained by calculating the change rate of wavelet spectrums can better reflect the characteristics of effective A-P layers (Figure 10.25a), although the seismic reflection frequency is related to stratum thickness, reservoir petrophysical properties, and oil-bearing properties. In general, seismic reflection signals are highly irregular. If the seismic trace is considered a random time series, the phase space reconstruction method can be used to measure its fractal dimension and obtain a correlation dimension parameter that directly reflects hydrocarbon distribution (Figure 10.25b). The analysis of the No. 1 volcanic massif in the CC gas reservoir shows that the correlation between the absorption coefficient/correlation dimension attribute and the natural gas distribution can exceed 70%.

Predicting the planar distribution of A-P units using integrated attribute analysis

A seismic attribute or attribute combination is characterized by multiple solutions under the influence of various factors. The method of integrated attribute analysis is effective for reducing the number of solutions and the uncertainty of prediction. The method is summarized as follows:

1. Applying single attribute to predict the possible planar distribution of A-P units (Figures 10.26a and 10.26b) based on the characteristics of individual seismic attributes or attribute combination, in combination with the analysis of static and dynamic data from the study area

2. Superposing the prediction results of all sensitive attributes and determining the confidence level for the distribution area of A-P units by superposition times (Figure 10.26c)

3. Performing a comprehensive analysis of static and dynamic data from the study area, and effectively predicting the most likely planar distribution of A-P units under the guidance of geological rules (Figure 10.26c)

1) Selecting the survey location

Based on the geological survey and regional geological analysis, a survey location is selected where an outcrop of the target stratum is available [20]. Interval 1 of the Yingcheng Formation is one of major gas-bearing layers in the volcanic gas reservoir of the Songliao basin. Geological surveys show that this set of volcanic strata crops out in the Jiutai area of Jilin Province. After a field survey, the Guanma Mountain-Tuanjie village area with good field exposures is selected for a detailed field outcrop survey (identified by the blue dashed line in Figure 10.29).

2) Plotting the surveyed geological profile and establishing an architectural model of volcanic edifice

Nine trench faces (1735 m) are surveyed by arranging exploratory trenches in the survey area. Based on the 20 selected samples for chemical analysis, 588 field photographs, 191 rock thin sections, and 725 micrographs, as well as the contact relationships observed from the exposed volcanic strata, the surveyed geological profile (Figure 10.30) is plotted and a geological model reflecting the architectural characteristics of the volcanic edifice is established for the target strata (Figure 10.31).

3) Obtaining drill core and dividing A-P units in a borehole

One cored well (Well Ying-D1) is drilled in a key location (i.e., the center of flank) of the volcanic edifice. Based on the lithological and lithofacies analyses as well as porosity-permeability measurement, the A-P units are classified and divided in a borehole according to the principle of upper cap rock, middle reservoir, and lower basal bed (Figure 10.32).

4) Ascertaining distribution characteristics and establishing distribution models of A-P units

Under the architectural constraints of the volcanic edifice, the spatial distribution of A-P units is predicted and the distribution modes of A-P units for this type of volcanic edifice is established on the basis of the division of A-P units in boreholes, exploratory trench measurement, and field survey of geological profiles (Figure 10.33).

5) Characterizing the shape and scale of A-P units

Outcrop survey results show that most effusive facies A-P units are bedded, explosive facies A-P units lenticular, and A-P units in mixed facies irregular. In volcanic gas reservoirs, most A-P units are 0.2 to 1.7 km long, 0.1 to 1.4 km wide, and 10 to 50 m thick, covering an area of 0.1 to 0.9 km², with different scales and poor continuity (Figure 10.34).

1) Geometry of A-P units

In profiles, A-P units in near-crater explosive facies commonly have a massive structure with large thickness and small extension; and A-P units in off-crater effusive facies are mainly bedded, with small thickness and large extension (Figure 10.35).

