Properties Of Carbonate Reservoirs Research Articles

Enhancement Petrophysical Properties of Carbonate Reservoirs Using Plasma Channel Technology, Case Study: Yamama Formation, South of Iraq

Carbonate reservoirs are significant hydrocarbon productions, but their complex and heterogeneous nature often poses challenges in maximizing oil and gas recovery. Traditional methods for enhancing petrophysical properties have shown limited success in carbonate formations. However, new advancements in Plasma Channel Technology have shown promising results in improving the reservoir's petrophysical properties. The application of Plasma Channel Technology is considered as a novel approach to enhance the petrophysical properties of carbonate reservoirs. Plasma channel technology involves the controlled application of high-voltage electrical discharges to create conductive channels (new porosity) within the reservoir units. These channels serve as preferential pathways for fluid flow (permeability) and facilitate enhanced oil and gas recovery. The main objective of this study is to show the key aspects of the enhancement of the Yamama reservoir properties using Plasma Channel Technology minimizes the need for chemical additives to reduce operational costs and environmental impact. Therefore, Yamama Formation is studied in two oil wells Snd-1 and Snd-2, six core samples from reservoir units within the Yamama succession were studied to determine porosity using a Scanning Electron Microscope which provides valuable insights into their internal structure and porosity characteristics in these samples. A comparison had been done between porosities within rock samples before and after application of Plasma channel technology, where the porosity was ranging from micropores (10 nm) to mesopores (10-50 nm) in some samples before voltage electrical discharges, while it reached mesopores (20-50 nm) to macropores (more than 500 nm) after voltage electrical discharges.These pores have been increased in size if there was higher voltage which means enhancement in the petrophysical properties in the reservoir.

Reinterpretation of Salodik Group Performance Based on Facies and Diagenesis Approaches to Classify Reservoir Quality

An integrated study of several methods to characterize a carbonate reservoir provides a comprehensive result on carbonate facies and diagenesis. Therefore, a study on the Tertiary carbonate reservoir at the East Arm of Sulawesi area, Banggai – Sula Basin, is intended to determine the Salodik Group carbonate reservoir facies and diagenesis. Moreover, the purpose of this study is to determine the intensity and implication of syn-depositional or post-depositional processes, and to provide the reservoir final properties values, such as porosity and permeability to classify the reservoir quality. The methods of the study are based on carbonate fieldwork mapping to obtain samples on distributed traverses. Further analyses are the laboratory works to determine facies and diagenesis features on a more detailed scale. This study shows several facies, dominantly classified into reefal packstone, planktonic foraminifera packstone-wackestone, large foram packstone-mudstone, skeletal packstone-wackestone, planktonic-large foraminifera packstone-wackestone, and large foraminifera-red algae packstone. The dominant diagenesis processes are cementation and dissolution. Moreover, meteoric vadose and meteoric phreatic are predominantly the main diagenesis environment of Salodik Group formations. Furthermore, the dominant diagenesis stage is Eogenetic. The post-depositional process or the diagenetic processes, provides a more significant impact on carbonate reservoir properties than the syn-depositional process as shown on lithofacies texture. However, in general, from the younger to the oldest formation of Salodik Group, this shows a decreasing trend of reservoir properties due to the dominantcementation process in the oldest formation. Overall, the reservoir quality referring to porosity value is classified as negligible to excellent. While, based on permeability data, it is classified as the tight to good reservoir.

Time-calibration of carbonate diagenesis and regional tectonism in the Norwegian Barents Sea

