Dolomite Reservoir Research Articles

Dolomite reservoir fluid discrimination method based on fluid identification factor

The complexity of the pore structure of dolomite leads to strong heterogeneity of the reservoir distribution and the inability to accurately describe the rock-electric parameters, which makes it difficult to distinguish the properties of reservoir fluids based on electrical logging. In this paper, fluid identification factors are applied to dolomite reservoirs for the first time, which enriches the methods of non-electric logging to identify reservoir fluid properties. Based on the inductive analysis of existing fluid identification factors, according to the number of times that logging data participates in the calculation, they are divided into three types of fluid identification factors: direct, indirect and composite; composite fluid identification factors can reflect changes in reservoir lithology, and reconstruct composite fluid identification factor H considering the difference between the lateral deformation coefficient of dolomite and other lithologies; the sensitivity of the composite fluid identification factor to gas saturation is tested, and fluid factors used in dolomite reservoirs were optimized. Select the array acoustic logging and density logging data from three wells in the subsalt dolomite in the Tarim Basin, China, calculate the parameters of the compressional wave and shear wave impedances (Ip, Is), λρ, μρ, ρf, F, H and etc., and use the two-dimensional chart method to analyze the effect of the fluid identification factors above, and λρ-μρ, F-σ, ρf-σ, H-μ and other charts are selected to establish logging fluid discriminant curves. The application shows that the fluid identification factor H is more suitable for distinguishing gas layers, and F is more suitable for distinguishing water layers.

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.

Dolomite recrystallization revealed by Δ47/U-Pb thermochronometry in the Upper Jurassic Arab Formation, United Arab Emirates

Abstract The process of recrystallization affecting dolomitic successions remains a longstanding enigma in carbonate research. Recrystallization influences the accuracy of genetic dolomitization models as well as the prediction of porosity and permeability distribution within dolomitic reservoirs. We investigate early-formed dolomites of the Upper Jurassic Arab Formation reservoir (Arabian Platform, United Arab Emirates), where recrystallization is not easily ascertained based on petrographic and O-C-Sr isotope analyses. Conversely, the application of Δ47/U-Pb thermochronometry revealed the occurrence of burial recrystallization over a temperature-time interval of ~45 °C/45 m.y. during the Early and Late Cretaceous. The process was initially driven by Late Jurassic mixed marine-meteoric fluids, which evolved during burial in a closed hydrologic system and remained in thermal equilibrium with the host rocks. Recrystallization was a stepwise process affecting the succession heterogeneously, so that samples only few meters apart presently record different temperature-time stages of the process that stopped when hydrocarbons migrated into the reservoir. Our results illustrate how Δ47/U-Pb thermochronometry may provide a novel approach to unravel dolomite recrystallization and to precisely determine the timing and physicochemical conditions (temperature and δ18Ow) that characterized the process. Therefore, this study paves the way for better appraisal of recrystallization in dolomitic reservoirs.

In Situ Combustion of Heavy Oil within a Vuggy Carbonate Reservoir: Part I—Feasibility Study

Worldwide, the known recoverable heavy oil and bitumen reserves make up more than 64% of the total reserves, of which more than 60% are trapped in carbonates. Air injection has immense potential for hydrocarbon recovery from various reservoirs. While most of the successful air-based techniques are performed within carbonate reservoirs containing light oil, theoretically, in situ combustion (ISC) has also shown great potential for recovering heavy oil and bitumen. Carbonates are complex in terms of geology and are often associated with fractures and vugs that affect the fluid flow, pressure propagation and progression of the ISC reactions. This paper describes the first experiment in which the triple-porosity concept was applied to simulate heterogeneity through artificially induced vugs, core matrix and fractures. This approach was used to study the feasibility of the ISC recovery technique for heavy oil (14° API) within a dolomite reservoir using a combustion tube (CT) in an experiment performed at 1740 psig. The combustion front advanced through 78% of the core length prior to the termination of air injection, producing 80% of the initial oil. To differentiate between the various sources of the CO2 gas (a product of carbonate decomposition vs. the combustion reaction) the atomic ratios of (CO2 + CO)/CO = 6 and (CO2 + CO)/N2 = 0.21 were applied. Additionally, partial upgrading of the produced heavy oil was observed.

