Abstract:

The transitional zone at the south margin of Ordos Basin is structurally complicated and unique in terms of reservoiring conditions with its well-developed faults and fractures serving as better channels for hydrocarbon seepage and providing more space for hydrocarbon accumulation in tight and low permeability layers.Based on the determination of regional structural stress and fault-fracture evolving characteristics in the zone, technologies for identifying and characterizing these faults and fractures are developed through a combination of core, drilling and logging data with 3D seismic analysis of fine coherence, amplitude change, curvature attributes and etc.Considering the fact that fractured reservoirs are widely distributed in the zone, the study proposes the concept of fault-fracture bodies to describe the reservoirs.The fault-fracture bodies are defined as composite reservoirs composed of faults, associated brittle fractured belts and low-permeability sandstone layers modified by the belts.They are usually capped or flanked by impermeable argillaceous rocks or tight layers.The discovery of several oil reservoirs in the bodies of the zone has already verified their exploration value.The identification and description of the bodies indicates that the distribution of large-to-medium-scale fracture assemblages determine the skeletal morphology of the bodies, and the superimposition of small-to-micro-scale fracture and sand body boundaries determines the ranges of the bodies.Meanwhile, a set of identification and description procedures is formed to characterize the geology of the bodies.Practices have proven that the procedures are of guiding value to the exploration and development of low abundance and low permeability reservoirs in the zone.

Key words: fault-fracture body, identification method, tight oil reservoir, transitional zone at the south margin, Ordos Basin