Fractures development characteristics and distribution prediction of carbonate buried hills in Nanpu Sag, Bohai Bay Basin, China

Abstract

The natural fracture system plays a key role in the formation of hydrocarbon reservoirs in the carbonate buried hill of the Nanpu Sag in the Bohai Bay Basin, affecting the distribution of high-quality reservoirs and the migration and accumulation of hydrocarbons. Using data from outcrops, cores, thin sections, and image logs, a quantitative analysis was conducted on the development patterns of fractures both in vertical and horizontal directions, and the main controlling factors for fracture development were identified. On this basis, numerical simulation techniques were applied to quantitatively predict the development patterns of fractures in the carbonate reservoirs of the ancient buried hills in Nanpu Sag. Four types of fractures were identified in the study area: structural fractures, diagenetic fractures, weathering fractures, and dissolution fractures, with structural fractures being the most predominant. The fractures show a low degree of filling, with 59% being effective, indicating good fracture effectiveness. The linear density of structural fractures ranges from 3 to 10 m⁻¹, with an average of 5.6 m⁻¹. The height of structural fractures is generally less than 30 cm, mainly distributed between 5 and 20 cm. The microscopic fracture areal density ranges from 25 to 50 cm/cm², with an average of 32.3 cm/cm², and the porosity of micro-fractures ranges from 0.24% to 0.69%, averaging at 0.55%. These micro-fractures provide essential storage space in tight reservoirs and enhance pore connectivity, facilitating hydrocarbon migration and accumulation. Three primary fracture groups were identified in the study area: nearly E–W trending fractures, NE–SW trending fractures, and NW–SE trending fractures, with the first two groups being the most developed. The degree of fracture development in the study area is mainly affected by lithology, rock mechanical layers, and faults. Fractures are most abundant in dolomite and dolomitic limestone, but less developed in mudstone. Different rock mechanical interfaces affect the geometry, scale, and intensity of fracture development. Stratigraphy-bound fractures are generally vertical and terminate at rock mechanical interfaces, while throughgoing fractures usually span multiple mechanical layers and are controlled by more extensive mechanical interfaces. Faults are important factor in fracture heterogeneity, with fracture intensity being highest near fault cores, especially at fault tips, overlaps, intersections, and the hinges of fault-associated folds. The number of fractures decreases as the distance from the fault zone increases.