High-quality Dolomite Reservoirs Research Articles

Dissolution kinetics and heterogeneous evolution of dolomite with different pore structures

Mesogenetic dissolution is a critical process in the evolution and preservation of pore spaces in carbonate reservoirs. While limestone dissolution has been extensively studied, there is a lack of research on the dissolution kinetics of dolomite reservoirs, particularly regarding differences in dissolution mechanisms and heterogeneity evolution across various pore structures. This study aims to investigate diagenesis processes and heterogeneity evolution patterns in dolomites with different pore structures by simulating reservoir dissolution using organic acids generated during kerogen cracking. Flow system dissolution experiments were conducted on four dolomites with varying pore structures in 0.2% acetic acid under high-temperature (T = 40–160 °C) and high-pressure (P = 10–50 MPa) conditions. The chemical composition of the fluid and pore structure images were analyzed using ICP-OES and X-ray computed microtomography, respectively. Pore size distribution and evolution were assessed through digital cores based on Micro-CT analysis, while fractal and multifractal analyses were employed to quantify the evolution of pore structure heterogeneity. The findings highlight the importance of an effective combination of early material base and subsequent organic acid dissolution in the formation and maintenance of deep, high-quality dolomite reservoirs.

Diagenetic evolution and cementation mechanism in deep Carbonate reservoirs: A case study of Dengying Fm. 2 in Penglai, Sichuan Basin, China

The Dengying Formation in the Penglai region of the Sichuan Basin is renowned for hosting high-quality dolomite reservoirs. However, the understanding of their formation mechanisms and spatial distribution patterns remains enigmatic. This study undertakes a comprehensive petrographic analysis of the second member of the Dengying Formation, primarily focusing on its burial history, reservoir characteristics, and an examination of the types, stages, and developmental patterns of cementation within the dolomite reservoir. This is achieved through the application of isotopic geochemistry, inclusion temperature analysis, and laser U-Pb dating. The findings reveal that cements within the second member of the Dengying Formation predominantly occur in pores and fractures, often exhibiting euhedral and subhedral morphologies. These cements also exhibit evidence of replacing original non-skeletal grains and micrites in semi-euhedral and other forms. Broadly, the cements can be classified into four types: calcium cement (the most abundant), dolomite cement, quartz, and pyrite. The formation of calcareous and siliceous cements closely aligns with the sedimentation and burial processes of the Dengying Formation as a whole. The early to late stages of early-middle diagenesis play a crucial role in the formation of dolomite within the second member. Notably, the degree of euhedral dolomite formation directly correlates with the specific diagenetic stage. Our research indicates that the development of high-quality reservoirs in deep-to-ultra-deep microbial dolomites is a result of complex multi-factor interactions and multi-stage discontinuous modifications, leading to strong heterogeneity within the reservoirs. Microbial dolomite carbonates inherently exhibit high initial porosity, with the favorable sedimentary facies serving as the fundamental basis for reservoir development. Dissolution processes during the syngenetic and quasi-syngenetic periods, in conjunction with epigenetic karstification, further augment porosity and expand the reservoir's capacity. During burial and deep burial stages, various acidic fluids play a pivotal role in preserving and modifying the early-formed pores. In the deep environment characterized by high temperature and pressure, microbial dolomite reservoirs undergo intricate diagenetic evolution and possess complex pore preservation mechanisms. Remarkably, even under exceptional ultra-deep conditions, high-quality microbial dolomite carbonate reservoirs can still develop and be preserved, presenting significant potential for oil and gas exploration.

Research advances on the mechanisms of reservoir formation and hydrocarbon accumulation and the oil and gas development methods of deep and ultra-deep marine carbonates