Diagenesis plays a crucial role in carbonate reservoir properties, for example through the dissolution or precipitation of carbonate minerals, with burial history and fluid migration thought to play an important role in the timing of these events. To better understand these relationships and the local manifestation of regional events, we study carbonate sedimentary rocks and associated diagenetic cements from the Loppa High and Finnmark Platform using in-situ U–Pb carbonate geochronology and C–O stable isotope ratios, combined with burial history modelling. The results indicate a complex history of diagenesis: analyzed dolomicrite samples from the Loppa High typically yield ages that are older than their biostratigraphic age, in contrast to dolomicrite samples from the Finnmark Platform that yielded younger ages; this regional offset may be reflective of different styles of early diagenesis, as well as heterogeneous and re-deposited origin of some studied materials. While many diagenetic calcite cements coincide with modelled burial or uplift events, other events have no cements associated with them, although the possibility that some diagenetic carbonate phases were unsampled cannot be ruled out. Some calcite cements are not associated with burial events at all and may instead be related to hydrocarbon ‘charging’, supported by strongly negative δ13C values recorded in these cements. Broadly, these results highlight the value of integrating petrographic observations, burial history modelling, carbonate U–Pb geochronology, and C–O isotope ratios, as well as the complexity of untangling diagenetic histories.

Structural and seismic attribute analysis of the Paleocene carbonate reservoir from the Balkassar Field, Potwar Plateau, Pakistan

The structural and seismic properties of the Paleocene carbonate reservoir are re-evaluated, using 3D seismic data multi-attributes, to improve reservoir prediction accuracy rarely documented earlier. This study highlights a number of relevant post-stack seismic attributes for understanding the structural setup and seismic properties of a carbonate reservoir. The structural analysis was carried out using seismic interpretation, combined with multi-attribute analysis for prospect actualization and generation of a 3D structural model to determine the structural aspect. The structural interpretation reveals the presence of faults and an asymmetrical anticlinal structure demonstrating a four-way closure, confirming compressional tectonics. The high amplitude anomaly (-6 to 12) combined with the RMS attribute (23 to 32) suggests bright spots in the Paleocene carbonate reservoir and gas saturated zone, the variance attribute (0.05 to 0.03) highlights the presence of two major faults, and the lower instantaneous frequency demonstrates the presence of hydrocarbons. The attribute results indicate that the Lockhart Limestone possesses good reservoir potential as revealed by DHI (bright spot), gas saturated zone and a trapping mechanism supported by four-way structure closure in the region and can help to understand analogous carbonate reservoirs in similar geological settings around the world.

WITHDRAWN: The impact of stylolitization on the elastic properties of carbonate reservoirs using laboratory measurements

Stylolitization is one of the most important sedimentary features occurring during diagenetic processes in carbonate rocks. This study experimentally explored how stylolitization and stylolite types affect the elastic behavior of carbonate rocks using hydrostatic tests and ultrasonic measurements. Moreover, petrophysical and well data, including logs (image logs, density, porosity, compressional sonic wave, and shear sonic wave), and Computed Tomography (CT) scans have been used for the selection of appropriate plugs and stylolite classification based on their morphological properties. The observed stylolites were subdivided into four groups: rectangular, seismogram, suture (sharp), and wave-like. To develop new insight into the relationship between stylolites and elastic properties, hydrostatic laboratory tests were conducted under two different saturation conditions: drained (zero pore pressure) and undrained (constant pore pressure). Our results indicate that stylolite-free samples have higher porosity, which suggests that they may have lower velocities. Furthermore, we observed that the stylolitified samples had a higher gradient of VP changes compared with the VS changes, with a strong dependency on the stress state variations (confining and effective pressures). These samples have more potential to contain crack porosities than stylolite-free samples, and stylolites may act as weak surfaces. These weak surfaces can cause different velocity behaviors in the stylolite-bearing samples at low and high pressures (room and reservoir pressures). Our findings indicate that the opening or filling material within the stylolites may have contributed to the increase or decrease in the sonic velocities. If the stylolites are open, then their behaviors are similar to those of crack porosities and can decrease the velocities. However, the filled stylolites showed a positive effect on the velocities depending on the filling materials, which were dense. Finally, this study presents a mechanical anisotropy analysis to identify the origin and type of stylolite.

