Introduction
The proven natural gas reserves of the Sinian-Cambrian carbonate reservoirs in Gaoshiti and Moxi areas (hereinafter referred to as "Gaomo area") in the central Sichuan Basin exceed one trillion cubic meters. The single-scale and overall scale of the Anyue giant marine carbonate gas field in the basin are currently the largest in gas fields in China, marking a new level in the exploration of marine carbonate rocks in the Sichuan Basin [1⇓⇓-4]. The Sinian Dengying Formation, as one of the main producing targets in carbonate rocks, is distributed in different regions of the basin. The northern Sichuan Basin, where no breakthrough has been achieved in current exploration, has a similar geological setting to the Gaomo area in central Sichuan Basin. Through field geological surveys and sampling analysis of the Dengying Formation in northern Sichuan Basin and surrounding areas, it was found that there are microbial dolomites with extremely developed pores and vugs (including numerous bitumen veins) in the Dengying Formation, which indicates that the deep Dengying Formation in this area has good prospects for hydrocarbon accumulation [5], which may become a new frontier for increasing natural gas reserves in deep and ultra-deep carbonate rocks in the future. According to the exploration experience in the Gaomo area, the development and distribution of reservoirs are closely related to the distribution of sedimentary facies. The reservoir characteristics are different at the platform margin and within the platform, leading to distinct petrophysical properties. It is important for seismic exploration to systematically clarify the relationship between the reservoir characteristics and seismic elastic parameters of the Dengying Formation in different areas and different sedimentary facies through petrophysical experiments.
Previous petrophysical studies on carbonate reservoirs mostly focus on the pore structure and the fluid properties and distribution, as well as their influences on elastic wave velocity, anisotropy and dispersion characteristics [6]. However, these studies are mostly based on physical models, lacking geological significance. As we all know, the diagenesis processes such as cementation, compaction, dissolution, and dolomitization control the texture, porosity, pore shape, etc. of carbonate rocks, and these parameters in turn determine the elastic properties of the rocks [7]. Especially for ancient carbonate formations such as the Dengying Formation, the rock texture is extremely complex due to multi-stage diagenesis. For example, dolomitization can change the mineral composition, and dissolution can change the pore shape. It is quite difficult to define or establish reliable correlations between these parameters and elastic properties, allowing the determination of regional variation in seismic elastic properties to be challenging. Many scholars have conducted geological studies on the carbonate reservoirs of the Dengying Formation in the Sichuan Basin [8⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-19]. However, there is still a lack of systematic studies through petrophysical experiments, and the analysis by petrophysical experiment with consideration to the diagenetic evolution process of the target interval under the geological setting has not been reported.
In this study, a systematic experiment was performed on carbonate core samples from the fourth member of the Dengying Formation (hereinafter referred to as "Deng-4 Member") in the platform margin and restricted platform to determine the petrology, mineralogy, pore structure, and petrophysical elastic properties. Combined with the latest geological data, the deposition and diagenetic evolution processes of carbonate rocks in different sedimentary facies and their impacts on the seismic petrophysical properties were discussed, with a view to providing reliable petrophysical evidences for the seismic exploration of natural gas in carbonate rocks in the Dengying Formation of the Sichuan Basin.
1. Geological setting
The study area is located in the Northern Sichuan low-lying zone in North Sichuan) and Tongnan District in northwestern Chongqing (tectonically located in the central Sichuan Basin flat zone) (Fig. 1 ). The formation and development of the Sichuan Basin have undergone multiple tectonic movements, dominated by subsidence. The Sinian was a stage of carbonate sedimentation and tectonic evolution in the marine craton basin [9]. The Dengying Formation deposition inherited the sedimentary pattern of the Doushantuo Formation [20], with expanded transgression that formed a transgressive retrogradational sedimentary sequence. During the sedimentation of the Deng-1 Member, extensive transgression occurred to form an epicontinental sea. In this period, the Sichuan Basin had a relatively flat basement, with no significant difference in hydrodynamic conditions, and each study area deposited a set of algal-poor muddy-/powder-crystal dolomite in conformable contact with the underlying Doushantuo Formation (Fig. 2b ). The Deng-2 and Deng-1 members were deposited continuously. The transgressive scale peaked in the early sedimentary stage of Deng-2 Member, followed by a gradual decline in sea level. Moreover, due to tectonic processes, a platform-basin sedimentary environment was formed in this period. The study areas were in platform margin and restricted platform environments, where algae-rich microbial dolomite was prosperous, followed by muddy-/powder-crystal dolomite and a small amount of psammitic dolomite and dolomitic karst breccia. The Episode I of the Tongwan Movement in the late sedimentary stage of the Deng-2 Member caused the overall uplift, weathering, and erosion of the Deng-2 Member, making it in unconformable or disconformable contact with the Deng-3 Member. During the sedimentation of the Deng-3 Member, the sea level rose again, and relatively thin and dark mudstone and siltstone were developed in the areas, in conformable contact with the Deng-4 Member. During the sedimentation of the Deng-4 Member, the relative sea level further declined, and the sedimentary pattern of the study areas was similar to that in the sedimentary period of the Deng-2 Member, with muddy-/powder-crystal dolomite at the bottom, algae-rich microbial dolomite and psammitic dolomite in the middle and upper parts, and banded siliceous dolomite occasionally. The Episode II of the Tongwan Movement in the late sedimentary period of the Deng-4 Member led to varying degrees of weathering and erosion to the Deng-4 Member. In the platform region, the Deng-4 Member was in unconformable contact with the Cambrian Qiongzhusi Formation[21⇓-23]. Controlled by differences in the basin basement, the sedimentary landform was low, the water bodies were deep, and the platform margin was completely preserved in northern Sichuan Basin [24].
