Development Of High-quality Reservoirs Research Articles

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.

Impact of sedimentation and diagenesis on deeply buried sandy conglomerate reservoirs quality in nearshore sublacustrine fan: A case study of lower Member of the Eocene Shahejie Formation in Dongying Sag, Bohai Bay Basin (East China)

The sandy conglomerates of the lower member of Shahejie (Es4x) in the north of Dongying Sag, Bohai Bay Basin, were investigated to understand the formation mechanism of the good reservoir and controlling factors in deeply buried strata. This research is based on seismic, logging, and core data and comprehensively utilizes methods such as core description, petrographic analysis, cathodoluminescence image analysis, SEM-EDS analysis, and mercury intrusion. The sandy conglomerates are predominantly lithic arkose and litharenite. The reservoirs in the Es4x have experienced three diagenetic processes, compaction, dissolution, and cementation. Compaction remains the major factor affecting the reservoir quality, mainly in the progradation sequence, and cementation has a destructive effect on reservoirs, principally in the retrogradation sequence. The quantification of the compaction by the compaction index revealed that mechanical compaction is higher in the progradation sequence (0.68) than in the retrogradation sequences (0.59). Compaction is suppressed by carbonate cementation, but the extensive cementation significantly degrades the reservoir quality, especially in the retrogradation sequence. Dissolution of different rock fragments, feldspar, and the whole dissolution, in general, enhance the reservoir's quality in both intervals but more in the progradation sequence. Three types of reservoirs representing type I, type II, and Type III mercury intrusion capillary pressure curves were encountered in the Es4x. The braided channels in the progradation and retrogradation sets encompass the greatest reservoir quality and are characterized by type I and II reservoirs with good petrophysical properties. The preeminent reservoirs appear in the braided channels of the progradation set. The development of high-quality reservoirs is governed by the depositional subfacies, compaction, and dissolution in the Es4x. This study will contribute to comprehending the origin of the genesis mechanism of high-quality in deep sublacustrine sandy conglomerate and will help the exploration and pre-drill for favorable reservoirs in similar basins around the world.

Study on fluid mobility in sandwich-type shale oil reservoir using two-dimensional nuclear magnetic resonance approaches

ABSTRACT Regarding sandwich-type shale oil reservoir characterized by diversified lithologies and complex fluid mobility, we combined one- and two-dimensional (1D and 2D) nuclear magnetic resonance (NMR) core analysis techniques to quantitatively study the movable oil and fluid saturations of rock samples with different lithologies. First, we used 2D T1-T2 NMR to analyze the spectrum curve of the dolomitic mudstone and siltstone samples from a sandwich-type shale oil reservoir at initial and oil-saturated states. We evaluated the occurrence of movable oil saturations in source and reservoir rocks. Then, we analyzed samples of the siltstone serving as the reservoir rock using a centrifuge combined with 1D NMR approach to evaluate the characteristics of movable fluid saturations inside the reservoir. The results showed that the samples of dolomitic mudstone serving as the source rock in the sandwich-type shale oil reservoir in the Lucaogou Formation in Jimsar had the occurrence of kerogen and bitumen signals, the development of small- and medium-scale organic matter pores, and an average movable oil saturation of 15.6%. The siltstone serving as the reservoir rock had an average movable oil saturation of 53.8%, and its movable fluid saturation was positively correlated with porosity and permeability. For samples whose porosity was greater than 12% and permeability was greater than 0.1 mD, their movable fluid saturations usually were higher than 40%. Therefore, siltstone was the preferable diagenetic facies for the development of high-quality reservoirs.

An integrated pore size distribution measurement method of small angle neutron scattering and mercury intrusion capillary pressure

Small-angle neutron scattering and high-pressure mercury intrusion capillary pressure testing are integrated to analyze the pore size distribution of the broad sense shale oil reservoir samples of the Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin, China. The results show that, compared with the measurement method integrating gas adsorption and mercury intrusion, combination of small-angle neutron scattering and mercury intrusion can more accurately characterize full-scale pore size distribution. The full-scale pore size distribution curve of the rock samples in the study area includes two types: the declining type and submicron pore-dominated type. The declining type is mainly found with silty mudstone and dolomitic mudstone, and most of its pores are smaller than 80 nm. Silt-fine sandstones and dolarenite are mostly of the submicron pores-dominated type, with most pores smaller than 500 nm. They also present large specific pore volumes and average pore diameters of macropores and are the favorable lithogenous facies for development of high-quality reservoirs.

