Characteristics and influencing factors of Cretaceous reservoir in eastern depression of North Yellow Sea basin

Cretaceous is the key exploration target layer in the eastern depression of North Yellow Sea basin, which has a good prospect for oil and gas exploration. Its huge oil and gas resource potential has attracted great attention from petroleum geologists. In this study, the main rock types, reservoir space types, petrophysical characteristics and main controlling factors of Cretaceous reservoir are studied through core observation, thin section identification, petrophysical analysis and scanning electron microscope observation. The results indicate that the main rock types of Cretaceous reservoir in the eastern depression of North Yellow Sea basin are lithic arkose, feldspar lithic sandstone, some feldspar sandstone and a small amount of lithic sandstone. The average porosity is 6.9%, and the average permeability is 0.46 × 10−3 μm, so Cretaceous reservoir in the study area has poor petrophysical characteristics and belongs to low porosity and low permeability reservoir. Cretaceous reservoirs in the study area mainly develop in secondary pores, which are dominated by dissolution pores (including intragranular pores, intercrystalline pores and cleavage pores), followed by fractures. The main factors affecting petrophysical characteristics of Cretaceous reservoir in the study area are provenance properties, sedimentation, diagenesis (including compaction, cementation and dissolution) and tectonism. The provenance properties and sedimentation are the prerequisite conditions affecting petrophysical characteristics. Petrophysical characteristics of feldspar sandstone of Cretaceous reservoir in the study area and lithic arkose are better than that of feldspar lithic sandstone. Both compaction and cementation reduce the porosity and permeability of Cretaceous reservoir in the study area and make petrophysical characteristics become poor, whereas the dissolution and tectonism play an important role in improving petrophysical characteristics of Cretaceous reservoir.

Lamination Texture and Its Effects on Reservoir and Geochemical Properties of the Palaeogene Kongdian Formation in the Cangdong Sag, Bohai Bay Basin, China

The characteristics of laminae are critical to lacustrine shale strata. They are the keys to the quality of source rocks and reservoirs, as well as engineering operations in shale plays. This study uses organic geochemistry, thin section identification, X-ray diffraction, field emission scanning electron microscopy, and other analytical methods, to reveal the detailed lamination texture and vertical distribution of laminae in the second Member of the Kongdian Formation in Cangdong Sag. The principal results are as follows: (1) A classification of laminae is proposed to characterize reservoir and geochemical properties. The five types of laminae are as follows: feldspar-quartz laminae (FQL), clay laminae (CLL), carbonate laminae (CAL), organic matter laminae (OML), and bioclastic laminae (BCL). There are also four significant lamina combinations (with the increasing TOC values): FQL-CLL combination, FQL-CLL-BCL combination, FQL-CLL-OML combination, and FQL-CAL-CLL-OML combination; (2) differences between laminae occur because of the variability in pore types and structures. There appears to be a greater abundance of intercrystalline pores of clay minerals in the FQL, CAL, BCL, and OML, and well-developed organic pores in the CAL and CLL, and the counterparts of intragranular pores of bioclastic material in the BCL. This detailed characterization provides the following comparative quantification of the thin section porosity of laminae in the second Member of the Kongdian Formation can be differentiated: CAL > FQL > OML > BCL > CLL; (3) differentiation between vertical distributions of laminae is carried out in a single well. The FQL and CLL are widely distributed in all the samples, while the BCL is concentrated in the upper part of the second Member of the Kongdian Formation, and CAL is concentrated in the lower part. This detailed classification method, using geochemical analysis and vertical distribution descriptions, offers a detailed understanding of lamination texture and its effects on reservoir and geochemical properties, which will provide a scientific guidance and technical support to better estimate reservoir quality and to identify new sweet spots in the second Member of the Kongdian Formation in the Cangdong Sag.

