The Distribution and Formation Mechanism of Dolomite in Triassic Feixianguan Formation in JianNan Area

Wang, Guangwei; Li, Pingping; Chen, Gang; Yu, Xinya

Gas reservoirs of the Triassic Feixianguan Formation in JianNan area are mainly composed of limestone with low porosity, parts of which were dolomitized to form high porous reservoirs. However, the distribution and formation mechanism of dolomite are poorly documented. In order to recognize the distribution law of dolomite, the percentage of dolomite in carbonate rocks was predicated on the basis of cores, lithological logs and seismic attributes derived from 3-D seismic volume. By integrating whole-rock δ13C (3.99 to 5.48‰, PDB), δ18O (-7.18 to -5.44‰, PDB), 87Sr/86Sr (0.707391 to 0.706963) values, trace elements of dolomite with sedimentary facies, characteristics of dolomitizing fluid, dolomite formation temperatures, sources of magnesium are discussed. The results are: (1) Five most effective attributes were selected. These attributes are Amplitude Weighted Frequency, Amplitude Weighted Cosine Phase, Amplitude Envelope, Quadrature Trace and Instantaneous Phase. The distribution of dolomite is mapped, and dolomite occurs along the margins of a trough. (2) The order degree of dolomite ranges from 0.77 to 0.94 (average 0.87). Formation temperatures calculated from δ18O values vary from 44 to 54°C. Two features above reflect dolomite formed in shallow burial environment. (3) Sr2+/Ca2+ ratio (0.028 to 0.078) of dolomitizing fluid and Z (132.1 to 135.6) calculated from δ13C and δ18O suggest dolomite precipitated from more salty fluid than connate seawater. (4) Rare earth element patterns of dolomite are similar to these of limestone recorded geochemistry of the seawater in which it precipitated; 87Sr/86Sr of dolomite within the range of values of the Triassic seawater indicates dolomitizing fluid is seawater-derived brine. Some conclusions can be obtained as follows: (1) Dolomite occurs mainly along the rims of the trough and is controlled by sedimentary facies. (2) Dolomite formed in the shallow burial environment. Dolomitizing fluids were generated by compactional flow of modified-marine pore fluids. Mg2+ came from seawater-derived brine and clay mineral transformations in marlstones deposited in the trough. Mg-rich burial fluids driven by sediment compaction passed laterally through adjacent porous margin-shoals to form replacement dolomite. (3) Looking for relatively high porous reservoirs, shoals along margins of the trough can be recognized as important exploration targets.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013