Reservoir Characteristics of Buried-hill Draping Zone in L Oilfield, Offshore China

The reservoir in different parts of buried-hill draping zone is often quite different, so it is of great significance to clarify the reservoir characteristics for exploration and development. Based on core, well logging, seismic data and production data, reservoir characteristics of oil layer Ⅱ in the lower second member of Dongying Formation of L oilfield, Bohai Bay Basin, offshore eastern China are systematically studied. Analyses of seismic facies, well-seismic combination, paleogeomorphology, and sedimentary characteristics are carried out. Sediment source supply, lake level and buried hill basement geomorphology all contribute to reservoir quality. The research suggests that the different parts of buried-hill draping zone can be divided into four types. Reservoir thickness and physical properties vary. The area where the provenance direction is consistent with the ancient valley direction is a favorable location for the development of high-quality reservoirs. Under the guidance of the results, oilfield production practices in L oilfield offshore China are successful. Knowledge gained from study of L oilfield has application to the development of other similar fields.

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

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

Orderly distribution and differential enrichment of hydrocarbon in oil-rich sags: A case study of Dongying Sag, Jiyang Depression, Bohai Bay Basin, East China

Based on fine geological modeling and taking the evolution of key reservoir-forming elements as the main line, the mechanism of orderly distribution and differential enrichment of hydrocarbon was revealed by studying the joint evolution and coupling effect of pressure, fluid and reservoir properties. Orderly development of sedimentary systems in a rift basin is the base of orderly distribution of reservoirs, and the continuity of pressure structure is the key to controlling orderly distribution of reservoir. From the sag center to the margin, in the sag, second-order sequence and large-scale sedimentary system, the reservoirs appear in an orderly distribution from lithologic reservoir to structural reservoir to stratigraphic reservoir. Alternative acid and alkaline actions controlled the development of high quality reservoirs in the mid-fan sandy conglomerate bodies in the steep slope, resulting in the oil and gas accumulation pattern of sealing at the root fan and enrichment at the mid-fan, in which the breakthrough pressure difference between root fan and mid-fan determines the reservoir enrichment level. The action of acidic fluid controlled the development of high quality reservoirs in beach bar sand and turbidite. The pressure difference between high-pressure source rock caused by pressurization of hydrocarbon generation and low-pressure reservoir caused by reservoir improvement provided driving force for oil and gas charging, giving rise to the pressing-absorbing oil and gas charging mechanism controlled by source rock overpressure. The research results have guidance and reference significance for fine exploration in mature exploration areas.

Paleo-environment and reservoir evolution of the Middle Ordovician Kelimoli Formation in the northern Tianhuan area, Ordos Basin, China: implications for high-quality reservoir development

ABSTRACTThe Middle Ordovician Kelimoli Formation carbonate reservoirs in the northern Tianhuan area, Ordos Basin, China are main exploration targets. Subsurface core samples, logging, drilling and production data from the area were used to characterise the carbonate reservoir and to construct a genetic model for exploration. The sedimentary facies identified include trough–continental rise, upper–lower slope, platform margin reef-shoal, open platform and evaporation-restricted platform. The slope and platform margin facies are potential zones for high-quality reservoirs. Porosity in the study area comprises intergrain, intercrystal, intragrain and intracrystal pores, fractures and vughs. The Sr/Ba (0.40–4.87) and V/(V + Ni) (0.64–0.97) ratios indicate deposition in a brackish water-dominated environment under reducing conditions, associated with sea-level fluctuations during the deposition of the Kelimoli Formation. The Sr/Ba, V/(V + Ni), 87Sr/86Sr ratios, δ18O values and crystal texture of dolomite samples suggest that meteoric water was involved in the diagenetic fluid in the near-surface depositional environment. Isotopes (δ13C and δ18O) and trace elements (Fe and Mn) allowed the identification of seepage-reflux and mixed-water dolomitisation. The crystal textures of the samples consist of micritic, bioclastic and reefal limestones, and dolomite with gypsum, which were easily dissolved during the early diagenetic stage. The epidiagenetic stage was the key period for the development of high-quality reservoirs because of large-scale bedding-parallel karstification from meteoric water. The reservoir zones, dominated by partially filled and unfilled vughs, and fractures, are favourable exploration targets in the northern Tianhuan area.

