Formation mechanisms of good-quality clastic reservoirs in deep formations in rifted basins: A case study of Raoyang sag in Bohai Bay Basin, East China

Formation mechanisms of good-quality clastic reservoirs in deep formations in rifted basins: A case study of Raoyang sag in Bohai Bay Basin, East China

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 2, April 2018 Online English edition of the Chinese language journal Cite this article as: PETR...

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PETROLEUM EXPLORATION AND DEVELOPMENT Volume 45, Issue 2, April 2018 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2018, 45(2): 264–272.

RESEARCH PAPER

Formation mechanisms of good-quality clastic reservoirs in deep formations in rifted basins: A case study of Raoyang sag in Bohai Bay Basin, East China JIN Fengming1, ZHANG Kaixun2, 3, *, WANG Quan4, NIU Xinjie4, YU Zuogang4, BAI Guoping3, ZHAO Xuan5 1. PetroChina Dagang Company, Tianjin 300280, China; 2. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China; 3. College of Geosciences, China University of Petroleum, Beijing 102249, China; 4. PetroChina Huabei Company, Renqiu 062552, China; 5. CNPC Bohai Drilling Engineering Company Limited, Renqiu 062552, China

Abstract: In order to reveal the development mechanism of high-quality clastic rock reservoir, the basic characteristics of Sha-3 Member of the Shahejie Formation in the Raoyang sag, Bohai Bay Basin, are analyzed based on cores observation, thin-sections and SEM images, and petrophysical properties measurements as well. It is found that high-mature composition and texture, early oil charging, and dissolution are the main factors controlling the formation and preservation of pores in deep reservoirs. Compaction is the major factor destructing pores, whereas formation overpressure is conducive to the preservation of original pores, high compositional and medium textural maturity can enhance the resistance capacity to compaction and protect primary pores. Early oil charging could lead to temporary cessation of diagenesis and thus inhibit the cementation. When organic acids entered reservoir formations, considerable amounts of secondary pores were formed, leading to the local improvement of petrophysical properties. When predicting good quality belt in exploration of deep basin, it is recommended that the superimposing effects of the multiple factors (overpressure, early oil charging, compositional and textural maturity, diagenesis) be taken into consideration. Key words: Bohai Bay Basin; Raoyang sag; rifted basin; Shahejie Formation; diagenesis; high porosity zone; dissolution

Introduction The formation mechanism of pores in deep reservoirs is an important issue for hydrocarbon exploration[13]. Effective reservoirs have been discovered in the deep formations of Raoyang sag (with depth of over 3 500 m and formation temperature of 100 C), Bohai Bay Basin, and we need to understand the formation mechanisms and factors controlling the development and distribution of high-quality reservoirs[45]. Researchers have reached many understandings on the formation mechanisms of deep sandstone reservoirs[617], e.g., during the formation of deep reservoirs in the Dongying Sag, sandstone thickness has a crucial impact on the reservoir property; whereas the dissolution of unstable minerals, such as feldspar would lead to the redistribution of minerals and formation of micropores, resulting in drop of permeability[6].

Therefore, when examining the formation mechanisms of good reservoirs, the dissolution which increases porosity shouldn’t be considered alone, rather multiple factors and their comprehensive effect should be taken into consideration. Taking the Sha-3 Member of the Shahejie Formation in the Raoyang sag as an example, thin sections of 193 core samples from 25 wells were observed to select typical ones, physical property measurement, scanning electron microscopy observation (SEM), X-ray diffraction analysis, and cathodoluminescence spectrum were conducted on these typical samples. Based on the results of these experiments, the formation mechanisms and controlling factors of Paleogenetic potential reservoirs were analyzed.

1.

