| Citation: |
Qin Na, Hong Xu, Da-peng Su, Meng Tao, Wei-wei Zhang, Zhao-peng Ji, Qing Wang, 2019. Microscopic characteristics and geological significance of tight dolomite in well Xike-1, Xisha Islands, China, China Geology, 2, 458-466. doi: 10.31035/cg2018089
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Microscopic characteristics and geological significance of tight dolomite in well Xike-1, Xisha Islands, China
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Abstract
A recent study suggests that the deep dolomites in well Xike-1 located more than 1000 m deep have a very low porosity, and no permeability for most of the samples. Given that the largest oil and gas fields in the South China Sea are all composed of biohermal dolomites while the deep dolomites of Xike-1 serve as the caprock formations rather than the reservoirs, this represents the first discovery and is of great theoretical importance. In this paper, core photos of the tight dolomites are presented. Hand specimens description and systematic porosity-permeability reveal a very low porosity, and no permeability for most of the samples. PLM, SEM and XRPD analysis discovered that the dolostones in the deep tight dolomites are quite large in size and euhedral or subhedral in shape. Alizarin red-stained sections are mostly pink in color and have a full coverage, which are considered as evidence for multiple calcite intrusions after the formation of ankerite dolomites. Biological relict textures are developed. The order degree is medium to low. The dolomites could be the product of penecontemporaneous dolomitization.
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Keywords:
- Permeability /
- Order degree /
- Tight dolomite /
- Rock types /
- Gas oil exploration engineering /
- Xike-1 of Shi Island /
- China
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References
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Qin Na, Hong Xu, Da-peng Su, Meng Tao, Wei-wei Zhang, Zhao-peng Ji, Qing Wang, 2019. Microscopic characteristics and geological significance of tight dolomite in well Xike-1, Xisha Islands, China, China Geology, 2, 458-466. doi: 10.31035/cg2018089
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Figure 1.
The location of the well Xike-l (Modified from Zhu WL and Mi LJ, 2010; Wang ZF et al., 2015; The basemap according to China Map, China Map Press, 2013)
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Figure 2.
Photos of the core samples from the intervals of 1044.13–1089.42 m and 1100.42–1173.62 m in well Xike-1.
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Figure 3.
SEM images of the 1044.13–1089.42 m dolomite interval in well Xike-1. a–Depth 1045.00 m, consists mostly of crystal powder dolostone; b–depth 1050.46 m, sparry dolostone with developed intercrystalline pores; c–depth 1065.00 m, saccharoidal dolostone with intercrystalline pores; d–depth 1076.68 m, sparry dolostone with developed dissolution pores; e–depth 1086.50 m, sparry dolostone with developed intercrystalline pores; f–depth 1089.45 m, sparry dolostone with developed intercrystalline pores.
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Figure 4.
SEM images of the 1100.42–1128.62 m dolomite interval in well Xike-1. a–Depth 1100.49 m, sparry dolostone with intercrystalline pores; b–depth 1109.27 m, saccharoidal dolostone; c–depth 1111.60 m, sparry dolostone with crystal plane dissolution micropores; d–depth 1112.90 m, sparry dolostone with no intercrystalline pores; e–depth 1117.32 m, sparry dolostone with no intercrystalline pores; f–depth 1119.90 m, sparry dolostone, the dolostone grains are embedded into the adjacent crystals due to limited growth spaces, intercrystalline pores are observed; g–depth 1123.80 m, sparry dolostone with no intercrystalline pores; h–depth 1127.72 m, sparry dolostone, euhedral crystals; i–depth 1128.62 m, sparry dolostone with intercrystalline pores.
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Figure 5.
SEM images of the 1138.22–1173.62 m dolomite interval in well Xike-1. a–Depth 1138.22 m, sparry dolostone with no intercrystalline pores; b–depth 1139.00 m, sparry dolostone with no intercrystalline pores; c–depth 1141.42 m, consists mostly of sparrydolostone, some of the crystals are incompletely shaped or are embedded into the adjacent crystals due to limited growth spaces, growth laminates are detected across the dolostone section; d–depth 1144.42 m, consists mostly of sparry dolostone with intercrystalline pores; e–depth 1146.32 m, consists mostly of sparry dolostone, euhedral, with presence of intercrystalline pores; f–depth 1147.60 m, consists mostly of sparry dolostone, growth laminates are detected across the section, with planar contacts being the prevailing form of contact; g–depth 1149.92 m, sparry dolostone with presence of intercrystalline pores and crystal plane dissolution micropores; h–depth 1167.50 m, sparry dolostone and a little micritic dolostone with presence of intercrystalline pores; i–depth 1173.20 m, sparry dolostone with presence of a few intercrystalline pores
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Figure 6.
Microscopic characteristics of the various kinds of dolomite in well Xike-1. a–Depth 1109.27 m, sparry bioclastic dolomite with integral grain texture, PPL (4X); b–depth 1144.4 2 m, showing obvious grain profiles, PPL (4X); c–depth 1127.72 m, bioclast-bearing sparry dolomite, PPL (4X); d–depth 1065.00 m, fine crystal calcareous dolomite, PPL (4X); e–depth 1065.00 m, fine crystal calcareous dolomite, PPL (10X); f-depth 1076.68 m, powder crystal dolomite, PPL (4X).
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Figure 7.
Microscopic characteristics of the dolomite interstitial materials in well Xike-1. a–Depth 1168.00 m, euhedral dolostone; b–depth 1122.80 m, micritic crystal anhedral dolostone; c–depth 1134.27 m, flaky clay minerals distributed between the dolostone; d–depth 1111.60 m, spherical fillings.