Utilization mechanism of foam flooding and distribution situation of residual oil in fractured-vuggy carbonate reservoirs

The development of fractured-vuggy carbonate reservoirs is extremely difficult because of the complex fractured-vuggy structure and strong heterogeneity.Foam flooding is a potential enhanced oil recovery(EOR)technology in fractured-vuggy carbonate reservoirs.Based on the similarity criterion,three types of 2D visual physical models of the fractured-vuggy structure were made by laser ablation technique,and a 3D visual physical model of the fractured-vuggy reservoir was made by 3D printing technology.Then the physical analog experiments of foam flooding were carried out in these models.The experimental results show that foam can effectively improve the mobility ratio,control the flow velocity of the fluid in different directions,and sweep complex fracture networks.The effect of foam flooding in fractures can be improved by increasing foam strength and enhancing foam stability.The effect of foam flooding in vugs can be improved by reducing the density of the foam and the interfacial tension between foam and oil.Three types of microscopic residual oil and three types of macroscopic residual oil can be displaced by foam flooding.This study verifies the EOR of foam flooding in the fractured-vuggy reservoir and provides theoretical support for the application of foam flooding in fractured-vuggy reservoirs.

Lost circulation material for abnormally high temperature and pressure fractured-vuggy carbonate reservoirs in Tazhong block, Tarim Basin, NW China

To effectively solve the problem of lost circulation and well kick frequently occurring during the drilling of abnormally high temperature and pressure fractured-vuggy reservoirs in the Tazhong block, a rigid particle material, GZD, with high temperature tolerance, high rigidity(> 8 MPa) and low abrasiveness has been selected based on geological characteristics of the theft zones in the reservoirs. Through static pressure sealing experiments, its dosage when used alone and when used in combination with lignin fiber, elastic material SQD-98 and calcium carbonate were optimized, and the formula of a new type(SXM-I) of compound lost circulation material with high temperature tolerance and high strength was formed. Its performance was evaluated by compatibility test, static sealing experiment and sand bed plugging experiment. The test results show that it has good compatibility with drilling fluid used commonly and is able to plug fractures and vugs, the sealed fractures are able to withstand the static pressure of more than 9 MPa and the cumulative leakage is 13.4 mL. The mud filtrate invasion depth is only 2.5 cm in 30 min when the sand bed is made of particles with sizes between 10 mesh and 20 mesh. Overall, with good sealing property and high temperature and high pressure tolerance, the lost circulation material provides strong technical support for the safety drilling in the block.

Inter-well interferences and their influencing factors during water flooding in fractured-vuggy carbonate reservoirs

Based on the characteristics of injection-production units in fractured-vuggy carbonate reservoirs,nine groups of experiments were designed and performed to analyze the interference characteristics and their influencing factors during water flooding.Based on percolation theory,an inversion model for simulating waterflooding interferences was proposed to study the influence laws of different factors on interference characteristics.The results show that well spacing,permeability ratio,cave size,and cave location all affect the interference characteristics of water flooding.When the cave is located in high permeability fractures,or in the small well spacing direction,or close to the producer in an injection-production unit,the effects of water flooding are much better.When the large cave is located in the high-permeability or small well spacing direction,the well in the direction with lower permeability or smaller well spacing will see water breakthrough earlier.When the cave is in the higher permeability direction and the reserves between the water injector and producer differ greatly,the conductivity differences in different injection-production directions are favorable for water flooding.When the injection-production well pattern is constructed or recombined,it’s better to make the reserves of caves in different injection-production directions proportional to permeability,and inversely proportional to the well spacing.The well close to the cave should be a producer,and the well far from the cave should be an injector.Different ratios of cave reserves to fracture reserves correspond to different optimal well spacings and optimal permeability ratios.Moreover,both optimal well spacing and optimal permeability ratio increase as the ratio of cave reserves to fracture reserves increases.

