High-quality Reservoirs Research Articles

Experimental research on the influence of heterogeneous distribution of oil and water on calcite cementation from external sources during multiple oil emplacements in clastic reservoirs

The influence of oil emplacement on calcite cementation from external sources is a topic of significant concern that is surrounded by controversy. Microscopic heterogeneity of oil-water distribution is probably the crux of the controversy. However, the influence of the heterogeneous distribution of oil and water on calcite cementation has not been directly observed and demonstrated. In this study, visual physical simulation experiments of multistage oil emplacement and calcite precipitation were conducted in glass micromodels. The processes of multistage oil emplacement and calcite precipitation occurred alternately. Characteristics of heterogeneous distribution of oil and water and the calcite precipitation processes were clearly observed and recorded in real-time under a microscope during the experiments. Three important factors, including heterogeneous distribution of the sizes of pore-throat spaces, the previously charged oil, and the presence of precipitated oil-wet calcites, jointly control the heterogeneous distribution of oil and water in the pore-throat spaces. With the supply of materials from external sources, relatively rapid material advection resulted in abundant calcites precipitation in the interconnected pore-throat spaces completely filled with water. With the gradual increase in the proportion of oil to water in pores and throats, a gradual decrease occurred in the diffusion rate and relatively low-content calcites precipitated in water-bearing position in pores and water films. Calcites scarcely precipitated in the pores completely filled with oil. Experimental results are of significance for the processes of carbonate precipitation after oil emplacement in clastic reservoirs. The reservoirs with high paleo-oil saturation after first-stage oil emplacement are conducive to formation of high-quality reservoirs.

Main control factors and distribution of carbonate reservoirs in the Hexiwu structural belt

The hydrocarbon accumulation conditions of carbonate buried hills in Hexiwu structural belt are superior, and the hydrocarbon accumulation is controlled by multiple factors such as stratum, structure and reservoir, so it is the focus of oil and gas exploration to find high quality reservoirs. Through the analysis of drilling, logging and core data, it is clear that the carbonate reservoir space in Hexiwu structural belt is mainly composed of intergranular pore, tectonic dissolution fracture and tectonic fracture. Sedimentary facies, diagenesis and tectonic movement are the main controlling factors of carbonate buried hill reservoir formation. The favorable sedimentary facies belt is the foundation of reservoir development, the favorable diagenesis is the guarantee of reservoir development, and the favorable tectonic movement is the key to reservoir development. Through the stratified section to carry out comprehensive reservoir prediction, the accuracy of reservoir prediction results is significantly improved, which provides a foundation for oil and gas development.

Multistage Acid-fracturing Increases the Dolomite Production in High Temperature Deep Horizontal Well

The Sinian dolomite reservoirs in Southwest Oil and Gas Field, China, are characterized by HTHP, high stress, low porosity and permeability, large thickness, and presence of natural fractures and holes. Highly deviated well multistage acid fracturing can help expand the contacting area between wellbore and reservoir, thereby increasing the well production. Through experiments, acid fracturing simulations, staging optimization and field applications, some conclusions were obtained. Large-scale physical modelling experiments and numerical simulations show that, given the high horizontal stress difference, presence of natural fractures, can induce a complex fractures networks. The acid systems with different viscosities, such as self-generating acid, gelled acid and diverting acid, are optimized to, in conjunction with highrate injection and temporary diverting, improve the stimulated reservoir volume and conductivity of acid-fractures in the dolomite reservoirs. The combination of mechanical packers and sliding sleeves facilitate the effective isolation of HTHP dolomite reservoir. The positions of the sliding sleeves are designed according to the high-quality reservoir intervals determined by logging interpretation. The lateral of deviated well is 480m-970m long and stimulated by acid fracturing in 3-6 stages. The post-fracturing tracer testing proves the effectiveness of isolation. This technology was applied for 6 wells, with a daily gas production of (21–110) × 104m3/d after acid fracturing. It provides a new method to increase the production of highly deviated wells or horizontal wells in deep carbonate reservoirs.

