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DOI:10.1306/10132523091
Effects of hydrothermal fluids on carbonate reservoirs: Case study from the lower Permian Sichuan Basin (Qixia Formation), China
Di Xiao1 , Jian Cao2 , Xiucheng Tan1 , Long Wen3 , Ran Liu3 , Feifan Lu1 , Minglong Li1 , Feng Liang4 , and Jianfeng Zheng4
1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, PetroChina Key Laboratory of Carbonate Reservoirs, Sichuan Natural Gas Geology Key Laboratory, Southwest Petroleum University, Chengdu, Sichuan 610500, China
2 State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
3 PetroChina Southwest Oil and Gas Field Company, Chengdu, Sichuan 610051, China
4 CNPC Key Laboratory of Carbonate Reservoirs, PetroChina Hangzhou Research Institute of Geology, Hangzhou, 310023, China
Ahead of Print Abstract
The effects of hydrothermal fluids on carbonate reservoirs are a critical issue in petroleum geology. Here we present a comprehensive study of the lower Permian Qixia Formation in the Sichuan Basin, southwestern China, based on petrology, geochemistry, and geochronology. We identified universal effects of hydrothermal fluids on sucrosic dolomites of the Qixia Formation. These dolomites are classified on the basis of petrologic characteristics as anhedral medium- to coarse-crystalline dolomites (Dol-1), euhedral medium- to coarse-crystalline dolomites (Dol-2), and saddle-shaped dolomites (SD). Dol-2 is further subclassified as dark-red Dol-2a and bright-rimmed Dol-2b according to the cathodoluminescence. Dol-1 and Dol-2a, which respectively originated from remnant-grain-bearing matrix limestones and those modified by eogenetic karstification, are the products of horizontal thermal convective dolomitization by marine-related fluids during the shallow burialjust after deposition (from 274.5 ± 9.9 to 269.1 ± 12.0 Ma). They are both geochemically characterized by left-leaning REE patterns with positive Y anomalies and relatively low fluid-inclusion homogenization temperatures (70–150 °C). Dol-2a have flatter REE patterns and lower Y/Ho values than Dol-1. Dol-2b are the products of Dol-2a superimposed by fault-controlled thermal convective dolomitization by marine-related fluids during the Emeishan Large Igneous Province (from 263.2 ± 10.3 to 258.7 ± 8.1 Ma). SD are the cements of the dolomitizing fluids. Dol-2b and SD are both geochemically characterized by left-leaning REE patterns with positive Eu and Y anomalies and relatively high fluid-inclusion homogenization temperatures (120–220 °C). SD have more positive Eu anomalies than Dol-2b. Hydrothermal dolomitization in the Qixia Formation made a limited contribution to reservoirs, which were inherited and modified during the early stage of hydrothermal dolomitization and infilled and destroyed during the late stage. Our study highlights that hydrothermal effects on reservoirs cannot be generalized, and that the origin and properties of hydrothermal fluids and modified lithologic associations should be comprehensively considered, as they produce diverse hydrothermal effects. Whether or not high-quality carbonate reservoirs develop in deep to ultra-deep zones is highly dependent on the occurrence of original favorable sedimentary facies and early constructive diagenetic modification. Burial diagenesis during the late stage may improve reservoirs conditionally.
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