Effect of Pore Structure on Reservoir Quality and Oiliness in

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Effect of Pore Structure on Reservoir Quality and Oiliness in Eocene Dongying Formation Sandstones in Nanpu Sag, Bohai Bay Basin, Eastern China Jin Lai,*,†,‡ Guiwen Wang,*,† Xiaojiao Pang,‡ Xuechun Fan,‡ Zhenglong Zhou,‡ Zhaowei Si,§ Weibiao Xie,§ and Ziqiang Qin∥

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†

State Key Laboratory of Petroleum Resources and Prospecting, and ‡College of Geosciences, China University of Petroleum, Beijing 102249, China § PetroChina Jidong Oilfield Company, Tangshan 063004, China ∥ Department of Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071-2000, United States ABSTRACT: Routine core analysis, thin sections under transited and ultraviolet light, scanning electron microscope (SEM) analysis, and nuclear magnetic resonance (NMR) measurements were used to investigate the pore structure of the Eocene Dongying Formation sandstones in the Nanpu Sag, Bohai Bay Basin. Regression analysis was performed to build up the relationships between microscopic pore structures and macroscopic behaviors. The pore systems mainly include primary intergranular pores, intergranular and intragranular dissolution pores, and clay-dominated micropores. Most of the intergranular pore bodies are connected by effective necking and sheet like pore throats, while the intragranular pores and micropores are connected by bending-flake and narrow cluster pore throat. Unimodal, bimodal, or multimodal T2 (transverse relaxation time) distributions can be observed due to the wide ranges of pore bodies and throats in Dongying sandstones. Consequently, wide variations of NMR parameters are encountered, and high-quality reservoirs contain abundant intergranular pores connected by large pore throats, and therefore have high T2gm (the geometric mean of the T2 distribution) but low BVI (bulk volume of immovable fluid) values. The intragranular dissolution pores and clay-dominated micropores display strong fluorescences, while only the edge of the intergranular pores emits weak fluorescence, and no fluorescence was identified in the intergranular pores. The sandstones generally lack fluorescence when they are tightly compacted or cemented by carbonate cements. NMR T2 distributions also reveal that the main oil-bearing pore size distribution is in the small pore realm (1−100 ms), which is in good agreement with fluorescent thin sections. The results help improve the knowledge of the effects of pore structure on reservoir quality and oil-bearing property in sandstones, and could provide insights into enhancing oil recovery.

1. INTRODUCTION The Nanpu Sag, which is located at the northeastern part of Huanghua Depression within the Bohai Bay Basin (Figure 1),1−3 is a well-known hydrocarbon producing province (Figure 1).4−7 The Eocene Dongying Formation, which is interpreted to be the deposits of braided delta, fan delta, and lacustrine facies, as well as subaqueous gravity flows,8−11 is an important hydrocarbon-bearing stratigraphic unit in Nanpu Sag. 12 Recently, many large oil fields including the Gaoshangpu, Liuzan, Laoyemiao, and Beipu oil fields have been found in the Nanpu Sag.2,13,14 However, the reservoirs have heterogeneous pore structures due to the complex depositional settings and the various types and degrees the sandstones experienced during the geological history. Pore structure, which is defined as pore throat size and distribution, the geometric shape, and their connectivity,15−19 determines the reservoir quality as well as hydrocarbon charging, migration, and accumulation in sandstones.19−21 Investigation of the controls of microscopic pore structure on reservoir quality and oiliness is therefore of great importance for hydrocarbon exploration and efficient development.15,19,22−27 This study investigates the macroscopic reservoir quality and microscopic pore structure of the Dongying sandstones, and aims to provide insights into the effect of pore structure on © XXXX American Chemical Society

reservoir quality and oiliness using a combination of thin section, SEM, and NMR measurements. The reservoir quality, pore systems, and pore throats were first investigated by routine core analysis, thin section, and SEM analysis. The oilbearing property then was determined from cores under sealed coring conditions and was identified with microfluorescence. Pore size distributions were then determined from the NMR T2 spectrum, and the relationships between reservoir quality (permeability) and NMR parameters (T2gm and BVI) were investigated. The relationships between microscopic pore structures and macroscopic behaviors are discussed, and the effect of the pore structure on oiliness. This study helps improve our understanding of the pore structure characteristics and provides insights into the effect of the pore structure on oiliness in sandstones with similar geological backgrounds, and may have implications in reflecting the microscopic oil charging path or storage/occurrence space. Received: June 7, 2018 Revised: July 17, 2018 Published: August 21, 2018 A