A Tiny-Fault Detection Strategy Based on Phase Congruency—An Example of Carbonate Reservoir in Ordos Basin, China

Tiny-fault detection plays a very important role in the research on the tight oil and gas reservoir in well area X in Ordos Basin, China. In this study, the target formation is the Majiagou dolomite reservoir section under the Ordovician salt with low-amplitude structures generally developed. The conventional attributes extracted from migrated seismic data could not achieve ideal results in detecting hidden faults with small displacement due to slight travel time differences and weak amplitude disturbances. To address this challenge, a segment and fusion strategy was adopted to highlight tiny faults in this region. First, the phase congruency analysis method was used to extract the local edges of coherence to locate the faults. Second, in the extraction process, the coherence was divided into segments according to the fault scales, and then enhanced segment by segment and fused. Third, the empirical formula of the new fault indicator was constructed by the phase congruency features, which can be used to accurately characterize tiny faults. This strategy performs well in both model tests and the migrated seismic data.

Multiphase dolomitization mechanisms of the Cambrian upper Changping Formation, North China Platform, China

The Cambrian dolomite reservoirs of the North China Platform offer good exploration prospects, but their occurrence and the impacts of dolomitization on reservoir quality are not yet clearly understood. The study herein assesses outcrop samples in the Cambrian upper Changping Formation, Dingjiatan area, and highlights the important role of multiphase dolomitization in the formation of paleo-reservoirs of acceptable porosity, where oil pools and fields may be discovered. A combination of petrology, fluid inclusion microthermometry, isotopes, and trace element compositions has been used to explain multiphase dolomitization mechanisms and their impacts on dolomite reservoirs. Five types of dolomites are identified through outcrop observation, thin section identification via transmitted light, and cathodoluminescence. The following geochemical analyses reveal various dolomitization mechanisms. In the (pene) contemporaneous stage, microbial dolomite is commonly related to microbial metabolic activities with significant carbon isotopic depletion compared to the Cambrian seawater values. With the influence of microbial dolomitization, dolomicrite corresponds to the sabkha dolomitization mode in a low-salinity seawater environment and early-stage dolomitization. The structureless dolomite (ssD) in the early highstand systems tract (EHST), characterized by elevated 87Sr/88Sr ratios and low oxygen isotopic values, forms from stratum brine water in the shallow-middle burial stage rather than in the (pene) contemporaneous stage. In contrast, ssD in the late highstand systems tract (LHST) undergoes (pene) contemporaneous dolomitization at salinities between 6% and 28% before later pore water transformation, with the participation of atmospheric freshwater through faults and unconformities exhibiting the lowest inclusion temperature and salinity values. The medium to coarse crystalline dolomite (MCD) in the LHST and the saddle dolomite (SD) in the EHST with low REE values are atypical hydrothermal dolomites caused by a combined superposition of middle-deep burial hydrothermal fluids at temperatures >150°C and stratigraphic brines. The MCD is also influenced by terrigenous water characterized by relatively low Eu anomaly values. Finally, the mechanisms of porosity increase are investigated, and it is concluded that the pore increase caused by the (pene) contemporaneous reflux interaction and the later pore retention both lead to better ssD reservoirs in the LHST than in the EHST.

