Eogenetic Karst Control of Carbonate Reservoirs during a Transient Exposure: A Case Study of the Ordovician Yingshan Formation in the Northern Slope of the Tazhong Uplift, Tarim Basin, China
Abstract
:1. Introduction
2. Geologic Setting
3. Methods
3.1. Rock Thin-Section Preparation and Identification
3.2. Cathode Luminescence Analysis
3.3. Electron Microprobe Analysis
4. Paleokarst Characteristics
4.1. Dissolution Characteristics in the Core
4.2. Dissolution Characteristics in FMI Logs
5. Development Characteristics of Paleokarst Cave Systems
5.1. Statistics of Pore-Cave Development
- (1)
- Drilling and mud logging data indicate some 25 intervals of blow-off/loss/overflow in more than 90 Yingshan Formation wells. About 92% of these intervals had vertical blow-off lengths of <3 m, and only two intervals had vertical blow-off lengths of about 4 m (the first and second intervals had a vertical blow-off length of 4.2 m and 4.3 m, respectively).
- (2)
- Interpretation of some 72 wells with conventional logging data (e.g., GR and Rt curves) indicated a total of 213 poor-quality reservoir intervals. The majority of reservoirs (about 54%) in the Yingshan Formation were found at least 50 m below the unconformity, and about 31% of the reservoir intervals were found 50–150 m below the unconformity. About 51% of the reservoir intervals had a thickness of <5 m (Figure 8a). Seventy-eight high-quality reservoir intervals were identified out of a total of 213 reservoir intervals, including 16 unfilled cave reservoirs and 60 unfilled fracture-cave reservoir intervals. Based on log analysis, 56 high-GR-filled intervals with heights ranging from 1–15 m were found. Each high-GR filling interval was a dissolution zone containing at least one smaller filled karst caves in the vertical direction, where the average height of a single small karst cave was <3 m. Pore-caves in the study area were mostly filled and displayed high GR on logs. Additionally, statistical analysis shows that reservoir intervals were generally found in the lower parts of filled intervals (Figure 8b).
- (3)
- FMI data from 19 wells indicated that there were five wells with >40 cm-sized caves. There were 14 wells that intersected 34 caves; 91% of the caves were <3 m high, and 50% of the caves were <1 m high (Figure 9a). The caves were found over discrete intervals below the top of the Yingshan Formation: 17 caves within 30–50 m, 6 caves within 0–10 m, and 3 caves within 50 m below the formation top.
- (4)
- FMI data from six horizontal wells indicated that two wells did not intersect >40 cm-sized caves, and four wells intersected 65 caves (including intermittently distributed caves in one horizontal well). Most of the caves had widths between 1–2 m, and several caves had widths or lengths greater than 10 m, and even up to 100 m (Figure 9b).
5.2. Distribution Characteristics of Paleokarst Pores and Caves
6. Geochemical Characteristics of Paleokarst Cave Fillings
6.1. Cathode Luminescence Characteristics of Pore Fillings and Identification of Paleokarst Environment
6.2. Trace Elements Testing of Fillings and Identification of Paleokarst Environments
7. Paleokarst Types and Genetic Model of the Yingshan Formation Karst in the Northern Slope of the Tazhong Area
7.1. Comparison between the Yingshan Formation Karst Characteristics and Modern Karst Types
7.2. Development Model of Pores and Caves Controlled by Eogenetic Karsts
8. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Spot ID | Analyzing Item | MgO (wt %) | Al2O3 (wt %) | SiO2 (wt %) | K2O (wt %) | CaO (wt %) | Mn (ppm) | Fe (ppm) | Fe/Mn | Sr (ppm) | Ba (ppm) | Sr/Ba | Identification of Paleokarst Environment | ||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Sr/Ba | Fe/Mn | Environments | |||||||||||||
a-1 | Calcite in karst caves | 0.042 | 0.392 | 0.604 | 0.199 | 55.050 | 0 | 529 | / | 0 | 0 | / | / | <1 | Cannot identify |
a-2 | 0.018 | 0.032 | 0.037 | 0.040 | 55.649 | 403 | 54 | 0.13 | 0 | 179 | 0 | ||||
a-3 | 0.019 | 0.042 | 0.136 | 0.059 | 55.746 | 279 | 443 | 1.59 | 0 | 0 | / | ||||
a-4 | Limestone grains in karst caves | 0.194 | 0.094 | 0.070 | 0.028 | 55.546 | 116 | 0 | 0 | 152 | 0 | / | >1 | / | Seawater |
a-5 | 0.197 | 0.343 | 0.590 | 0.102 | 55.027 | 62 | 342 | 5.52 | 0 | 18 | 0 | <1 | ≈5 | Freshwater | |
a-6 | 0.404 | 0.038 | 0.132 | 0.063 | 55.269 | 0 | 490 | / | 381 | 0 | / | >1 | / | Seawater | |
a-7 | Clay in karst caves | 2.040 | 28.133 | 41.72 | 10.722 | 0.766 | 318 | 12,903 | 40.58 | 0 | 152 | 0 | <1 | >5 | |
a-8 | 2.069 | 26.849 | 39.381 | 9.815 | 1.213 | 372 | 6510 | 17.50 | 0 | 152 | 0 | Freshwater | |||
a-9 | 1.927 | 27.916 | 41.316 | 10.239 | 1.262 | 225 | 6844 | 30.42 | 0 | 242 | 0 | ||||
b-1 | Calcite in karst caves | 0.198 | 1.010 | 1.661 | 0.267 | 53.479 | 0 | 1680 | / | 0 | 0 | / | >1 | ≈1 | |
b-2 | 0.076 | 0.153 | 0.203 | 0.058 | 55.497 | 0 | 529 | / | 0 | 0 | / | Seawater | |||
b-3 | 0.132 | 0.566 | 0.267 | 0.018 | 55.082 | 132 | 210 | 1.59 | 676 | 0 | / | ||||
c-1 | Calcite in karst caves | 0.070 | 0.081 | 0.187 | 0.046 | 55.593 | 0 | 319 | / | 617 | 125 | 4.94 | >1 | / | |
c-2 | 0.018 | 0.039 | 0.180 | 0.014 | 55.785 | 0 | 506 | / | 0 | 0 | / | Seawater | |||
c-3 | 0.086 | 0.197 | 0.409 | 0.041 | 55.246 | 0 | 1291 | / | 0 | 0 | / | ||||
d-1 | Calcite cement of calcarenite in upper karst caves | 0.150 | 0.065 | 0.172 | 0.012 | 55.525 | 0 | 630 | / | 592 | 0 | / | >1 | ≈1 | |
d-2 | 0.088 | 0.112 | 0.079 | 0.014 | 55.696 | 0 | 482 | / | 0 | 0 | / | Seawater | |||
d-3 | 0.132 | 0.566 | 0.267 | 0.018 | 55.082 | 132 | 210 | 1.59 | 676 | 0 | / | ||||
e-1 | Arenite grains of calcarenite in upper karst caves | 0.208 | 0.097 | 0.153 | 0 | 55.462 | 0 | 179 | / | 0 | 0 | / | >1 | ≈5 | |
e-2 | 0.168 | 0.574 | 0.385 | 0 | 55.034 | 85 | 389 | 4.58 | 101 | 215 | 0.47 | Freshwater | |||
e-3 | 0.179 | 0.033 | 0.119 | 0.010 | 55.648 | 0 | 0 | / | 0 | 0 | / |
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Dan, Y.; Lin, L.; Liang, B.; Zhang, Q.; Yu, Y.; Cao, J.; Li, J. Eogenetic Karst Control of Carbonate Reservoirs during a Transient Exposure: A Case Study of the Ordovician Yingshan Formation in the Northern Slope of the Tazhong Uplift, Tarim Basin, China. Minerals 2018, 8, 345. https://doi.org/10.3390/min8080345
Dan Y, Lin L, Liang B, Zhang Q, Yu Y, Cao J, Li J. Eogenetic Karst Control of Carbonate Reservoirs during a Transient Exposure: A Case Study of the Ordovician Yingshan Formation in the Northern Slope of the Tazhong Uplift, Tarim Basin, China. Minerals. 2018; 8(8):345. https://doi.org/10.3390/min8080345
Chicago/Turabian StyleDan, Yong, Liangbiao Lin, Bin Liang, Qingyu Zhang, Yu Yu, Jianwen Cao, and Jingrui Li. 2018. "Eogenetic Karst Control of Carbonate Reservoirs during a Transient Exposure: A Case Study of the Ordovician Yingshan Formation in the Northern Slope of the Tazhong Uplift, Tarim Basin, China" Minerals 8, no. 8: 345. https://doi.org/10.3390/min8080345