Fig. 1. Location of the study area. |
Fig. 2. Comprehensive geological maps of the central Sichuan. |
2. Experiments
The samples for this study were obtained from drilling cores and fresh and unweathered rocks as outcrop sections. A total of 146 plunger samples with diameter of 2.54 cm and length of 5-7 cm were obtained by drilling and cutting the core samples, and then they were ground and cleaned for subsequent physical property measurement and acoustic experiment. For physical property measurement, the CMS-300 core measuring system was used to measure the porosity (with a range of 0.01%-40.00%) according to the Boyle's law, and the permeability (with a range of (0.000 05-15 000)×10−3 µm2) using the pressure pulse attenuation method. For ultrasonic velocity measurement, by using the ultrasonic pulse penetration method, the propagation time of pulse vibration penetrating a sample with known length was measured accurately to calculate the acoustic velocity. The P-wave velocity was measured using a probe with a main frequency of 800 kHz, while the S-wave velocity was measured using a probe with a main frequency of 350 kHz, with an absolute error of less than 1%. Moreover, typical samples were selected for low-frequency petrophysical measurement by the stress-strain method, so that the acoustic properties of the rock samples were obtained by directly recording the forced deformation of the rock samples. The remnants of samples after drilling and cutting were used for petrological experiments. The mineralogical characteristics of the rocks were identified by X-ray diffraction (XRD) of the minerals in at least 5 g samples with a diameter of less than 0.075 mm after grinding. For this purpose, four dominant minerals (calcite, quartz, clay and dolomite) of carbonate rocks in the study areas were considered. The analysis of rock texture was mainly completed by using a polarizing microscope to observe (rock and cast) thin sections. To ensure that different lithologies and pore structures in the study areas are covered, the samples for making thin sections were selected based on hand specimen observation and mud logging data. The cast thin sections were prepared by the method of vacuum infusion of impregnating agent, which were ground to about 30 µm thick for visible light transmission. The experiments were all conducted in the State Key Laboratory of Oil and Gas Reservoir Geology and Development Engineering at Chengdu University of Technology. Combined with existing geological data, the diagenetic evolution process was analyzed and the impact of diagenesis on the petrophysical properties of carbonate reservoirs was discussed.
3. Reservoir
3.1. Petrology
The core description and microscopic thin section identification indicate a complex lithology of the Deng-4 Member reservoirs in the study areas (Fig. 3 ). The platform margin mainly consists of thrombolite dolomite, algal-laminated dolomite, algal-stromatolitic dolomite and algal-psammitic dolomite of platform-margin mound- shoal facies, as well as silty-fine-crystal dolomite, muddy dolomite, siliceous dolomite, and calcareous dolomite (dolomitic limestone) of inter-mound-shoal bottomland facies. The thrombolite dolomite, light gray to white gray, is mainly distributed in the upper part of the Deng-4 Member, often coexisting with algal-stromatolitic dolomite. The hand specimens show obvious irregular dark gray thrombolites, and the clots observed under the microscope are mainly formed by the adhesion of muddy- crystal dolomite by dark microorganisms (algae). The algal-laminated dolomite is laminated in gray color, and widespread in the Deng-4 Member. The alternating light and dark laminated structure is nearly straight, with the dark laminae rich in algal and the bright laminae dominated by dolosparite. Microscopically, it shows that the diameter of dolomite grains is mostly 0.03-0.25 mm, with a hypautomorphic-xenomorphic mosaic texture. The algal-stromatolitic dolomite is gray in color, with obvious wavy laminae in hand specimens, and mainly distributed in the middle and upper parts of the Deng-4 Member. A single wavy stromatolitic lamina varies greatly in thickness, with good lateral continuity. Under the microscope, the fine- and medium-crystal dolomite grains mainly exhibit an automorphic-hypautomorphic mosaic texture. There are also moundy stromatolitic dolomites in limited scale, with stromatolite laminae in poor lateral continuity. The algal-psammitic dolomite is gray to light gray, and sparsely developed in the Deng-4 Member. Under the microscope, psammitic particles of different sizes can be seen, with traces of microbial bonding. These particles are mainly composed of muddy- to silty-crystal dolomites, and they are moderately sorted and well rounded (mostly circular and elliptical). The silty- to fine-crystal dolomite is light gray, and developed at multiple locations in the Deng-4 Member. It is mainly composed of particles with a size of 10-50 µm and a hypautomorphic-xenomorphic mosaic texture. The surface of the dolomite is dirty, containing terrestrial quartz and clay debris. The quartz is well sorted and rounded, and can form siliceous and muddy dolomites. The calcareous dolomite is dark gray, with dense contact between particles. Dark silty-/fine- crystal dolomite is often bounded by the stylolite with micritic calcite, or the dolomite crystals with fog core and bright edge are distributed in patches.
Fig. 3. Petrological characteristics of carbonate rocks in Deng-4 Member. |
In the rock samples collected in this study, the color of carbonate rocks in the restricted platform is relatively darker (Fig. 3b ). The mound-shoal inside the restricted platform (hereinafter referred to as the intra-platform mound-shoal) mainly develops algal-thrombolite dolomite, algal-laminated dolomite, algal-stromatolitic dolomite and algal-psammitic dolomite. Dolomitic lagoon mainly develops muddy-crystal dolomite, muddy dolomite, siliceous dolomite and calcareous dolomite (dolomitic limestone). The rock characteristics are similar to those in the platform margin, but the scale of microbial dolomite (mainly in the upper part of the Deng-4 Member) is smaller than that in the platform margin mound- shoal.