Feasibility of foamed acid treatment in upper stimulation of fractured-vuggy dolomite reservoirs with bottom water

Bottom water breakthrough, which is posed by the failure in an initial water control during acid treatment, is a complex problem in fractured-vuggy dolomite reservoirs with bottom water. To analyze the feasibility of upper stimulation during foamed acid treatment, various experiments were performed, including the analysis of acid flow and etching exclusively in fractures and that in fractures between caves (or reservoirs). The results confirm the abilities of the retarding and upper stimulation of foamed acid. Foamed acid penetrates into the upper fractures and replaces the oil in this part of the reservoir, whereas thickened acid invariably displaces into fractures present in the lower part. During treatment with foamed acid, the displacement rate of oil increases with an increase in the upper fracture connectivity. Furthermore, upper stimulation is suitable for reservoirs with developed upper and undeveloped lower fractures. For field applications, the flow rate should not be extremely high when using foamed acid for upper stimulation of fractured-vuggy dolomite reservoirs with bottom water, and should be maintained at the minimum while ensuring construction efficiency; otherwise, it might result in bottom water breakthrough.

Constrained 3D gravity modelling of a multilayered aquifer system in an arid region: Case of Sebkhat El Bhira basin (Central Tunisia)

In arid regions, the significant and increasing demand for water has caused intensive exploitation of groundwater resources. The Sebkhat El Bhira basin, shared between Kairouan and Sidi Bouzid governorates in Central Tunisia, presents a multi-layer aquifer system composed of four main reservoirs: the sandy Mio-Plio-Quaternary (MPQ), the Paleogene, the Upper Cretaceous limestone and the Lower Cretaceous limestone, sand and dolomite reservoirs. The intensive exploitation of the shallower aquifer has led to a drawdown of the water table and a deterioration of water quality. The purpose of this work is to characterize the geometry of these potential reservoirs for sustainable water management, by applying geophysical methods.We used land gravity data covering 1387 Km2, twelve seismic reflection profiles with a total length of approximately 308 Km, two petroleum wells and 39 water wells.The processing of the gravity data shows the subdivision of Sebkhat El Bhira basin into four sub-basins and the identification of principal and minor lineaments oriented N-S, NE-SW, NNW-SSE, and E-W. The interpretation of reflection seismic lines allowed the identification of five main horizons: the bottom of MPQ, the bottom of Paleogene, the bottom of Upper Cretaceous, the bottom of Lower Cretaceous and the bottom of Jurassic (Top Triassic). For a better understanding of the subsurface structuring, we built twelve two-dimensional (2D) forward models of density. These latter are constrained in the area inside the plain by the converted time-to-depth seismic reflection data and wells. The interpolation of the 2D models allowed us to visualize the lateral and vertical thickness variations of each of the chrono-stratigraphic units and to produce a three-dimensional (3D) density model. The geophysical modelling allowed us to highlight the post-Jurassic tectonic and the Triassic halokinesis activities responsible for the actual subsurface geometry.The Triassic evaporites, in the study area, flow and rise through the fault's subvertical branches. It outcrops locally at the North-South Axis mountains range (NOSA). Inside the plain, the evaporitic uprising shows different schemes like diapir, wall and pillow. This Triassic halokinesis, with the tectonics, has shaped the deep morphology of the basin and managed the available space for the deposition of the overlying stratigraphic formations.The general structure of the potential aquifers is characterized by a depth which increases from the East to the West and towards the central part of the plain. Inside the basin, the structure of these aquifers is complex.

Origin of dolomites in the Permian dolomitic reservoirs of Fengcheng Formation in Mahu Sag, Junggar Basin, NW China

Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear. Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections, scanning electron microscope (SEM), electron microprobe (EMP), C, O and Sr isotopes analysis, and other techniques. (1) Dolomites were mainly precipitated in three stages: penecontemporaneous–shallow burial stage (early stage of the Middle Permian), middle burial stage (middle stage of the Middle Permian), and middle–deep burial stage, with the former two stages in dominance. (2) Dolomitization fluid was high-salinity brine originating from alkaline lake. In the penecontemporaneous–shallow burial stage, Mg2+ was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass. In the middle burial stage, Mg2+ mainly came from the transformation of clay minerals, devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar. (3) Regular changes of Mg, Mn, Fe, Sr, Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid, and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial. (4) In the penecontemporaneous stage, induced by alkaline-lake microorganisms, the micritic–microcrystalline dolomites were formed by primary precipitation, replacement of aragonite and high-Mg calcite, and other processes; in the shallow burial stage, the silt-sized dolomites were formed by continuous growth of micritic–microcrystalline dolomite and replacement of calcites, tuffs and other substances; in the middle burial stage, the dolomites, mainly silt- and fine-sized, were formed by replacement of volcanic materials. The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.

