Enrichment of tight oil and its controlling factors in central and western China

Abstract

Taking the tight oil of the Zhongnan sag in the Ordos Basin, Jimusar sag in the Junggar Basin and Qingxi sag in the Jiuquan Basin as study objects, based on field survey, dissection of tight oil reservoirs, sample test, modeling experiment and comprehensive analysis, this study reveals that the tight oil accumulates at start-up pressure, advances under differential pressure, diffuses at alternating fast and low speeds, charges in stepped large area and migrates rapidly through fractures, and enriches in dominant fractures and pores. The root cause of ladder-like charge is the multiple scales of pores. The widespread source rock with high hydrocarbon generation intensity is the material basis for tight oil enrichment; the dominant source reservoir assemblage is the basic unit for tight oil enrichment; fractures and beddings are conducive to local rapid migration of tight oil; fractures and pores work together to control the enrichment of tight oil. Two typical accumulation models of tight oil are established, namely “source reservoir in coexistence, four optimal factors controlling enrichment around central area, and large-scale continuous distribution” for a large freshwater lake clastic rock basin and “source reservoir integration, four optimal factors controlling enrichment, central area distribution, small in size but high in enrichment degree “ for a small saline lake diamictite depression.