Diagenetic alterations related to sedimentary architecture of deltaic distributary channels in red beds of the Cretaceous Yaojia Formation, Songliao Basin

Fine-scale diagenetic alterations linked to architectural patterns of the sandstones were studied in meter-scale deltaic distributary channels of the Upper Cretaceous Yaojia Formation, Songliao Basin. The deltaic reservoirs of the Yaojia Formation are mostly developed in three types of distributary channels: major distributary channels, crevasse channels, and terminal distributary channels. The three types of distributary channels have different sedimentary architecture and lithofacies. The distribution of reservoir intervals varies greatly in the upward fining distributary channel depositional cycles (meter-scale). From the diagenetic alterations in the distributary channels successions, compaction, calcareous cementation, grain-coating clays with hematite in the red bed sandstones, and dissolution of feldspar or rock fragments are main control factors of high-quality reservoirs. More porosity loss is generated by strong compaction near the top and bottom part of the distributary channel succession. This could be caused by the high clay content and mud clasts which contribute much to the porosity loss in the sandstones. Calcareous cementation is mainly developed near the clay-rich intercalations. Dissolution pores in the sandstones without hematite cement and grain-coating clays are well developed in the middle position of the successions. The cementation is restricted in sandstones with unique hematite cement and grain-coating clays in the reddish crevasse distributary channels. The distribution pattern of high-quality reservoirs within the three different types of distributary channels in red beds is proposed by linking the diagenetic processes to depositional environments and architecture. High-quality hydrocarbon reservoirs are mostly preserved in the massive and cross-bedded sandstones from the middle part of the major distributary channel successions.

Diagenesis and reservoir quality of deep-lacustrine sandy-debris-flow tight sandstones in Upper Triassic Yanchang Formation, Ordos Basin, China: Implications for reservoir heterogeneity and hydrocarbon accumulation

Tight sandstone in the Upper Triassic Yanchang Formation, Ordos Basin, contains large accumulations of hydrocarbon resources. This study presents an investigation of the deep-lacustrine sandy-debris-flow tight sandstones in the Yanchang Formation, based on petrographic, geochemical and petrophysical analysis. It reveals that reservoir heterogeneity and hydrocarbon accumulation of tight sandstones were affected by diagenetic alteration. These tight sandstones are dominantly lithic arkoses and feldspathic litharenites which are characterized by moderate to good sorting, fine to medium grain, subangular to subrounded and poor reservoir properties (average porosity and permeability are 10.5% and 1.23 mD, respectively). Significant diagenesis including compaction, feldspar dissolution and cementation by carbonate, quartz and authigenic clay have altered the studied tight sandstones. Compaction and carbonate cementation are the dominant destructive diagenetic processes. Carbonate cements are mainly developed at the edge of sandstone (distance to the sandstone-mudstone boundary mostly < 1 m) due to the limited transportation distance from adjacent mudstone. The relative high-quality reservoir intervals will preferentially develop in the middle of sandstone (distance to the sandstone-mudstone boundary mostly > 1 m). Feldspar dissolution has no significant impact on reservoir quality, as it results in pore space redistribution. There is little variation due to the primary intergranular pores being converted to dissolution pores and intercrystal micropores of authigenic clay. The lower level of compaction in the middle sandstone is due to cemented marginal sandstone. Moreover the preservation and redistribution of the pore space of the middle sandstones is generally due to carbonate cementation and feldspar dissolution. Different petrophysical properties between marginal and central sandstones lead to reservoir heterogeneity. Reservoir petrophysical properties display a positive correlation with oil-bearing capacity, which reveals that hydrocarbon accumulation can also be affected by reservoir property. Ultimately, tight sandstones with porosity >6%, permeability >0.1mD and thickness >2 m can be regarded as effective reservoirs in the Yanchang Formation. The results will contribute to the understanding of the origin of deep-lacustrine sandy-debris-flow tight sandstones reservoirs, which will enable further exploration for effective reservoirs in lacustrine sandy debrites across the globe, predominantly in those sharing the same geological features.