Geological setting The Jizhong Depression in the west Bohai Bay Basin re-

Received date: 25 Oct. 2017; Revised date: 24 Feb. 2018. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05006-005). Copyright © 2018, Research Institute of Petroleum Exploration & Development, PetroChina. Publishing Services provided by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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ceived a succession of Mesozoic and Cenozoic deposits. This depression can be divided into 12 sags and 7 uplifts, among them, the Raoyang sag is located in the middle of the Jizhong Depression (Fig. 1) with widespread NNE-trending normal faults[1820]. The strata in the Raoyang sag primarily consist of the Paleogene, Neogene and Quaternary sequences with large thickness. The strata encountered in drilling from bottom to top (Fig. 1) are the Paleogene Kongdian Formation, Shahejie Formation and Dongying Formation, the Neogene Guantao Formation and Minghuazhen Formation, and the Quaternary. The Paleogene–Neogene is characterized by coarse-finecoarse sedimentary cycle, e.g., the bottom is dominated by the fluvial deposits during the early stage of lake, the middle is filled by the lacustrine argillaceous sediments during the deep lake period, and the upper is primarily fluvial and diluvial deposits during the lake shrinkage[2021].

Fig. 1.

2. 2.1.

Lithology and reservoir Lithology

The sandstone of Sha-3 Member of the Shahejie Formation in the Raoyang sag is dominated by low compositional mature and medium textural mature lithic arkose and arkose. The sandstone is made up of quartz (with an average content of 46.5%), feldspar (with an average content of 38.24%) and lithics (with an average content of 15.26%), respectively. The sandstone is fine to medium in grain size, medium to well sorted and sub-rounded, with grains in point-line contact. The content of matrix, mainly clay-minerals, is from 1.33% 21.67%, on average 3.23% (Fig. 2a). Cements comprised of carbonate (Fig. 2b-2c), silicic (quartz overgrowth and authigenic quartz) (Fig. 2c-2d) and secondary clay minerals have a wide content range of 1.63%38.37%. Clay minerals are dominated by illite (Fig. 2d-2f), with minor illite/smectite

Location and structure units of the study area.

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Fig. 2. Images showing the diagenetic minerals in the Sha-3 Member of the Shahejie Formation. (a) Well Liu 101, 3 697.04 m, false matrices (Pm) and authigenic illite (I), thin-section; (b) Well Lu 43, 3 570.25 m, calcite (C), thin-section; (c) Well Liu 101, 3 695.44 m, quartz secondary overgrowth (Qv), dolomite (D) and ankerite (An), thin-section; (d) Well Liu 101, 3 696.64 m, dissolved feldspar (Df) covered by chlorite (Cc), authigenic quartz (Eq), thin-section; (e) Well Liu 101, 3 642.02 m, filamentous illite (I) in dissolved pores (Df), thin-section; (f) Well Liu 107, 3 674.50 m, arkose, illite (I), SEM image; (g) Well Liu 498, 3 778.50 m, arkose, illite/smectite mixed layer (I/S), SEM image; (h) Well Liu 101, 3 572.50 m, arkose, kaolinite (K), SEM image; (i) Well Liu 107, 3 775.50 m, lithic arkose, chlorite (Ch), SEM image.

mixed-layer (Fig. 2g), while kaolinite and chlorite can be seen in intergranular pores (Fig. 2h-i). SEM images show that the most part of clay minerals fills in pores (Fig. 2d-2i), while part of them appears as rim or film around grains. Sha-3 sandstone currently at middle diagenetic stage A experienced mechanical compaction, cementation and dissolution successively[2124]. 2.2.

Reservoir property

Core analysis suggests that Sha-3 Member is characterized by low porosity (<15%), low-permeability and strong heterogeneity. The reservoirs with medium porosity of 15%25%, low porosity of 10%15%, extra low porosity of 5%10% and super low porosity of less than 5%, account for 9.11%, 55.86%, 24.75% and 10.13%, respectively, so reservoirs with low porosity and extra-low porosity take the majority. Also, permeability varies widely from (0.01267.00)×103 μm2, with an average value of 9.60×103 μm2. The reservoirs with medium permeability of (50.00500.00)×103 μm2, low permeability of (1050)×103 μm2, extra-low permeability of

(110)×103 μm2 and super-low permeability of less than 1.00× 103 μm2 account for 3.37%, 15.84%, 39.30% and 33.00%, respectively, which show that the porosity is in positive correlation with the permeability. 2.3.