Practice and knowledge of volumetric development of deep fractured-vuggy carbonate reservoirs in Tarim Basin, NW China

Different from the continental layered sandstone and fracture-pore carbonate reservoirs, the fractured-vuggy carbonate reservoirs in the Tarim Basin are mainly composed of fractured-vuggy bodies of different sizes and shapes. Based on years of study on the geological features, flow mechanisms, high-precision depiction and the recovery mode of fractured-vuggy bodies, the idea of “volumetric development” is proposed and put into practice. A “body by body” production methodology is established with respect to volumetric unit of fractures and vugs based on vuggy body’s spatial allocation and reserves. A variety of development wells, various technological methods, and multi-type injection media are used to develop this type of reservoirs in an all-around way. As a result, the resource and production structures of the Tahe oilfield are significantly improved and a highly efficient development is achieved.

“Second quantitative characterization”and its application in fractured-vuggy carbonate reservoirs

The Ordovician fracture-vug carbonate reservoirs of Tarim Basin,are featured by developed vugs,caves and fractures.The strong heterogeneity results in huge uncertainty when these reservoirs are quantitatively characterized using merely static seismic data.The effective quantitative characterization of the reservoirs has been an urgent problem to be solved.This study creatively proposes the"second quantitative characterization"technique with the combination of dynamic and static data based on the primary static quantitative characterization and fully considering lots of key influence factors when conducting characterization.In this technique,dynamic analysis methods such as well testing,production rate transient analysis,dynamic reserve evaluation and dynamic connectivity evaluation are used to get understandings on this kind of reservoir.These understandings are used as statistical parameters to constrain the inversion of seismic wave impedance to improve the relationship between wave impedance and porosity and determine the fracture-vug morphology,calculate dynamic reserves,and then a more accurate fracture-vugmodel can be selected and used to calculate the oil-water contact inversely based on the results of"second quantitative characterization".This method can lower the uncertainties in the primary quantitative characterization of fracture-vug reservoirs,enhance the accuracy of characterization results significantly,and has achieved good application results in the fracture-vug carbonate reservoirs of Tarim Basin.

Impact of effective stress on permeability for carbonate fractured-vuggy rocks

To gain insight into the flow mechanisms and stress sensitivity for fractured-vuggy reservoirs,several core models with different structural characteristics were designed and fabricated to investigate the impact of effective stress on permeability for carbonate fractured-vuggy rocks(CFVR).It shows that the permeability performance curves under different pore and confining pressures(i.e.altered stress conditions)for the fractured core models and the vuggy core models have similar change patterns.The ranges of permeability variation are significantly wider at high pore pressures,indicating that permeability reduction is the most significant during the early stage of development for fractured-vuggy reservoirs.Since each obtained effective stress coefficient for permeability(ESCP)varies with the changes in confining pressure and pore pressure,the effective stresses for permeability of four representative CFVR show obvious nonlinear characteristics,and the variation ranges of ESCP are all between 0 and 1.Meanwhile,a comprehensive ESCP mathematical model considering triple media,including matrix pores,fractures,and dissolved vugs,was proposed.It is proved theoretically that the ESCP of CFVR generally varies between 0 and 1.Additionally,the regression results showed that the power model ranked highest among the four empirical models mainly applied in stress sensitivity characterization,followed by the logarithmic model,exponential model,and binomial model.The concept of“permeability decline rate”was introduced to better evaluate the stress sensitivity performance for CFVR,in which the one-fracture rock is the strongest,followed by the fracture-vug rock and two-horizontalfracture rock;the through-hole rock is the weakest.In general,this study provides a theoretical basis to guide the design of development and adjustment programs for carbonate fractured-vuggy reservoirs.