Geochemical characteristics of hydrogen, oxygen, carbon isotopes and REE of cenozoic dolomites in Well Xike 1, Xisha Islands, South China sea and the significance for dolomitization in island-reef areas

Due to the special tectonic background and complex geological evolution characteristics of the South China Sea, reef dolomite reservoirs (sush as Well Xike 1) are widely developed. Based on the drilling core data of Well Xike 1, the structure and geochemical characteristics of dolomite reservoirs, including carbon, oxygen, hydrogen isotopes and REE were systematically studied using geochemical and petrological methods. It is found that the geochemical characteristics of REE show that the main diagenetic environment of dolomites is a low-temperature alkaline semi open oxidation environment; the carbon and oxygen isotopes of the dolomites are generally lack of correlation, the δD value is significantly lower than the hydrogen isotope value of seawater. Meanwhile, the oxygen isotope value of deep dolomites is negatively biased, which may be due to the increase or decrease of pore water temperature caused by deep thermal convection that related to the regional tectonic movements of the South China Sea. The δ18O value is also consistent with the geological reality of increasing saddle dolomite content in deep dolomites. The distribution of the δ13C value indicates that the dolomite inherited the carbon of the original limestone during dolomitization, while the characteristic of the δD value shows that it may be affected by the mixing of atmospheric precipitation and concentrated seawater in the quasi contemporaneous period. Based on the comprehensive analysis of the geochemical characteristics of the Well Xike 1, it is considered that the higher diagenetic temperature could be an important factor leading to the huge differences between the diagenetic model of deep and shallow dolomites. The geochemical characteristics of the shallow dolomites show that it is mainly reflux infiltration dolomitization under the micro evaporation and concentration sea water environments, while the deep dolomite is transformed by the hot water fluids in the epigenetic diagenetic evolution stage.

Effects of diagenesis on quality of dengying formation deep dolomite reservoir, Central Sichuan Basin, China: Insights from petrology, geochemistry and in situ U-Pb dating

The Ediacaran Dengying Formation in Central Sichuan Basin is the deep dolomite gas reservoir with the largest natural gas reserves in China, providing an excellent example for understanding the effect of diagenesis evolution on deep dolomite reservoir quality. By integrating petrology, geochemistry and in situ U-Pb dating, this study aims to reveal the genesis of different rock fabrics and their corresponding diagenetic events, and to discuss the temporal relationship of diagenetic events and their effects on the reservoir quality of Dengying Formation. Two phases of dolomite and three phases of dolomite cement are identified in Ediacaran Dengying Formation deep dolomite reservoirs of Central Sichuan Basin as follows: 1) matrix dolomites (MD), 2) fine-medium crystalline dolomites (FMD), 3) fibrous dolomites (FD), 4) medium-coarse crystalline dolomites (MCD), and 5) saddle dolomite (SD). By analyzing petrographic and temporal relationships between these fabrics, it is suggested that the deep dolomite reservoir of Dengying Formation in Central Sichuan Basin has experienced six diagenetic stages, including 1) syndiagenetic stage, 2) the first-time early diagenetic stage, 3) epidiagenetic stage, 4) the second-time early diagenetic stage, 5) middle diagenetic stage and 6) late diagenetic stage. Microbial dolomitization during syndiagenetic stage produced limited increase in reservoir porosity, but it is of great significance for preservation of reservoir spaces. Silicification is most unfavorable for reservoir formation, but its duration and influence range are very limited. At epidiagenetic stage, the physical properties of Dengying Formation dolomite reservoir have been greatly improved. Two-time early diagenetic stages are both destructive for preservation of reservoir spaces. During middle diagenetic stage, two-stage of hydrothermal alterations occurred, corresponding to Late Silurian to Devonian and Late Permian, respectively. Hydrothermal alteration of the MD is both constructive and destructive, but overall it is constructive, and the improvement of physical properties is limited. For the karstified MD, both two-stage hydrothermal alterations were destructive, resulting in damage of reservoir physical properties caused by cementation by two-stage SD and one-stage MVT minerals.