3.2. Reservoir performance of sedimentary microfacies
The reservoirs of platform margin and restricted platform subfacies are similar in diagenetic evolution, and the reservoirs of platform margin and intra-platform mound-shoal microfacies are deposited in geomorphic highlands (Fig. 2c ), where strong hydrodynamic conditions and abundant nutrient components promoted the rapid growth of microorganisms such as blue-green algae (bacteria). Extracellular polymers produced by microbial activities led to low-temperature dolomite precipitation [5,25], forming microbial dolomites such as algal-stromatolitic dolomite and algal-laminated dolomite, which evolved into vertically stacked multi-stage mound-shoal complexes with uplifted landform due to periodic sea level fluctuations. These microbial dolomites have well-developed algal lattice pores and intergranular pores. During the same period, foliated dolomite formed by early low-temperature dolomitization processes such as evaporative seepage-reflux, and capillary concentration filled the primary pores. After this diagenetic stage, the sediments were basically consolidated, and the reservoir space was mainly composed of residual algal lattice pores and intergranular pores. Due to frequent and continuous exposure to the sea surface, the early low-temperature dolomite (with low degree of ordering), and incompletely consolidated and dolomitized easily soluble minerals (e.g. calcareous minerals and aragonite) were dissolved by atmospheric fresh water during penecontemporaneous dissolution. Dissolved vugs are distributed along beds in the cores (Fig. 4a ), and mostly filled with bitumen. Affected by the differential uplift of Episode II of Tongwan Movement, severe uplift-induced erosion occurred within the platform in central Sichuan Basin, and a certain scale of karst fractures and vugs were formed during the late stage of epigenetic karstification, which had a limited influence on northern Sichuan Basin. As the strata subsided, muddy-crystal dolomite underwent recrystallization and secondary enlargement due to burial dolomitization, and some clastic or microbial dolomites formed fine- to medium-crystal dolomites without residual original rock fabric, or silty-/fine-crystal dolomites with residual original rock fabric. During this period, the fault system formed by multiple tectonic ruptures in the reservoir, providing a pathway for the activity of organic acids generated by basement hydrothermal fluids and the cracking of the Qiongzhusi Formation source rock. In such a closed system, the products of hydrothermal fluid or organic acid dissolution could not flow out, and the formed precipitates, such as saddle dolomite, pyrite and quartz, would damage pores. The reservoir space remained stable as a whole. Burial dissolution could adjust the pore system. And the areas with better porosity and permeability (where acidic fluids are more likely to flow in) have a large specific surface area of dissolution and more developed dissolution pores, thereby optimizing the distribution of reservoir spaces. The difference between the platform margin mound-shoal and the intra-platform mound-shoal mainly lies in the scale of the mound-shoal complexes and the intensity of penecontemporaneous dissolution. The former was deposited under stronger hydrodynamic conditions, resulting in larger mound-shoal complex, longer exposure time to the sea surface, and stronger penecontemporaneous dissolution. The latter, distributed in relatively high positions within restricted platforms, was deposited under weaker hydrodynamic conditions, resulting in mound-shoal complexes with smaller vertical scale and horizontal range, and weaker penecontemporaneous dissolution.
Fig. 4. Characteristics of reservoir spaces in Deng-4 Member carbonates. (a) Bed-parallel dissolution pores, core, Ningqiang Hujiaba section; (b) Dissolution pores, filled or partly filled with bitumen, cast thin section, Well MX8, 5257.5 m; (c) Pores filled with bitumen and secondary dolomite, SEM, Well MX8, 5257.5 m; (d) Dissolution pores, SEM, Well MX8, 5262.8 m; (e) Tectonic fractures, ordinary thin section, Well MX8, 5275.8 m; (f) Dissolution fractures, cast thin section, Well HS2, 5721.3 m; (g) Intercrystal dissolution pores filled with bitumen, cast thin section, Well MX8, 5337.8 m; (h) Intercrystal pores, SEM, Well HS2, 5485.3 m; (i) Intergranular dissolution pores, cast thin section, Well HS2, 5567.9 m. |
The inter-mound-shoal bottomlands and dolomitic lagoons are mainly distributed in flat bottomland between various mound-shoals, to an extent controlled by tectonic landforms. Due to the shielding of the surrounding geomorphic highlands of the mound-shoals, the dolomitic lagoons and the inter-mound-shoal bottomlands are significantly weakened in hydrodynamic conditions, where microorganisms are not developed. During the sedimentation, the aggregates of fine-grained carbonate particles were mainly deposited. The relatively deeper seawater environment resulted in weak low-temperature dolomitization processes such as seepage-reflux and capillary concentration, and the sediments were mainly located below sea level, thus they were weakly affected by the dissolution of penecontemporaneous atmospheric fresh water. During the shallow burial period, the sediments that were not modified by the near surface diagenetic environment were in an unconsolidated state. Under strong compaction, the primary pores reduced significantly, and suture structures formed by pressure dissolution can be seen in the rocks (Fig. 3b ). The sediments in the inter-mound-shoal bottomlands contain relatively less algae and muddy substances, which is more conducive to burial dolomitization. Dolomite crystals are recrystallized and become coarser and automorphic to form silty-/fine-crystal dolomite. The intercrystal pores generated in this process can improve the effective porosity and permeability of the rock. The later hydrothermal fluids invaded the interior of the rocks by structural rupture, optimizing the reservoir space to a certain extent. The accumulated fine-grained carbonate rocks in dolomitic lagoons generally contain a certain amount of muddy, siliceous, and dispersed algae, which limits recrystallization [5]. Hydrothermal fluids cannot effectively flow in tight muddy-crystal dolomite, muddy dolomite, or siliceous dolomite. Moreover, due to farther distance from the source rock, limited supply of organic acids makes dissolution impossible, leading to underdeveloped pores.