Research on Quantitative Identification of Three-Dimensional Connectivity of Fractured-Vuggy Reservoirs

The fractured-vuggy carbonate oil resources in the western basin of China are extremely rich.The connectivity of carbonate reservoirs is complex,and there is still a lack of clear understanding of the development and topological structure of the pore space in fractured-vuggy reservoirs.Thus,effective prediction of fractured-vuggy reservoirs is difficult.In view of this,this work employs adaptive point cloud technology to reproduce the shape and capture the characteristics of a fractured-vuggy reservoir.To identify the complex connectivity among pores,fractures,and vugs,a simplified one-dimensional connectivity model is established by using the meshless connection element method(CEM).Considering that different types of connection units have different flow characteristics,a sequential coupling calculation method that can efficiently calculate reservoir pressure and saturation is developed.By automatic history matching,the dynamic production data is fitted in real-time,and the characteristic parameters of the connection unit are inverted.Simulation results show that the three-dimensional connectivity model of the fractured-vuggy reservoir built in this work is as close as 90%of the fine grid model,while the dynamic simulation efficiency is much higher with good accuracy.

Quantitative characterization of deep fault-karst carbonate reservoirs: A case study of the Yuejin block in the Tahe oilfield

The Ordovician reservoirs in the Tahe oilfield are dominated by fractured-vuggy carbonate reservoirs, of which fault-karst reservoirs are a hot topic in recent years. Fault-karst reservoirs feature high production, large burial depth, and strong heterogeneity under the control of faulting and karstification. Based on geological, logging, and seismic data, this study classified the Ordovician fault-karst reservoirs in the Yuejin block of the Tahe oilfield into three types, namely karst-cave, dissolved-vug, and fractured types, and established the integrated identification criteria of the three types of reservoirs. This study characterized karst caves, dissolved vugs, and multi-scale faults through seismic wave impedance inversion and frequency-domain detection of multi-scale faults. 3D geological models of different types of reservoirs were built using the combined deterministic and stochastic methods and characterized the spatial distribution of multi-scale faults, karst caves, dissolved vugs, and physical property parameters of reservoir. This study established the method for the geological modeling of fault-karst reservoirs, achieved the quantitative characterization and revealed the heterogeneity of fault-karst reservoirs. The karst-cave and dissolved-vug types are high in porosity and act as reservoirs, while the fractured type is high in permeability and act as flow pathway. This study lays the foundation for the development index prediction, well emplacement, and efficient development of the fault-karst carbonate reservoirs.

Characteristics,Formation Periods,and Controlling Factors of Tectonic Fractures in Carbonate Geothermal Reservoirs:A Case Study of the Jixianian System in the Xiong'an New Area,China

As typical carbonate geothermal reservoirs with low porosity in northern China,the Jixianian System in the Xiong’an New Area is the main target for geothermal fluid exploration.The Jixianian System comprises the Gaoyuzhuang,Yangzhuang,Wumishan,Hongshuizhuang,and Tieling formations.The characteristics,formation periods,and controlling factors of reservoir tectonic fractures have been determined based on analyses of outcrops,cores,thin sections,and image logs.The results show that unfilled fractures account for over 87% and most tectonic fractures are high-angle shear fractures with angles concentrated at 40°to 70°and the fracture porosity increases linearly with an increased fracture aperture.Within the same tectonic setting and stress field,the lithology and layer thickness are the dominant factors governing the development of tectonic fractures,which are the most developed in dolomites and thin layers.Tectonic fractures were most likely formed in regions near faults or areas with larger stress gradients.The tectonic fractures in the carbonate geothermal reservoirs are roughly divided into four sets:NNW-SSE and NNE-SSW oriented‘X’-conjugated shear fractures formed from the Paleozoic to the pre-Yanshanian Movement;NE-SW-oriented shear fractures,formed in episode B of the Yanshanian Movement,occurred at the Early Cretaceous;nearly E-W-oriented tensional fractures formed in the late Yanshanian Movement at the Late Cretaceous to Paleogene,and NEE-SW-oriented shear fractures formed during the Himalayan movement.