A new insight on relative permeability modifier as conformance control in carbonate reservoir

The primary issue of high-water cuts in exploiting carbonate fields poses significant challenges and requires an immediate solution. The approach to tackle the root cause of this problem is reservoir conformance control. Conventional conformance control treatments, such as polymer-based materials, are ineffective and incompatible with the unique properties of carbonate reservoirs. Textural heterogeneity in carbonate leads to heterogeneity in the distribution of storage and flow properties that may govern changes in saturation patterns. The percolation of gas-water flow in carbonate highly depends on the pore structure. Conformance control refers to managing fluid movement within a reservoir to optimize production and reduce unwanted fluid production. The major challenge of water conformance control for carbonates using chemical treatment is the compatibility of the polymer system with the rock and fluid settings. This paper reviews current research on conformance control methods, including chemical and mechanical techniques. The effectiveness of various conformance control methods was evaluated based on their ability to improve sweep efficiency, reduce water production, and increase hydrocarbon recovery in different reservoir types. The review also discusses the challenges of implementing conformance control techniques, such as formation damage and fluid mobility issues. Overall, the paper concludes that successful implementation of conformance control strategies requires a thorough understanding of reservoir characteristics and planning to ensure optimal results. The findings of this study provide valuable insights into developing effective conformance control techniques that can improve hydrocarbon recovery and optimize reservoir performance. The results of this study provide critical insight into using efficient and operationally low-risk materials to reduce water cuts in the strong water drive carbonate reservoir.

Geo-Body and Geostatistical Modelling of Carbonate Reservoir Facies Architecture and Characterization

Carbonate reservoirs present significant challenges in characterizing and extracting hydrocarbons due to their low permeability, matrix heterogeneities, fractures, and dissolution patterns. Accurately predicting the facies architecture and reservoir properties in such complex formations has been a persistent challenge for geoscientists. This paper proposes an integrated approach that combines geo-body extraction and geostatistical modeling to accurately predict the facies architecture and reservoir properties in carbonate reservoirs. The methodology begins by generating 3D seismic root mean square amplitude (RMS) attributes, which are then used to extract geo-bodies along the pay sequences. The extracted geo-bodies are then subjected to geostatistical modeling to analyze reservoir properties to facilitate the optimization of drilling and production strategies. To validate the effectiveness of the proposed approach, a small field in the Mumbai offshore basin is chosen as a case study. This field is located on the Mumbai High-Deep Continental Shelf and exhibits westerly dipping structures. Structural mapping confirms the presence of an antiformal structure, with one particular well (D-8) at the crest showing the absence of hydrocarbons. The proposed approach mapped two seismic reflectors within the reservoir zones and generated window-based 3D seismic RMS attributes to extract three geo-bodies within the reservoir. Facies and property modeling revealed the presence of distinct non-reservoir facies with poor reservoir properties near dry wells (D-8, D-4, and D-7), which is in line with the production performance observed in the drilled wells. The proposed integrated approach of geo-body extraction and geostatistical modeling is effective in delineating the facies architecture and reservoir heterogeneity of carbonate reservoirs. It enables the identification of favorable reservoir facies and facilitates a comprehensive assessment of the remaining potential.

Multi-Well Characterisation of Complex Carbonate Reservoirs Using S-Wave Impedance and P-Wave Impedance: Mission Canyon Carbonates (Mississippian), Williston Basin, US, as a Test Case

The complexity of carbonate reservoirs is, in part, caused by the multiple rock-types and porosity types that may occur in any carbonate reservoir. Consequently, a persistent concern has been how to derive accurate quantitative estimates of carbonate reservoir properties from borehole and seismic data. Recent developments in this pursuit involve combined use of S-wave and P-wave data, together with petrophysical definitions of their impedances in a manner that lends them geologic character and significance. S-wave impedance and P-wave impedance are linearly related with their respective velocities. Both relationships are rock-typing criteria since their slope and intercept terms consist of geology-dependent coefficients. Linear-linear crossplots of S-wave impedance versus S-wave velocity and P-wave impedance versus P-wave velocity, using core scale measurements in four wells, support the linearity, pattern of data points, and character of the lines predicted for brine and air/gas pore fluids. The slopes and intercepts obtained by the method of linear regression analysis also show values that are location-specific, as well as exhibit variabilities that are indicative of reservoir heterogeneities from one well to another. These results show strong potential in using impedance-velocity relationships for quantitative interpretations of well logs and reflection seismic data.

