Science China Earth Sciences

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

This paper investigates the origin of ultra-deep dolostone and the factors influencing large-scale dolostone reservoirsfocusimng on the Sinian Dengying Formation and the Cambrian Longwangmiao Formation in the Sichuan Basin. The studtinvolves petrology, microscale X-ray diffraction, trace element analysis, and C-0-Sr-Mg isotope experiments to provide adetailed analysis. The research findings indicate that the Dengying and Longwangmiao formations comprise six types of matrixdolostone and four types of cement. The Dengying Formation, which developed under a sedimentary background of a restrictedplatform, contains special microbial and microcrystalline dolostones. The dolomite grains are small (<30 um) and have a loworder degree (Min=0.55), with large unit cell parameters and an extremely high Na content (Max=788 ppm). The "'sr''sr valueof th趨系 ざ汞髃罈偹退飑拭stone塬骊華蚶奇韪ю鲊称onsistent with contemporaneous seawater, while the s’'c and s"$0 values are lower than those of thecontemporaneous seawater. The s"Mg value is small (Min=-2.319). Powder crystal, fine-crystalline, and calcite dolostoneswith coarser an廋砗 more ordered crystals exhibit similar s"'C and "'Sr/'sr values to microbial and microcrystalline dolostoneDuring the sedimentary period of the Dengying Formation, ancient marine conditions were favorable for microbial survival.Microorganisms induced the direct precipitation of primary dolomite in seawater, forming microbial and microcrystallinedolostones during the seawater diagenesis period. During the subsequent diagenesis period, dolostones underwent the efiects ofdissolution-recrystalization, structures, and hydrothermal fuids. This resulted in the formation of dolostone with coarsercrystals, a high拡淘刑晔岳性臌斯猝黌窗ree of order, and various types of cement. The Longwangmiao Formation was developed in an inter.platform beach characterized by special paticle dolostone. The particle dolostone has a large grain size (>30 ju), high ordedegree (Min=0.7), small umit cell parameters, high Na content (Max=432 ppm), and low Fe and Mn content. The s"g and s'”'cvalues are consistent with the contemporaneous seawater, while the o"0 and "'Sr'sr values are higher than those of thecontemporaneous seawater. There is mutual coupling between multiple-period varying o"Mg values and sedimentary cyclesThe dolostone in the Longwangmiao Formation resulted from the metasomatism of limestone by evaporated seawater. Thethickness and scale of the dolostone in the Longwangmiao Formation are controlled by the periodic changes in sea level. Theperiod of dolostone development from the Sinian to the Cambrian coincides with the transition from Rodinia's breakup toGondwana's convergence. These events have resulted in vastly different marine properties, microbial activities, and sedimentaryclimate backgrounds between the Simian and the Cambrian. These diferences may be the findamental factors leading to thedistinct origins of dolostone formed in the two periods. The distribution of sedimentary facies and deep tectonic activities in theSichuan Basin ftom the Sinian to the Cambrian is infuenced by the breakup and convergence of the supercontinent. This processplays a k壓喏赛y rol波傖銳挾枼蝿效 detemining the distribution, pore formation, preservation, and adiustment mechanisms of ultra-deepdolostone reservoirs. To effectively analyze the genesis and reservoir mechanisms of ultra-deep dolostone in other regions oiavers, especially during the specifc period of sunercontiment breakup and convergence, it is cicial to consider the comorehensive characteristics of seawater properties, microbial activities, sedimentary environment, and fault systems driven bvtectonic activities. This can help predict the distribution of high-quality and large-scale ulra-deep dolostone reservoirs.

67

7

2352-2382

1869-1897

SCI