Discovery and theoretical and technical innovations of Yuanba gas field in Sichuan Basin, SW China

Abstract To solve the difficulties in exploration and development in Yuanba ultra-deep gas field in Sichuan Basin, SW China, the article studies the mechanism of quality reef reservoirs development and gas accumulation and innovates techniques in ultra-deep seismic exploration, drilling, completion and testing. Through the reconstruction of dynamic depositional evolution process and regional depositional framework of homoclinal ramp-rimmed platform in Upper Permian, three theories are put forward: first, “early beach-late reef, multiple stacking, arrangement in rows and belts” is the sedimentary mode for the reservoirs in the Changxing Formation of Yuanba area; second, “dissolution in early exposure stage and dolomitization during shallow burial giving rise to the pores in matrix, overpressure caused by cracking of liquid hydrocarbon during deep burial inducing fractures” is the reservoirs development mechanisms; third, “coupling of pores and fractures” controls the development of high quality reservoirs in deep formations. From correlation of oil and source rock, it is concluded that the Wujiaping Formation and Dalong Formation of deep-water continental shelf are the major source rocks in the Permian of northern Sichuan Basin. The hydrocarbon accumulation mode in ultra-deep formations of low-deformation zones is characterized by “three-micro (micro-fault, micro-fracture interbed crack) migration, near-source enrichment, and persistent preservation”. Through seismic inversion using the pore structure parameters of pore-fracture diadactic structure model, the high production gas enrichment area in Yuanba gas field is 98.5 km2. Moreover, special well structure and unconventional well structure were used to deal with multiple pressure systems and sealing of complex formations. A kind of integral, high pressure resistant FF-level gas wellhead and ground safety linkage device was developed to accomplish safe and environmentally friendly gas production.

Reservoir quality, natural fractures, and gas productivity of upper Triassic Xujiahe tight gas sandstones in western Sichuan Basin, China

The main target for hydrocarbon exploration in the western Sichuan Basin in China is the tight gas sandstone reservoir of the Upper Triassic Xujiahe Formation. Sandstones of the Xujiahe Formation are characterized as tight with ultralow porosity, ultralow permeability, and intensive heterogeneity. In this study, various techniques, such as scanning electron microscopy, X-ray diffraction, cathodoluminescence microscopy, casting thin-section, and fluid inclusion thermometry, were employed to investigate the composition, sedimentary facies, diagenesis history, diagenetic intensity, natural fracture characteristics, and their impact on reservoir quality. The results showed that subaqueous distributary channels and mouth bars with low rock fragment content are favorable for pore preservation and secondary pore development. Except for the medium to strong dissolution intensity, chlorite rim growth is also important for the development of high quality reservoirs. Intraformational shear fractures and horizontal shear fractures associated with tectonic movement increase pore space and improve the capability of transmitting fluids. Differences in gas productivity could be attributed to differences in reservoir quality, which were controlled by the sedimentary environment, diagenetic intensity, and fracture characteristics. A significantly positive correlation was found between total porosity and gas productivity of the tight sandstones. The high density fracture distribution was found owing to the fault system and structural deformation zone, which resulted in high gas productivity. The research results were successfully applied to improve production efficiency in the Xujiahe tight gas sandstones, and could be applicable to other similar fields involving tight gas.

The Early Cambrian Mianyang-Changning Intracratonic Sag and Its Control on Petroleum Accumulation in the Sichuan Basin, China

The older and deeper hydrocarbon accumulations receive increasing attention across the world, providing more technical and commercial challenges to hydrocarbon exploration. We present a study of an asymmetrical, N-S striking intracratonic sag which developed across the Sichuan basin, south China, from Late Ediacaran to Early Cambrian times. The Mianyang-Changning intracratonic sag is ~50 km wide, with its steepest part in the basin center. In particular the eastern margin shows its greatest steepness. Five episodes in the evolutions of the sag can be recognized. It begins in the Late Ediacaran with an uplift and erosion correlated to Tongwan movement. Initial extension occurred during the Early Cambrian Maidiping period, when more strata of the Maidiping Formation were deposited across the sag. Subsequently, maximum extension occurred during the Early Cambrian Qiongzhusi period that resulted in 450–1700 m thick Maidiping-Canglangpu Formations being deposited in the sag. Then, the sag disappeared at the Longwangmiao period, as it was infilled by the sediments. The intracratonic sag has significant influence on the development of high-quality reservoirs in the Dengying and Longwangmiao Formations and source-rock of the Niutitang Formation. It thus indicates that a high probability for oil/gas accumulation exists along the intracratonic sag, across the central Sichuan basin.