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Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
3. (May), Pages 486-498
Diagenesis of Analcime-Bearing Reservoir Sandstones: The Upper Permian
Pingdiquan Formation, Junggar Basin, Northwest China
Zhaohui Tang (1), John Parnell (2), Fred J. Longstaffe
The Junggar Basin is one of the largest and most important oil-producing
basins in China, in which Upper Permian lacustrine oil shales are among
the thickest and richest source rocks in the world. The Upper Permian Pingdiquan
Formation was deposited predominantly in fan-delta sequences within a lacustrine
setting. In the northeastern Junggar Basin, the Pingdiquan Formation sandstones
(volcanic and feldspathic litharenites) constitute the principal oil reservoirs,
whereas the interbedded black shales are the predominant oil source rocks.
The early diagenetic minerals in the sandstones include siderite, pyrite,
analcime, albite, calcite, and trace amounts of halite. Late diagenetic
minerals include K-feldspar, ankerite, and minor mixed-layer clay minerals.
A similar diagenetic sequence as recognized in the intercalated mudrocks,
which contain abundant analcime, albite, and microcrystalline dolomite.
Early authigenic mineral formation (e.g., calcite versus analcime/albite)
was controlled by alternating periods of lower and higher salinity in the
lacustrine environment. Cementation by siderite, analcime, calcite, and
albite substantially occluded sandstone porosity early in diagenesis. However,
extensive dissolution of analcime cement and detrital feldspars during
burial produced significant secondary porosity in the sandstones, improving
their reservoir potential. Organic acids generated during oil-shale maturation
may be responsible for secondary porosity in the interbedded sandstones.
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