Due to the special tectonic background and complex geological evolution characteristics of the South China Sea, reef dolomite reservoirs (sush as Well Xike 1) are widely developed. Based on the drilling core data of Well Xike 1, the structure and geochemical characteristics of dolomite reservoirs, including carbon, oxygen, hydrogen isotopes and REE were systematically studied using geochemical and petrological methods. It is found that the geochemical characteristics of REE show that the main diagenetic environment of dolomites is a low-temperature alkaline semi open oxidation environment; the carbon and oxygen isotopes of the dolomites are generally lack of correlation, the δD value is significantly lower than the hydrogen isotope value of seawater. Meanwhile, the oxygen isotope value of deep dolomites is negatively biased, which may be due to the increase or decrease of pore water temperature caused by deep thermal convection that related to the regional tectonic movements of the South China Sea. The δ¹⁸O value is also consistent with the geological reality of increasing saddle dolomite content in deep dolomites. The distribution of the δ¹³C value indicates that the dolomite inherited the carbon of the original limestone during dolomitization, while the characteristic of the δD value shows that it may be affected by the mixing of atmospheric precipitation and concentrated seawater in the quasi contemporaneous period. Based on the comprehensive analysis of the geochemical characteristics of the Well Xike 1, it is considered that the higher diagenetic temperature could be an important factor leading to the huge differences between the diagenetic model of deep and shallow dolomites. The geochemical characteristics of the shallow dolomites show that it is mainly reflux infiltration dolomitization under the micro evaporation and concentration sea water environments, while the deep dolomite is transformed by the hot water fluids in the epigenetic diagenetic evolution stage.

The Lower Permian Shanxi Formation in the Eastern Ordos Basin is a set of transitional shale, and it is also a key target for shale gas exploration in China. Three sets of organic-rich transitional shale intervals (Lower shale, Middle shale and Upper shale) developed in Shan 2³ Submember of Shanxi Formation. Based on TOC test, X-diffraction, porosity, in-situ gas content experiment and NMR experiments with gradient centrifugation and drying temperature, the reservoir characteristics and pore fluid distribution of the three sets of organic-rich transitional shale are studied. The results show that: 1) The Middle and Lower shales have higher TOC content, brittleness index and gas content, reflecting better reservoir quality, while the Upper shales have lower gas content and fracturing ability. The total gas content of shale in the Middle and Lower shales is high, and the lost gas and desorbed gas account for 80% of the total gas content. 2) The Middle shale has the highest movable water content (32.58%), while the Lower shale has the highest capillary bound water content (57.52%). In general, the capillary bound water content of marine-continental transitional shale in the Shan 2³ Submember of the study area is high, ranging from 39.96% to 57.52%. 3) Based on pore fluid flow capacity, shale pores are divided into movable pores, bound pores and immovable pores. The Middle shale and the Lower shale have high movable pores, with the porosity ratio up to 27%, and the lower limit of exploitable pore size is 10 nm. The movable pore content of upper shale is 25%, and the lower limit of pore size is 12.6 nm. It is suggested that the Lower and Middle shales have more development potential under the associated development technology.