Diagenetic alteration and geochemical evolution during sandstones bleaching of deep red-bed induced by methane migration in petroliferous basins

Bleaching of red-bed sandstones is an important process related to the organic-inorganic interactions and diagenetic alteration, playing an important role in reservoir heterogeneity and carbon cycle in petroliferous basins. Distinguishing from the aeolian red-bed in the outcrop, a case study of deep red-bed of fluvial facies was conducted on the Permian Xiashihezi Formation, NE Ordos Basin, China. Four diagenetic facies can be identified as red, brown, green and white sandstones (RS, BS, GS, WS) with typical diagenetic alteration and geochemical characteristics. Thereinto, diagenetic alterations are dominated by clay minerals and carbonate cements including calcite and siderite. Carbon and oxygen isotopes showed a synergistic decreasing trend from RS, BS to WS with the increase of altered degree. The unaltered RS retained abundant early hematite, smectite and calcite, while strongly bleached WS had lowest feldspar but extensive kaolinite and illite, and manganese-rich calcite. Medium-altered BS and GS are characterized by late magnesium-rich siderite and chlorite coating respectively. The isotopic data, fluid inclusions and modeling indicated that CH4 is the important causal fluid for sandstones bleaching. Diagenetic alteration and geochemical evolution in deep red-bed of fluvial facies was influenced by upward-fining sedimentary sequence, open/closed diagenetic system and natural gas migration. The geochemical behaviors of Fe are responsible for the diversity of diagenetic facies. Fe2+ from redox between Fe3+ and CH4, integrated Mg2+ released from the transformation of smectite to illite into magnesium-rich siderite, or into chlorite in coarser sandstones in relatively open system. Extensive CH4-rich reducing fluids were oxidized to acidic fluids in the more open diagenetic system with coarser grained sandstones, resulting in the intense dissolution and the high-quality reservoirs distributed in the lower sequence of fluvial facies. Furthermore, CH4 oxidation by Fe(III) in deep red-bed can be extremely important CH4 sink and expected to potential sites of geological carbon sequestration.

Determination of Paleocurrent Directions Based on Well Logging Technology Aiming at the Lower Third Member of the Shahejie Formation in the Chezhen Depression and Its Implications

The Bohai Bay basin, mainly formed in the Cenozoic, is an important storehouse of groundwater in the North China Plain. The sedimentary deposits transported by paleocurrents often provided favorable conditions for the enrichment of modern liquid reservoirs. However, due to limited seismic and well logging data, studies focused on the macroscopic directions of paleocurrents are scarce. In this study, we obtained a series of well logging data for the sedimentary layers of Es3L Formation in the Chezhen depression. The results indicate the sources of paleocurrents from the northeast, northwest, and west to a center of subsidence in the northern Chezhen depression at that time. Based on the well testing data, the physical properties of the layers from Es3L Formation in this region were generally poor, but two abnormal overpressure zones were found at 3700–3800 m and 4100–4300 m deep intervals, suggesting potential high-quality underground liquid reservoirs. By combining with other geological evidence, we suggest that the Pacific Plate was retreating and changing its direction from NE–SE to W–E and the Bohai–Luxi block was suffering an extrusion from NE induced by the Lan–Liao and Tan–Lu strike-slip faults in the early Paleogene.

Diagenetic history and reservoir evolution of tight sandstones in the second member of the Upper Triassic Xujiahe Formation, western Sichuan Basin, China