Study of void space structure and its influence on carbonate reservoir properties: X-ray microtomography, electron microscopy, and well testing

Many carbonate reservoirs containing significant hydrocarbon resources are characterized by complex pore structures and, as a result, heterogeneous distributions of fluids in their pore networks. Accurately assessing the microscopic pore structure characteristics of carbonate rocks and their influence on reservoir properties is essential for the successful development of oil fields. In the studied carbonate reservoirs of three oil fields in the Perm Krai, Russia, complex tectonic activity and multi-stage diagenetic modification have resulted in the formation of a heterogeneous pore network spectrum ranging from macropores to nanometer-scale (<1 μm) pores. Thin sections and computed tomography were applied to obtain the petrography of carbonate rocks, 2D images of a wide range of pores, and a 3D representation of the pore network. Well testing was used to study the pore network structure at the macrolevel. Combining well testing and laboratory-based core studies allowed the structural features of the reservoirs of the three fields under consideration to be characterized in high detail. The presence of magnesium in the mineral composition of the studied limestone decreased its capacitive characteristics. We identify three fundamentally different void space scenarios: (i) void spaces formed only by primary intercrystalline pores, (ii) much larger secondary pores are also present, and (iii) primary and secondary voids connected by a network of partially healed fractures. Such variations in the structure of void space contribute to varying reservoir behavior. The presence of larger voids and, accordingly, higher initial permeability contributes to intense deformation of the void space and permeability decreases with decreasing pressure. The established patterns in this work explain the characteristic dynamics of well flow rates during field development.

Control on carbonate reservoir properties along a shallowing-upward sequence: The middle oxfordian jura inner platform predictive model

The study of the Bonnevaux section from the Middle Oxfordian Jura inner platform (France) highlights that a shallowing-upward sequence showing a large variability of original microfacies linked to various depositional environments (open sea to lagoon) can be exclusively composed of microporous limestones. A spatial evolution of reservoir qualities and transport properties of these microporous limestones is recognized at the platform scale: a correlated increase in total porosity (from 2.8 to 21.3%) and permeability (from 0.0001 to 16 mD) and a decrease in trapped porosity (from 100 to ∼50%) is observed from the upper offshore to the back-shoal lagoon environments. The increase in permeability observed at the platform scale is correlated with an evolution of the unimodal distribution of pore access radii and an increase in threshold radius (from 0.033 to 0.570 μm). It results from a diachronism in the control of meteoric pore-fluid flows by the microporosity (vs macroporosity) during the burial diagenesis. A later control of meteoric pore-fluid flows by the microporosity related to differences in cementation (1) reduces the fluid-rock interactions in the micrite areas and consequently promotes open fabrics with larger pore access radii and (2) allows to preserve larger pore access radii in the intercrystalline pore network of cements. In the case of reservoir qualities, this later control (1) promotes a higher total porosity in the micrite areas in relation with more open fabrics and (2) allows to preserve larger pore access inducing a lower trapped porosity. The macroporosity can played a key role when the blocky cementation of the pore space is partial. This study also reveals that a submarine lithification can prevent the petrophysical impact of a pore-fluid meteoric input during the burial diagenesis.

Controls of fracturing on porosity in pre-salt carbonate reservoirs

This work aims to improve the understanding of how fracture zones affect carbonate reservoir properties based on observations of a pre-salt well located in the Santos Basin, Brazil. The identification of fracture zones allowed for the observation of a relationship between the occurrence of rock fractures and the silicification, as the latter plays an important role in determining porosity (higher silica content may increase brittleness of the rocks therefore increasing the likelihood of creating fractures zones and fractures may be filled up reducing the total porosity). To support the proposed observation, an integrated study was conducted using borehole imaging, spectroscopy logs, and sidewall core samples. The porosities were defined using nuclear magnetic resonance log analysis, alongside sidewall core samples, and thin sections. The integration of rock samples and well data with seismic analysis was performed to analyze the presence of a regional fault system that could explain high fracture densities as well as observed silica content characteristics. The results show how different types of cement filling up the formation pores affect fracture densities and total porosity. Furthermore, it was possible to infer that the amount of silica content observed in well logs and thin sections relates to hydrothermal fluids reaching out the reservoir through regional fault systems detected in the seismic section. Therefore, this paper supports the comprehension of how diagenetic processes can significantly affect the properties of pre-salt reservoirs.