Ultra-deep carbonate basement reservoirs formed by polyphase fracture-related karstification in the Offshore Bohai Bay Basin, China

The Palaeozoic carbonate basement of the Offshore Bohai Bay Basin (OBBB) presents considerable potential for hydrocarbon exploration. However, the multistage tectonism and complex superimposed palaeo-karstification in the area are unclear, which leads to a lack of understanding on the formation mechanism and distribution of the deep carbonate basement reservoirs. In this study, the occurrence of a fracture-vug network and its fillings in carbonate reservoirs were investigated based on borehole cores, thin sections, and image logs from the southwestern slope of the OBBB's Bozhong Sag. Then the diagenetic fluid properties of the carbonate matrix and fillings were analysed via the data of carbon, oxygen, and strontium isotopes, and major, rare elements from coring intervals. The results revealed that fracture-related karst reservoirs have lithologic selectivity inclined toward dolomite strata. The intersecting relationships, widths, and strikes of the fractures and the regional tectonic background indicate three structural fracture families: NW-, NNE-, and NNW- trending, related to the Indosinian, middle Yanshanian, and late Yanshanian orogeny, respectively. The Indosinian NW- and end-Mesozoic NNE-trending fractures produced by compressional tectonic stress mainly contributed to the formation of the basement reservoirs. The geochemistry of the calcite veins filling these fractures suggests two main types of diagenetic fluids. The fluid of autogenic recharge related to the earlier fills is karstification diffuse flow dominated by internal runoff from rainfall in the highland setting of the Indosinian thrusting orogenic belt. The other fluid of allogenic recharge related to the later fills is the main lateral freshwater flow dominated by external runoff from the catchment in the setting of the horst-lowland within the rifting basin, induced by the Yanshanian destruction of the North China Craton. Finally, the relationship between the three fracture families and two kinds of related fluids is revealed. This allows us to propose a model to understand the polyphase-superimposed fracture-related karst reservoir complexes within the deep carbonate basement of tilting fault blocks that neighbour the Bozhong hydrocarbon kitchen and predict the formation of potential plays with high accuracy.

Control of strike-slip faults on Sinian carbonate reservoirs in Anyue gas field, Sichuan Basin, SW China

The largest Precambrian gas field (Anyue gas field) in China has been discovered in the central Sichuan Basin. However, the deep ancient Ediacaran (Sinian) dolomite presents a substantial challenge due to their tightness and heterogeneity, rather than assumed large-area stratified reservoirs controlled by mound-shoal microfacies. This complicates the characterization of “sweet spot” reservoirs crucial for efficient gas exploitation. By analyzing compiled geological, geophysical and production data, this study investigates the impact of strike-slip fault on the development and distribution of high-quality “sweet spot” (fractured-vuggy) reservoirs in the Ediacaran dolomite of the Anyue gas field. The dolomite matrix reservoir exhibits low porosity (less than 4%) and low permeability (less than 0.5×10^(-3) μm^(2)). Contrarily, fractures and their dissolution processes along strike-slip fault zone significantly enhance matrix permeability by more than one order of magnitude and matrix porosity by more than one time. Widespread “sweet spot” fracture-vuggy reservoirs are found along the strike-slip fault zone, formed at the end of the Ediacaran. These fractured reservoirs are controlled by the coupling mechanisms of sedimentary microfacies, fracturing and karstification. Karstification prevails at the platform margin, while both fracturing and karstification control high-quality reservoirs in the intraplatform, resulting in reservoir diversity in terms of scale, assemblage and type. The architecture of the strike-slip fault zone governed the differential distribution of fracture zones and the fault-controlled “sweet spot” reservoirs, leading to wide fractured-vuggy reservoirs across the strike-slip fault zone. In conclusion, the intracratonic weak strike-slip fault can play a crucial role in improving tight carbonate reservoir, and the strike-slip fault-related “sweet spot” reservoir emerges as a unique and promising target for the efficient development of deep hydrocarbon resources. Tailored development strategies need to be implemented for these reservoirs, considering the diverse and differential impacts exerted by strike-slip faults on the reservoirs.