3.3. Reservoir space
Multiple stages of diagenesis have resulted in disappearance of most primary pores in the carbonate rocks of the Deng-4 Member. The reservoir spaces are mainly composed of medium-small pores (intercrystal pores, intercrystal dissolution pores, and intergranular dissolution pores), vugs and fractures caused by secondary diagenesis. Specifically, fractures have a low contribution to the reservoir space and can mainly connect various types of vugs to improve the permeability of the reservoirs. The reservoir spaces of carbonate rocks in the Deng-4 Member in platform margin are mainly composed of algal lattice dissolution pores/vugs, followed by intercrystal pores, intercrystal dissolution pores and intergranular dissolution pores. The algal lattice dissolution pores/vugs are mostly developed in the algal-stromatolitic and thrombolite dolomites in the upper part of the Deng-4 Member, showing obvious bed-parallel distribution characteristics. These pores/vugs mainly expand along the dissolution of algae, and most are filled or partially filled with bitumen and secondary dolomite. The better-preserved dissolution vugs are still good reservoir spaces in the study areas (Fig. 4a -4d). Intercrystal pores and intercrystal dissolution pores are mainly distributed in crystal dolomite and silty-/fine-crystal dolomite with residual particle texture, and partially filled with quartz and bitumen (Fig. 4g-4h ). The intergranular dissolution pores, sharing a relatively small proportion of the reservoir space in the study areas, are mainly distributed in algal-psammitic dolomite (Fig. 4i ). In addition, algal-rich dolomites develop bed-parallel dissolution vugs due to selective dissolution, and form irregular bed-parallel dissolution fractures, which have certain positive significance for reservoir space, although they are partially filled with bitumen, dolomite, and quartz (Fig. 4f ). Due to multiphase tectonic movements, the rocks develop tectonic fractures, which are mostly filled with dolomite cement, making it almost impossible to form effective reservoir space (Fig. 4e ). The carbonate rocks of restricted platform and platform margin are generally consistent in reservoir space types, with the main difference in the scale of different pore types. Compared to carbonate rocks of restricted platform subfacies, the carbonate rocks of platform margin subfacies have more dissolution pores and fractures.
3.4. Physical properties of reservoirs
Reservoirs of different sedimentary facies in the Deng-4 Member are different in physical properties. The platform margin samples exhibit the porosity of 0.35%- 12.29% and dominantly 2%-8% (about 10% of the samples have porosity greater than 10%, averaging 4.89%), and the permeability of (0.000 1-20.900 0)×10−3 µm2 and averaging 1.24×10−3 µm2. The restricted platform samples have the porosity of 0.30%-6.67% (averaging 3.62%) and permeability of (0.000 06-14.500 00)×10−3 µm2 (averaging 1.12×10−3 µm2). The samples exhibit medium-low porosity, and generally low permeability.
The samples demonstrate a cloud distribution of porosity versus permeability, which is clearly zonal according to the classification by sedimentary microfacies (Fig. 5a-5b ). The platform margin mound-shoal samples have a wide range of porosity and permeability, with some porosity exceeding 7%. The porosity and permeability of the inter-mound-shoal bottomlands are relatively low. Similar distribution characteristics are observed in the intra-platform mound-shoal and dolomitic lagoon samples in restricted platform, which indicates that the porosity and permeability of the samples are closely related to sedimentary facies. The mound-shoal complexes provide materials for reservoir development, and penecontemporaneous dissolution controls the development of dissolution vugs in the reservoirs. In addition, a small amount of data points overlap between platform margin mound-shoal and inter-mound-shoal bottomland, or between intra-platform mound-shoal and dolomitic lagoon. This may be related to the adjustment and optimization process of reservoir space. In the parts with worse porosity and permeability in the mound-shoals, hydrothermal fluids and organic acid corrosion products cannot be discharged, thus the formed sediments fill the pores, resulting in decreased porosity and permeability. Fig. 5c-5d shows the influence of pore structure on porosity and permeability. Based on the differences in the development characteristics of carbonate reservoir space with varying lithologies in the Deng-4 Member, the pore combinations can be divided into four types: crack-dissolution vug, dissolution vug, crack-pore, and pore. The reservoirs of crack-dissolution vug type and dissolution vug type are mainly developed in algal dolomite and granular dolomite of mound-shoal facies. The reservoirs of crack- pore type and pore type are mainly developed in the dolomites of inter-mound-shoal bottomland and dolomitic lagoon facies. The samples of crack-dissolution vug type exhibit high permeability. The samples of dissolution vug type are favorable for reservoirs with the highest porosity, but their porosity varies widely. The samples of pore type have the lowest porosity and permeability. The cracks generated by tectonic rupture can communicate pores, so the permeability of samples with well-developed cracks is usually higher, specifically manifested as the permeability of the crack-dissolution vug type being 1-3 orders of magnitude higher than the other two types of samples.
