Petroleum Science

Origin of dolostone,Mg-C-0 isotopes,Sea level changes,Burial adjustment dolomitization,Buqu formation,Plateau basin

The origin of dolostone in the Middle lurassic Buqu Formation of the Plateau Basin has been a subject oprolonged debate. This study combines detailed petrological observations with analyses of Mg-C-0isotopes and elements to constrain the origin of dolostones in the Buqu Formation. Petrography andcathodoluminescence (CL) examination identified three types of matrix dolostones: very finely to finelycrystalline dolostone (D1), finely to medium crystalline dolostone (D2), and medium to coarsely crys.talline dolostone (D3). The analysis of the diagenesis sequence reveals that D1 originated from thedolomitization of grainstone in the early diagenetic phase, whereas D2 and D3 resulted from therecrystallization of D1 during the later burial phase. The presence of high Na (>100 ppm), low Fe(<1000 ppm), low Mn (<250 ppm), positive Ce anomaly, LREE enrichment, stable 826Mg (-2.28%to -2.04%), and 8’3c (1.02%-2.95%) indicates that the early dolomitization fluid was oxidized seawater.As the crystal size increases (D1-D2-D3), the progressively rising Mn content and significantlynegative 8'80(-10.72%to-7.81%) suggest that the dolostone has experienced modification andalteration by buried pore water in the later stages. "he fluctuations in relative sea level during thesedimentary deposition of the Buqu Formation were reconstructed through the utilization of Na, Sr/CuSr/Ba, Rb/Sr, >REE, and $'3c, it was observed that the 826Mg of dolostone closely mirrored the variationsin sea level. The consistent trend of change confrms that sea level fluctuations control the formation anddistribution of early dolostone. Frequent sea level rise and fall prompted the limestone deposited on thecarbonate platform to be continuously transformed into dolostone, which accumulates over a longperiod to form large-scale thick dolostone. After the formation entered the burial stage, under thecombined action of high Mg/Ca ratio pore water, high temperature, and high pressure, the early dolo.stone experienced the adjustment of burial dolomitization. This research offers a typical case study onthe application of Mg-C-0 isotope and elements to determine the origin of dolostone, This will aid in amore comprehensive understanding of the formation process of dolostone in ancient rock records.

22

2

574-587

1995-8226

SCI