Conditions of XJ-A structure of the Paleogene reservoir in the Xijiang Depression and their influence on hydrocarbon accumulation
The Xijiang Depression is located in the southwest of the Zhuyi Depression in the north of the Pearl River Mouth Basin.It has been poorly explored in research compared with the adjacent Huizhou Depression.Recent research has revealed that the Enping and Wenchang Formations in the Xijiang Depression from the Paleogene have adequate potential for hydrocarbon generation,expulsion,and exploration and development.To clarify the impact of the conditions of the Paleogene reservoir on oil and gas accumulation in the Xijiang Depression,we considered the XJ-A structure located in the main depression of Xijiang,and used data on the physical properties of the reservoir,sedimentary facies,the identification of thin sections,and inclusion testing to analyze its macroscopic and microscopic characteristics.The results showed that the Paleogene sedimentary facies of the XJ-A structure were dominated by shore shallow-lake,middle deep-lake,and braided river-delta facies.The Enping Formation developed a large volume of mudstone that was dominated by shore shallow-lake facies,while the Wenchang Formation was dominated by sandstone from the braided river-delta facies such that the former was rich in mud while the latter was rich in sand.The sandstone of the Enping Formation primarily consisted of feldspar and quartz sandstone,while the Wenchang Formation mainly featured feldspar sandstone.The porosity of sandstone in the Enping Formation ranged from 5%~15%,that of sandstone in the Wenchang Formation ranged from 10%~15%,and their permeabilities ranged from(1~10)× 10-3μm2.They all belonged to reservoirs with low/ultra-low porosity and low permeability within the squared-meter range.The diagenesis of the reservoir mainly featured compaction,the precipitation of authigenic minerals,dissolution,and replacement and filling.In conjunction with the homogenization temperature of inclusions captured by the sandstone reservoir in this area,the vitrinite reflectance of organic matter,and the percentage content of montmorillonite in the illite/montmorillonite mixed layer,showed that the sandstone reservoir of the XJ-A structure was in the A2 stage of intermediate diagenesis.Our analysis showed that the XJ-A structure had suitable reservoir-related conditions for the development of the Paleogene system,and the Paleogene sand body provided an effective pathway for the lateral migration of oil and gas.It also cooperated with faults in the area to transport oil and gas,thus guaranteeing their accumulation in the trap on the slope belt.