Pore types

The deep Sha-3 reservoir in Raoyang sag has primary pores (Fig. 3b), secondary pores (Fig. 3c-3g) and a small number of micro fractures (Fig. 3h). The secondary dissolved pores are primarily dissolved intragranular pores and dissolved intergranular pores, and a small number of dissolved pores in interstitial material. The intragranular dissolved pores and intergranular dissolved pores constitute primary storage space of the reservoirs, which are attributed to dissolution of feldspar and lithic fragment. This can be demonstrated by moldic pores in Fig. 3e-3f. Bitumen can be seen between quartz overgrowth and quartz grains in large pores (Fig. 3i), while some carbonaceous bitumen covers authigenic quartz, indicating two stages of hydrocarbon migration/accumulation[2122].

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Fig. 3. Characteristic of pores and fractures in the deep-buried reservoirs of the Roayang sag. (a) Well Chu 22, 4 119.18 m, primary pores (PP), angular-shaped pore edges, thin-section; (b) Well Liu 101, 3 643.62 m, residual primary pores (Rpp), SEM image; (c) Well liu 412, 3 397.00 m, feldspar dissolved pores (Dpe), thin-section; (d) Well Chu 22, 4 119.63 m, extra-large pores due to the feldspar dissolution (Sp), dissolution of dissolvable components in lithic fragment, thin-section; (e) Well Chu 22, 4 119.18 m, moldic pores due to the feldspar dissolution (Mp), thin-section; (f) Well Liu 101, 3 760.08 m, intragranular dissolved pores in feldspar (Inp), thin-section; (g) Well Liu 101, 3 643.62 m, feldspar dissolution along cleavage, SEM image; (h) Well Ning 70X, 3 753.50 m, fractures filled by bitumen (F), thin-section; (i) Well Liu 101, 3 811.54 m, intergranular pores (Ip) partially filled by bitumen, thin-section.

3.

Formation of high-quality reservoir

Investigating formation mechanism of potential reservoir is of great importance for finding the favorable reservoir zones[2324], because the reservoir evolution can be more complicated with the increase of depth. Physical properties of cores show there are two abnormally-high porosity zones in the Sha-3 Member, at the depths of 3 5003 700 m, and 4 0004 100 m (Fig. 4a). The occurrence of these zones can be attributed to the differential diagenetic processes in vertical direction. Based on previous studies[11,13,15,1819,25] and exploration practices in the Raoyang sag, it is believed that the high-quality reservoirs in the study area are controlled by multiple processes, including overpressure, early hydrocarbon charging, original rock components and structural maturity, as well as diagenesis, etc. 3.1.

Overpressure-preserved primary pores

Deep reservoirs in the Raoyang sag are mostly weak-me-

dium high-pressure, with ultrahigh pressure at the local positions[2526]. And the overpressure tends to decrease from the center to the margins. Fig. 4a-4b suggests that the reservoir physical property is positively correlated with its pressure, indicating that overpressure can prohibit the destructive effect of overburden pressure to the pores. Several high porosity zones occur in the deep formations (>3 300 m) of the Raoyang sag (Fig. 4a), while overpressure is also widespread in these zones (>3 500 m) with pressure coefficient higher than 1.1 (Fig. 4c). Evidence shows that overpressure in the Raoyang sag is caused by undercompaction, since the deposition rates of the Paleogene, Neogene and Quaternary there are 140, 129 and 260 m/Ma, respectively[2526]. Thin-section observation suggests that compaction has the strongest damage to pores in the reservoirs (Fig. 5). Therefore, weak compaction is an indicator of potential reservoirs. Abnormal high pressure helped the preservation of primary pores,

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Fig. 4.

Variation of reservoir property and formation pressure in the southern Raoyang sag.