In plan view, volcanic A-P units have three distribution shapes (Figure 10.36): (1) a potato-shaped A-P unit, developed in explosive facies of isolated volcanic edifice, with a small amount of erupted magma and small scale; (2) a striped A-P unit, developed in effusive facies and controlled by palaeotopography, with a large planar distribution; and (3) a sheetlike A-P unit, common in both explosive and effusive facies, formed from the superposition of A-P units of multiple volcanic edifices, and relatively large in scale.

2) Geometric parameters and scale of a-p units

As is shown by the CC gas field example (Figure 10.37), most volcanic A-P units have a discrete distribution in the gas field. Their lateral extension radius is 300 to 2100 m, with an average of 537 m, and their thickness is 9 to 124 m with an average of 20 m. As a result, they have drastically different vertical and horizontal scales.

A-P units have different scales for various lithofacies. Explosive facies A-P units are mostly 15 to 100 m thick and 200 to 800 m wide, whereas effusive facies A-P units are mostly 5 to 45 m thick and 400 to 1000 m wide.

1) Dividing A-P units in wells

In terms of the identification markers of A-P units, coring, logging, testing, and production performance data are integrated to identify A-P layers, barriers, and interbeds and to divide A-P units in wells.

2) Establishing profile distribution models of A-P units

The typical cross-well skeleton or framework profile is plotted for the dense well grid area (Figure 10.38). The profile distribution model of A-P units is established through the identification of multiwell profiles and interwell combination of A-P units, taking into consideration the influences of faults, water-gas relationships, and pressure systems along with the constraints of volcanic architecture (Figure 10.39).

3) Predicting spatial distribution and characterizing A-P unit shape and scale

The spatial distribution of A-P units is predicted through the combination of techniques of A-P unit interface tracing, 3D attribute volume spatial assemblage, and seismic inversion data volume spatial assemblage. It provides a foundation for characterizing the shape and scale A-P units.

Research results show that most A-P units in the dense well area are lenticular or lamellar, bounded by lithofacies, subfacies, or petrophysical properties. They are 0.5 to 2.8 km long, 0.3 to 2.5 km wide, and 10 to 50 m thick, covering an area of 0.4 to 3 km² with highly variable scales (Figure 10.40).

1) Superposition relationships

According to their locations, A-P units are classified into three types: vertical superposition, lateral connection, and isolated types (Figure 10.41). Vertical superposition mainly occurs between A-P units of two adjacent volcanic massifs (or stages or lithofacies) within the same volcanic edifice; lateral connection is found mainly between two volcanic edifices or volcanic massifs with lateral superposition; the isolated type, with a lenticular morphology, occurs within a discrete volcanic edifice or in a relatively independent volcanic massif within a volcanic edifice.

2) Connection modes

A-P units are connected by open fractures, structural fractures, large columnar joint fractures, and multifracture network passages. Among them, open fractures have the longest connection distance, multifracture network passages rank second, and the columnar joint has the shortest connection distance, limited within a single volcanic edifice.

3) Connection types

In accordance with the general classification principle of “superposition relationships + connection modes,” the connections of A-P units are classified into 10 types, including vertical-superposition opening fracture connection type, lateral-superposition structural fracture connection type, isolated multifracture network passage, and so on. Among them, some assemblages only occur in specific settings. For example, the isolated columnar joint fracture connection type only forms among A-P units of two independent volcanic massifs within a volcanic edifice.

1) Accumulation capacity

Accumulation capacity is mainly influenced by the scale and internal effective porosity of A-P units. In general, it is characterized by the thickness and distribution area of A-P units, well-controlled reserves, the type of reservoir space, pore and fracture assemblage, A-P mode, and porosity. These can be shown using the XX gas field as an example:

1. Type I A-P unit. This type of A-P unit has the strongest accumulation capacity, with large scale and well-controlled reserves, multiple types of reservoir space and pore-fracture assemblage, appropriate A-P configuration, and high porosity. Its distribution area is larger than 2 km²; net pay thickness is greater than 20 m, and well-controlled reserve is larger than 5 × 10⁸ m³. Reservoir space is composed mainly of intergranular pores, vesicles, dissolution pores, and fractures. The pore-fracture assemblage includes intergranular pore type, vesicle type, fracture-intergranular pore type, fracture-vesicle type, and fracture-dissolution pore type, amounting to an effective porosity higher than 10%.