Pore Structure and Connectivity of Mixed Siliciclastic-Carbonate Tight Reservoirs in the Palaeogene from Qaidam Basin, NW China

The pore structure and connectivity in petroleum reservoirs are controlled in part by their petrological properties. Mixed siliciclastic-carbonate rocks have complex compositions and heterogeneous spatial distributions of the various minerals. As a result, the study of the pore structure and connectivity of mixed siliciclastic-carbonate tight reservoirs has been limited. In this study, methods such as thin section microscopy, X-ray diffraction, X-ray computed tomography, low pressure N2 adsorption, and spontaneous imbibition were adopted to comprehensively analyze the petrological properties, pore structure, and connectivity of the mixed siliciclastic-carbonate tight reservoirs in the upper member of the Xiaganchaigou Formation in the Yingxi Area, Qaidam Basin. The results showed that micrometer-sized pores in mixed siliciclastic-carbonate tight reservoirs are mainly dissolution pores, and that the spatial distribution of the pores is highly heterogeneous. The average pore radius range, average throat radius range, and average coordination number range of micronmeter-sized pores are 2.09~3.42 μm, 1.32~2.19 μm, and 0.48~1.49, respectively. Restricted by the concentrated distribution of local anhydrite, the connectivity of micronmeter-sized pores develops well only in the anhydrite, showing negligible contribution to the overall reservoir connectivity. In contrast, nanometer-sized pores in the mixed siliciclastic-carbonate tight reservoirs are mainly intercrystalline pores in dolomite. The range of nanometer-sized pores diameters is mainly distributed in 1.73-31.47 nm. The pores have a smooth surface, simple structure, and relatively homogeneous spatial distribution. The dissolution of dolomite intercrystalline pores by acidic fluids increases the connectivity of the nanometer-sized pores. This paper presents genetic models for microscopic pore structures and connectivity of mixed siliciclastic-carbonate rocks, making possible the evaluation on the quality of the mixed siliciclastic-carbonate tight reservoirs.

Characteristics of Micro/Nano Pores and Hydrocarbon Accumulation in a Continental Shale Oil Reservoir—A Case Study of the Lucaogou Formation in the Jimsar Sag, Junggar Basin, Northwest China

This paper comprehensively studies the micro- and nanometer-scale pore characteristics and structure of the Lucaogou Formation shale oil reservoir in the Jimsar Sag using high-pressure mercury analysis, field emission scanning electron microscopy and nano-CT scanning technology. In addition, the occurrence states of crude oil in pores are analyzed combined with macro–micro characteristics. The results show that there are various reservoir types; the main reservoir pore structure is on the micron and nanometer levels, with other void spaces including intergranular pores, interparticle dissolution pores, intercrystalline pores and microfissures. Nanopores are generally oil-bearing and mostly in the adsorption state, which changes the traditional understanding that micron pores are the only microscopic pores in the reservoir and confirms that shale oil exists in ‘sweet spots’ and mud-shale sections of the Lucaogou Formation.

Analysis of logging parameter model of Jurassic complex reservoir in Xinjiang Shenquan Oilfield

The study discusses the characteristics of Jurassic reservoir in Shenquan Oilfield in Xinjiang. The reservoir of Qiketai Sanjianfang formation mainly develops delta front facies and shore shallow lake facies sand bodies.The lithological characteristics of the reservoir are mainly fine sandstone and siltstone, and the physical properties of the target layer are mainly medium porosity and medium permeability. The Jurassic reservoir space in Shenquan Oilfield is sandstone pore type, mainly composed of secondary pores, mainly including intergranular dissolved pores, intragranular dissolved pores and intercrystalline pores, with good connectivity between pores. According to the analysis data of well logging, core analysis, production performance and DST in Shenquan Oilfield, porosity, permeability and water saturation models are established respectively:The porosity interpretation model is established by using acoustic wave and shale content; the permeability parameter interpretation model is realized by applying graph based cluster analysis method and neural network algorithm. The oil saturation interpretation model is obtained by Archie formula. According to the logging interpretation model and interpretation results, it can meet the needs of reservoir evaluation, oil-water distribution and original geological reserve parameters. After analyzing the logging parameter model and reservoir parameter calculation method, it is considered that the Jurassic in the Shenquan Oilfield in Xinjiang has better physical properties.

