Hydrocarbon accumulation conditions of Chang7 section, Yanchang Formation, Ordos Basin

The Ordos Lake in Chang7 phase was largest and deepest in Yanchang period, and deposited high-quality and widely-distributed source rocks with some sandstones of delta front facies and turbidites facies. The analyses of deposition environment, reservoir feature, diagenetic facies and the assemblage of source, reservoir and cap showed that the Chang7 section had favorable source conditions. The favorable reservoirs mainly included turbidites of underwater distributary channel at delta front and semi-deep/deep lake facies. The sandstones were mainly lithic arkose sandstones and feldspathic lithic sandstones with low compositional and textural maturities. The reservoirs had low porosity (8.45% in average) and low permeability (0.12×10^-3 μm² in average). Chang7 sandstones were divided into 5 diagenetic facies types, among which the most favorable type was the cemetation-micropore intergranular pore and dissolution-dissloved pore and micropore diagenetic facies, but the distribution area was limited. The diagenetic facies with worst reservoir physical properties was the compacted cement-microporous diagenetic facies, which had broader distribution area. The semi-deep/deep lake deposition environment led to poor plane connectivity in Chang7 section. Hydrocarbon mainly migrated vertically, or laterally for short distance, and accumulated when blocked by mudstones or tight sandstones.