Stress sensitivity of carbonate gas reservoirs and its microscopic mechanism

In order to evaluate the stress sensitivity of carbonate reservoirs,a series of rock stress sensitivity tests were carried out under in-situ formation temperature and stress condition.Based on the calibration of capillary pressure curve,the variable fractal dimension was introduced to establish the conversion formula between relaxation time and pore size.By using the nuclear magnetic resonance(NMR)method,the pore volume loss caused by stress sensitivity within different scales of pore throat was quantitatively analyzed,and the microscopic mechanism of stress sensitivity of carbonate gas reservoirs was clarified.The results show that fractures can significantly affect the stress sensitivity of carbonate reservoirs.With the increase of initial permeability,the stress sensitivity coefficient decreases and then increases for porous reservoirs,but increases monotonously for fractured-porous reservoirs.The pore volume loss caused by stress sensitivity mainly occurs for mesopores(0.02–0.50μm),contributing more than 50%of the total volume loss.Single high-angle fracture contributes 9.6%of the stress sensitivity and 15.7%of the irreversible damage.The microscopic mechanism of the stress sensitivity of carbonate gas reservoirs can be concluded as fracture closure,elastic contraction of pores and plastic deformation of rock skeleton.

Characteristics,classification and KNN-based evaluation of paleokarst carbonate reservoirs:A case study of Feixianguan Formation in northeastern Sichuan Basin,China

The Feixianguan Formation reservoirs in northeastern Sichuan are mainly a suite of carbonate platform deposits.The reservoir types are diverse with high heterogeneity and complex genetic mechanisms.Pores,vugs and fractures of different genetic mechanisms and scales are often developed in association,and it is difficult to classify reservoir types merely based on static data such as outcrop observation,and cores and logging data.In the study,the reservoirs in the Feixianguan Formation are grouped into five types by combining dynamic and static data,that is,karst breccia-residual vuggy type,solution-enhanced vuggy type,fractured-vuggy type,fractured type and matrix type(non-reservoir).Based on conventional logging data,core data and formation microscanner image(FMI)data of the Qilibei block,northeastern Sichuan Basin,the reservoirs are classified in accordance with fracture-vug matching relationship.Based on the principle of cluster analysis,K-Nearest Neighbor(KNN)classification templates are established,and the applicability of the model is verified by using the reservoir data from wells uninvolved in modeling.Following the analysis of the results of reservoir type discrimination and the production of corresponding reservoir intervals,the contributions of various reservoir types to production are evaluated and the reliability of reservoir type classification is verified.The results show that the solution-enhanced vuggy type is of high-quality sweet spot reservoir in the study area with good physical property and high gas production,followed by the fractured-vuggy type,and the fractured and karst breccia-residual vuggy types are the least promising.

Fracture characterization of Asmari Formation carbonate reservoirs in G Oilfield, Zagros Basin, Middle East

The Asmari Formation in the G oilfield on the Iran-Iraq border is a fractured-porous multi-lithology mixed reservoir, for which fracture is an important factor affecting oil productivity and water cut. The characterization and modeling of fractures in the carbonate reservoir of G oilfield are challenging due to weak conventional well log responses of fractures and a lack of specific logs, such as image logs. This study proposes an integrated approach for characterizing and modeling fractures in the carbonate reservoir. The features, formation mechanism, influencing factors, and prediction methods of fractures in the Asmari Formation carbonate reservoirs of G oilfield were studied using core observation, thin section, image log, cross-dipole acoustic log (CDAL), geomechanics numerical simulation (GNS), and production data. According to CDAL-based fracture density interpretation, GNS-based fracture intensity prediction between wells, and DFN-based rock fracture properties modeling, the quantitative fracture characterization for G oilfield was realized. This research shows that the fractures in the Asamri Formation are mainly medium-to high-angle shear fractures. The substantial compression stress during the Miocene played a major role in the formation of the prominent fractures and determined their trend in the region, with primary trends of NNW-SSE and NNE-SSW. The fracture distribution has regularity, and the fractures in zone A dolomites are more highly developed than that in zone B limestones vertically. Horizontally, fractures intensity is mainly controlled by faults and structural location. The results of this study may benefit the optimization of well design during field development. From 2019 to 2021, three horizontal wells pilot tests were deployed in the fractures belt in zone A, and these fractures prominently increased the permeability of tight dolomite reservoirs. The initial production of the wells is four to five times the average production of other wells in the area, showing a good development effect. Meanwhile, the updated numerical simulation validates that the history match accuracy of water cut based on the dual-porosity model is significantly improved, proving the fracture evaluation and prediction results to be relatively reliable and applicable.