Quantitative characterization and origin of differences in pore parameter distribution: A case study of the lower cambrian longwangmiao formation in the gaoshiti area of central Sichuan Basin

The research on the developmental characteristics and evolution process of pores in dolomite reservoirs is a hot spot in petroleum geology. In this paper, taking the dolomite reservoirs of the Longwangmiao Formation in the Gaoshiti area as an example, the differential distribution and evolution of pore parameters have been systematically studied combined with downhole cores, logging, cast thin sections, and scanning electron microscope data. We used statistical methods and image analysis techniques to quantitatively characterize the pore distribution of the Longwangmiao Formation in the study area. Moreover, the factors controlling the distribution of pores in the dolomite of the Longwangmiao Formation were identified, namely, lithology, sedimentary facies and diagenesis. Finally, the evolution mode of pores of the Longwangmiao Formation was constructed. The results show that the main types of pores developed in the Longwangmiao Formation in the study area include intergranular and intragranular dissolved pores, intercrystalline dissolved pores and mold pores. Among them, the intergranular dissolved pores are the main pore type. The pores with surface porosity higher than 0.5% in the Long 2 Member in the study area account for 46% of the total pores, while those in the Long 1 Member account for 25.5%. Therefore, the degree of development of pores in the Long 2 Member is better than the Long 1 Member. In addition, the granular dolomite in the grain shoal facies is favorable for the formation of large-scale pores, and the oolitic dolomite is the most favorable reservoir type. The most favorable factors for the development of pores are the atmospheric freshwater dissolution and dolomitization in the quasi-contemporaneous period; the former caused the formation of a large number of intergranular dissolved pores, while the dolomitized particles and their ring-edge cements in the latter promoted the retention of residual intergranular dissolved pores. The dissolution, dolomite cementation and hydrothermal action in the burial stage promoted the reformation of pores.

The effect of meteoric water on the very fine crystalline dolomite reservoir in the shallow burial zone: A case study of the Ma55 submember of Majiagou Formation in Ordos Basin

The meteoric water has obviously changed the physical properties of dolostone reservoirs in the vertical vadose zone and the horizontal phreatic zone, but its influence on the dolostone reservoirs in the shallow burial zone beneath the phreatic surface is still unclear. This study aims to reveal the effect of meteoric water on the dolostone reservoirs in the shallow burial zone through X-ray diffraction, cathodoluminescence, C, O, and Sr isotope using the sample from Majiagou Formation in the Daniudi gas field, Ordos Basin. The diagenesis and paragenesis of the Ma 55 submember were identified and interpreted through petrological study, combined with data from electron probe, X-ray diffraction analysis, and geochemical parameters of diagenetic minerals. The color of the very fine crystalline dolomite under the cathodeluminescence is dark red and red. The order degree of dolomite ranges from 0.54 to 0.91, showing the origin of early seepage-reflux dolomitization. There are a large number of different calcite cements as fills within the pores and fractures. The color of the calcite cement under the cathodoluminescence is orange-yellow, with a zonal structure. Hydrothermal fluid during late diagenesis could be identified by the authigenic fluorite filling in the fractures. According to the assembly of diagenetic minerals, the very fine crystalline dolostones have experienced the seepage-reflux dolomitization, meteoric water dissolution, shallow burial cementation and late cementation. The void spaces of the very fine crystalline dolostones are intercrystalline pores and microfractures. Although a large number of dissolved pores and caves developed in the period of meteoric water dissolution, these caves and dissolved pores has been mostly filled by multi-stages of cementation. Therefore, the effect of meteoric water on dolostone reservoirs in the shallow burial zone beneath the phreatic surface is not obvious. The main controlling factor for the quality of dolostone reservoir was dolomitization. This study provides a new understanding of the influence of meteoric water on reservoir quality in the shallow burial zone during the paleokarst period.