Based on the new data of drilling, seismic, logging, test and experiments, the key scientific problems in reservoir formation, hydrocarbon accumulation and efficient oil and gas development methods of deep and ultra-deep marine carbonate strata in the central and western superimposed basin in China have been continuously studied. (1) The fault-controlled carbonate reservoir and the ancient dolomite reservoir are two important types of reservoirs in the deep and ultra-deep marine carbonates. According to the formation origin, the large-scale fault-controlled reservoir can be further divided into three types: fracture-cavity reservoir formed by tectonic rupture, fault and fluid-controlled reservoir, and shoal and mound reservoir modified by fault and fluid. The Sinian microbial dolomites are developed in the aragonite-dolomite sea. The predominant mound-shoal facies, early dolomitization and dissolution, acidic fluid environment, anhydrite capping and overpressure are the key factors for the formation and preservation of high-quality dolomite reservoirs. (2) The organic-rich shale of the marine carbonate strata in the superimposed basins of central and western China are mainly developed in the sedimentary environments of deep-water shelf of passive continental margin and carbonate ramp. The tectonic-thermal system is the important factor controlling the hydrocarbon phase in deep and ultra-deep reservoirs, and the reformed dynamic field controls oil and gas accumulation and distribution in deep and ultra-deep marine carbonates. (3) During the development of high-sulfur gas fields such as Puguang, sulfur precipitation blocks the wellbore. The application of sulfur solvent combined with coiled tubing has a significant effect on removing sulfur blockage. The integrated technology of dual-medium modeling and numerical simulation based on sedimentary simulation can accurately characterize the spatial distribution and changes of the water invasion front. Afterward, water control strategies for the entire life cycle of gas wells are proposed, including flow rate management, water drainage and plugging. (4) In the development of ultra-deep fault-controlled fractured-cavity reservoirs, well production declines rapidly due to the permeability reduction, which is a consequence of reservoir stress-sensitivity. The rapid phase change in condensate gas reservoir and pressure decline significantly affect the recovery of condensate oil. Innovative development methods such as gravity drive through water and natural gas injection, and natural gas drive through top injection and bottom production for ultra-deep fault-controlled condensate gas reservoirs are proposed. By adopting the hierarchical geological modeling and the fluid-solid-thermal coupled numerical simulation, the accuracy of producing performance prediction in oil and gas reservoirs has been effectively improved.

A novel evaluation method of dolomite reservoir using electrical image logs: The Cambrian dolomites in Tarim Basin, China

The Cambrian dolomites in Tarim basin are important hydrocarbon exploration targets. However, these ancient dolostone reservoirs are deeply buried, and the lithology and reservoir space are complex, therefore the logging evaluation of reservoir effectiveness is difficult. This study analyzes the log response characteristics using core data and electric image logs, and divides the dolomite reservoir into four types: pore type, fracture type, vug type and fracture-vug type. Among them, the vug type and fracture-vug type are the main reservoir types with the highest industrial production. Then, this study innovatively proposes the method of point-by-point calibration of electrical image log, which avoids the difference in response characteristics of various electrical image log instruments, and reduces the uncertainty caused by human factors of conventional calibration. The point-by-point calibration method uses local data for comparison and calculates calibration coefficient automatically. The calibrated electrical image log data can maintain better matching and consistency with conventional resistivity in high and low resistivity simultaneously, and it is more truly reflect the borehole-wall resistivity, consequently, can establish a foundation for subsequent reservoir evaluation. Finally, according to the calibrated electrical image log data and the 360° roll-scan core photos, the vugs in-plane porosity on electrical image, the vug connectivity and the porosity spectrum are used to identify reservoir types and evaluate reservoir effectiveness. The vugs in-plane porosity on electrical image can truly reflect the development of fractures and vugs, and quantitatively evaluate the vugs and fractures of the whole borehole-wall by calibrating with core. The vug connectivity index can indicate the isolated vugs and connected vugs, and quantitatively evaluate the connectivity of vugs and fractures, in addition the porosity spectrum can reflect the pore structure of reservoir and identify the reservoir with secondary porosity. The coupling of the three methods can accurately identify high-quality dolomite reservoirs with vugs and good connectivity. This novel technology adopts the idea of classification and identification before quantitative evaluation and establishes a new bridge from qualitative analysis to quantitative evaluation. The method proposed helps improve the interpretation accuracy and coincidence rate and provides a reference for the comprehensive evaluation of ultra-deep dolomite reservoirs in the Tarim basin and worldwide.

