Characteristics and main controlling factors of tight oil reservoirs in Chang 7 Member of Yanchang Formation in Ordos Basin, North Shaanxi

To explore the mechanism of tight-sandstone reservoir development in the Chang 7 Member of the Yanchang Formation in the Ordos Basin, North Shaanxi, the characteristics of the sedimentary rocks and the main control factors were analyzed by using the core data, thin section of cast, X-ray diffraction, scanning electron microscopy, and high-pressure mercury compression. Results show that the sedimentary rocks of Chang 7 Member are rich in feldspar, and mainly are feldspathic and lithic feldspar sandstones. The pore types of tight sandstone reservoir are mostly intra-grain dissolution pores and inter-grain dissolution pores of feldspar, and intergranular micropores and microfractures are also developed. The Chang 7 Period in the study area was mainly a deltaic foreland and shallow lakeside depositional environment with weak hydrodynamics as reflected by high mud content in the sandstones. The medium-intensity compaction and early calcite cementation caused the primary pore space to disappear, and the cementation further made the pore space smaller. Due to the high content of feldspar and clay, they were subject to dissolution and then formed dissolution pores and improved the reservoir quality, which is also the main controlling factors on the development of dense-sandstone reservoirs in the Chang 7 Member of the study area.