The formation mechanism of authigenic chlorite in tight sandstone and its effect on tight oil adsorption during hydrocarbon filling

Authigenic chlorite, which is frequently found in sandstone, has a controlling effect on the reservoirs in which tight oil is adsorbed during hydrocarbon filling. In this study, the content, occurrence state, timing, mechanism and influence of authigenic chlorite on the micro-occurrence states of tight oil were studied using Thin Section (TS), Fluorescent Thin Section (FTS), X-Ray Diffraction (XRD), Field Emission-Scanning Electron Microscopy (FE-SEM), Environmental Scanning Electron Microscopy (ESEM), and Energy Dispersive Spectroscopy (EDS). The results indicate: (1) a spatial coupling between chlorite development, a brackish water delta front facies depositional environment, and biotite-rich arkosic sandstone. (2) Authigenic chlorite can be divided into three types: grain-coating chlorite, pore-lining chlorite, and rosette chlorite. Chlorite forms after early compaction but before other diagenetic phases, and grows via precipitation from pore waters that contain products released during the dissolution of volcanic rock fragments and biotites. Porewater is also pressure-released from feldspars and mudstone. (3) The micro-occurrence states of tight oil can be divided into five types: emulsion form, cluster form, throat form, thin-film form, and the isolated or agglomerated particle form. (4) During hydrocarbon filling, tight oil mainly occurs on the surface of grain-coating and pore-lining chlorite in the form of a thin film, the granular or agglomerated forms are mainly enriched within the intercrystalline pores within the authigenic chlorite, and the cluster forms are mainly enriched in dissolution pores. Isolated or agglomerated particles of tight oil primarily occur in the intercrystalline pores of the rosette chlorite. (5) The specific surface area and the authigenic chlorite’s adsorption potential of authigenic chlorite control the micro-occurrence of tight oil on the surface of the chlorite and in intercrystalline pores. The adsorption capacity of chlorite lies in the following order: pore-lining chlorite intercrystalline pores > rosette chlorite > chlorite in feldspar dissolution pores > pore-lining chlorite surface > grain-coating chlorite intercrystalline pores > grain-coating chlorite surface.

Micron-Nano Pore Structures and Microscopic Pore Distribution of Oil Shale in the Shahejie Formation of the Dongying Depression, Bohai Bay Basin, Using the Argon-Scanning Electron Microscope Method

To investigate the pore structure of shale oil reservoirs, seven organic-rich shales from the Shahejie Formation in the Dongying Depression were studied by rock pyrolysis analysis, X-ray diffraction analysis and scanning electron microscopy with argon ion beam polishing. The proportions of the different types of pores at different scales, the statistical relationships between the mineral contents and pore development, and the development of pores in the mineral laminae combination were discussed. The results demonstrated that the nanometer to micron pore structures were divided into intraparticle pores, interparticle pores, intercrystalline pores, organic pores, microfissures and diagenetic contraction fractures. Interparticle pores and intraparticle pores are both essential components of the sample pores. The original residual pores are mainly small pores and mesopores, while the secondary dissolution pores are primarily mesopores and macropores. For different pore sizes, there are small surface pore contributions of diagenetic contraction fractures, microfissures, intercrystalline pores, and organic pores. The carbonate minerals content controls the oil storage capacity of shale by dominating the development of the various pore types, while the clay mineral bands content can affect the permeability of shale by influencing the development of large-scale microfissures. In the laminae development sample, surface pore development is closely related to the sedimentary mineral microcircle, which consists of a falling semicycle and a rising semicircle. The total surface porosity, including microfissures, diagenetic contraction fractures, interparticle pores and intraparticle pores, mainly developed at the intersection of the rising semicircle and the falling semicircle, and this development corresponds to the highest level of a cycle. In summary, interparticle pores and intraparticle pores are the main components of pores developed in low thermal maturity shale and shale laminae where their heterogeneity is influenced by mineral composition and laminae microcircles.

Origin of Bedding-Parallel Calcite Veins from Lacustrine Shale in the Eocene Dongying Depression, Bohai Bay Basin, China