Mg isotopic geochemistry and origin of Early Ordovician dolomite and implications for the formation of high-quality reservoir in the Tabei area, Tarim Basin, NW China

The source and driving mechanism of Mg-rich fluids in the dolomitization process of the Lower Ordovician Penglaiba Formation in the Tarim Basin have not been effectively traced, limiting the accuracy of the prediction of high-quality dolomite reservoirs. In this study, based on Mg isotopes combined with petrology, trace elements, and C-O-Sr isotopes, the powder crystalline dolomite (D1) and fine crystalline dolomite (D2) of the Penglaiba Formation were found to have high Na and Sr contents and low Fe and Mn contents. The REE, δ13C, δ18O and 87Sr/86Sr ratios ranges were similar to those of seawater, and δ26Mg shows a steady downward trend. The dolomitization fluids of D1 and D2 were derived from seawater and migrated vertically downward. Penecontemporaneous dolomitization is beneficial for the inheritance and preservation of original rock pores. The Na and Sr contents of medium crystalline dolomite (D3) and coarse rystalline dolomite (D4) were low, whereas the Fe and Mn contents were high. The REE, δ18O and 87Sr/86Sr ratios differed from those of seawater, and δ26Mg shows a fluctuating trend. The fluids for the precipitation of D3 and D4 were sourced from concentrated marine pore water during burial, , with the dolomitization fluid migrating through intergranular pores and structural fractures. Burial dolomitization has preservation, adjustment, and transformation effects on dolomite reservoirs. Deep high-quality dolomite reservoirs are not only controlled by the original sedimentary environment, but is also affected by late-stage or subsequent burial transformation.

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.

Exploration field analysis and zone optimization of sinian, Tarim Basin, China

Carbonate rock from the Late Proterozoic to the Early Paleozoic is an important field of oil and gas exploration. The Sinian carbonate rock series in Tarim Basin are old, deep buried, with few drilling wells and various geological problems are unclear that restrict the exploration of deep carbonate rock. Based on the analysis of Sinian geological structure, by means of stratigraphic correlation and seismic prediction, the distribution of Sinian favorable source rocks, deep reservoir facies belts, and reservoir forming assemblages are studied to comb the exploration fields and favorable zones of Sinian in Tarim Basin. The findings reveal that slope-basin facies source rocks developed in Sinian. In the upper part of the Qigebulake Formation, high-quality dolomite reservoirs evolved, and mudstone from the Yuertus Formation at the foot of Cambrian can form favorable reservoir cap assemblage. The south slope of Tabei Uplift and the north slope of Tazhong Uplift are the most favorable exploration zones for Sinian dolomite, favorable exploration area of approximately 31,000 km2. The findings can provide a certain reference for the Sinian carbonate oil and gas exploration in the Tarim Basin.

Characteristics and Formation Mechanism of the Lower Paleozoic Dolomite Reservoirs in the Dongying Depression, Bohai Bay Basin

The Lower Paleozoic carbonate strata experience multi-stage tectonic activity and post-depositional volcanic activity in the Dongying Depression, Bohai Bay basin. These tectonic and magmatic activities have caused the reservoir to undergo severe diagenesis, resulting in strong reservoir heterogeneity. This study aims to identify the characteristics of dolomite, various reservoir spaces’ characteristics, the origin of different types of dolomite, and the porosity evolution. According to crystal size and morphology, dolomites can be divided into three kinds of matrix dolomites and four kinds of dolomite cements. The petrology and geochemistry of the dolomite suggests that matrix dolomite is formed from seawater. The medium-to-coarse-crystalline dolomite cement (D3) has a higher 87Sr/86Sr ratio (0.7119 to 0.7129) and a higher homogenization temperature (>125 °C), suggesting that the fluid for the precipitation of D3 is a mixed fluid formed by hydrothermal fluid eroding the 87Sr-rich feldspar sandstone. The strikingly negative δ18O values (−23.7 to −25.7‰ VPDB) of saddle dolomite (D4) indicate that D4 precipitated from hydrothermal fluids and the Mg2+ source may be due to dissolution of the host dolomite that formed in the evaporation environment. The reservoir spaces of the target strata in the study area mainly include fractures, dissolution vugs, intercrystalline pores, and moldic pores. Dissolution is the basis for forming high-quality dolomite reservoirs. The faults and fractures provided favorable conditions for dissolution. Hydrothermal fluid and organic acid were the main dissolution fluids for the dolomite reservoir, which were beneficial to the development of secondary pores. In the study area, organic acid dissolution was shown to contribute more than hydrothermal dissolution in the study area.