Fig. 5. Relationship between porosity and permeability of Deng-4 reservoirs of different sedimentary facies. |
4. Petrophysical characteristics of the carbonate rocks
The elastic properties of rocks are mainly obtained by measuring the P-wave and S-wave velocities of the samples in the ultrasonic frequency band. The measurement was conducted at the effective stress of 2-60 MPa, which initially increased from 2 MPa to 5 MPa, and subsequently at an interval of 5 MPa. In addition to analyzing the variation of the elastic wave velocity with pressure, the elastic parameters of the samples at 60 MPa were selected for subsequent analysis to correspond to the effective stress (average value) under reservoir conditions.
4.1. Variation of elastic wave velocity with pressure
There are always two types of pressures in a reservoir: the overburden pressure (also known as surrounding rock pressure), and the reservoir pressure (also known as fluid pressure or pore pressure), the difference between which is referred to as the effective pressure. The depth range of reservoirs in different structural parts of the study areas varies, which affects the effective pressure of the reservoirs. In addition, when the oil or gas reservoir is put into production, the distribution of formation pressure will change accordingly. The relationship between rock elastic wave velocity and effective pressure can provide a physical basis for reservoir prediction and seismic monitoring of oil and gas production processes. Fig. 6 shows the variation of P-wave and S-wave velocities with effective pressure in typical carbonate samples with different pore types under dry and water saturated conditions. The porosities of pore type, crack-pore type, vug type, and crack-vug type samples are 1.8%, 2.3%, 4.3% and 4.7%, respectively. It can be seen that the pore structure controls the trend of dry rock velocity variation with pressure. For crack-pore type and crack-vug type samples, the porosity increases remarkably with increasing pressure, and shows a high pressure effect, that is, rapid increase in low-pressure interval and slowing down in high-pressure interval. The pore type samples exhibit a low pressure effect that the velocity slowly increases with increasing pressure. This is because that highly compressible soft pores such as microcracks are more sensitive to pressure variation. As such cracks tend to close during progressive loading, equiaxed pores or vugs have high stiffness and can effectively resist pressure. The samples of pore-vug type may have intercrystal pores and fractures and fewer pore throats, with a trend of pressure-velocity variation between pore type and "dual porosity" samples, which can be considered as a transitional state of pressure-velocity relationship. Compared to dry samples, the water-saturated samples generally show an increasing trend of P-wave velocity, but in different amplitude. The crack-vug type and crack-pore type water-saturated samples with soft pores demonstrate more significant increase in velocity, indicating that the fluid can more significantly improve the mechanical properties of cracks. The water-saturated samples reflect similar increase trend of S-wave velocity to that of P-wave. For crack-vug type and crack-pore type water-saturated samples, however, the increase in S-wave velocity is much weaker than that in P-wave velocity, due to the insensitivity of shear modulus to fluids. In theory, according to the Gassmann relationship, the water-saturated samples maintain a constant shear modulus, while the S-wave velocity decreases as the rock density increases. The increase in S-wave velocity is due to the measured frequency being in the ultrasonic frequency range, and the fluid related dispersion effect causes the increase in S-wave velocity.
Fig. 6. Velocity variation with effective pressure and microscopic characteristics of typical samples in Deng-4 Member. |
4.2. Influence of fluids on rock elastic characteristics
Fig. 7. Relationship between P-wave velocity and water saturation effect and SEM images of different lithologies in dry rock samples of Deng-4 Member. (a) Relationship between P-wave velocity and water saturation effect of dry rocks; (b) Grain dolomite, dolomite crystals closely contact with each other, Well MX8, 5227.9 m; (c) Muddy dolomite, dolomite crystals contact with illite crystal boundaries, Well HS2, 5509.4 m; (d) Siliceous dolomite, quartz crystals are embedded in dolomite crystals, Well HS2, 5581.8 m; (e) Calcareous dolomite, calcite crystals are distributed in dolomite with a porphyritic manner, Well HS2, 5485.8 m. |
A few of muddy, siliceous, and calcareous dolomite samples deviate from the dominant trend, but exhibit low P-wave velocity and low water saturation effect. Carbonate sediments containing terrestrial quartz and clay in low-energy deep-water environments are affected by compaction. Quartz and clay fill pores, and form stable structures between fine-grained carbonate particles. After later dolomitization, siliceous and muddy dolomites with dolomite and quartz (or dolomite and clay) as the stress skeletons are formed (Fig. 7c-7d ), allowing the elastic wave velocity to be sensitive to the elastic properties of quartz and clay minerals. Carbonate sediments in low-energy environments underwent incomplete dolomitization during diagenesis to form calcareous dolomite. In which, undolomitized calcite and dolomite together serve as the rock stress skeleton, and most calcite is distributed in patches (Fig. 7e ). Compared to dolomite, minerals such as clay, quartz, and calcite have lower P-wave velocities. Hence, the low P-wave velocity characteristics of rocks are mainly attributed to mineralogy, rather than the role of soft pores such as microcracks. The difference in water environment and the complex diagenesis during the sedimentary period led to complex mineral composition in the samples from the Deng-4 Member. The hydrodynamic conditions at the platform margin are relatively stronger, and there are less muddy and siliceous debris in the sediments. In addition, the carbonate rocks at platform margin develop more dissolution vugs under strong penecontemporaneous dissolution, which has a certain positive effect on subsequent burial dolomitization. These factors can comprehensively affect the mineralogical characteristics of rocks in the target intervals in the study areas. The regional tectonic rupture produced different distribution characteristics of cracks, and coupling with other diagenetic processes, formed reservoirs of different pore types. This induces different water saturation effects in the samples. Moreover, the differential reservoir spaces and the different fluid states (gas or liquid) formed during geological diagenesis can affect fluid distribution.
