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A Comparative Study of Cambrian Hydrothermal Dolomite and Associated Dolomite Reservoirs in Western Canada Sedimentary Basin and Tarim Basin in NW China

Hairuo Qing1, Daizhao Chen2, Zhijun Jin3, and Ping Luo4
¹University of Regina, Regina, SK, Canada
²Institute of Geology and Geophysics, Beijing, China
³SEPRI, Sinopec, Beijing, China
4RIPED, PetroChina, Beijing, China

The process of dolomitization and formation of dolomite reservoirs have been a controversial topic for a long time. Numerous dolomitizing models of have been proposed to explain the occurrence of massive dolomite in the rock record. However, if these models were applied blindly without much scrutiny, a dolomite bandwagon is produced, such as seepage/reflux model in the 60’s and mixing zone model in the 70’s-80’s. Since late 80’s, a number of detailed case studies from the Western Canada Sedimentary Basin (WCSB) suggested that hydrothermal fluids played a vital role in formation of massive dolomite and associated reservoirs. In recent years, hydrothermal dolomitization has received so much attention and is becoming a new dolomite bandwagon, which has been applied indiscriminately to some case studies in China where the differences between these so called hydrothermal dolomite were ignored.

Our comparative study and analyses of Cambrian hydrothermal dolomites from the WCSB versa those from the Tarim Basin indicate a fundamental difference between the hydrothermal dolomite in these two basins. The hydrothermal dolomites from the WCSB generally occurred in an open system that delivered Mg as well as heat to the reaction site; whereas most of hydrothermal dolomites in the Tarim Basin occurred in a closed or semi closed system, where Mg is mostly derived via chemical compaction of precursor dolomite. This fundamental difference resulted in a number of unique petrographic and geochemical characteristics of hydrothermal dolomites in two different basins:

(1) The WCSB is characterized by high water/rock ratio of an open system; the Tarim Basin illustrates low water/rock ratio, typically of closed, semi-closed system.

(2) In the WCSB, hydrothermal fluids precipitated large amount of dolomite cement and replaced host precursor limestone; in the Tarim Basin hydrothermal fluids mostly modified earlier dolomites with minor amount dolomite cements.

(3) In the WCSB, the geochemical signatures of hydrothermal dolomite are distinctly different from those of host carbonate rocks; in the Tarim Basin the geochemical signatures of hydrothermal are similar to host carbonate rocks.

(4) In the WCSB, hydrothermal fluids is mostly related to deeply seated fault system; in the Tarim Basin hydrothermal fluids is mostly like linked to the igneous activities.

The fundamental difference of the origin of hydrothermal dolomite from the WCSB and from the Tarim Basin requires a different exploration strategy applied to these two basins. In the WCSB, explorations of hydrothermal dolomite should be focused along regional conduit system along regional faults and reef chains. In the Tarim Basin, hydrothermal dolomite reservoir preferentially occurs around igneous intrusions and/or local fault systems.

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013