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.

TSR, deep oil cracking and exploration potential in the Hetianhe gas field, Tarim Basin, China

Oil and gas in deep and ultra-deep strata has been a hot topic in both research and exploration. The understanding of reservoirs in relatively shallow strata may be of significance for deep exploration. The Hetianhe gas field is the only large gas field discovered yet in the southwest region of the Tarim Basin, with a small quantity of condensate oil penetrated and a burial depth ranging from 1035 m to 2885 m, and the origin and source of the oil and gas has long been unclear. By using comprehensive 2D gas chromatography/time of flight mass spectrometry (GC × GC-TOFMS), abundant diamondoids and some thiadiamondoids were detected in condensate oil samples collected from 3 wells, additionally, the features, including dry gas with a trace amount of hydrogen sulfide, heavy carbon isotopes, and similar sulfur isotopes in hydrogen sulfide and Cambrian gypsum, indicate that the oil and gas has undergone the thermochemical sulfate reduction (TSR) and thermal cracking. The current reservoir temperature, from 35 °C to 83 °C, does not meet the condition for the onset of these secondary geochemical alterations, therefore, the oil and gas may originate from deep strata. According to the reconstruction of the geologic evolution, deep oil and gas may have migrated vertically along faults and accumulated in the Hetianhe structure formed after 10 Ma to form the secondary gas reservoirs; hence it is inferred that favorable petroleum plays may exist in deep strata. The thick evaporite in the Middle Cambrian and the high-quality dolomite reservoirs in the Lower Cambrian constitute a favorable reservoir-seal assemblage, with the current depth between 8500 m and 10000 m and reservoir temperature above 200 °C, where the TSR and thermal cracking may occur. The sulfur isotope of hydrogen sulfide also demonstrates the occurrence of TSR process in the Cambrian strata. Therefore, large-scale of oil and gas reserves may be preserved in the stable zones in the Cambrian sub-salt strata, and it is suggested to strengthen hydrocarbon exploration in the Cambrian sub-salt strata in the southwest region of the Tarim Basin.

Origins and Geochemistry of Dolomites and Their Dissolution in the Middle Triassic Leikoupo Formation, Western Sichuan Basin, China

Triassic dolomites occur pervasively in the Western Sichuan Basin. Although these strata have been deeply buried and affected by multiple phases of dolomitization and dissolution, some intervals in the upper part of the Leikoupo Formation have developed high porosity. Based on their petrographic and geochemical features, three major types of dolomite fabrics are recognized, namely, dolomicrite, fabric-retentive dolomite, and fabric-destructive dolomite. Geochemical evidence indicates that the dolomicrite formed following the Sabkha model in a low-temperature hypersaline environment, as these rocks exhibit abnormally high Sr and Na contents, lower Fe and Mn contents, δ18O values generally ranging from −1.70‰ to −1.67‰ (with an average value of −1.69‰), and higher Mg/Ca ratios. The fabric-retentive dolomite formed following the seepage-reflux model in a shallow burial environment, and these rocks exhibit the highest 87Sr/86Sr ratios, δ18O values generally ranging from −6.10‰ to −2.50‰ (with an average value of −3.98‰), and a wide range of Fe and Mn contents, indicating that they may have been altered by meteoric water. The fabric-destructive dolomite formed following the burial model at elevated temperatures; these rocks exhibit the lowest Sr and Na contents, δ18O values generally ranging from −7.01‰ to −6.62‰ (with an average value of −6.79‰), relatively higher Mg/Ca values, and lower 87Sr/86Sr ratios. The early Sabkha, seepage-reflux dolomitization and penecontemporaneous periodic meteoric freshwater selective dissolution processes formed multi-period, overlapping moldic pores, algal framework pores, and intragranular dissolution pores. The superposition of organic acid dissolution during the burial period is the main controlling factor of the formation of deeply buried, high-quality dolomite reservoirs in the Leikoupo Formation.

