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Petrography and Geochemistry of Late Cambrian Reservoirs, Tabei Uplift, Northern Tarim Basin, NW China

Siyang Zhang1, Hairuo Qing1, Shaonan Zhang2,3, and Qingyu Huang4,5
1University of Regina, Regina, SK, Canada
2Southwest Petroleum University, Chengdu, China
3State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China
4Chengdu University of Technology, Chengdu, China
5State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu, China

The late Cambrian strata in Tabei Uplift, northern Tarim Basin, NW China contains a succession of restricted platform and platform margin carbonates, which are mostly dolomitized. Based on obtained petrography and geochemistry data, these dolomites are interpreted as: penecontemporaneous dolomite, burial dolomite, and late stage dolomite. Those are associated with four crystalline dolomite types: (1) very finely – finely crystalline replacement dolomite; (2) fine – medium crystalline replacement dolomite; (3) dolomite cement and (4) saddle dolomite cement. Very finely – finely crystalline replacement dolomite is associated with mudstone, wackestone and packstone with well preserved the precursor texture of limestone. It is typically composed of un-planar to planar-s dolomite with noneweak luminescent under CL. Fine – medium crystalline replacement dolomite is preferentially occurring in late Cambrian carbonate reservoirs. It consists of mainly planar-s crystal mosaics, and displays the bright red luminescence rim in the crystal margin with none/dull red luminescence in the crystal center. Fine – medium crystalline replacement dolomite has subhedral crystal shape with sucrosic texture, but the precursor limestone texture was not preserved. Rhomb dolomite cement consists of medium – coarsely, planar-s to planar-e crystals with a successively CL zonation from mottled, to bright, to dull, to bright luminescent. It occurs preferentially in vugs and fractures. Saddle dolomite, minor dolomite type with reddish colour, shows strong wave extinction and a mottled texture without zonation under cathodoluminescence.

All four types of dolomite have similar trace element distribution with enrich in K, Fe, and Al. Very finely – finely crystalline dolomite displays the δ18O value ranging from -5.7 to - 10.0 (‰ vPDB) (mean -7.3‰), δ13C value from +1.4 to -2.0 (‰ vPDB) (mean -0.5‰) slightly higher than those of the Cambrian calcite precipitated from seawater (δ18O and δ13C values between -7.0 to -10.0 and 0.0 to -2.0 (‰ vPDB), respectively). Very finely – finely crystalline dolomite has 87Sr/86Sr ratios from 0.7082 to 0.7091 that mostly overlap with the Sr isotope value of coeval seawater (0.7086-0.7092). Fine – medium crystalline replacement dolomite exhibits a wide range of δ18O values from -5.8 to -11.4 (‰ vPDB) (mean -8.3‰); but a narrow range of δ13C values from -0.9 to -1.4 (‰ vPDB) (mean -0.6‰), and 87Sr/86Sr ratios from 0.7075 to 0.7108. Both types of dolomite cements have δ13C values similar to that of the Cambrian marine calcite, δ18O values of saddle dolomite varies from -9.9 to -13.0 (‰ vPDB), which is lower than those of rhomb dolomite cement -8.7 to –9.9 (‰ vPDB). In addition, saddle dolomite has the 87Sr/86Sr ratios from 0.7094 to 0.7102 is slightly higher than those of rhomb dolomite cement.

Penecontemporaneous dolomite consists of very finely-finely crystalline dolomite with relict texture that was formed by penesaline seawater from slightly evaporative environment; finemedium crystalline replacement dolomite is related to brine-reflux at increased temperature during shallow burial; dolomite cements formed last in fractures and dissolution vugs by basinal fluids or, residual brine-water with high radiogenic 87Sr concentration and elevated temperature.

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