DOI: 10.1021/acs.energyfuels.8b01989 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Location map of the Nanpu Sag in Bohai Bay Basin within Eastern China and its structural division map showing all of the structural belts (Dong et al., 2010; Guo et al., 2013; Chen et al., 2017). The mineralogy, surface morphology, various types of clay minerals, and the related pore throat systems were analyzed on the freshly broken gold-coated samples using a high-voltage scanning electron microscope (SEM) instrument. NMR measurement, which is sensitive to hydrogen fluids, can be used for characterizing the pore structure and petrophysical properties of reservoir rocks in a nondestructive way.21,28−30 Laboratory NMR analysis was also performed on the 175 core plug samples at saturated and centrifuged status, respectively, to obtain the transversal relaxation time (T2) spectrum, and determination of pore size distribution. The core samples were first fully (100%) saturated with NaCl brine with a salinity of 50 000 mg/L for 48 h, and then the T2 spectrum at the saturated status was measured. The core samples were kept in a centrifugal machine with a rotation speed of 9000 r/min for 1 h to remove the free water; therefore, the T2 spectrum at the centrifugal status can be obtained. The NMR apparatus provides 2 MHz frequency of the main magnetic field, waiting time of 6000 ms,

2. SAMPLES AND METHODS Routine core analysis was performed on a total of 175 standard core plugs (2.5 cm diameter and 5.0 cm length) using CMS-300 Core Measurement system to obtain the helium porosity and air permeability with a confining pressure of 6 MPa. Oil and water saturation was measured on the core under sealed coring conditions. Before thin sectioning, the samples were impregnated with blue resin to highlight porosity. Thin sections (30 μm thickness) were half stained with Alizarin Red S and K-ferricyanide for recognition of carbonates. The thin sections were then observed using a polarizing microscope under the under plane-polarized and cross-polarized light to identify the detrital components, diagenetic minerals, and pore systems. Thin sections used for fluorescence observation of hydrocarbons in pores were not impregnated with blue epoxy. The fluorescence of hydrocarbons in the thin section was checked by ultraviolet light using a ZEISS microscope fitted with a mercury lamp. B


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Figure 2. Cross-plot of permeability and porosity for Dongying sandstones. Note that there is an order of magnitude variation in permeability for the same porosity. signal superposition time of 128, and echo spacing Te of 0.2 ms. The NMR experiment processes follow the Chinese Oil and Gas Industry Standard (SYT6490-2007).31 A total of five samples were taken from the sealing core data using the pressure-maintaining core barrel method, and there exists the volatilization of movable hydrocarbons to some extent. The core is drilled to the core within 2 h, and sent to the laboratory for core analysis as soon as possible. The sealed core plugs were first saturated with water (the amounts of residual oil displaced by saturated water can be neglected), and then NMR measurements were performed to obtain the T2 spectrum. The samples then were saturated with the MnCl2 brine with a salinity of 15 000 mg/L, the signals of water were fully removed, and the NMR measurements were also performed to obtain the T2 distribution of the residual oil in the core plugs under sealed coring conditions. NMR measures the interaction between the nucleus and the magnetic field,21,32 and the transverse relaxation time T2 can be simplified as

1 1 S a = =ρ =ρ T2 T2S V r

while intergranular dissolution pores due to dissolution along the edges of the grains are recognized as the embay-like morphology (Figure 3B). The chemically unstable framework grains (feldspars and rock fragments) experienced various degrees of dissolution, resulting in a high amount of dissolution pore content in Dongying sandstones (Figure 3C). Additionally, remnants or even kaolinites can be observed in the intragranular pores (Figure 3C). When the framework grains are completely dissolved, the Modic pores were formed (Figure 3D). SEM analysis also reveals the various degrees of dissolution/alteration of feldspars, and the intragranular dissolution pores are commonly observed (Figure 3E,F). The clay minerals revealed by XRD and SEM analyses are kaolinite, illite, and illite−smectite mixed layer (Figure 3). Kaolinite displaying as pseudohexagonal platelets, and vermicular or booklet texture, and hair-like or sheet-like illite or illite− smectite mixed layer occurs as pore-filling and pore-lining cement (Figure 3). Vermicular kaolinites as well as the webby mixed layer illite/smectite contain abundant micropores (100 ms) (Figure 10), which indicates that the large pore realms associated with long T2 components do not contain oils. The T2 spectra of the two samples are characterized by bimodal behaviors, indicating the geometrical arrangement composed of small (micropores and intragranular dissolution pores) to large (intergranular pores) H


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Figure 10. NMR T2 spectrum for oil-bearing samples and related thin sections.

Figure 11. T2 spectrum and related thin sections of Type I pore structure. I


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Figure 12. T2 spectrum and related thin sections of Type II pore structure.

Figure 13. T2 spectrum and related thin sections of Type III pore structure.