Fig. 5. Thin-sections showing the pore structures in overpressure zone and normal pressure zone in the Raoyang sag. (a) Well Liu 101, 3 900.20 m, no overpressure zone, strong compaction, no primary pores; (b) Well Liu 101, 3 547.78 m, overpressure zone, well-preserved primary pores, and dissolved pores.

which would provide conditions for later dissolution. Fig. 5a shows in areas with no abnormal high pressure, the compaction is stronger, so the grains are in concavo-convex contact, the primary pores are poorly-preserved, and the later dissolution is not obvious. But undercompaction leads to widespread abnormal high pressure in the central subsag of the Raoyang sag, which prohibited the compaction and preserved some primary pores, and these primary pores are conducive to dissolution of the reservoir by the organic acid. Thus, overpressure not only preserved primary pores, but also contributed to the dissolution in the later stage. 3.2. Impact of early hydrocarbon charging on the high-quality reservoirs Hydrocarbon charging in the early stage can delay the cementation through decreasing the supplement of required ions, also, it can alter the rock wettability from water-wet into oil-wet, providing smooth migration paths for hydrocarbon charging in the later stage[27].

Two stages of bitumen can be found in the Sha-3 Member in the Raoyang sag, the first occurs between quartz overgrowth and quartz grains, while the second exists at the surface or the inside of the quartz grains, indicating two periods of hydrocarbon charging. Homogenization temperature in Fig. 6 also shows two hydrocarbon charging processes with peak temperatures of 100 C and 120 C, respectively. The diagenesis evolution model of the Raoyang sag was established using parameters from Well Liu 101, borehole temperature from Well Liu 425, and ancient thermal heat flow and rock thermal conductivity in the Bohai Bay Basin[2831] (Fig. 7). In the early syngenetic–quasi-syngenetic stage, having not suffered diagenesis, clastic grains in the reservoirs of Raoyang sag were in point-suspension contact, then chlorite and early carbonate cement (mainly micritic and sparry calcite) developed (Fig. 7a-7b), and the carbonate cement filled the primary pores (Fig. 7b). With the increase of depth, compaction became more and more intense, leading to reduction of the reservoir porosity, and grains in the reservoir turned into

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point-line contact (Fig. 7a, 7c), however, support derived from overpressure and rigid particles preserved some of the primary pores. Meanwhile, organic acid from source rocks dissolved feldspar particle along cleavage cracks (Fig. 7d), resulting in the occurrence of kaolinite and quartz overgrowth (Fig. 2d). The first stage of hydrocarbon charging occurred later, which is evidenced by the bitumen on the surface of the quartz overgrowth and primary pores (Fig. 3i). Ferrodolomite cementation occurred during this period because of iron- and magnesium-rich fluid brought about by fault activity, the carbonate cement of this stage filled primary pores and feldspar dissolved pores (Fig. 2e). Finally, the second stage of hydrocarbon charging occurred, forming bitumen on the surface of automorphic quartz and in fractures (Fig. 3h). The two times of hydrocarbon charging in the Shahejie Formation happened at 27 Ma and 3 Ma, respectively. The first time of hydrocarbon charging decreased water saturation, inhibited the cementation and preserved some pores (Fig. 3h and 3i), otherwise, without hydrocarbon charging, ferrocalcite

and ferrodolomite would have filled considerable pores, resulting in reservoir densification (Fig. 7e). Therefore, the early stage of hydrocarbon charging contributed greatly to the high-quality reservoirs.

Fig. 6. Histogram of homogenization temperatures of coeval brine inclusions from the Sha-3 Member in the Raoyang sag.

Fig. 7. Diagenetic evolution sequence and porosity evolution model of the Sha-3 Member in the Raoyang sag. Es3—Sha-3 Member of the Shahejie Formation; Es2—Sha-2 Member of the Shahejie Formation; Es1—Sha-1 Member of the Shahejie Formation; Ed—Dongying Formation; Ng—Guantao Formation; Nm—Minghuazhen Formation; Qp—Quaternary Pingyuan Formation.

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Fig. 8.

Rock components and structure differentiation (298 samples).

3.3. The impact of rock components and structure on high-quality reservoirs 3.3.1.

Rock components

Generally, reservoir property varies with rock components, e.g., high content of rigid particles (quartz) can increase the anti-compaction capacity of the rock, thus allowing some primary pores to be preserved, whereas, high contents of plastic particles or matrix would bring about stronger compaction, more loss of porosity and thus poorer reservoir physical properties (Fig. 8a). 3.3.2.

