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Diagenetic controls on the reservoir quality of fine-grained ‘tight’ sandstones: a case study based on NMR analysis Hai hua Zhu, Dakang Zhong, Tingshan Zhang, Guangcheng Liu, Jingli Yao, and Chuanhang He Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03734 • Publication Date (Web): 14 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018
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Diagenetic controls on the reservoir quality of finegrained ‘tight’ sandstones: a case study based on NMR analysis Haihua Zhu*†‡, Dakang Zhong §, Tingshan Zhang †‡, Guangcheng Liu †‡, Jingli Yao
‖
and
Chuanhang He⊥ †Stake Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu, 610500, China;
‡School of Geoscience and technology, Southwest Petroleum University, Chengdu, 610500, China;
§School of Earth Science, China University of Petroleum, Beijing 102249, China;
ǁNational Engineering Laboratory for Exploration and Development of Low Permeability Oil & Gas Fields, Xi’an 710018, China;
⊥BGP
Inc., China National Petroleum Corporation, Zhuozhou. Hebei, 072751, China
ABSTRACT Accurate description of diagenetic controls on reservoir quality in ‘tight’ sandstones can be difficult because of the inherent fine grain size and complex components of such oil reservoirs. In this study, petrological techniques and nuclear magnetic resonance (NMR) analysis were applied to fine-grained tight sandstones with varying grain sizes in order to reveal the diagenetic controls on reservoir quality. Results show that macropores in tight sandstones occur mainly as intergranular and dissolution pores, whereas micropores are distributed within ductile rock fragments, clay and mica minerals, as well as occurring as dissolution micropores. Pore size distribution (PSD) / T2 spectrum display three distribution patterns: i) a macropore-
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dominant bimodal distribution, ii) a macropore-micropore bimodal distribution, and iii) a micropore-dominant skewed distribution. A decrease in grain size correlates with weaker framework support of particles and thus more intensive mechanical compaction, resulting in the loss of both macroporosity and microporosity. Consequently, PSD change from macroporedominant bimodal distributions to micropore-dominant skewed distributions as the pore type shifts from being dominated by macropores to intragranular micropores. In fine-grained sandstones, an increase in the abundance of ductile components corresponds to a loss of total porosity, related to the decrease in abundance of macropores, whereas the change in micropore abundance is negligible. This change is reflected in PSD by a shift from macropore-dominant bimodal distributions to macro-micropore bimodal distributions. The authigenic minerals in tight sandstone reservoirs occur mainly as late-stage carbonate minerals, and the precipitation of this carbonate cement preferentially occurs within macropores. When carbonate cement content is low, it has a limited influence on total porosity. However, it does significantly reduce the connectivity of the pore system, which is different to what might be expected in conventional sandstone reservoirs. Therefore, particle grain size, the abundance of ductile components and late-stage cementation all contribute to the prediction of reservoir quality in oil-bearing tight sandstones.