Calcite veins, which developed parallel to the bedding, are widespread in laminated source rocks in the Eocene Dongying Depression. However, there is a lack of systematic description and classification of the veins. This study presents a systematic characterization of the calcite veins, host rocks, and micritic carbonate laminae by applying petrographic and geochemical methods to understand vein-forming mechanisms. Antitaxial and syntaxial veins are examined. Antitaxial veins contain typical fibrous crystals with the most intense fluorescence, and the median zone of these veins is often the micritic carbonate. Calcite crystals in syntaxial veins develop a blocky morphology of various sizes, indicating obvious growth competition. Data of rare earth elements and trace elements obtained from the micritic laminae, host rocks, and calcite veins are very similar. This indicates that the vein-forming nutrients originated from the carbonate in the host rocks and micritic laminae. The minor difference in C and Sr isotopes between calcite veins and micritic carbonate within the host rock and the negative shift in O isotopes in the veins are caused by ion exchange and dehydration of swelling clay minerals in the burial environment. This further proves that the calcite veins are formed in a closed system. Geochemical analysis suggests that the rocks are in the oil window and have good hydrocarbon potential. Thermal evolution of the acidic fluids generated from organic matter (OM) resulted in the dissolution of carbonate and formed fluid overpressure in the rocks. Fluid overpressure induced the formation of fractures in the interlayer and expanded the veins with the force of crystallization due to fibrous calcite growth. Blocky crystals grow in the fractures from the margins toward the center. Hydrocarbon expulsed via OM maturation in the host rock fills the intercrystalline pores. Moreover, shale with bedding-parallel calcite has the characteristics of high-quality shale oil reservoirs. These characteristics will probably provide guidance for shale oil exploration.

Describing the Full Pore Size Distribution of Tight Sandstone and Analyzing the Impact of Clay Type on Pore Size Distribution

Understanding the pore size distribution (PSD) of tight sandstone and the effect of clay minerals on the PSD is important for reservoir evaluation. Due to the complex shape of clay minerals, the multiscale pore size of tight sandstone, and the limitation of different experimental methods, it is hard to characterize the full PSD of tight sandstone, especially the point of connection (POC) of different derived PSD curves. In this paper, a more comprehensive technique integrated different precision methods of N2/CO2 low-pressure adsorption isotherms (N2/CO2-LPAI), mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), and synchrotron X-ray computed tomography (XCT) to investigate the full PSD for three typical tight sandstones in China. Two different forms of PSD data presentations, differential pore volume versus diameter (dV/dR) and the log differential pore volume versus diameter (dV/dlogR), were firstly used to determine the POC. The full integrated PSD curves and scanning electron microscopy (SEM) images were carried out on the different clay-rich tight sandstones. The results show that the pores are classified into three types: intercrystalline pores (less than 0.01 μm), clay-related pores and residual intergranular pores (0.01 μm to 10 μm), and microfractures and dissolution pores (greater than 10 μm). The percentage of intercrystalline pores has a small relation on the porosity and connectivity, while there is a strong correlation among microfractures, dissolution pores, porosity, and especially connectivity. The microfractures and dissolution pores are the main connection channels, so a little change of the main connection channels will have a great effect on the permeability of the tight sandstones.

Evaluation of the matrix influence on the microscopic pore-throat structures of deep-water tight sandstone: A case study from the Upper Triassic Chang 6 oil group of the Yanchang Formation in the Huaqing area, Ordos Basin, China

Former studies have suggested that the matrix of clastic rocks is unfavorable for the storage-percolation of reservoirs. However, the contribution of the matrix to the microscopic pore-throat structures in deep-water tight sandstone cannot be ignored. Aiming at the deep-water tight sandstone of the Chang 6 reservoir in the Ordos Basin (China), we have evaluated the characteristics of the matrix and the secondary pores in the matrix based on a multiscale microscopic identification and testing method, to reveal the influence of the matrix on the types, distribution, and heterogeneity of the pore throats. The results show that, unlike cements, the composition of the matrix is complex, characterized by its poor crystal form with no cement generation relationship. The structure of the matrix components is not completely dense. Intercrystalline pores and dissolved matrix pores are developed in the matrix after diagenetic modification, with a pore diameter of 20–1000 nm. These pores form a complex matrix secondary pore network. The matrix controls the number and volume of 0–1 μm pore throats. The matrix is constructive to the distribution of pore throats when its content is ≤7%. This positive effect gradually decreases with the increase of the matrix content, intensifying the compaction of the reservoir. The matrix controls the heterogeneities of the pore throat structures in deep-water tight sandstone. Large pore throats are gradually separated and disintegrated into a large number of micro-/nanopores by the matrix with the increase of the matrix content. Meanwhile, the fractal dimensions approached 3, increasing the complexity of the pore structures. Therefore, the matrix is favorable and unfavorable to the microscopic pore throat structures of the reservoir. The matrix not only results in the loss of intergranular pores but also generates a large number of secondary micro-/nanopore throat networks with complex structures, constituting an effective space for hydrocarbon accumulation and percolation in deep-water tight sandstone.