Key factors affecting hydrocarbon accumulation in ancient dolomite gas reservoirs of Xixiangchi formation (Southern Sichuan Basin, China)

The Upper Cambrian Xixiangchi Formation in the Southern Sichuan Basin, China, has favorable hydrocarbon accumulation conditions. The accumulation factors and enrichment conditions of this formation were key considerations in this study. By analyzing core, thin section, seismic, and geochemical data, the research shows that there are many sets of granular and crystalline dolomite reservoirs in the Xixiangchi Formation vertically, with thin thickness of single-reservoir. During the transformation of karst and tectonism, dissolution pores and fractures developed to form an ideal reservoir space. The reservoir of the Xixiangchi Formation is connected to the Lower Cambrian source rock through a fault system. The high-energy shoal facies of the Xixiangchi Formation are located on the oil and gas migration path, providing an appropriate reservoir space for forming the source reservoir configuration relationship between the lower generation and upper reservoir. The key factors affecting hydrocarbon accumulation in the Xixiangchi Formation are sufficient oil and gas supply, development of inherited paleo-uplift, effective transportation system, and favorable reservoir-forming combination. The inherited paleo-uplift controls the distribution of gas reservoirs. Owing to the short migration distance of oil and gas, hydrocarbon is found near source hydrocarbon accumulation, and the paleo-uplift slope area should be targeted for exploration in future studies.

Formation and preservation of ultra-deep high-quality dolomite reservoirs under the coupling of sedimentation and diagenesis in the central Tarim Basin, NW China

The formation mechanism of high-quality reservoirs is a key issue for ultra-deep (>6000 m) oil and gas exploration. The Lower Cambrian Xiaoerbrak dolomite with a burial depth of more than 7000 m in the Tazhong area of the Tarim Basin still keeps a maximum porosity of up to 20%, which is rare in the world. Based on the identification of meter-scale sedimentary cycles in cored intervals, the main controlling factors on the formation of high-quality reservoirs were discussed. The upper part of the sedimentary cycle is mainly composed of stromatolitic dolomite and oolitic dolomite. Comparatively, quartz-bearing dolomite and algal-bonded dolomite, as the product of low energy depositional environment are dominant in the lower part of each sedimentary cycle. Vugs formed by meteoric dissolution widely exist in the middle-upper part of the sedimentary cycle due to brief to moderate long-term subaerial exposure, whereas the lower part shows low porosity with a few intercrystalline pores. It indicates that subaerial exposure dissolution caused by periodic sea level decline is the main source of pores. The stylolite density and thickness of insoluble residue decrease rapidly from the lower to upper part of each sedimentary cycle. The content of aluminosilicate minerals in dolomite is positively related to the stylolite density, indicating that the aluminosilicate minerals play a promoting role in the pressure solution process. Pure dolomites with the least aluminosilicate minerals content in the upper of the sedimentary cycles have the strongest resistance to pressure solution, which is beneficial to ultra-deep high-quality reservoir preservation. Coupling of sedimentation and diagenesis to illustrate the origin of ultra-deep high-quality dolomite reservoirs is proposed.

Early dolomitization and subsequent hydrothermal modification of the middle Permian Qixia Formation carbonate in the northwest Sichuan Basin