The second member of the Upper Triassic Xujiahe Formation is an important target for tight gas exploration in the western Sichuan Basin. The diagenetic history and reservoir evolution were investigated using integrated analyses of petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), cathodoluminescence (CL), fluid inclusion microthermometry and stable carbon and oxygen isotopes. The sandstones consist mainly of litharenites, sublitharenites and feldsparthic litharenites with fine-to medium-sand grain sizes, moderate to good sorting and subangular to subrounded roundness. The reservoir properties of the sandstones are generally poor with low porosity and matrix permeability, small pore-throat radii, and high displacement pressure. Eodiagenesis is composed mainly of mechanical compaction, precipitation of siderite, stage-I quartz overgrowths, chlorite, calcite and ferroan calcite. Mesodiagenesis consists mainly of mechanical and chemical compaction, calcite and ferroan calcite precipitation, stage-II and stage-III quartz cementation, feldspar dissolution and precipitation of illite, dolomite and ankerite. Three types of sandstone lithofacies are defined according to sandstone textures and composition, diagenetic minerals, grain sizes, sorting and pore types: lithofacies I, II and III. Compaction appears to reduce the reservoir quality more significantly compared to cementation. Authigenic chlorite coats were constructive in preserving primary porosity, while carbonate cementation decreased reservoir quality. Diagenetic evolution pathways and reservoir quality prediction models for the Upper Triassic Xujiahe Formation sandstones have been proposed to shed light on the different reservoir quality evolution patterns of various sandstone lithofacies. Lithofacies I ─ medium and coarse-grained sandstones characterized by extensive chlorite coating, feldspar dissolution and a low content of clay matrix and cement, appears to have the best reservoir properties and may serve as a high-quality reservoir facies in these tight sandstone reservoirs.

Genetic mechanism of high-quality shale gas reservoirs in the Wufeng–LongmaxiFms in the Sichuan Basin

The Upper Ordovician WufengFm and the Lower Silurian LongmaxiFm are important strata for shale gas exploration and development in the Sichuan Basin, but the genetic mechanism, evolutionary history and the controlling effect of mineral diagenetic evolution on the formation of shale gas reservoirs are not clear. In this paper, the evolution history of organic matter pores and the diagenetic evolution of minerals were analyzed based on the analysis of petrology, mineralogy and organic geochemistry, combined with basin simulation and practical shale gas exploration and development. Then, the types and genetic mechanisms of high-quality shale gas reservoirs were discussed, and the development intervals of high-quality shale gas reservoirs were determined. And the following research results are obtained. First, the shale gas development intervals of Wufeng–LongmaxiFms in the Sichuan Basin are mainly dominated by siliceous shale, limy siliceous shale and clayey shale. Rock type has an important controlling effect on the types and characteristics of shale reservoir space. Siliceous shale and limy siliceous shale have the highest reservoir capacity with the most developed organic pores. Second, the diagenetic evolution of minerals controls the formation of shale gas reservoirs. Biogenic silica, formed in the early diagenetic stage, together with terrestrial detrital silica and pyrite, constitutes particle support lattices in the form of microcrystalline aggregates, so as to resist the compaction effectively and preserve a great number of residual intergranular pores, which is beneficial to the formation of high-quality shale gas reservoirs. Third, siliceous shale in the WF2–LM4 graptolite zone (from WufengFm to the bottom of LongmaxiFm) presents a high-quality reservoir genetic mechanism of “multicellular algae controlling hydrocarbon source, biogenic silica controlling framework, and co-evolution controlling a high-quality reservoir”. In conclusion, the siliceous shale and limy siliceous shale in the WF2–LM4 graptolite zones are the main development intervals of high-quality shale gas reservoirs in the Sichuan Basin. It is also a valuable reference for the Upper Ordovician-Lower Silurianshale gas exploration and development in other countries and regions worldwide.

Seismic stratigraphy analysis of main controlling factors of Lower Wuerhe Formation reservoir in Manan area

Fan delta sedimentary reservoirs developed in the Lower Wuerhe Formation of the Manan Slope in the Mahu Sag of the Jungger Basin have good prospects for oil and gas exploration and development. In order to clarify the high-quality reservoir system and the distribution range of the reservoir plane, a seismic stratigraphic interpretation of the Lower Wuerhe Formation is carried out in combination with geology, logging and 3D seismic data. The Lower Wuerhe Formation is divided into three stratigraphic series, a total of 8 four-level sequences, and 8 water body lifting cycles. Through seismic wave group and sedimentary evolution analysis, combined with pseudo-acoustic impedance amplitude and production test analysis, the high-quality oil-bearing reservoirs of the Lower Wuerhe Formation are mainly the third member of the Lower Wuerhe Formation. Oil-bearing reservoirs are mainly controlled by sedimentary facies belts caused by the ups and downs of water bodies. The well-developed oil-bearing reservoirs are developed in the fan delta plain-fan delta front-shore shallow lake sedimentary system.