Simulation study of supercritical carbon dioxide jet fracturing for carbonate geothermal reservoir based on fluid-thermo-mechanical coupling model

Geothermal energy is a kind of renewable,sustainable and clean energy resource.Geothermal energy is abundant in carbonate reservoirs.However,low matrix permeability limits its exploitation.The super-critical carbon dioxide(SC-CO_(2))jet fracturing is expected to efficiently stimulate the carbonate geothermal reservoirs and achieve the storage of CO_(2) simultaneously.In this paper,we established a transient seepage and fluid-thermo-mechanical coupled model to analyze the impact performance of sc-CO_(2) jet fracturing.The mesh-based parallel code coupling interface was employed to couple the fluid and solid domains by exchanging the data through the mesh interface.The physical properties change of sC-CO_(2) with temperature were considered in the numerical model.Results showed that SC-CO_(2) jet frac-turing is superior to water-jet fracturing with respect to jetting velocity,particle trajectory and pene-trability.Besides,stress distribution on the carbonate rock showed that the tensile and shear failure would more easily occur by SC-CO_(2) jet than that by water jet.Moreover,pressure and temperature control the jet field and seepage field of sC-CO_(2) simultaneously.Increasing the jet temperature can effectively enhance the impingement effect and seepage process by decreasing the viscosity and density of SC-CO_(2).The key findings are expected to provide a theoretical basis and design reference for applying SC-CO_(2) jet fracturing in carbonate geothermal reservoirs.

Origin of carbonate minerals and impacts on reservoir quality of the Wufeng and Longmaxi Shale, Sichuan Basin

The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ^(13)C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ^(13)C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 ℃ to 182.90 ℃. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.

Quantitative Method of Classification and Discrimination of a Porous Carbonate Reservoir Integrating K-means Clustering and Bayesian Theory

Reservoir classification is a key link in reservoir evaluation.However,traditional manual means are inefficient,subjective,and classification standards are not uniform.Therefore,taking the Mishrif Formation of the Western Iraq as an example,a new reservoir classification and discrimination method is established by using the K-means clustering method and the Bayesian discrimination method.These methods are applied to non-cored wells to calculate the discrimination accuracy of the reservoir type,and thus the main reasons for low accuracy of reservoir discrimination are clarified.The results show that the discrimination accuracy of reservoir type based on K-means clustering and Bayesian stepwise discrimination is strongly related to the accuracy of the core data.The discrimination accuracy rate of TypeⅠ,TypeⅡ,and TypeⅤreservoirs is found to be significantly higher than that of TypeⅢand TypeⅣreservoirs using the method of combining K-means clustering and Bayesian theory based on logging data.Although the recognition accuracy of the new methodology for the TypeⅣreservoir is low,with average accuracy the new method has reached more than 82%in the entire study area,which lays a good foundation for rapid and accurate discrimination of reservoir types and the fine evaluation of a reservoir.