4.3. Relationship between P-wave impedance and Poisson ratio
The P-wave impedance and Poisson ratio are indicative parameters in quantitative seismic interpretation. The relationship between the P-wave impedance and Poisson's ratio variation of the platform margin samples and the restricted platform samples show similar trends. In this experiment, the P-wave impedance distribution range of the platform margin samples is larger, and the Poisson ratio distribution range is smaller. The relationship between P-wave impedance and Poisson ratio in the study areas is also influenced by pore structure, pore fluids, and mineral composition (Fig. 8 ). For dry samples, the vug type samples have much higher P-wave impedance and Poisson ratio than that of the crack-vug type and crack-pore type samples. This is because microcracks cause a differential decrease in P-wave and S-wave velocities (the decrease in P-wave velocity is more pronounced). As the crack proportion increases, the P-wave impedance and Poisson ratio gradually decrease. For water-saturated samples, the variation trends are different. After the cracks are saturated with water, the stiffness significantly increases. Coupling with the dispersion effect related to jet flow, the P-wave velocity increases more greatly than the S-wave velocity. Therefore, for the crack-vug type and crack-pore type samples, the P-wave impedance gradually decreases with the increase of cracks, while the Poisson ratio increases with the increase of cracks. After dissolution vugs or pores are saturated with water, the stiffness changes little, and the changes in P-wave and S-wave velocities are small. In contrast, the P-wave impedance and Poisson ratio of vug-type samples do not change obviously. The decreasing trend of P-wave impedance and Poisson ratio caused by siliceous material is similar to that of cracks. Siliceous dolomites in the study areas can be divided into two types: siliceous filled pores and siliceous bands. When pores are filled with siliceous material, some quartz and dolomite together serve as the stress skeleton of the rock (as described in Section 4.2). Affected by the low P-wave velocity and low Poisson ratio of quartz, the samples exhibit characteristics of low P-wave impedance and low Poisson ratio. When there are siliceous bands in the rock, it can be considered that dolomite and quartz interbeds are formed in the rocks, where quartz has a greater impact on the elastic properties, resulting in lower P-wave impedance and Poisson ratio. Muddy and calcareous dolomites are affected by the elastic properties of clay and calcite minerals (with higher Poisson ratios than dolomite), resulting in a decrease in P-wave impedance and an increase in Poisson ratio. It should be pointed out that the muddy, siliceous, and calcareous dolomite samples in Fig. 8 are all in dry state. Compared to the restricted platform samples, the platform margin samples contain lower proportions of muddy, calcareous, and siliceous dolomites. In seismic interpretation, if the influence of minerals (clay, quartz, and calcite) is excluded, the relationship between P-wave impedance and Poisson ratio can be used to distinguish crack-vug type and crack-pore type gas-bearing dolomite reservoirs, so as to facilitate the sweet spot evaluation for exploration.
Fig. 8. Relationship between P-wave impedance and Poisson ratio of Deng-4 Member samples of (a) platform margin and (b) restricted platform. |
In order to quantitatively explain the relationship between acoustic velocity and porosity, porous rocks are considered as a mixture of solid particles, spherical pores, and penny-shaped cracks, for purpose of simplification, and their effective elastic properties are expressed as a function of total porosity and crack density, where crack density is defined as a statistical summary of the porosity of non-spherical inclusions. The effective bulk modulus under dry conditions is expressed as [26-27]:
$\frac{K_{0}}{K_{\mathrm{d}}}=1+\frac{\rho_{\mathrm{c}}}{(1-\phi)} \frac{h}{\left(1-2 v_{0}\right)}\left(1-\frac{v_{\mathrm{d}}}{2}\right)$
where, h is an intermediate variable, expressed as:
The results show that as the porosity increases, the P-wave velocity decreases to varying degrees, and the aspect ratio of pores (γ) controls the decrease degree of velocity (Fig. 9 ). When γ=0.9, the porosity-velocity trend slightly decreases. When γ<0.2, the P-wave velocity sharply decreases. The crack-pore type and crack-vug type dolomite samples fall between the model lines of γ=0.01 and γ=0.2, and the vug type dolomite samples stay above the model line of γ=0.1, indicating differences in pore systems among different types of samples. Combined with the calculation results of Eqs. (1) and (2), the crack densities of the samples are 0.01-0.15, and the crack densities of the crack-vug type and crack-pore type samples are generally greater than 0.07, indicating a high degree of crack development. Specifically, the crack-vug type reservoirs have both hydrocarbon storage space and migration channels, especially the platform margin reservoirs which contain the most developed dissolution vugs as a result of strong penecontemporaneous dissolution, allowing them to be worthy of special attention in exploration. The crack-pore type reservoirs exhibit relatively low porosity, but can still be effective when they reach a large scale. The vug type reservoirs have favorable storage space, regardless of strong dissolution, but they have no hydrocarbon transportation channels for lack of cracks; thus, they need to be developed with the support of stimulation treatments. Muddy, calcareous, and siliceous dolomites fall in the regions below γ=0.03, possibly leading to an inappropriate interpretation that the low porosity-P-wave velocity trend is resulted from the increase in crack proportion. Essentially, the mineralogy of the model line considers the average of XRD results of all samples, but neglects the changes in mineral composition of such samples. From the above analysis, it can be seen that the elastic wave velocity variation of the rocks is a joint result of multiple factors, especially for carbonate rocks with complex diagenesis. It is difficult to simply associate the elastic properties of rocks with a certain parameter. The hydrodynamic conditions during sedimentation, the intensity of penecontemporaneous dissolution, regional tectonic rupture and burial dissolution can all affect the porosity-elastic wave velocity relationship of reservoir rocks.
