Abstract:

In the Ordos Basin, the effects of compaction and diagenesis are strong in the Chang 6 reservoir of the Jiyuan Oilfield, and the great heterogeneity has limited the oil and gas development. This paper first studied the microscopic characteristics of the reservoir via microcosmic experiments including conventional physical properties, image granularity, casting lamella, X-ray diffraction, high-pressure mercury injection, constant speed mercury injection and SEM. Taking time as the main factor and integrating with the depositional environment, burial depth, paleotemperature, tectonism and other minor factors, we established the porosity evolution simulation equations of tight sandstone reservoir at Chang 6 by using the diagenetic evolution characteristics and geological comprehensive effects. We analyzed the formation mechanism of the diagenesis effect and porosity evolution in tight sandstone reservoirs. Computing results of porosity evolution show that the initial porosity of the Chang 6 reservoir was 37.33%, the porosity loss by compaction effect was 22.47%, porosity loss by early phase cement-metasomatism was 3.69%, secondary porosity was 6.53%, the porosity loss by middle and late phase cement-metasomatism was 6.97%, and the porosity was 10.06% on average. We suggest that the differential diagenetic evolution process of tight sandstone reservoir is the fundamental cause of the physical property differences and micro-pore structures.

Key words: Jiyuan Oilfield, Chang 6 reservoir, tight sandstone, differential diagenesis, porosity evolution