Polymer efficiency and sulfate concentration for hybrid EOR application to an acidic carbonate reservoir

Polymers play an important role in hybrid enhanced oil recovery (EOR), which involves both a polymer and low-salinity water. Because the polymer commonly used for low-salinity polymer flooding (LSPF) is strongly sensitive to brine pH, its efficiency can deteriorate in carbonate reservoirs containing highly acidic formation water. In this study, polymer efficiency in an acidic carbonate reservoir was investigated experimentally for different salinity levels and SO42− concentrations. Results indicated that lowering salinity improved polymer stability, resulting in less polymer adsorption, greater wettability alteration, and ultimately, higher oil recovery. However, low salinity may not be desirable for LSPF if the injected fluid does not contain a sufficient number of sulfate (SO42−) ions. Analysis of polymer efficiency showed that more oil can be produced with the same polymer concentration by adjusting the SO42− content. Therefore, when river water, which is relatively easily available in onshore fields, is designed to be injected into an acidic carbonate reservoir, the LSPF method proposed in this study can be a reliable and environmentally friendly method with addition of a sufficient number of SO42− ions to river water.

Types of Karst-fractured and Porous Reservoirs in China's Carbonates and the Nature of the Tahe Oilfield in the Tarim Basin

Almost all the oil and gas reservoirs developed in marine sedimentary strata of China have undergone processes of multi-phase reservoir formation and later modification. The irregular reservoirs are classified into three types as the Naxi, Tahe and Renqiu ones, increasing successively in the development degree of karstificated pores and fissures and the connection degree of independent reservoirs. In these reservoirs, the unity in the fluid feature, pressure and oil-gas-water interface also increases successively from the Naxi to the Renqiu type. The main body of Ordovician reservoirs of the Tahe Oilfield in the Tarim Basin is a network pool rather than a stratified, massive, stratigraphically-unconformed or weathering-crust one. The fluid nature of oil, gas and water, the interface positions and the pressures, as well as the dynamic conditions of fluids within the reservoirs during the production are all different from those in stratified or massive oil and gas reservoirs. Carbonates in the Akekule uplift and the Tahe Oilfield are assemblages of various types of reservoirs, which have an overall oil-bearing potential and obvious uneven distribution. Testing and producing tests are the major means to evaluate this type of reservoirs and acid fracturing improvement is a key link in petroleum exploration and development.

Comparison of basic features and origins of oolitic shoal reservoirs between carbonate platform interior and platform margin locations in the Lower Triassic Feixianguan Formation of the Sichuan Basin,southwest China

The oolitic shoal reservoirs of the Lower Triassic Feixianguan Formation carbonates in the Sichuan Basin of southwest China are an important target for gas exploration in the basin.Their occurrence,like other cases worldwide,can be divided into two locations in general,i.e.,platform interior and platform margin locations.Their differences of reservoir features and origins,however,have not been investigated comprehensively due to different exploration degrees.This issue is addressed in this paper,to provide basic data and information for the basin＇s hydrocarbon exploration and for the study of carbonate platform sedimentology and reservoir geology worldwide.We compared the features of these two types of reservoirs in detail,including the depositional and diagenetic features,pore types and petrophysical features.Based on the comparison,the origin of the reservoirs was further discussed.It is shown that the reservoirs in platform interior and platform margin locations differ significantly.The interior carbonates were deposited in moderate to high energy settings and the dominant lithologic type was limestone,which was weakly compacted and intensely cemented and has undergone meteoric dissolution.Pore types include intragranular dissolution and moldic pores,with low porosities（6%） and low permeabilities（0.1 mD）.By contrast,the platform margin carbonates were deposited in relatively high energy settings and mainly consisted of dolostones with some limestones.The rocks were strongly compacted but incompletely cemented.As a result,some primary intergranular pores were preserved.Both meteoric solution and burial solution have taken place.There are various types of pore spaces including intergranular and intercrystalline solution pores and residual intergranular pores.This type of reservoir generally has better petrophysical properties（9% porosity and 0.1 mD permeability） and pore-throat structures than the interior reservoirs.These differences were influenced by both primary depositional features and secondary diagenesis.For the interior carbonate reservoirs,early meteoric dissolution,weak compaction and strong cementation are important controlling factors.By contrast,the factors controlling the formation of the margin carbonate reservoirs mainly include dolomitization,preservation of primary pores and burial dissolution.