1 INTRODUCTION ‘Tight oil’ is an important unconventional oil and gas resource in China. Oil held within the Chang-7 Member accounts for approximately 15% of total oil and gas resources in the Ordos Basin.1,
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Commercial productivity of tight oil is dependent on the successful application of
hydraulic fracturing techniques. However, due to the inherent poor reservoir quality and
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heterogeneous nature of such reservoirs, oil production following hydraulic fracturing can vary significantly. For example, in the PetroChina-operated Changqing oilfield, only 40% of wells achieve commercial production from the Chang-7 tight sandstone following hydraulic fracturing.3 Pore throat architecture plays a vital role in the storage and flow characteristics of these reservoirs, and also in the resultant hydrofracturing effects and ultimate recoverable reserves.4,5 Therefore, the prediction of reservoir quality still remains important issue that need to be addressed in regard to the exploration and development of tight oil. Diagenetic processes that contribute to the evolution of porosity in buried sandstones have been well studied.6-10 Many workers have discussed pore features and porosity-influencing factors in fine-grained tight sandstones. For example, Pitman et al. considered compaction and carbonate cementation to be the major cause of porosity loss within the Bakken tight sandstone.11 And for more deeply buried sandstones, hydrocarbon emplacement may also have an influence on reservoir porosity. It has also been suggested that the presence of clay minerals has a significant effect on the properties of the Bakken tight sandstone reservoirs, manifested by porosity decreasing with a corresponding increase in matrix clay content.12 Armitage et al. considered depositional illitic clays, local chlorite, and quartz cementation to be the main controls on the porosity of siltstone in the Krechaba oil field, Algeria.13 The tight sandstone of Chang-7 Member in the Ordos Basin was deposited within a lacustrine basin, with significant provenance variability, and under diverse sedimentary conditions.14 This resulted in a diversity of mineralogical composition and highly variable diagenetic evolution of the sandstones.15 Intense compaction, facilitated by fine grain size and high clay content, together with late-stage carbonate cementation, are deemed to be the major cause for porosity loss in the Chang-7
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sandstone.2,16,17 These conclusions are based on consideration of total porosity variation during digenetic processes, as in conventional reservoirs. However, the differences in diagenetic effects upon the pore system, between that of conventional porous sandstones and tight sandstones, have not been well addressed. For example, it is poorly understood whether the effect of compaction on porosity, pore types, PSD as well as connectivity in tight sandstones of different particle sizes and/or components remains constant. Nor is it clear whether carbonate cementation still mainly affects the total porosity of tight sandstones. Therefore, by selecting representative samples of tight sandstone, and analyzing their petrologic and diagenetic features, pore structures, this work aims to address two principal questions: (1) How do diagenetic processes influence the pore structures (porosity, PSD, pore connectivity) of fine-grained sandstone reservoirs? (2) What is the difference in diagenetic effects upon the pore structures, between that of conventional sandstones and tight sandstone? 2 SAMPLING AND METHODOLOGY Seven representative core samples (N75, B12, L189, Z129 Z191, H218 and S110) from the Chang-7 Member were selected in the core region of tight oil exploration of the Ordos Basin (Figure 1). These samples are derived from two principal provenance directions: samples N75, B12, L189, Z129 and Z191 are originated primarily from the Qinling and Liupanshan orogenic belts in the southwest, whereas the sands in sample H218 are derived from the Yinshan palaeocontinental terrain in the northeast. Sample S110, on the other hand, is believed to be of mixed provenance.14,
15, 18
These samples were subjected to thin section petrology, scanning
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electron microscope (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) and NMR analyses.
Figure 1. Location of the study area and distribution of samples.
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2.1 Rock & Mineral Analysis. Thin sections of seven samples were prepared for petrologic investigation. A thin sliver of sandstone was mounted on a glass slide and ground smooth to 30µm thick using finer abrasive grit. Then the thin sections were stained with Alizarin Red S and K-ferricyanide for carbonate mineral determination. Texture, detrital composition, and textural relationships among different cements were observed under a polarizing petrographic microscope using transmitted and reflected light. Sandstone modal composition was determined by counting 300 points per thin section. Clay minerals and pores with small size of studied samples are difficult to observe under petrographic microscope. Seven samples were further studied using FEI Quanta 200F SEM, which was equipped with an EDAX Energy dispersive spectroscopy. The testing was operated in vacuum chamber under 20 kV, 25 °C. Before SEM testing, samples were cut to 1cm×1cm× 3mm in size. The surfaces were cut as perpendicular to bedding as possible. Fresh surfaces were firstly mechanical polished using emery paper and then further milled by Ar-ion milling using Ion Milling Gatan 961.CS. The Ar-ion milling was performed at 5kV with rotation speed of 3rpm, operation angles of 4°. Smooth milling surfaces were Au-plated to obtain a conductive surface for SEM observation. Modal composition gained from thin section was calibrated with XRD results. Samples were crushed and milled to