Seismic prediction on the favorable efficient development areas of the Longwangmiao Fm gas reservoir in the Gaoshiti–Moxi area, Sichuan Basin

The Lower Cambrian Longwangmiao Fm gas reservoir in the Gaoshiti–Moxi area, the Sichuan Basin, is a super giant monoblock marine carbonate gas reservoir with its single size being the largest in China. The key to the realization of high and stable production gas wells in this gas reservoir is to identify accurately high-permeability zones where there are dissolved pores or dissolved pores are superimposed with fractures. However, high quality dolomite reservoirs are characterized by large burial depth and strong heterogeneity, so reservoir prediction is of difficult. In this paper, related seismic researches were carried out and supporting technologies were developed as follows. First, a geologic model was built after an analysis of the existing data and forward modeling was carried out to establish a reservoir seismic response model. Second, by virtue of well-oriented amplitude processing technology, spherical diffusion compensation factor was obtained based on VSP well logging data and the true amplitude of seismic data was recovered. Third, the resolution of deep seismic data was improved by using the well-oriented high-resolution frequency-expanding technology and prestack time migration data of high quality was acquired. And fourth, multiple shoal facies reservoirs were traced by using the global automatic seismic interpretation technology which is based on stratigraphic model, multiple reservoirs which are laterally continuous and vertically superimposed could be predicted, and the areal distribution of high quality reservoirs could be described accurately and efficiently. By virtue of the supporting technologies, drilling trajectory is positioned accurately, and the deployed development wells all have high yield. These technologies also promote the construction of a modern supergiant gas field of tens of billions of cubic meters.

Carbon and oxygen isotopic compositions, and palaeoenvironmental significance of saline lacustrine dolomite from the Qaidam Basin, Western China

Recently, significant progress has been made in tight reservoir exploration of petroleum in the western Eocene Qaidam Basin in Western China. Among these rocks, there are high-quality dolomite reservoirs, of which we studied the petrological, mineralogical, and geochemical characteristics. These analyzes revealed that the dolomite structures are mainly micritic, with a widely developed intercrystal pore. The pore radius measured less than 1 μm and the micritic dolomite δ13CPDB values ranged from −4.5‰ to 0.2‰, with an average value of −2.4‰. These values are negatively δ13C biased when compared to other lacustrine carbonate rock in China, indicating that the braided delta supplies light carbon-rich surface water. Over time, the dolomite carbon isotopic composition changed due to dissolved carbon from organic matter, which produced CO2 via oxidation disintegration. In the case of light carbon, the rich organic matter resulted in a lighter isotopic composition of the dissolved carbon and modified the lacustrine deposit dolomite carbon isotopic composition indirectly. In addition, the micritic dolomite δ18OPDB values ranged from −6.1‰ to 0.4‰, with an average value of −2.6‰. This value is positively δ18O biased compared to other lacustrine carbonate rock in China and was affected by the saline lacustrine environment. The oxygen and carbon isotope correlation, salinity, and temperature data reflect that the dolomite micritic formed in both a semi-open and semi-closed saline environment on a penecontemporaneous stage via calcite metasomatism in the sediment interface under a low-temperature. This type of penecontemporaneous metasomatism dolomite is widely distributed throughout the study area, and the intergranular pores possess a strong anti-compaction ability. This critical discovery of the plane distribution of the tight dolomite in the Qaidam Basin has provided multiple novel avenues of study within the field.

Formation mechanism of deep Cambrian dolomite reservoirs in the Tarim basin, northwestern China