Well - test analysis for wells with gas rich CO2 in carbonate reservoir Song Hong Basin

The evaluations of properties of carbonate reservoirs containing gas with very high carbon dioxide (CO2) content often face many difficulties. In fact, there is a significal difference in interpretation results compared to conventional gas well testing due to: there is a gas to liquid phase transition; noise, abnormal increase or decrease in pressure and temperature during flow period, etc. These abnormal behaviors are all directly related to the specificity of CO2 properties with high compressible than that of natural gas, which can vary greatly density and can reach supercritical liquid state, transitioning solid to liquid or liqid to gaseous states even when pressure and temperature changes within a small range. During the testing of gas wells with high CO2 content, the phenomenon of pressure drop while pressure buid-up often occurs, leading to the analysis of reservoir parameters according to the conventional pressure build-up analysis method usually not feasible. The content of the article is aimed at researching and applying the method of interpreting well test data in the drawdown period while multi-rate flowing to more accurately estimate reservoir properties and mitigate risks in the design of the field development plan to achieve the economic efficiency.

A qualitative-quantitative approach for studying the impact of facies and diagenesis control on the rudist biostrome of the Sarvak formation, Abadan plain, SW Iran

Carbonate reservoir properties and their heterogeneities are of the main issues in reservoir qualitative and quantitative studies. In this paper, the role of facies and diagenesis processes on reservoir quality in the Cenomanian-Early Turonian Sarvak formation of SW of Iran was studied using 1504 samples from the core and a set of full suite logs from three wells. The depositional setting ranges from grain-rich reefal to lagoonal facies with grain-dominated and mud-dominated textures. Several diagenetic realms such as meteoric vadose, phreatic (vadose and marine) and mixing were identified based on petrographic observations. Compared to the mud-dominated facies of the Sarvak formation, the grain-dominated facies were intensely subjected to diagenetic processes. Furthermore, dissolution and neomorphism resulting from the subaerial exposure during meteoric diagenesis enhanced the reservoir quality, while further cementation, compaction and micritization have a negative impact on reservoir characteristics. The quantitative approaches show the eustatic sea-level rise in the Cenomanian-Turonian and the regional Turonian unconformity provide a textural and diagenetic pathway towards enhancing/reducing the reservoir quality. Reservoir properties are highly dependent on the sedimentary processes of the Rudist biostromes where secondary diagenetic agents are probably contributed through the selective leaching of aragonitic grains and matrix under the meteoric water environment.

Control on Carbonate Reservoir Properties Along a Shallowing-Upward Sequence: The Middle Oxfordian Jura Inner Platform Predictive Model

Control on Carbonate Reservoir Properties Along a Shallowing-Upward Sequence: The Middle Oxfordian Jura Inner Platform Predictive Model

A novel triple-porosity model for fractured-vuggy reservoirs based on Maxwell-Garnett mixing rule

Under the new energy pattern, the petrophysical properties of carbonate reservoirs are still the research focus in the petroleum industry. Compared with conventional reservoirs, the calculation of water saturation is more complicated for carbonate reservoirs. One of the main challenges is that the accurate calculation of the porosity exponent of the reservoirs is influenced by the development of fractures and vugs. The existing triple-porosity model provides a way to calculate the porosity exponents; however, the existing model still has some problems: (1) The conduction theory adopted to establish the model varies from the actual conductivity process of the reservoir; (2) The model is deduced from the effective medium theory and regional empirical formula without considering the fracture shapes, thus it is not applicable for the carbonate reservoirs with fractures developed. Therefore, this paper proposed a novel triple-porosity model to calculate the porosity exponents by applying the Maxwell-Garnett mixing rule to the fractured-vuggy reservoir. This novel model can calculate the porosity exponents of reservoirs with the joint development of the vugs (non-connected vugs), fractures (fractures and connected vugs), and matrix pores. From the results of the novel model, the more developed the vugs, the greater the porosity exponents, and the more developed the fractures, the smaller the porosity exponent. Therefore, the porosity exponents of the reservoir can reflect the combined influence of the vugs, fractures and matrix pores. In order to effectively calculate the water saturation through this model, a method based on the binary processing of the Full-bore Formation Micro-Imager (FMI) logs was further proposed to calculate the vug porosity and fracture porosity. The field application demonstrated that the novel triple-porosity model can accurately calculate the water saturation of the fractured-vuggy reservoirs and provide guidance to the actual production.

