The Source, Distribution of Organic Acids in Oilfield Waters and Their Effects on Mineral Diagenesis in Tarim Basin

Cai Chunfang; Mei Bowen; Ma Ting; Zhao Hongjing; Fang Xiaolin

The source, distribution and effects of organic acid anions( OAA) in sedimentary basins were notsystematically studied and were poorly understood in China. The paper presented 185 OAA data and 100samples of dissolved Al, Si, Ba and other 12 elements in oilfield waters from Cambrian to Tertiary reser-voirs of the Tarim basin. The above data showed that OAA of high concentration (> 800× 10- 6) oc-curred in reservoirs deep than 4 300 m and nearby unconformities. OAA are thought to be sourced fromthermal maturation of kerogen biodegradation and thermochemical sulfate reduction of crude oils, whichis supported by high reservoir temperature( 115 ℃ to 139 ℃ ), abundance of dissolved H 2 S in oilfield waters( 600 × 10- 6to 780 × 10- 6), and occurrence of tas mats. OAA may contribute 30 % ～ 70 %, up to 92 % oftotal alkalinity; thus it controls pH values of oilfield waters. The concentration of dissolved aluminumranges up from 3. 1 mg /L to 5. 4 mg /L in oil- bearing reservoirs, and it indicates the existence of organ-ic- complexing agents.Selected mineral equilibrium calculation from Carboniferous, Triassic and Jurassic reservoirs wascarried out by computer code SOLMINEQ. 88 for geochemical modeling of water- rock interaction, andshowed that oligioclase is unsaturated and can be dissolved at 81 ℃ to 138 ℃ while kaolinate is over- sat-urated and tends to precipitate at the temperature lower than 132 ℃; quartz is near equilibrium; K-feldspar is stable between 81 ℃ to 105 ℃. From the above, it can be concluded that oligioclase dissolu-tion and kaolinite precipitation are the main diagenesis, and control secondary porosity. And it is consis-tent with the petrographic observation.

Jianghan Petroleum Institute, Jingsha 434102

Received Date: 1995-11-28

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Abstract: The source, distribution and effects of organic acid anions( OAA) in sedimentary basins were notsystematically studied and were poorly understood in China. The paper presented 185 OAA data and 100samples of dissolved Al, Si, Ba and other 12 elements in oilfield waters from Cambrian to Tertiary reser-voirs of the Tarim basin. The above data showed that OAA of high concentration (> 800× 10- 6) oc-curred in reservoirs deep than 4 300 m and nearby unconformities. OAA are thought to be sourced fromthermal maturation of kerogen biodegradation and thermochemical sulfate reduction of crude oils, whichis supported by high reservoir temperature( 115 ℃ to 139 ℃ ), abundance of dissolved H 2 S in oilfield waters( 600 × 10- 6to 780 × 10- 6), and occurrence of tas mats. OAA may contribute 30 % ～ 70 %, up to 92 % oftotal alkalinity; thus it controls pH values of oilfield waters. The concentration of dissolved aluminumranges up from 3. 1 mg /L to 5. 4 mg /L in oil- bearing reservoirs, and it indicates the existence of organ-ic- complexing agents.Selected mineral equilibrium calculation from Carboniferous, Triassic and Jurassic reservoirs wascarried out by computer code SOLMINEQ. 88 for geochemical modeling of water- rock interaction, andshowed that oligioclase is unsaturated and can be dissolved at 81 ℃ to 138 ℃ while kaolinate is over- sat-urated and tends to precipitate at the temperature lower than 132 ℃; quartz is near equilibrium; K-feldspar is stable between 81 ℃ to 105 ℃. From the above, it can be concluded that oligioclase dissolu-tion and kaolinite precipitation are the main diagenesis, and control secondary porosity. And it is consis-tent with the petrographic observation.

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The Source, Distribution of Organic Acids in Oilfield Waters and Their Effects on Mineral Diagenesis in Tarim Basin

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