Characteristics and main controlling factors of Ordovician deep subsalt reservoir in central and eastern Ordos Basin, China

In recent years, the subsalt dolomite reservoirs in the Ordovician strata of the Ordos Basin have indicated good exploration results and are expected to have a high hydrocarbon potential. This study targets the Ma 3 and Ma 4 reservoirs below the salt rock of the Ordovician in the central and eastern Ordos Basin. It investigates the reservoir characteristics and main controlling factors of high-potential reservoirs by combining core observations, cast slabs, scanning electron microscopy, physical property analysis, and mercury injection data. The results show that the Ma 3 reservoir is dominated by gypsum-bearing dolomite deposited in evaporation environments, whereas the Ma 4 reservoir is dominated by calcite dolomite under a transgressive environment. The types of Ma 4 reservoir space mainly include dissolved pores (dissolved holes), intra-crystalline pores, intra-crystalline dissolved pores, and microcracks. The types of Ma 3 reservoir space mainly include microcracks, gypsum mould pore, dissolved pores (dissolved holes), and intergranular pores. The pore structures of reservoirs are complex. The capillary pressure curves can be divided into four types: dissolved pore type, intra-crystalline (dissolved) pore type, microcrack type, and micro-porous type. Based on available information, the average porosity values of the Ma 3 and Ma 4 reservoirs are 2.1% and 2.3%, respectively, and the average permeability values are 0.19 × 10−3μm2 and 0.22 × 10−3μm2, respectively. The Central paleo-uplift, the Wushenqi secondary paleo-uplift, and the Shenmu low uplift controlled the sedimentary facies and favorable lithofacies distributions. Sedimentary microfacies control rock fabric, and the rock fabric affects the physical properties of the reservoirs. The limestone and dolomite reservoirs with gypsum and sand cutting texture have relatively good physical properties. Diagenesis is also a key factor in the development of sub-salt reservoirs. Intra-crystalline pores are mainly formed under various dolomization mechanisms. The surficial and buried diagenetic environments facilitate the formation of a large number of secondary pores, such as dissolved pores (dissolved holes), intra-crystalline dissolved pores, and gypsum mould pores. In addition, the Yanshan tectonic activity played a vital role in the development of reservoir microcracks. The microcracks can significantly improve reservoir physical properties. It is favored that the tectonic setting, rock fabric, diagenetic, and microcrack alterations collectively determine the sub-salt reservoirs' storage capacity and are the main controlling factors for forming high-quality reservoirs.

Reservoir characteristics and control factors of Tongbomiao formation in Hongqi sag

Hongqi sag is located in the north of Beiehur depression in Hailar Basin, which is a secondary structural unit in the peripheral depression of Hailar Basin, with great exploration potential. Based on SEM, conventional thin sheet, cast thin sheet, mercury pressure and physical property data, this paper discusses the reservoir characteristics, types and control factors of high quality reservoir in Tongbomiao formation, Hongqi sag, so as to provide a strong geological basis for the exploration of oil and gas in this area. The reservoir space of Hongqi sag is mainly composed of four types: primary intergranular pore, micro fracture, feldspar dissolution pore and debris dissolution pore. It is of the type of ultra low porosity and low permeability. Although Tongbomiao formation in Hongqi sag is of very low porosity and low permeability type, high quality reservoir is still developed, and its reason is mainly controlled by sedimentary environment, diagenesis and structural fracture.

Study of seismic attributes recognition method of high quality igneous rock reservoir

Aimed target area is deeply buried, complex lithology, dual media, reservoir development degree is controlled by a variety of factors, meanwhile, lateral thickness and lithofacies change rapidly, and strata formation is poor. Therefore, igneous rock reservoir has difficulty in predicting, since seismic is complicated to track trace, reservoir attribute analysis is hard to determine the time window, and inversion modeling requires sophisticated. By analyzing, the basalt in the target research area accounts for the principal component of the igneous rock, however, the igneous rocks with relatively developed reservoirs are mostly distributed in the trachyte breccia which has good productivity. The results of petrophysical study indicate that frequency-dependent AVO inversion method is an important means to identify fluid and reservoir prediction, notwithstanding it is difficult to distinguish high-quality reservoirs barely by P-wave impedance. Consequently, AVOF inversion method is appropriately proposed to identify igneous rock reservoir. Foremost, eliminating the effects of algorithm,frequency, spectrum balancing and other factors, then put the improved three-term Aki&amp;Richards frequency-dependent AVO inversion method applying to distinguish igneous reservoir fluid and lithology, for the purpose of carrying out the identification of high-quality reservoirs.