Sorting and roundness

Fig. 8b shows that the anti-compaction capacity has a negative relationship with sorting, suggesting that the reservoir quality is also controlled by sorting. The evaluation and prediction of deep-buried reservoirs need to consider the clastic composition (quartz content) and structure (sorting and roundness, etc.), which is helpful to predict the favorable zone by overlapping multiple factors. 3.4.

The impact of diagenesis on high-quality reservoirs

As widely accepted, diagenesis can determine reservoir quality, specifically, compaction is the major factor causing drop of porosity, and cementation is another destructive diagenesis process, whereas dissolution is an important constructive diagenesis process[32-33]. 3.4.1.

Compaction

Deep reservoirs in the study area are large in depth and deposited at a high speed. And near to the provenance, the reservoirs have lower content of rigid particles, so compaction has made a stronger impact on the densification of the reservoirs. The relationship of compaction and cementation with

Fig. 9. Cross-plot showing the porosity decrease in the Raoyang sag[34].

porosity reduction in Fig. 9 shows that the compaction can decrease porosity by 2570%, and therefore, compaction is regarded as a key destructive process. In other words, anticompaction capacity is essential for the high-quality reservoir. 3.4.2.

Cementation

Cements, especially carbonate cement in the later stage, can decrease porosity considerably. Fine-coarse grained carbonate cements, e.g., calcite, dolomite and ferrodolomite, can destroy primary and secondary pores. The carbonate cements in the Shahejie Formation can be divided into three phases. The first phase is mainly micritic calcite that occurred as basal cementation and porous cementation. The second phase is medium-coarse dolomite. Due to the variation of diagenetic environment in the middle diagenetic stage, calcite formed in the early stage was dissolved and siliceous cement occupied its place. The third phase is the ferrodolomite filling pores and

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often replacing framework grains. Cementation in the late stage often has decisive effect on reservoir property, because the charging of organic acid is often earlier than the late stage cementation and thus can’t dissolve cements in the late stage. Furthermore, cements of the late stage often fill tiny pores and throats, causing drastic drop of permeability. Fig. 9 shows that the cementation in the Raoyang sag can decrease porosity by 25%75%, indicating that the cementation is another destructive process there. 3.4.3.

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Dissolution

stone geochemical systems during diagenesis: Typical ex-

Dissolution is commonly seen in the Sha-3 Member of the Shahejie Formation, which is attributed to largely acid fluid derived from mature source rock (Fig. 2-3). The preservation of primary pores is also important to the dissolution, because the residual pores can be pathways for dissolving fluids. Thin-sections and SEM images suggest that the meteoric fresh water in the early stage dissolved feldspar along cleavages into kaolinite, which was further converted into illite in the later stage. Fig. 6 shows that the measured temperature of the deep-buried reservoirs are in the range of 95130 C. The acidity of organic acids can be kept effectively at lower temperature, whereas when the temperature rises, the organic acids would decompose into CO2, the formation water rich in CO2 maintains the acidity and dissolution capacity, enabling dissolution and porosity increase to go on.

4.

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Conclusions

[11] XUE Zong’an, ZHAO Yuhong, WU Yiping, et al. Characteris-

The Paleogene deep reservoirs in the Raoyang sag (>3 500 m deep) are dominated by gray medium-fine grained lithic arkose and arkose, and largely low in porosity and permeability. Pores in the reservoirs include intragranular dissolved pores, intergranular dissolution pores and residual primary pores, of which secondary pores take the majority. The formation of high-quality reservoirs is mainly controlled by overpressure, high compositional maturity and medium texture maturity, early hydrocarbon charging and dissolution. Overpressure can considerably inhibit compaction, and coarse sandstone with high compositional maturity and medium textural maturity resulted from deposition is conducive to preservation of pores, while hydrocarbon charging can inhibit cementation and protect pores; and charging of organic acid can give rise to the secondary pores and improve reservoir physical properties.

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