柴达木盆地英西地区渐新统湖相碳酸盐岩储层形成与演化

Formation and evolution of Oligocene lacustrine carbonate reservoirs in Yingxi area of Qaidam Basin

柴达木盆地英西地区渐新统下干柴沟组上段发育的咸化湖泊相的碳酸盐岩中已接连钻获一批高产油气井,具有较大油气勘探潜力。综合岩心观察、物性、岩石薄片、扫描电镜、CT扫描、碳—氧同位素等方法分析了下干柴沟组上段储层的矿物组分、岩石物性、储集空间,厘清了岩石组分对储层的影响及成岩演化序列。结果表明:研究区渐新统岩性以灰岩、白云岩和砂岩为主,发育晶间孔、溶蚀孔、微裂隙和铸模孔,白云岩储层物性相对较好;渐新统储层中白云石的发育能有效改善储层物性;白云岩沉积于强烈蒸发作用的封闭水体环境,为低温的准同生白云石化作用的产物;成岩作用包括胶结作用、压实作用、溶蚀作用、交代作用和破裂作用,建设性成岩作用为白云石化作用、早—中成岩阶段的淡水溶蚀、有机酸溶蚀和构造应力。研究成果为柴达木盆地英西地区渐新统湖相碳酸盐岩有利储层预测提供了科学依据。

The carbonate rocks of saline lacustrine developed in Upper Member of Oligocene Lower Ganchaigou Formation of Yingxi area in Qaidam Basin have considerable oil and gas exploration potentials for a number of high-yield wells have been successively drilled.With the comprehensive application of the core observations,physical property,thin sections,SEM,CT scanning and carbon-oxygen isotopes,the mineral composition,petrophysical properties and reservoir space were figured out for the Upper Member of Oligocene Lower Ganchaigou Formation,and the influence of the rock composition on the reservoir and the diagenetic evolution sequence were calibrated.The results show that in the study area,Oligocene lithologies mainly include limestone,dolomite and sandstone rocks with intergranular pores,dissolved pores,micro-fractures and moldic pores developed,and the physical properties of the dolomite reservoirs are relatively good.The development of dolomite in Oligocene reservoirs can effectively improve the physical properties of the reservoirs.The dolomite deposited in the closed and high-evaporation water environment is regarded as the product of low-temperature penecontemporaneous dolomitization.The diageneses include cementation,compaction,dissolution,metasmatism and fracture,and the constructive diagenesis includes dolomitization,fresh-water dissolution,organic acid dissolution and tectonic stress in the early-middle diagenesis stages.The study achievements have provided the scientific evidence for the prediction of the favorable reservoirs in Oligocene lacustrine carbonate rocks in Yingxi area of Qaidam Basin.