An integrated approach to distribute carbonate reservoir properties, using lithofacies and seismic attributes: a case study from SW of Iran

Complexity and heterogeneity of carbonate deposits make the understanding of the reservoir quality challenging. This study is a contribution to the understanding of the distribution of carbonate–evaporite reservoir property of the Permo–Triassic Kangan–Dalan succession in one of the giant gas fields in the southwest of Iran. To capture and describe the complexity and heterogeneity of the carbonate reservoir, we defined different rock properties approach to the management of quantitative data on carbonate reservoir which is based on well and seismic data with an integrated sedimentological investigation, petrophysical evaluation, and seismic analysis to determine the new reservoir lithofacies classification. A lithofacies-based reservoir characterization study was conducted carbonate reservoir to build a numerical 3-dimensional (3D) geological model including reservoir property prediction for reservoir flow simulation. To do this, full suite of logs from six wells, core from four wells in the studied interval and 3D seismic volume across the studied interval have been involved and investigated. Five lithofacies were identified from detailed analysis, and the relationship between lithofacies and reservoir property was considered and geostatistically propagated in the studied area. This study shows that an integrated study using seismic data and well logs can be used to basically divide the reservoir into relatively homogeneous zones based on core and petrophysical analysis, and then evaluate each zone to distinguish high-quality zones. This approach provides useful insight to Permo–Triassic carbonate reservoir and other similar reservoirs and can also be effectively used in areas which lack well or outcrop data.

Petrophysical properties variation of bitumen-bearing carbonates at increasing temperatures from laboratory to model

Variations in reservoir seismic properties can be correlated to changes in saturated-fluid properties. Thus, the determination of variation in petrophysical properties of carbonate-bearing rocks is of interest to the oil exploration industry because unconventional oils, such as bitumen (HHC), are emerging as an alternative hydrocarbon reserve. We have investigated the temperature effects on laboratory seismic wave velocities of HHC-bearing carbonate rocks belonging to the Bolognano Formation (Majella Mountain, central Italy), which can be defined as a natural laboratory to study carbonate reservoir properties. We conduct an initial characterization in terms of porosity and density for the carbonate-bearing samples and then density and viscosity measurements for the residual HHC, extracted by HCl dissolution of the hosting rock. Acoustic wave velocities are recorded from ambient temperature to 90°C. Our acoustic velocity data point out an inverse relationship with temperature, and compressional (P) and shear (S) wave velocities show a distinct trend with increasing temperature depending on the amount of HHC content. Indeed, samples with the highest HHC content show a larger gradient of velocity changes in the temperature range of approximately 50°C–60°C, suggesting that the bitumen can be in a fluid state. Conversely, below approximately 50°C, the velocity gradient is lower because, at this temperature, bitumen can change its phase in a solid state. We also propose a theoretical model to predict the P-wave velocity change at different initial porosities for HHC-saturated samples suggesting that the velocity change mainly is related to the absolute volume of HHC.