Well TS1, which is located in the north uplift of the Tarim basin, northwestern China, has a total depth of 8408 m and is the deepest petroleum well in the world. The well penetrates a high-quality Upper Cambrian dolomite reservoir from 6884 m to 8408 m. The porosity of this dolomite increases with increasing depth, reaching 9.1% at a depth of 8407.56 m. A large number of dissolution pores are present in the core retrieved from well TS1. Some of the pores are partially filled with coarse dolomite crystals, which display saddle shapes and undulose extinction. The pore-filling dolomites have high concentrations of Fe and Mn, averaging 3158.1 and 172.5 ppm, respectively. The concentration of Ba is up to 4000.3 ppm. The oxygen isotope composition of the pore-filling dolomites is relatively light, and the δ18O values range from −10.9‰ to −5.1‰. The 87Sr/86Sr values are relatively high, ranging from 0.709361 to 0.709975. The mineralogical and geochemical features demonstrate that the pore-filling dolomites were precipitated from hydrothermal fluids that significantly altered the deep Cambrian dolomite reservoirs during upward migration along faults and fractures in the Permian. According to petrologic, geochemical and seismic features, the dolomite reservoirs were first dissolved by meteoric water during or shortly after deposition and were then significantly altered by hydrothermal fluids during burial. A porosity variation model was proposed for the deep dolomite reservoirs that were influenced by hydrothermal fluids. The model of increasing porosity with increasing depth in well TS1 indicates that there may be high-quality dolomite reservoirs in the deep strata of basins where hydrothermal fluids are active. The small amount of liquid hydrocarbon obtained from cores at a depth of more than 8400 m demonstrates that the deep dolomite reservoir is still suitable for hydrocarbon accumulation due to an unusually low geothermal gradient of approximately 20 °C/km. The deep strata in basins with similar geological parameters may therefore be potential exploration targets elsewhere in the world.

Geological characteristics and forming conditions of the platform margin large reef-shoal gas province in the Sichuan Basin

Abstract The large gas province of platform margin reefs and shoals of the Permian-Triassic ChangxingFormation-Feixianguan Formation in the Sichuan Basin is studied as an example to describe the forming conditions and geological characteristics of large strip-like gas provinces of platform margin reefs and shoals. The Sichuan Basin experienced cratonic rifting and basement faulting during the Late Permian. Within a widespread regional ramp, a paleogeomorphology pattern of “three highs and three depressions” were formed. The three negative relieves are, respectively, Chengkou-E'xi trough, Kaijiang-Liangping trough, and Nanchong-Mianyang intraplatform depression, and the three topographic highs are Fengjie-Zhenba platform flat, Shizhu-Yilong platform flat, and Suining-Moxi platform flat. In the transitional areas between platform flats and troughs, large-scale platform margin reef-shoal complexes were developed under high energy environment. Diagenetic processes, such as early stage dissolution, seepage-reflux dolomitization and burial dissolution, contributed to porosity improvement in high-quality dolomite reservoirs along platform margins. High mature source rocks and faults developed within the platform margin reef-shoal complexes, forming better hydrocarbon accumulation conditions than those of intraplatform reefs and shoals. Therefore, the distribution of platform margins controlled that of gas accumulation zones. Paleogeography background of large platform, extensive dolomitization and solution in reef-shoal reservoirs, good underlying and lateral adjacent source rocks, strip assemblages of lithological, structural, and litho-structural oil and gas traps are the four basic elements that determine the formation and distribution of large strip-like gas provinces. The configuration of the four elements in the studied area shows promising exploration prospects of large platform margin oil and gas provinces.

The formation mechanism of high-quality dolomite reservoir in the deep of Puguang Gas Field

The discovery of Puguang Gas Field provides the exploration of China deep marine carbonate rock with important references. In Puguang Gas Field, the dolomite reservoirs discovered in the deep are the best in the present of China, which present big thickness and wide-range distribution, and develop abundant secondary porosity. The researches show that Puguang Gas Field bears the characteristics of early gas-filling time, deep burial, high matured organic matter and long-term interaction of hydrocarbon (oil and gas)-water-rock (carbonate reservoir). The developments of secondary pores in this area are affected by multiple diagenesis and their formation mechanisms are complicated. Through the research on depositional environment, sedimentary facies and reservoir porosity characters of Changxing and Feixianguan Formations, it is thought that high-quality dolomite reservoirs of Puguang Gas Field form on the favorable sedimentary facies belts, which are the integrate result affected by several factors including superficial corrosion, burial corrosion, overpressure and tectonic movement, among which burial corrosion of TSR to reservoir and overpressure formed by thermal evolution of organic matter have great effect on the formation of secondary porosity of Changxing and Feixianguan Formations.