Geological modeling of sandstone fractured reservoirs constrained by outcrop geological knowledge: a case study from the Yanchang formation reservoir in Jinghe oilfield, Ordos Basin

Abstract: [Objective] The internal structure of tight sandstone fault-fracture reservoirs is complex. However, due to the limited number of well data and the resolution constraints of seismic data, there is a lack of constraints for building high-precision 3D geological models of these reservoirs, which hinders sweet spot prediction and development planning.[Methods] This paper proposes a three-level modeling framework for fault-fracture reservoirs, focusing on their outline, internal structural zones, and internal attributes. The reservoir outline is constrained by integrating outcrop-based statistics of fault-fracture dimensions with 3D geological attributes. For modeling internal structural zones, a fluctuating decreasing function of fracture density is introduced, combined with density thresholds for different zones, to develop a 3D geological model of the internal structures. The internal fracture model is built using the discrete fracture network (DFN) method, based on statistical laws of fracture parameters derived from outcrops and imaging logs. In terms of matrix reservoir property modeling, the enhancing effect of fracture development on matrix properties is taken into account.[Results] The results show that: (1) The width of fault-fracture reservoirs in the Yanchang Formation of the southern Ordos Basin generally ranges between 80 and 160 m, and the width exhibits a log-linear relationship with fault displacement. (2) The internal part of fault-fracture reservoirs can be divided into a fractured zone, a fracture zone, and a matrix zone. The fractured zone typically extends 5-20 m, while the fracture zone generally spans 15-50 m. The fracture density within the reservoir follows a fluctuating decreasing function with increasing distance from the fault. (3) Fracture parameters of different internal structural units are consistent with statistics from outcrops and imaging logs. The increase in matrix porosity in fracture-developed zones is proportional to fracture density. [Conclusion] This study proposes a multi-source data integration modeling method constrained by an outcrop-based geological knowledge database. It addresses the challenge of high-precision 3D geological modeling of tight sandstone fault-fracture reservoirs under conditions of limited well data and insufficient seismic resolution. The method provides technical support for the exploration and development of such reservoirs.