Dolomitization Of The Triassic Feixianguan Formation, Northeastern Sichuan Basin

There are giant reserves of natural gas and high-quality reservoir rocks in the deeply buried Triassic Feixianguan Formation, Northeastern Sichuan Basin, which is rare in the domestic and international marine carbonate reservoirs. The natural gas exploration for the Triassic Feixianguan Formation, Northeastern Sichuan Basin relates to the high-quality dolomite reservoirs, which are represented as advantage reservoir facie. If the high-quality dolomite reservoirs could be found, and then the natural gas exploration will be hopeful. The dolomitization and diagenesis of the high-quality dolomite reservoirs in Triassic Feixianguan Formation, Northeastern Sichuan Basin have become the hot research field in recent years. Based on detailed core description, field work and sample collection, the research of petrography, geochemistry, temperature and origin of dolomitizing fluids, dolomitizing system and mechanism was systematiclly carried out in this study.The main dolomite types in Triassic Feixianguan Formation include micrite dolomite, grainy dolomite (with good original texture), crystalline dolomite (poor or non-original texture) and dolomite cement. Vertically, crystalline and grainy dolomites are more likely to distributed in the Member 1 and 2 of Feixianguan Formation, and they are non-strata bound and local distribution, whereas dolomites in the Member 4 of Feixianguan Formation are significantly strata bound and regional distribution in the Northeastern Sichuan Basin.Limestone samples in the Triassic Feixianguan Formation are dominated by calcite mineral, of which Mn contents are less than 500ppm and Sr contents are more than 500ppm. The strontium and carbon isotopic evolution curve of marine carbonate in the Triassic Feixianguan Formation are rather consistent with the data from global coeval seawater, indicating that the global geological events control the evolution of strontium and carbon isotopic composition of seawater, whereas oxygen isotopes have exchanged with meteoric water or other diagenetic fluids and could not represent the original composition of the seawater. The strontium isotopic composition of dolomites in the Triassic Feixianguan Formation is higher than that of the corresponding coeval seawater, but dolomitizing fluids is still close to the seawater. The dolomites in the Member 1 and 2 of Feixianguan Formation inherited the carbon from seawater, whereas a few dolomites in the Member 4 of Feixianguan Formation with heavy carbon isotope signatures relate to bacterial sulfate reduction (BSR). The oxygen isotopic composition of dolomites in the Feixianguan Formation are almost close to each other, indicating the similar oxygen isotope composition of dolomitizing fluids and dolomitizing temperatures, but the dolomites in the Member 4 of Feixianguan Formation with higher oxygen isotope composition relate to high-salinity lagoon water after the strong evaporation.The homogenization temperatures of some fluid inclusions in the large dolomite crystals (mainly fine to medium grain) indicate that hose dolomite crystals may precipitate at 100℃to 130℃, which are consistent with the temperatures of main calcite precipitation, so they belong to the high-temperature dolomitizing fluids. The other large dolomite crystals (including powder to medium grain, and their mixed grain), which has few fluid inclusions or only small size ones, may precipitate in less than 100℃, so they belong to low-temperature dolomitizing fluids. The calculated temperatures of most dolomites in the Triassic Feixianguan Formation are at temperatures of 40℃-140℃(within the internal oxygen isotope geothermometer) or 60℃-100℃(within the external oxygen isotope geothermometer), and most of them are particularly at <100℃. The high-temperature zone of the calculated temperatures is significantly consistent with homogenization temperature of fluid inclusions, indicating calculated temperatures of the oxygen isotope geothermometer may provide a reliable estimation of dolomitizing temperature. The calculated temperatures of the oxygen isotope geothermometer further support that dolomites may precipitate almost in the low-temperature range at 40℃-100℃.The Ca/Mg ratios of the dolomitizing fluids in the Triassic Feixianguan Formation are more likely less than 1.17 at temperatures of 40℃-100℃, and the Triassic seawater or Triassic marine fluids were sufficiency to this ratio. The grainy dolomites (with good original texture) and crystalline dolomites (with poor or non-original texture) contain low Mn, Fe contents, whereas micrite dolomites contain high Mn, Fe contents. The Sr contents of dolomites in the Triassic Feixianguan Formation are generally less than 200ppm. The carbon sources of crystalline dolomites in the Feixianguan Formation relate to in-situ or distal migration of the dissolved carbonate, and carbon sources of grainy dolomites in the Feixianguan Formation relate to coeval or non-coeval seawater. The carbon sources of micrite dolomites in the Feixianguan Formation are more complex. Most of them are impacted by the mixture of light carbon isotope from meteoric water, and a few of them are impacted by pore water with heavy carbon isotope from bacterial sulfate reduction (BSR). The crystalline dolomites and dolomite cements are more likely related to the deeply buried dolomitizing fluids with relatively high temperature, and grainy dolomites (with good original texture) and micrite dolomites are more likely related to the dolomitizing fluids near surface with relatively low temperature. Some micrite dolomites are susceptible to further oxygen isotopic re-equilibrium in the recrystallized process. The 87Sr/86Sr ratios of dolomites in the Triassic Feixianguan Formation are significantly greater than that of coeval seawater, but the seawater is still the most important dolomitizing fluid.The dolomitizing systems in the Triassic Feixianguan Formation can be summarized in two significantly different types by geochemistry: open system and closed system. The crystalline dolomites and dolomite cements are more likely associated with closed system that relate to high-salinity evaporative seawater from the Jialingjiang Formation, and correspond to burial dolomitization by non-coeval seawater. The grainy dolomites are more likely associated with open system that relate to lateral or vertical high-salinity evaporative lagoon water, and correspond to penecontemporaneous or shallow burial dolomitization (seepage reflux) by evaporative seawater. The micrite dolomites are more likely associated with open system that relate to mixture of meteoric water and seawater, and correspond to penecontemporaneous evaporation-pump dolomitization in the mixing-zone, whereas a few micrite dolomites in the Member 4 of Feixianguan Formation may correspond to penecontemporaneous microbes (bacteria) dolomitization in the anoxic reducing condition.