Low-amplitude structures and oil-gas enrichment on the Yishaan Slope, Ordos Basin

Considering the geological and tectonic characteristics and based on drilling data and production test results, a drawing method with a dense well pattern and large scale was used to analyze the low-amplitude structures and oil-gas enrichment in the Yishaan slope, Ordos Basin. Low-amplitude structures are developed regionally and intensively, extended directionally in array folding, and evolved inheritedly. Based on geometry, coverage and genesis, the structures are divided primarily. With tectonic stress as the driving factor, the low-amplitude structures contain basically nose-like units, in nearly E-W trend. The fluctuating attitude of strata at the break affects the driving force, speed and direction of oil-gas migration along the updip of the slope, making oil/gas/water differ in sections and oil/gas dissipate or cluster locally, so that the alternating oil-gas areas and the oil-water areas with more water are formed. A series of composite traps (lithologic-structural, structural-lithologic or stratigraphical-structural) are defined by combination of the endpoint connecting line of hinge zone in low amplitude tectonic slope break with the lithology or property pinch-out line of sedimentary facies belt side parts, or with local strata pinch-out line. In such traps, the hydrocarbons are accumulated. The advantageous configuration of low-amplitude structures and large sedimentary systems as well as favorable facies controls the oil and gas distribution in Mesozoic –Paleozoic formations in the Yishaan slope.