Characterization and probabilistic estimation of tight carbonate reservoir properties using quantitative geophysical approach: a case study from a mature gas field in the Middle Indus Basin of Pakistan

In this study a tight carbonate gas reservoir of early Eocene (S1 formation) is studiedfor litho-facies estimation and probabilistic estimation of reservoir properties predictionusing quantitative geophysical approach from a mature gas field in the Middle IndusBasin, onshore Pakistan. Quantitative seismic reservoir characterization approachrelied on well based litho-facies re-classification, Amplitude Variation with Offset (AVO)attributes analysis and Pre-Stack simultaneous inversion attributes constrained withcustomized well-log and seismic data (gathers) conditioning. Three main litho-facies(hydrocarbon bearing limestone, tight limestone and shale) are classified estimatedbased on the precise analysis of well data using petrophysical properties. AVOattributes (intercept and gradient) conveniently inspection for amplitude behavior(reflection coefficients) of the possible AVO (class I), fluids and lithologycharacteristics. Probable litho-facies (tight limestone and shale) are estimated usingwell based litho-facies classification and inverted seismic attributes (p-impedance anddensity) from pre-stack simultaneous inversion in a Bayesian framework. Additionally,petrophysical properties (clay volume and porosity) are derived from probabilisticneural network approach using well logs and pre-stack inverted attributes (pimpedanceand density) constrained with sample-based seismic attributes(instantaneous, windowed frequency, filters, derivatives, integrated and time).

Lower Permian Carbonate Buildups in the Northern Timan–Pechora Basin as the Main Hydrocarbon Exploration Object

Organogenic buildups named as carbonate mounds are widespread in Lower Permian deposits in the northern Timan–Pechora basin and are of great interest to the oil-and-gas industry as natural hydrocarbon reservoirs. Their distribution and textural features were controlled by seabed morphology, sea level changes, and specific paleoecology of the Early Permian period. In contrast to classic reefs, these buildups were not able to form large accumulative landforms that changed the hydrodynamics and paleogeography of sedimentary basins. The results of this study allowed us to divide the Lower Permian carbonate buildups into two types: (1) skeletal mounds formed primarily by algae and Palaeoaplysina communities; (2) microbial mounds composed of microbial assemblages. Identification of these mounds based on seismic data usually does not meet any difficulties. However, their texture, peculiarities of reservoir distribution, and variations of their properties are often unreflected in the characteristics of seismic waves due to limitations of the seismic survey resolution. The sedimentation modeling describes spatiotemporal relationships of the identified lithofacies characterized by different reservoir potential. Application of these results during the seismic interpretation increases significantly reliability of the forecast of carbonate reservoir properties.

Effects of micrite microtextures on the elastic and petrophysical properties of carbonate reservoirs

Apparent differences in sedimentation and diagenesis exist between carbonate reservoirs in different areas and affect their petrophysical and elastic properties. To elucidate the relevant mechanism, we study and analyze the characteristics of rock microstructure and elastic properties of carbonates and their variation regularity using 89 carbonate samples from the different areas The results show that the overall variation regularities of the physical and elastic properties of the carbonate rocks are controlled by the microtextures of the microcrystalline calcite, whereas the traditional classification of rock- and pore- structures is no longer applicable. The micrite microtextures can be divided, with respect to their morphological features, into porous micrite, compact micrite, and tight micrite. As the micrites evolves from the first to the last type, crystal boundaries are observed with increasingly close coalescence, the micritic intercrystalline porosity and pore-throat radius gradually decrease; meanwhile, the rigidity of the calcite microcrystalline particle boundary and elastic homogeneity are enhanced. As a result, the seismic elastic characteristics, such as permeability and velocity of samples, show a general trend of decreasing with the increase of porosity. For low-porosity rock samples (φ 5%) dominated by tight micrite, the micritic pores have limited contributions to porosity and permeability and the micrite elastic properties are similar to those of the rock matrix. In such cases, the macroscopic physical and elastic properties are more susceptible to the formation of cracks and dissolution pores, but these features are controlled by the pore structure. The pore aspect ratio can be used as a good indication of pore types. The bulk modulus aspect ratio for dissolution pores is greater than 0.2, whereas that of the intergranular pores ranges from 0.1 to 0.2. The porous and compact micrites are observed to have a bulk modulus aspect ratio less than 0.1, whereas the ratio of the tight micrite approaches 0.2.