Carbon Capture &amp;amp; Storage: Rock Experimental Characterization &amp;amp; Modelling Prospective

Geological storage of carbon dioxide is one of the options of greenhouse gas mitigation as it reduces the release of CO2 to the atmosphere. Eni is a major energy company in which decarbonization plays an essential role in its vision for a future with zero net emissions. H field carbon storage project comes in line with Eni strategy of carbon neutrality by 2050. Depleted gas fields are among the most suitable candidates for CO2 sequestration, with well-known high quality reservoir characteristics and good sealing. H is a depleted offshore gas field located in the North Sea; it has two main reservoirs: H1 and H2. Due to the massive size of both reservoirs, well-known reservoirs’ characterization, good reservoirs’ petrophysics, and the existence of thick and continuous cap layers (C1 & C2), H1 and H2 reservoirs have been nominated to be good candidates for a carbon sequestration project. The study objective is to build comprehensive mineralogical and geochemical models able to capture the physical and the chemical phenomena occurring in the short – medium – long terms after the CO2 injection in H1 and H2 formations. The proposed workflow relies on the following procedure: 1. laboratory experimental analyses (SEM-EDX, XRD, XRF) on core samples 2. data elaboration and integration 3. conceptual geological-geochemical models’ definition 4. 0D static batch models a. equilibrium approach b. kinetic approach Step 1, 2 and 3 are aimed at obtaining the mineralogical model, with attention devoted to the presence of the Fe-bearing minerals. In step 4, the rock-formation water geochemical system consistence is preliminary checked by performing a sequence of 0D equilibrium models with PHREEQC-V3 code, (USGS, 1999), allowing primary evaluations on mineral phases chemical stability (dissolution/precipitation). This step is also needed for obtaining a synthetic formation water composition at the equilibrium with the mineralogical model used in the subsequent reactive transport models (RTM), and to validate new-estimated site-specific minerals thermochemical parameters. Then, the CO2 injection is preliminary simulated by means of 0D kinetic models to assess the main acting geochemical processes triggered by rock-brine-CO2 interaction. The results show that, at the conditions considered in the project, the rock petrophysical properties (e.g. porosity) alteration does not seem to be an issue. However, the main criticalities/uncertainties might be related to the Fe-bearing minerals (carbonates, micas and clays) chemical behavior in presence of CO2. As a final remark, the simulation results obtained in this work cannot be regarded as values of general validity because they depend on the characteristics assigned to each reservoir model. Nevertheless, the study provides a novel geochemical modelling procedure, which implies building a detailed geochemical conceptual model modelled by means of a complex kinetic reaction network to simulate the mineralogical alterations. The presented procedure can be followed to preliminary assess the geochemical short- and long-term risks for a given carbon storage project.

Formation and diagenetic characteristics of tight sandstones in closed to semi-closed systems: Typical example from the Permian Sulige gas field