High-quality dolomite reservoir are widely developed in Permian Qixia Formation in northwest Sichuan Basin. Understanding the genesis of dolomite is significant to predicting high-quality reservoir distribution. Based on the petrographical and geochemical characteristics, three types of dolomites and one type of dolomite cement were identified in the Qixia Formation in northwest Sichuan Basin: micritic to fine-crystalline dolomites (D1), medium-crystalline dolomite (D2), medium-to coarse-crystalline dolomite (D3) and saddle dolomite cement.Type D1 dolomites are characterized by micritic to fine-sized crystals, lamellar structure, generally dull to no luminescence, and relatively high strontium (Sr) concentrations. These characteristics suggest that type D1 dolomites were initially formed during the penecontemporaneous phase. The wide range of depleted δ18O values, slightly higher 87Sr/86Sr ratios and slightly positive europium (Eu) anomalies indicate that type-D1 dolomites were subsequently subjected to hydrothermal modification. Type D2 dolomite is characterized by unimodal middle-sized crystals and crystal texture dominated by nonplanar crystals. It usually occurs as thin or thick layers in the study area. The δ18O and δ13C values of type D2 dolomite generally fall within the estimated Middle Permian marine dolomite and show a significantly positive offset compared to those of coeval seawater. The 87Sr/86Sr ratios of type D2 dolomite are slightly higher than the coeval seawater. These features combined with the flat rare earth element (REE) profile and relatively high Sr concentrations (both are similar to the host limestone) indicate that type D2 dolomites first precipitated in the slightly evaporated Middle Permian seawater during the near-surface to shallow burial phase. Type D3 dolomites are mainly characterized by interlocking mosaic crystals with crystal sizes varying from 300 μm to 900 μm. It mainly occurs as patches in type D2 dolomites or as a thick layer sandwiched by type D2 dolomites. These features combined with its high 87Sr/86Sr ratios, δ18O depletion and positive Eu anomalies (which are very similar to those of the saddle dolomites) demonstrate that it is formed by hydrothermal recrystallization upon type D2 dolomites. Saddle dolomite generally occurs as the filling cement of pores, vugs, and fractures and is usually combined with matrix dolomite to form zebra dolomite and hydrothermal dolomite breccia. It has a curve crystal face, demonstrates wavy extinction under cross-polarization and red luminescence under a cathodoluminescence microscope. These features, together with the highest Th, most negative δ18O value and highest 87Sr/86Sr ratio compared to those of all types of dolomites in the study area as well as the positive Eu anomalies suggest that it should have been formed by hydrothermal fluid precipitation.Type D1 and type D3 dolomites are volumetrically minor and the pores are poorly developed. Saddle dolomite cemented and filled the pores and vugs of matrix dolomite, resulting in reservoir damage. Type D2 dolomites constitute the main body of high-quality reservoir in Qixia Formation. It precipitated in the slightly evaporated penesaline Middle Permian seawater during the near-surface to shallow burial phase. Potential high-quality reservoirs may occur in the shoal around the limited inter-shoal sea.

Causes of variation in the concentration and distribution of H2S in carbonate reservoirs of the Feixianguan Formation, Lower Triassic in the eastern Sichuan Basin

Hydrogen sulfide (H2S) has been encountered in many oil and gas fields worldwide, causing potentially various health and safety issues; thus, it is crucial to assess its concentration before drilling. Natural gas from the Feixianguan carbonates in the Jiannan gas field is predominantly composed of methane with various H2S concentrations varying from very minor to 3.71%. However, the H2S concentrations display obvious difference, ranging from 0.18% to 0.81% in the southern zone and 0.21–3.71% in the northern zone, with high concentrations (>1%) occurring selectively in dolomitized reservoirs in the northern zone. Based on petrographic and geochemical data and reactive transport numerical modeling, the H2S origin and sulfate sources were analyzed for understanding factors controlling the H2S concentration and distribution.In the northern zone containing high H2S concentrations, there is a common mineral assemblage of pyrobitumens, burial calcite cements, pyrite, and hemispherical elemental sulfur in dolomite reservoirs. Burial calcite cements, often coated by pyrobitumens, have higher precipitation temperatures (>106 °C), and lighter δ13C ratios than host dolostones, suggesting the incorporation of organic carbon into the precipitation of these high-temperature burial calcites supplied by thermochemical sulfate reduction (TSR). Numerical modeling indicates the SO42− concentrations of pore waters would increase significantly in dolomitized domains during seawater-involved dolomitization, consisting with the fact of the higher SO42− concentrations of pore waters in the northern zone. The dolomitizing fluids delivered sulfates into reservoirs which were inevitably reduced by hydrocarbons, once the reaction temperature was reached. Comparatively, in the southern zone where carbonates were not subjected to intensive dolomitization and the SO42− concentrations of pore waters were low, the H2S originated only from the thermal decomposition of oil and organic matter (<1%). This study implies that hydrocarbon exploration in dolomitized reservoirs should be given greater caution because of the more favorable conditions for H2S generation.