(Figure 11). Conversely, only weak short T2 components are detected (Figure 11). Abundant movable water can be removed by the centrifugal machine, and the NMR signal amplitudes under saturated and centrifuged status are significantly differentiated (Figure 11). From the aspect of fluorescence thin sections, it can be concluded that, with high reservoir quality and favorable pore structure, the oil-bearing pore systems are only the intragranular pores because the intergranular pores lack fluorescence (Figure 11). The left-skewed (higher left peak but lower right peak) bimodal NMR T2 distributions are mainly attributed to the coexistence of micropores, intergranular, and intragranular dissolution pores (Figure 12), which are also associated with the fine-medium grained sandstones by microscopic observa-

Bimodal NMR incremental T2 distribution, which contains both long T2 components and short T2 components, is observed in high-quality sandstones (Figure 11). The highquality reservoirs may relate to the fine-medium grained sandstones, which are well sorted according to thin section petrography (Figure 11). The T2 spectrum is commonly characterized by right-skewed (lower left peak but higher right peak)44 distributions because of the dominance of large intergranular pore systems connected by large pore throats, and the intergranular pores as well as some intragranular pores can be observed from the thin sections under transited light. Most of the T2 components are higher than 100 ms, and therefore result in a high value of T2gm but low BVI values. Actually, there are long T2 components larger than 1000 ms J


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Figure 14. T2 spectrum and related thin sections of Type IV pore structure.

tion. Minor amounts of intergranular pores are detected in thin sections, and therefore the right peak is relatively low. However, the T2 spectrum is also wide, and occasionally tail distributions can be observed (Figure 12). The main peaks appear at T2 of 10−100 ms. Most of the T2 components are in the range from 1 to 100 ms, and contain low content of T2 components larger than 100 ms, and consequently result in a relatively low T2gm value (Figure 12). However, the NMR signal amplitudes under saturated and centrifuged status have significant differentiation. With the dominance of intergranular and intragranular dissolution pores, the oil-bearing potential is good because most of these intergranular and intragranular dissolution pores are fluorescent (Figure 12). One modal is presented in samples with unimodal T2 distributions, and this indicates a continuous range of pore size. The main T2 component displays as a dominant peak at the short T2 times. Most of the T2 components range from 1 to 100 ms, and contain no tail distributions. The signal amplitudes of the left peak, which correspond to pore bodies and throats with small pore sizes, are high (Figures 13 and 14). Only minor amounts of fluids can be removed by centrifugal machine, and most of them are irreducible in the small pore realm. No evident differentiation in the T2 spectrum under saturated and centrifuged conditions can be observed (Figures 13 and 14). High content of BVI indicates a poor pore connectivity and complicated pore systems.50,51 From the aspect of thin section observations, the pore systems mainly contain intragranular dissolution pores, while the intergranular pores are rarely observed (Figures 13 and 14). Carbonate cements and detrital clays fill the intergranular pores, and the pore systems are not connected by effective pore throats. Thin section petrography confirms that the relatively poor pore structure in Figures 13 and 14 corresponds to the tightly compacted sandstones or sandstones with extensive carbonate cements. The carbonate cemented pores are fluorescent-free, while the detrital clay matrix-associated pores can emit weak fluorescence (Figures 13 and 14). Fluorescent thin sections also clarify that the good oil-bearing potential is associated

with intragranular pores as well as micropores in detrital clay matrix or authigenic clays (Figure 5).

5. CONCLUSIONS The research work above leads to the following conclusions: (1) The pore systems of Dongying sandstones consist of primary intergranular pores, intergranular and intragranular dissolution pores, moldic pores, and clay mineral associated micropores, and the sandstones span a wide range of reservoir quality, pore body, and throats. (2) Oils are present as green to yellow fluorescence, and oils are observed to exist in the intragranular dissolution pores and clay-dominated micropores. The intergranular pores only emit weak fluorescence or no fluorescence. Carbonate cemented or tightly compacted sandstones generally lack fluorescence. (3) The sandstones display unimodal, bimodal, or multimodal NMR T2 distributions, and large variations in NMR parameters (T2cutoff, T2gm, and BVI) are observed. The better are the microscopic pore structures, the higher will be the values of T2gm and the lower will be the values of BVI. (4) NMR T2 signals of the oils are mainly associated with the short T2 components, and when the T2 values are larger than 100 ms, no oil signals are encountered. The intergranular pores connected by effective throats also correspond to the high-quality reservoirs; however, the best oil-bearing property is associated with the intergranular and intragranular dissolution pores as well as the clay-controlled pores. Investigation of the effects of pore structure on reservoir quality and oil-bearing property helps provide insights into enhancing oil recovery in sandstones.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +861089733435. Fax: +861089734158. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jin Lai: 0000-0002-5247-8837 K


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank PetroChina Jidong Oilfield Co. for providing samples and data access and the Research Institute of Petroleum Exploration and Development for permission to publish this work. This work is financially supported by the National Natural Science Foundation of China (Nno. 41472115), and the Science Foundation of China University of Petroleum, Beijing (no. 2462017YJRC023). We would like to express our deep thanks and gratitude to Dr. Jennifer Wilcox (Editor of Energy & Fuels) and the editorial staff. We also benefited from the critical but constructive comments and suggestions made by the three reviewers.

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Effect of Pore Structure on Reservoir Quality and Oiliness in

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