In this paper, the genesis of dolomite in Longwangmiao Formation is analyzed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ¹³C value, and higher δ¹⁸O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca²⁺ was consumed, resulting in an increase in the Mg²⁺/Ca²⁺ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata.

本文通过野外调查和地球化学方法分析了龙王庙组白云岩的成因，建立了白云石化模型，对下一步龙王庙组储层预测具有重要意义。根据晶体大小，龙王庙组白云岩可分为微晶白云岩（MID）和大晶白云岩（MAD）两种。两种类型的白云石的铝含量均高于石灰泥岩，这表明在龙王庙组沉积过程中，陆源物质不断混入沉积物中，这极大地影响了龙王庙组的地球化学特征。两种类型的白云岩的稀土元素（REE）成分与石灰泥岩（LM）的成分一致，具有明显的δCe正异常，没有δEu异常，这是海相碳酸盐沉积的典型分布模式。微晶白云石具有更高的Na元素含量，降低δ ¹³ C值，和更高的δ ¹⁸O值，表明它是在高度盐水蒸发的环境中形成的。大部分的大晶白云石具有残余的晶粒织构。白云岩的元素以及同位素地球化学特征与石灰泥岩一致，表明大晶白云岩形成较早，并继承了前身石灰岩的地球化学特征。我们认为龙王庙组的白云石化模型是蒸发浓度+回流渗流模型。在龙王庙组沉积后期，海平面下降，在限制和半限制台地上出现了大片泻湖。随着蒸发和浓缩持续，诸如高镁方解石和文石泥等前体矿物经历了化学沉淀。²⁺被消耗掉，导致Mg ²⁺ / Ca ²⁺比增加。然后，前体矿物经历白云石化，并开始形成微晶白云岩。同时，由于高盐度浓海水与早期石灰岩中存在的孔隙水之间的密度差异，浓海水流回并向下渗透，引起下伏地层的白云石化，然后形成巨晶白云岩。白云石化程度主要受颗粒滩的发育程度和地层厚度的影响。