Fig. 9. Variation characteristics of (a) P-wave velocity and (b) S-wave velocity with porosity and crack density (crack density is the ratio of porosity to crack aspect ratio). |
4.4. Velocity dispersion characteristics
In actual seismic exploration, the frequency of seismic waves is very low, and there is dispersion between the elastic wave velocity measured in the laboratory ultrasonic frequency band and the seismic frequency band. Fig. 10 shows the low-frequency petrophysical measurement results of the crack-vug type samples, and adds a model line based on the unified theory of elastic waves in porous and cracked media. It can describe the impact of cracks on the elasticity of the media, and simulate the attenuation and dispersion of elastic waves caused by the squeezing effect. The specific formula is as follows [27]:
$K=K_{\mathrm{d}}+\frac{\alpha}{\frac{\alpha-\phi}{K_{\mathrm{S}}}+\frac{\phi}{K_{\mathrm{f}}}+S(\omega)}$
$S(\omega)=\frac{\frac{8}{3} \pi \rho_{\mathrm{c}} \frac{(1-v)}{\mu} f(\zeta)\left[\frac{\frac{1}{K_{\mathrm{d}}}-\frac{1}{K_{\mathrm{s}}}}{\frac{1}{K_{\mathrm{d}}}-\frac{1}{K}}-f(\zeta)\right]}{1+\frac{4(1-v) K_{\mathrm{f}}}{3 \mu \gamma}[1-f(\zeta)]}$
Fig. 10. Dispersion of P-wave velocity in porous and cracked rocks. |
Theoretical calculations indicate that as the aspect ratio of cracks decreases, the P-wave velocity dispersion shifts towards the low-frequency direction. This is because under the same pressure, the smaller the aspect ratio of cracks, the more easily the cracks are closed, and the more easily the jet flow from cracks to pores occur. The low-frequency petrophysical measurement results also show that the frequency dispersion occurs in crack-vug type samples.
The analysis of the experimental results above reveals that the velocity dispersion of water-saturated samples is closely related to the crack proportion. Accordingly, the relationship between the crack volume proportion and the velocity dispersion of elastic waves is illustrated in Fig. 11 . The velocity dispersion is obtained from the ratio of the difference between the measured ultrasonic velocity of water-saturated samples at high frequency and the velocity at low frequency to the low-frequency velocity. Especially, the low-frequency velocity is calculated from the water-saturated bulk modulus and shear modulus obtained by replacing the Gassmann equation fluid [28].
$K=K_{\mathrm{d}}+\frac{\left(1-\frac{K_{\mathrm{d}}}{K_{\mathrm{S}}}\right)}{\frac{\phi}{K_{\mathrm{f}}}+\frac{1-\phi}{K_{\mathrm{S}}}+\frac{K_{\mathrm{d}}}{K_{\mathrm{S}}^{2}}}$
Fig. 11. Varation of (a) P-wave velocity dispersion and (b) S-wave velocity dispersion with crack proportion. |
It is found that the difference between the theoretical values and the velocity obtained from low-frequency petrophysical experiments is very small (Fig. 10 ). Therefore, when the frequency is low enough, it is feasible to analyze the velocity dispersion characteristics by calculating the water-saturated rock velocity predicted by the Gassmann equation.
The crack volume proportion is the ratio of crack porosity to total porosity, and the crack porosity is calculated by the following formula [29]:
$\phi_{\mathrm{c}}=\frac{4 \pi \gamma \rho_{\mathrm{c}}}{3}$
The model lines in Fig. 10 were constructed according to the dispersion theory of porous-cracked rocks [30]. The maximum dispersion of P-wave velocity occurs when the crack volume proportion is around 0.3. The pore type samples have extremely low crack proportion, and exhibits the smallest dispersion of P-wave velocity. The crack-vug type and crack-pore type samples demonstrate the highest dispersion of P-wave velocity. When elastic waves compress on a rock, the pore pressure generated by soft pores such as cracks is much greater than that generated by rigid pores such as intercrystal pores or dissolution vugs. In the ultrasonic frequency range, there is no sufficient time for pore pressure to balance, resulting in local fluid pressure difference. In the low frequency range, the excess fluid pressure in the cracks is released through fluid flow to the pores, resulting in large velocity dispersion. When the crack volume proportion of the crack-pore type samples exceeds 0.7, the dispersion of P-wave velocity decreases. This type of samples is extremely tight, with low porosity and a high proportion of cracks. Moreover, the fluid pressure difference between randomly distributed cracks is very small, similar to the situation where the reservoir space is completely composed of pores or vugs, so there is no dispersion of bulk modulus. The dispersion of S-wave velocity is different from that of P-wave velocity in areas with high crack proportion, where the former continues to increase as the crack proportion increases. Essentially, for nearly spherical pores, pure shear strain is always present and fluid pressure remains constant; for cracks, shear stress generates different fluid pressures with the changes in the crack direction [29], resulting in shear dispersion. The platform margin reservoirs contain more abundant dissolution vugs, and the crack volume proportion is generally lower than that of the restricted platform reservoirs. Due to the causal relationship between frequency dispersion and attenuation, high-quality reservoirs of crack-vug type and crack-pore type in the Deng-4 Member may also exhibit strong attenuation characteristics.