Characteristics and origin of dolomite reservoir in the fifth member of the Ordovician Majiagou Formation in the Ordos Basin, China

Characteristics and origin of dolomite reservoir in the fifth member of the Ordovician Majiagou Formation in the Ordos Basin, China

A petroliferous Ediacaran microbial-dominated carbonate reservoir play in the central Sichuan Basin, China: Characteristics and diagenetic evolution

Some of the largest gas accumulations in the central Sichuan Basin, south China are hosted in the Ediacaran microbial-dominated dolomite reservoirs. However, recent exploration of the Secondary Member of the Ediacaran Dengying Formation (Z2dn2) has been hampered by an overall lack of understanding of the microbial-dominated dolomite reservoir and its diagenetic evolution process due to its prolonged burial history and complex diagenetic alternation. On the basis of detailed petrographic and geochemical analysis, we have conducted an in-depth investigation on the characteristics, diagenetic sequence and pore evolution process of the microbial-dominated dolomite reservoirs. The main reservoir comprises thromatolitic dolomite, stromatolitic dolomite and straticulate dolomite. Five generations of cement are identified in the microbial dolomite lithofacies. Multiple dissolution, cementation, differential dolomitization, recrystallization and silicification are recognized. Based on elemental geochemistry, carbon and oxygen isotope characteristics, as well as fluid inclusion homogenization temperatures, cements are attributed to six diagenetic stages. The coupling of high-energy deposition, early supergene karsting and dolomitization processes are found to be essential for developing some world-class favorable high-quality microbial-dominated dolomite reservoirs in Z2dn2 in the central Sichuan Basin.

Sequence-lithofacies paleogeographic characteristics and petroleum geological significance of Lower Permian Qixia Stage in Sichuan Basin and its adjacent areas, SW China

Through the analysis of logging, field outcrops, cores and geochemical data, and based on the study of the relationships between sea level changes, sequence filling, paleo-geomorphy and lithofacies, the sequence lithofacies paleo-geography and evolution process of the Lower Permian Liangshan-Qixia Formation (Qixia Stage for short) in Sichuan Basin and its surrounding areas are restored. The Qixia Stage can be divided into three third-order sequences, in which SQ0, SQ1 and SQ2 are developed in the depression area, and SQ1 and SQ2 are only developed in other areas. The paleo-geomorphy reflected by the thickness of each sequence indicates that before the deposition of the Qixia Stage in the Early Permian, the areas surrounding the Sichuan Basin are characterized by “four uplifts and four depressions”, namely, four paleo-uplifts/paleo-lands of Kangdian, Hannan, Shennongjia and Xuefeng Mountain, and four depressions of Chengdu-Mianyang, Kangdian front, Jiangkou and Yichang; while the interior of the basin is characterized by “secondary uplifts, secondary depressions and alternating convex-concave”. SQ2 is the main shoal forming period of the Qixia Formation, and the high-energy mound shoal facies mainly developed in the highs of sedimentary paleo-geomorphy and the relative slope break zones. The distribution of dolomitic reservoirs (dolomite, limy dolomite and dolomitic limestone) has a good correlation with the sedimentary geomorphic highs and slope break zones. The favorable mound-shoal and dolomitic reservoirs are distributed around depressions at platform-margin and along highs and around sags in the basin. It is pointed out that the platform-margin area in western Sichuan Basin is still the key area for exploration at present; while areas around Chengdu-Mianyang depression and Guangwang secondary depression inside the platform and areas around sags in central Sichuan-southern Sichuan are favorable exploration areas for dolomitic reservoirs of the Qixia Formation in the next step.