2. Type II A-P unit. This type of A-P unit has a relatively strong accumulation capacity, with a distribution area larger than 1.5 km², effective thickness greater than 15 m, and well-controlled reserves ranging from 2 × 10⁸ m³ to 5 × 10⁸ m³. The reservoir space is composed mainly of intergranular pores, vesicles, dissolution pores, and fractures. The pore-fracture assemblage includes intergranular pore type, vesicle type, fracture-intergranular pore type, fracture-vesicle type, and dissolution pore type, which amount to an effective porosity greater than 5%.

3. Type III A-P unit. This type of A-P unit has relatively poor accumulation capacity with a distribution area larger than 0.5 km², effective thickness greater than 5 m, and well-controlled reserves ranging from 0.5 × 10⁸ m³ to 2 × 10⁸ m³. The reservoir space is composed mainly of intergranular pores, vesicles, micropores, and fractures. The pore-fracture assemblage includes intergranular pore type, vesicle type, fracture-micropore type, fracture-dissolution pore type, and fracture type. These amount to an effective porosity higher than 3%.

4. Type IV A-P unit. This type of A-P units has poor accumulation capacity, characterized by small scale and well-controlled reserves, limited reservoir space and pore-fracture assemblage, unfavorable A-P configuration, and low porosity. Its distribution area is less than 1.5 km², effective thickness is less than 15 m, and well-controlled reserve is smaller than 0.5 × 10⁸ m³. The reservoir space is composed mainly of intergranular pores, isolated vesicles, micropores, and fractures, and the pore-fracture assemblage includes intergranular pore type, micropore type, and fracture-micropore type, amounting to an effective porosity higher than 3%.

2) Permeation capacity

Permeation capacity is influenced by matrix pore structures and the developmental characteristics of interbeds and fractures. Generally, it is characterized with throat radius, displacement pressure, permeability, single-well deliverability, and stable production capability.

1. Type I A-P unit. This type of A-P unit has the highest permeation capacity, referred to as the “sweet spot” of gas reservoir. It is characterized by a large throat radius, low displacement pressure, high permeability, an underdeveloped internal interbed, high single-well deliverability, and strong stable production capability, and it has a throat radius larger than 0.3 μm, displacement pressure lower than 2 MPa, permeability less than 1 mD, interbed density lower than 10 m/100 m, interbed frequency less than 2 layers/100 m, high single-well natural production or high postfracturing production, and strong stable production capability.

2. Type II A-P unit. This type of A-P unit has moderate permeation capacity with a throat radius larger than 0.1 μm, displacement pressure lower than 6 MPa, permeability higher than 0.1 mD, interbed density lower than 15 m/100 m, interbed frequency less than 3 layers/100 m, moderate single-well postfracturing production, and moderate stable production capability.

3. Type III A-P unit. This type of A-P unit has relatively poor permeation capacity with a throat radius larger than 0.01 μm, displacement pressure lower than 12 MPa, permeability higher than 0.01 mD, interbed density lower than 30 m/100 m, interbed frequency less than 5 layers/100 m, low single-well postfracturing production, and poor stable production capability.

4. Type IV A-P unit. This type of A-P unit has poor permeation capacity. It is characterized by a small throat radius, high displacement pressure, low permeability, well-developed internal interbed, low single-well deliverability, and weak stable production capability, and it has a throat radius larger than 0.01 μm, displacement pressure lower than 12 MPa, permeability higher than 0.01 mD, interbed density lower than 50 m/100 m, interbed frequency less than 10 layers/100 m, single-well gas production lower than the criteria of a commercial gas well, and weak stable production capability.