5. Influence of diagenesis on the elastic properties of rocks
Carbonate sediments are mostly consolidated by cementation before deep burial, resulting in less significant subsequent physical diagenesis than terrigenous clastic rocks [31]. However, the distinct near-surface diagenesis of different sedimentary facies affects subsequent series of diagenesis (e.g. burial dolomitization). Fig. 12a illustrates a simplified diagenetic process of crack-vug type microbial dolomite reservoirs. (1) The microbial dolomite of the Deng-4 Member, which contains algal lattice pores and intergranular pores in platform margin and intra-platform mound-shoals, has an initial porosity of up to 30%[32], low P-wave impedance, and the Poisson ratio close to 0.3. This is because the P-wave impedance is related to the P-wave velocity and density. The high-porosity rocks often have low P-wave velocity, which coupling with low rock density results in low P-wave impedance. The low- temperature dolomite sediments generated by microbial activity initially formed a rock skeleton composed of muddy-crystal dolomite crystals, and the Poisson ratio of the rock is close to that of the dolomite minerals. (2) The foliated dolomite cements generated by low-temperature dolomitization processes, such as seepage-reflux and capillary concentration, result in a systematic loss of vugs (the original porosity is reduced by about 60%), so that the rock density increases significantly and the P-wave impedance increases continuously. As this process does not lead to variations of cracks and lithology, there is no significant change in Poisson ratio. (3) Frequent and sustained penecontemporaneous dissolution formed bed-parallel dissolution vugs, resulting in an increase in reservoir porosity and a decrease in P-wave impedance. (4) The development of cracks by multi-stage tectonic rupture has little impact on density and porosity. However, the presence of cracks significantly reduces elastic wave velocity and exhibits differential changes in P-wave and S-wave velocities, ultimately resulting in a significant decrease in both P-wave impedance and Poisson ratio. (5) On the basis of tectonic rupture, burial dissolution can form dissolution vugs, but saddle dolomite and quartz of hydrothermal origin were precipitated. During this period, the reservoir intervals with poor porosity and permeability might be filled with sediments, while pores of high-quality reservoir intervals were further optimized, forming crack-vug type dolomite reservoir. In the process of reservoir space adjustment, density, porosity, and pore structure changed just slightly, so did P-wave impedance and Poisson ratio. The significant increase in P-wave impedance shown in the figure is the result of pore reduction with the effect of burial stage.
Fig. 12. Changes in seismic elastic properties of main reservoir type during diagenetic evolution in Deng-4 Member. |
6. Conclusions
The petrophysical study on carbonate rocks of the Deng-4 Member in different sedimentary facies of the Sichuan Basin reveals that the physical properties and elastic characteristics of rocks are controlled by sedimentary environment and diagenetic process. The sedimentary facies in combination with dissolution controls the physical properties of reservoir rocks, and favorable facies determine the material basis for reservoir development. Penecontemporaneous dissolution is an important factor in the formation of reservoir spaces, and tectonic rupture-hydrothermal dissolution optimizes the reservoir spaces. The reservoirs can be ranked as platform margin mound-shoal complex, restricted platform mound-shoal complex, inter-mound-shoal bottomland silty-/fine-crystal dolomites, and dolomitic lagoon carbonates, in a descending order of physical properties.
The experimental results show that the platform margin and restricted platform samples have similar variation laws of elastic properties, both controlled by mineralogy and pore structure, and closely related to sedimentary facies and diagenesis. Due to the hydrodynamic conditions during sedimentation and differential diagenesis, the proportions of muddy, siliceous, and calcareous dolomites in the platform margin are relatively low. The development degree of cracks is decided by the intensity and frequency of regional tectonic movements. The elastic properties of fine-grained sediments such as quartz, clay, and calcite in the low-energy facies result in a significant decrease in P-wave velocity and an increase or decrease in Poisson ratio. Cracks cause rocks to exhibit high pressure and high water saturation effects. In addition, due to the influence of cracks on the difference in P-wave and S-wave velocities, the P-wave impedance and Poisson ratio of dry samples show a decreasing trend with the increase of cracks. Fluid-related dispersion is also affected by pore structure. The crack-vug type dolomite samples exhibit high dispersion and high attenuation characteristics, while samples without or with high crack proportion have the smallest P-wave velocity dispersion. By utilizing dispersion and attenuation properties, high-quality reservoirs can be identified.
Nomenclature
f—frequency, Hz;
f(ζ)—frequency variation factor, dimensionless;
GR—Gamma ray, API;
h—intermediate variable, dimensionless;
K—bulk modulus of saturated samples, Pa;
K0—bulk modulus of solid matrix without crack (only embedding pores), Pa;
Kd—bulk modulus of dry sample, Pa;
Kf—bulk modulus of fluid, Pa;
K2—bulk modulus of crack or vug, Pa;
Ks—bulk modulus of matrix, Pa;
K*—effective bulk modulus, Pa;
γ—aspect ratio of pores, dimensionless;
T1(γ), T2(γ)—geomatrical factor of crack or vug, dimensionless;
μ—shear modulus of saturated sample, Pa;
μd—shear modulus of dry sample, Pa;
μ2—shear modulus of crack or vug, Pa;
μ*—effective shear modulus, Pa;
α—effective stress hole elasticity coefficient, dimensionless;
υ—Poisson's ratio of saturated sample, dimensionless;
υ0—Poisson's ratio of solid matrix without crack, dimensionless;
υd—Poisson's ratio of dry sample, dimensionless;
ϕ—total porosity of sample, %;
ϕc—crack porosity, %;
ρc—crack density, dimensionless;
ζ—argument, dimensionless;
ω—angle frequency, rad/s.