In a closed to semi-closed diagenetic environment, the migration efficiency of diagenetic fluid is low, material exchange is weak, and the diagenetic processes of dissolution and cementation differ greatly from those in an open diagenetic environment. Previous research on closed to semi-closed diagenetic systems has focused on the formation mechanism of tight sandstones and the factors that control the formation of high-quality reservoirs. In this study, we obtained data from measured physical properties, petrographic microscope images, scanning electron microscope images, nuclear magnetic resonance, and electronic CT scans from the Permian Sulige gas field, located in the Ordos Basin, North China. We comprehensively analyzed the diagenetic characteristics, diagenetic evolution, compaction mechanism of tight sandstones in closed and semi-closed diagenetic environments, as well as the relationship between compaction and accumulation within this environment. The findings showed that sandstone densification in this area is characterized by multiple stages of siliceous and calcareous cementation. The diagenetic evolution of the Sulige field suggests that the sandstone densification process was not the result of conventional compaction/reduction of porosity and simple cementation, but rather was caused by compaction and quasi-continuous solution-cementation. The migration of organic acids in this process was instrumental for dissolution and pore development, and induced the densification of sandstone. During the dissolution process within the reservoir, the dissolving fluid was retained insitu, thus forming different cements that corresponded to particular periods. We propose that the quasi-continuous dissolution and cementation that occurred during diagenesis was the main cause of densification in the sandstone reservoirs of the Sulige field closed to semi-closed systems. The findings of our study provide a new direction for ongoing research in the field of development of high-quality reservoirs.

Influence of internal textures in fracture development in dolostones: A case study in the Mesoproterozoic Wumishan Formation in the Jizhong Depression, Bohai Bay Basin, north China

Fractures provide pore spaces for the preservation and migration of hydrocarbons and control the quality of dolomitic reservoirs. Textures caused by sedimentary and tectonic processes control the mechanical properties of dolostones and the development and distribution of fractures in them. An understanding of the influence of textures present in dolostones in the development of fractures is crucial. Therefore, massive micritic, brecciated, columnar stromatolitic, and siliceous dolostones and laminites present in the early Mesoproterozoic Wumishan Formation (between ca. 1.5 Ga and ca. 1.45 Ga) in the Jizhong Depression with varying internal textures were analyzed based on observations of outcrops, core, and microstructures to determine their influence on reservoir quality. The development characteristics of fractures in the dolostones were investigated, and their petrophysical properties were measured. Uniaxial and triaxial compression tests were utilized to explore the impacts of textures on the distribution patterns of fracture in the dolostones. The fracture density, porosity, and permeability are 5.4/m, 1.55%, and 17.10 mD in the homogeneous and massive micritic dolostone, with fracture oriented parallel or at an acute angle to the maximum principal stress direction. The fracture density, porosity, and permeability, respectively, are characterized by averages of 8.9/m, 6.39%, and 106.68 mD, and are significantly better in the brecciated dolostones, and the fractures develop along the preexisting weakly cemented planes between the brecciated textures. The fracture density of the siliceous dolostone is 9.8/m, and its porosity and permeability are 2.74% and 29.68 mD. The extension of high-angle fractures is usually restrained by the low-angle bedding fractures, and intensive microcracks are developed in the siliceous textures. The occurrence of algal laminae in dolostones increases the proportion of low-angle fractures; these fractures can extend along the maximum principal stress direction and rupture along the weak algal laminae. The fracture density and petrophysical properties of columnar stromatolitic dolostones (7.4/m, 4.39%, and 87.75 mD) are better than those of laminites (4.3/m, 1.21%, and 14.57 mD). Therefore, the formative environment of textures in dolostones controls the quality of the reservoirs in the Wumishan Formation. The reservoirs in brecciated dolostones that are located in structural highs and fracture zones and columnar stromatolitic dolostones deposited in high-energy intertidal to upper subtidal zones have the highest reservoir quality. The quality of the reservoirs in siliceous banded dolostones deposited in mid-energy intertidal zones is average, and that of massive micritic dolostones and laminites deposited in low-energy lower-subtidal zones is poor.

Permeability and porosity prediction using logging data in a heterogeneous dolomite reservoir: An integrated approach

The accurate prediction of permeability and porosity is an important foundation for high-quality reservoir identification and geological modelling. However, the strong heterogeneity, complex lithology and diagenesis in carbonates have brought great challenges to the accurate evaluation of reservoir permeability and porosity. In this study, 253 core samples with logging data from the Lower Cambrian dolomite reservoir in the Tarim Basin were collected to establish reliable methods for permeability and porosity prediction. Five typical permeability-porosity correlations and six machine learning methods were applied to the core data and logging data to evaluate the applicability or prediction performance of different methods. By comparison, the flow zone indicator (FZI) and FZI* combined with discrete rock type (DRT) analysis in the five conventional models had a stronger ability to classify samples, and each DRT/DRT* had a special permeability-porosity relationship. Hence, an integrated indirect permeability prediction method was developed by combining the petrophysical rock typing methods (FZI or FZI*) with the PSO-SVM algorithm that hybridized the particle swarm optimization (PSO) and support vector machines (SVM). Compared with the direct machine learning prediction methods, the proposed integrated approach greatly improved the permeability prediction accuracy with the highest R2 of 0.869, indicating that the combination of the conventional permeability model and machine learning algorithm had the potential to reduce the influence of heterogeneity on permeability prediction. The superior performance of the integrated approach in permeability and porosity prediction lays a good theoretical foundation for the identification of high-quality dolomite reservoirs.

E&amp;P Notes (December 2020)

E&amp;P Notes (December 2020)

Using characteristics of pressure-temperature induced permeability variation of typical carbonate rock to determine its performance as reservoir or cap rock

The current research on carbonate rock often focuses on the storage space for oil and gas, and exploration of carbonate rock in petroliferous basins also mainly searches for high-quality reservoir rock for hydrocarbons. However, the importance of effective carbonate reservoir-cap rock configuration is increasingly highlighted, as hydrocarbon exploration advances towards deeper and older strata. Exploration practice shows that the favorable facies belt of carbonate rock, though deeply buried, still develops highly desirable reservoir rock with high porosity and permeability. Therefore, the presence of an effective carbonate cap rock collaborating with this reservoir rock becomes key to hydrocarbon exploration in deep carbonate rock. This research, for the first time, systematically and thoroughly carries out permeability measurement through the whole triaxial compression process of typical carbonate rock at varied temperatures, and captures the permeability evolution during deformation and failure, affected by temperatures and confining pressures. Test results show that typical dolomite and limestone present varied permeability variation characteristics in a triaxial loading state. Dolomite appears to be more of reservoir rock. It presents higher critical confining pressure for brittle–ductile transition, and is more prone to fracturing. Its permeability grows with temperature elevation. As for limestone, it has better performance as cap rock, which is embodied as the lower critical confining pressure for brittle–ductile transition, high sealing capacity, and permeability reduction with elevated temperatures. To sum up, vertical stacking of dolomite and limestone can form an effective reservoir-cap rock assemblage for hydrocarbon accumulation.

Quantitative evaluation methods of tight reservoirs based on multi-feature fusion: A case study of the fourth member of Shahejie Formation in Liaohe Depression

The formation and development of tight oil reservoirs in the fourth member of the Shahejie Formation (Es4) in the Leijia region of the western sag of the Liaohe Depression are highly heterogeneous and influenced by several factors. The relationship between these variables is extremely complex. In order to avoid deviations caused by single factors and logging data for less drilling when regional tight oil reservoirs have been assessed, quantitative evaluation methods for tight reservoirs based on multi-feature fusion have been proposed. Hand-drawn maps of brittleness index, carbonate rocks thickness, fracture density, mean permeability, and mean porosity are gridded by the Kriging model accompanied by k-means clustering to determine the types of reservoir and classification interval. After that, two quantitative evaluation methods, the overlapping favourability method (OFM) and the principal component analysis (PCA), were used to analyze the five influence factors of the tight oil reservoirs. Finally, reservoir evaluation maps were generated by dividing the interval values into classes. Results show that the weights of the five variables calculated by the hybrid weight method (HWM) are 0.1100,0.4929, 0.0803, 0.2889, and 0.0279. The reservoirs in the study area are finally divided into four types, with the best reservoir types primarily distributed in well areas, such as A99, A82, A91, A78, and A9. At the same time, the main controlling factors for the creation of the reservoir have been analyzed; however, the thickness of carbonate rock in the study area plays a decisive role in the development of high-quality reservoirs. The brittleness index and fracture density can significantly improve the efficiency of the reservoir. Quantitative approaches can effectively and scientifically evaluate the tight reservoirs, which can be widely used.